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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenSat, 13 Apr 2024 01:50:57 +0000Observations of Perfect Potential Flow and Critical Velocities in Superfluid Helium ll
https://resolver.caltech.edu/CaltechETD:etd-02022006-081545
Authors: {'items': [{'id': 'Craig-Paul-Palmer', 'name': {'family': 'Craig', 'given': 'Paul Palmer'}, 'show_email': 'NO'}]}
Year: 1959
DOI: 10.7907/AA2G-EP61
The lift on an airfoil placed in a velocity field of pure superfluid has been found to vanish (i.e., to be at least two orders of magnitude less than the classically expected value) for sufficiently low flow velocities. This result indicates that superfluid helium II can undergo pure potential flow without dissipation. The classical viscosity boundary condition at the trailing edge (Kuta condition) does not apply. Above a (temperature dependent) critical velocity, lift appears. However, even at the highest velocities obtained the observed lift lies far below that expected classically. The critical velocity found from this experiment rises near the lambda point in qualitative agreement with comparable experiments, but fails to show agreement in certain other respects. A "superfluid wind tunnel" capable of developing the required flow is described.https://thesis.library.caltech.edu/id/eprint/455Velocity measurement with a new probe in inhomogeneous turbulent jets
https://resolver.caltech.edu/CaltechETD:etd-03312009-110134
Authors: {'items': [{'id': 'Tombach-I-H', 'name': {'family': 'Tombach', 'given': 'Ivar Harald'}, 'show_email': 'NO'}]}
Year: 1969
DOI: 10.7907/V2CR-0Q19
Velocity profiles have been measured in subsonic, inhomogeneous, axisymmetric turbulent jets, using a new velocity measuring probe. This probe creates a train of heat pulses at one point in the flow and measures the time interval between the time a pulse is created and the time at which it is convected by the flow past a sensing wire a short distance downstream. In a turbulent flow the detected pulses are highly disturbed, but reconstruction of a mean pulse, by a digital computer, from several hundred pulses enabled calculation of a mean velocity and a velocity fluctuation level.
Measurements were made with this probe from near the nozzle to 48 diameters downstream in several combinations of jet gas and ambient gas (air-air, air-He, He-air, and He-SF[subfield 6]) having a range of jet/ambient density ratios from 0.03 to 7.25. Axial profiles of the variation with the density ratio of the jet width, mean velocity, and turbulence level were obtained from these measurements, showing the entrainment and approach to homogeneity of the inhomogeneous jet. A jet which is less dense than the ambient fluid is seen to entrain the ambient fluid more vigorously than the denser jet, as is demonstrated by its higher turbulence levels, more rapid growth, and more rapid axial decay of the mean velocity.
https://thesis.library.caltech.edu/id/eprint/1231Freezing phycomyces sporangiophores in superfluid helium for ultrastructure studies
https://resolver.caltech.edu/CaltechETD:etd-08082006-142417
Authors: {'items': [{'email': 'burke@life.illinois.edu', 'id': 'Burke-Patricia-Virginia', 'name': {'family': 'Burke', 'given': 'Patricia Virginia'}, 'show_email': 'YES'}]}
Year: 1971
DOI: 10.7907/PY8S-BH09
<p>Knowledge of the ultrastructure of the growing zone of Phycomyces sporangiophores is a desirable adjunct to studies of their sensory physiology. This includes discovery of the type of organelles present and their spatial arrangement. Chemical fixation preserves some of the cell's organelles satisfactorily, but considerable disruption and dislocation occur. Physical fixation by freezing should preserve the spatial distribution of organelles within the cell if the freezing is rapid enough to prevent ice crystal formation. Ice crystal inhibitors do not penetrate the cell satisfactorily, so that crystal size is determined only by the freezing rate.</p>
<p>Liquid He II exhibits a quantum mechanical mechanism of heat transfer which is much more efficient than the normal, classical heat conduction in fluid. The heat transfer rate is qualitatively described by the Landau equations for liquid He II with additional terms (Gorter-Mellink mutual friction) to account for frictional forces within the liquid. For the case of a cylindrical heater, the maximum steady state heat transfer rate before film boiling occurs depends on the depth of the heater below the liquid surface (hydrostatic head), the pressure of the gas above the liquid, and the liquid temperature. For a gas pressure of 20 Torr or more in excess of the equilibrium vapor pressure, the appearance of the film boiling changes from a uniform gas film to a fine haze, presumably of tiny gas bubbles. If He<sup>4</sup> gas is used to pressurize the liquid, the temperature of the liquid He bath rises to about 2 K and a layer of liquid He I forms at the gas-liquid interface. This layer of the He I grows at the expense of the bulk He II as heat is supplied by the warm gas. The phase boundary between the two liquid phases moves down through the liquid at about 5 cm/min. The conditions for optimal heat transfer occur immediately after the pressure increase. These conditions are a pressure excess of 20 Torr or more and a bath temperature of 1.9 - 2 K.</p>
<p>Sporangiophores are suspended by an iron filing from an electromagnet in a special chamber (room temperature) above a helium cryostat. As soon as the pressure excess in the cryostat exceeds 20-60 Torr they are released and fall freely into the superfluid helium through a tube of heated gas. The sporangiophores are collected in a plastic beaker at the bottom of the cryostat and transferred to a liquid nitrogen storage dewar. Frozen sporangiophores are prepared for electron microscopy using freeze-substitution and freeze-fracture techniques.</p>
<p>The thin sections and the freeze-fracture replicas show extensive ice crystal damage to stage IV sporangiophores which have a large central vacuole. Ice crystal damage is considerably less in stage I sporangiophores which have a much smaller vacuole. Presumably, the vacuole is responsible for the poor preservation of the ultra-structure.</p>
https://thesis.library.caltech.edu/id/eprint/3050Determination of the interaction potential of the noble gases from shock wave structure experiments. Feasibility of a modified electron beam densitometer technique to measure diffusive separation in shock waves in helium-argon mixtures
https://resolver.caltech.edu/CaltechETD:etd-04022009-092506
Authors: {'items': [{'id': 'Barcelo-B-T', 'name': {'family': 'Barcelo', 'given': 'Brian Thomas'}, 'show_email': 'NO'}]}
Year: 1971
DOI: 10.7907/QH9Z-BH10
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Shock wave experiments were conducted in the GALCIT* seventeen-inch low density shock tube to measure the interaction potential of the following nonradioactive noble gases: neon, argon, krypton, and xenon. The experimental shock profiles obtained by employing the electron beam densitometer technique were compared to the Monte Carlo numerical simulation of the shock wave structure. The comparison determined the intermolecular potential for these gases, a potential assumed to be of the form [...] = const/r[superscript s]. The values resulting for the free parameter s in the inverse repulsive power law were 9, 10, 11, and 12 for xenon, krypton, argon, and neon, respectively.
In a second phase of the experiments, the feasibility of a modified electron beam densitometer technique was investigated for measuring the shock wave structure in a binary mixture of helium and argon. It was desired to obtain both the argon and helium density profiles through the shock wave by varying the electron beam energy in two experiments of identical shock conditions (gas concentrations, Mach number, initial pressure, etc.). Theoretical calculations of the collision cross-section indicated only a slight possibility of separating the density profiles of the two species with the range of electron energy possible in the experimental apparatus (7000 to 15000 volts). Experiments conducted with initial gas concentrations of 10%, 20%, 50%, and 80% argon in shock waves with a Mach number of approximately four confirmed these suspicions but permitted qualitative conclusions in agreement with other similar investigations.
*Graduate Aeronautical Laboratories California Institute of Technology.
https://thesis.library.caltech.edu/id/eprint/1253Electron beam measurements of the shock wave structure. Part I. The inference of intermolecular potentials from shock structure experiments. Part II. The influence of accommodation on reflecting shock waves
https://resolver.caltech.edu/CaltechETD:etd-09042007-094927
Authors: {'items': [{'id': 'Steinhilper-E-A', 'name': {'family': 'Steinhilper', 'given': 'Eric A.'}, 'show_email': 'NO'}]}
Year: 1972
DOI: 10.7907/AWTA-PA12
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
A technique has been developed in which accurate measurements of shock wave structure and an exact molecular theory of shock waves are used to determine intermolecular potentials. Shock wave density profiles in neon, argon, krypton, and xenon are measured in the GALCIT 17-inch diameter shock tube. The theory is a numerical molecular simulation technique (developed by G. Bird of the University of Sidney) in which the only adjustable parameter is the intermolecular potential. Parameters for the exp-6 and Lennard-Jones potentials are determined by matching the experimental shock wave density profiles with those predicted by the Monte Carlo simulation technique. The experimental data are taken at shock Mach number of about 8; consequently, these results fall in an energy range midway between the molecular beam measurements and low temperature transport property results.
After the potentials for neon, argon, krypton, and xenon have been determined, they are tested for conformity to the Law of Corresponding States. Plots of the potentials in corresponding states coordinates, [...] vs.[...], show that the exp-6 potential model issuperior to the Lennard-Jones. This is an important result, because for the first time this statement can be made on the basis of one set of measurements. Previously it had been necessary to adduce molecular beam results in order toprove that the inverse twelfth-power repulsive part of the Lennard-Jones potential is too strong. Comparisons show that the exp-6, Lennard-Jones, simple repulsive, and hard sphere molecular potentials predict the experimental shock structure with successively decreasing accuracy. However, their accuracy is sufficient that any one of the potentials would predict any flow accurately enough to give an indication of the relative importance of the parameters governing the flow. This point is emphasized by the need for both the most precise experimental measurements and the use of the Law of Corresponding States in order to provide the basis for ranking the potentials. Moreover, changing the potentials has given a better understanding of the mechanisms by which intermolecular forces influence shock structure.
Measurements of density profiles during the reflection of thick shock waves in argon from the end wall of the GALCIT 17-inch diameter shock tube were reported previously. A mass balance using these profiles had revealed that as much as 20% of the gas which should have been between the end wall and the reflected shock was simply not present. Comparison with theory was not possible because no theory incorporated a loss of mass. Currently available theories for the reflection process include a Monte Carlo flow simulation technique for a thermally accommodating wall.
It is found that this technique can correctly predict either the reflected shock trajectory or the thermal layer near the wall, but the inability to duplicate both implies that there is a second important effect which we assume to be adsorption.
Additional experiments are conducted in neon which has a lower thermal accommodation coefficient than argon. If thermal accommodation is the only wall boundary condition, then according to the Monte Carlo calculations the shock should reflect faster in neon, and the thermal layer should be thinner. However, the measured density profiles show that the reflected shock trajectory is nearly the same as in argon but that there is only half as much "missing" mass. Thus, the neon results provide the most significant confirmation of adsorption.
Because this unexpected violation of the continuity equation was observed, a comprehensive review of instrumental effects and the data reduction technique is made. Several hypothetical effects are shown to have no influence on the loss of mass. However, improving the mass balance calculations accounts for approximately 25% of the missing mass. Correcting for multiple scattering of the electron beam accounts for another 10%, but this correction applies only at the highest densities. Therefore, the "missing" mass of the previous experiment is verified but is reduced somewhat in magnitude.https://thesis.library.caltech.edu/id/eprint/3324Part I. Investigation of Strong Shock Waves in a Conical Convergent Channel. Part II. Spectroscopic Investigation of Strong Shockwaves in a Conical, Convergent Channel
https://resolver.caltech.edu/CaltechETD:etd-09042007-104952
Authors: {'items': [{'id': 'Storm-Erik', 'name': {'family': 'Storm', 'given': 'Erik'}, 'show_email': 'NO'}]}
Year: 1973
DOI: 10.7907/TJFS-6964
<p>ABSTRACT OF PART I:</p>
<p>The behavior of an initially plane, strong shock wave propagating into a conical convergence is investigated experimentally and theoretically. In the experiment a 10° half-angle cone is mounted on the end of a pressure-driven shock tube. Shock waves with initial Mach numbers varying from 6.0 to 10.2 are generated in argon at a pressure of 1.5 torr. During each run local shock velocities at several positions along the cone axis are measured using a thin, multi-crystal piezoelectric probe inserted from the vertex. This technique produces accurate velocity data for both the incident and reflected shock waves. In the corresponding analysis, a simplified characteristics method is used to obtain an approximate solution of the axisymmetric diffraction equations derived by Whitham (1959).</p>
<p>Both the shock velocity measurements and the axisymmetric diffraction solution confirm that the incident shock behavior is dominated by cyclic diffraction processes which originate at the entrance of the cone. Each diffraction cycle is characterized by Mach reflection on the cone wall followed by Mach reflection on the axis. These cycles evidently persist until the shock reaches the cone vertex, where the measured velocity has increased by as much as a factor of three. Real-gas effects, enhanced in the experiment by increasing the initial Mach number and decreasing the pressure, apparently alter the shock wave behavior only in the region near the vertex. Velocity measurements for the reflected shock within the cone show that the shock velocity is nearly constant throughout most of the convergence length.</p>
<p>ABSTRACT OF PART II:</p>
<p>The thermodynamic conditions behind the incident and reflected shock wave close to the vertex of a convergent channel are investigated spectroscopically. The investigation was initiated in order to better determine the possible uses of such a geometrical device as a tool for high temperature plasma research. Using argon at an initial pressure of 1.5 torr, the shock Mach number prior to the entrance of the cone is 10.2. Two windows are mounted at x/1 = 0.9 in the cone, where the Mach number has increased to 24, and the emitted radiation is monitored for both time-resolved (Monochromator - Photomultiplier) and time-integrated (Spectrograph) analysis. The relative line intensity method is used to measure the electron temperature. The Stark broadened profile of the 6965.4 Å neutral argon line, and continuum intensity measurements are used to determine the electron number density.</p>
<p>From initial values of 13200°K and 1.4 x 10¹⁷ cm⁻³, both the electron temperature and number density profiles behind the incident shock are dominated by the previous shock diffraction processes. The general trend is a gradual increase, presumably due to the continuing compression of the gas shocked at successively earlier times. Superimposed upon this is the effect of hot slugs of gas from previous localized regions of very high Mach number. The reflected shock wave heats and compresses the gas even further. The subsequent expansion results in a series of rapid exponential decreases in temperature, density and pressure. Immediately after the reflected shock wave has passed, the gas appears to be in a nonequilibrium state with a population inversion among the upper excited atomic energy levels. There is an indication of the presence of a second reflected wave. The effects of self-absorption on Stark broadened lines is studied. An equation is derived, demonstrating the effect of individual corrections that are necessary before accurate interpretations of measured quantities can be made. Simple self-absorption correction schemes are demonstrated and shown to be self-consistent.</p>https://thesis.library.caltech.edu/id/eprint/3325Flow induced vibration of bluff structures
https://resolver.caltech.edu/CaltechETD:etd-09302005-082356
Authors: {'items': [{'id': 'Blevins-R-D', 'name': {'family': 'Blevins', 'given': 'Robert D.'}, 'show_email': 'NO'}]}
Year: 1974
DOI: 10.7907/F3D4-5Q27
Models are developed for both multi-degree-of-freedom aerodynamic galloping and vortex induced oscillation of bluff structures. These models are useful in the analysis of elastic structures exposed to a steady fluid flow.
An asymptotic method, based on the approximation of Bogoliubov and Mitropolsky, is developed for the analysis of the autonomous, internally resonant, nonlinear differential equations produced by the models. It is shown that the solutions of these systems can be divided into two classes by the nature of the secular terms arising in the perturbation equations.
A model for multi-degree-of-freedom galloping is developed by modeling the aerodynamic forces on the structure as dependent only on the relative magnitude and velocity of the flow to the structure. A simple criterion for the stability of the zero solution is presented. Examples are made with a noninertially coupled system with the torsion and plunge degrees-of-freedom and a cubic curve fit to the aerodynamic coefficients. Examples show that the system is dominated by either a torsion or a plunge mode except when the natural frequencies of these modes are in certain integer multiples. In these cases the two modes interact strongly and they achieve first order limit cycles simultaneously.
A model for vortex induced vibration of elastic structures is produced from a control volume approach to the vortex shedding process. The model features both fluid and structural oscillators. The model parameters are determined from experimental data by matching the model response to experimental observation for the cases of fixed and forced cylinder motion. A frequency entrainment effect is produced by the model for an elastically mounted cylinder resonating with vortex shedding. The resonant amplitude of an elastically mounted cylinder predicted by the model is in good agreement with experimental data.https://thesis.library.caltech.edu/id/eprint/3835Experiments on thin airfoils spanning a transonic shear flow
https://resolver.caltech.edu/CaltechETD:etd-11292006-134453
Authors: {'items': [{'id': 'Cosner-R-R', 'name': {'family': 'Cosner', 'given': 'Raymond Robert'}, 'show_email': 'NO'}]}
Year: 1976
DOI: 10.7907/YJNQ-ZP43
A wind tunnel was built at the California Institute of Technology to provide two uniform coflowing streams at Mach numbers of 0.6 and l.4 with a plane mixing layer in between. Preliminary studies were made of this free shear layer, indicating a region of self-similar behavior and general agreement in growth rate with previous studies.
A program of experimental and theoretical work was completed in which wedges were installed in the supersonic stream to create shock waves incident on the shear layer. Similar studies were performed with wedges in the subsonic stream. Good agreement was found between the analytic first-order theory and experiment in showing that the upstream propagation of pressure disturbances ahead of their source in the uniform subsonic stream is limited to about 0.6 subsonic layer thicknesses.
The problem of an airfoil in spanwise-varying transonic shear was studied experimentally with the goal of understanding the interactions in the shear region between the supersonic and subsonic streams, especially in terms of deviations from quasi-two-dimensional behavior. The effect of modest angle of attack was also examined.
https://thesis.library.caltech.edu/id/eprint/4685A chemical reaction in a turbulent jet
https://resolver.caltech.edu/CaltechETD:etd-11282006-152216
Authors: {'items': [{'email': 'jshea@ida.org', 'id': 'Shea-J-R', 'name': {'family': 'Shea', 'given': 'John R.'}, 'show_email': 'YES'}]}
Year: 1976
DOI: 10.7907/BJQT-9A50
<p>The turbulent mixing and subsequent chemical reaction of gases is an essential part of many technological processes ranging from gas furnaces to chemical lasers. Surprisingly, there is very little information, either theoretical or experimental, about the actual rate of the chemical reaction in such processes. Generally the chemical kinetics are well understood, but the process of turbulent mixing is not. Many measurements of mixing in turbulent jets have been made in the past, but they have generally failed to distinguish essentially unmixed gas in the turbulent mixing zone from gas which is mixed on a molecular scale. Knowledge of where turbulent fluid is mixed on a molecular scale is critical for predicting chemical reaction rates in the flow.</p>
<p>In this experiment the rate of a chemical reaction in an axisymmetric turbulent jet is studied, and the results are used to determine the rate of molecular mixing in the jet. A turbulent jet containing dilute ozone in an inert mixture of nitrogen and oxygen flows into a stagnant tank of nitric oxide and nitrogen. When the gases mix on a molecular scale, the ozone and nitric oxide rapidly react to produce oxygen and nitrogen dioxide. The rate at which the mixing and chemical reaction proceeds is determined by using an ultraviolet light absorption technique to measure the time averaged ozone concentration at points throughout the jets mixing zone.</p>
<p>The experiment establishes a criterion for determining when a reaction of known chemical kinetics is sufficiently rapid that chemical nonequilibrium has a negligible effect on the mean reactant profile. When a reacting jet satisfies this criterion for equilibrium chemistry, the reactant profiles are found to be independent of jet Reynolds numbers from 4,000 to 32,000 based on the nozzle diameter.</p>
<p>In addition, a mixing fraction, η, is defined to measure the extent of local molecular scale mixing independently of a chemical reaction occurring in the jet. The fraction assumes values of unity in the unmixed primary jet, zero in unmixed ambient fluid, and intermediate fractions for mixtures of all proportions. Points on nonreacting jet profiles are related to time averages of η. A limiting highly reacting ozone profile, found when a large excess of nitric oxide is present in the ambient fluid, is related to the time average of an intermittency function, J(η), defined equal to unity when η is within a specified neighborhood of one and zero elsewhere. Thus the experimental measurements of ozone profiles are directly related to the statistics of molecular scale mixing in the jet.</p>https://thesis.library.caltech.edu/id/eprint/4675Surface roughness effects on the hypersonic turbulent boundary layer
https://resolver.caltech.edu/CaltechETD:etd-11142006-154140
Authors: {'items': [{'id': 'Berg-D-E', 'name': {'family': 'Berg', 'given': 'Dale Evan'}, 'show_email': 'NO'}]}
Year: 1977
DOI: 10.7907/2S44-JV38
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
An experimental investigation of the response of a hypersonic turbulent boundary layer to a step change in surface roughness has been performed. The boundary layer on a flat nozzle wall of a Mach 6 wind tunnel was subjected to abrupt changes in surface roughness and its adjustment to the new surface conditions was examined. Both mean and fluctuating flow properties were acquired for smooth-to-rough and rough-to-smooth surface configurations.
The boundary layer was found to respond gradually and to attain new equilibrium profiles, for both the mean and the fluctuating properties, some 10 to 25 [...] downstream of the step change. Mean flow self-similarity was the first to establish itself, followed by the mass flux fluctuations, followed in turn by the total temperature fluctuations.
Use of a modified Van Driest transformation resulted in good correlation of smooth and rough wall data in the form of the incompressible law of the wall. This is true even in the nonequilibrium vicinity of the step for small roughness heights.
The present data are found to correlate well with previously published roughness effect data from low and high speed flows when the roughnesses are characterized by an equivalent sand grain roughness height.
Existing correlations based on low speed data were found to be unsuccessful in predicting the effect of this roughness on the skin friction and velocity profile. The indiscriminate use of low speed roughness effects correlations to predict the effects of roughness on supersonic and hypersonic flows must therefore be regarded as a procedure subject to gross errors.
Significant pressure and temperature history effects were observed throughout the boundary layer. The existence of these effects was found to create a nozzle wall boundary layer whose properties were far different than those in a boundary layer on a flat plate in the freestream, raising questions about the validity of simulating the flat plate boundary layer with the nozzle wall boundary layer.https://thesis.library.caltech.edu/id/eprint/4555Weak shocks in open-ended ducts with complex geometry
https://resolver.caltech.edu/CaltechETD:etd-11302006-133224
Authors: {'items': [{'id': 'Craig-J-E', 'name': {'family': 'Craig', 'given': 'James Eldon'}, 'show_email': 'NO'}]}
Year: 1977
DOI: 10.7907/QAFR-PS29
The dynamics of weak shocks in ducts of complex geometry and the sound radiation produced by the reflection of a weak shock from the open end of a duct have been investigated. Duct geometries include expansion chambers with and without inlet or outlet tubes extended and enclosed perforated tubes. Internal and external pressure histories of the interaction of weak shocks with simple muffler elements have been recorded using a standard one-shot shock tube and a resonating shock tube. The excitation shock Mach number ranged from 1.05 to 1.55. Analytical investigations, including a synthesis of existing works on internal weak-shock interactions of an acoustic treatment of the sound radiation produced by weak shock waves, are presented. Combining the above analyses, models for the reduction in radiated sound per unit of incident shock amplitude, as a result of inserting a muffler between the source and the tailpipe exit, are developed.
For expansion chambers with and without extensions, the dependence of the transmitted and reflected waves and of the radiated sound on area ratio is compared with predictions. In particular, measured transmission coefficients for expansion chambers agree reasonably well with the predictions for all shock strengths; however, for large area ratios, the predicted sound attenuation is not observed, as waves diffracted at the upstream junction cause more sound to be radiated. For expansion chambers with internal extensions, sound attenuation is increased for low incident shock strengths; while for increasing incident shock strength, the internal transmission characteristics deteriorate, the reducing the sound attenuation.
For enclosed perforated tubes, the dependence of the transmitted and reflected waves and of the radiated sound on the perforated area ratio and incident shock strength is compared with predictions. For perforated tubes with infinite enclosure, the transmission and reflection coefficients depend on both incident shock strength and perforated area ratio, as predicted. However, agreement with data is obtained only after inserting a perforated discharge coefficient with the perforated area ratio in the theory. The reduction of sound radiation with perforated area ratio is measured for one incident shock strength and then compared with predictions. For small area ratios, there is agreement but for large area ratios the measurements show that less sound is radiated than predicted. For large area ratios, gradual compressions with smooth fronts (not shock fronts) are transmitted, resulting in less radiated sound. Enclosures have no effect on the sound attenuation for small perforate area ratios; however, as the perforate area ratio increases, the enclosure eventually inhibits further increase in sound attenuation.
https://thesis.library.caltech.edu/id/eprint/4694Experimental investigation on axisymmetric turbulent wakes with zero momentum defect
https://resolver.caltech.edu/CaltechETD:etd-11282006-111227
Authors: {'items': [{'id': 'Higuchi-H', 'name': {'family': 'Higuchi', 'given': 'Hiroshi'}, 'show_email': 'NO'}]}
Year: 1977
DOI: 10.7907/121B-VN50
An experimental investigation of a turbulent axisymmetric wake with zero momentum defect was carried out. The experiment was conducted in a low speed wind tunnel with a circular tube mounted parallel to the stream. A controlled amount of air was injected into the stream at the end of the model to cancel the drag produced by the turbulent boundary layer on the model. The measurements on the mean flow and the fluctuation quantities were carried up to 90 diameters downstream. By adjusting the strength of the injections the behavior of the pure wake, the co-flowing jet and the matched injection were examined, and the self similar properties both in the mean velocity and the turbulent intensity were found to exist in these cases. Rapid decays off the centerline velocity and the maximum turbulent intensity were observed in the matched injection case. The effect of the initial condition was studied by artificially thickening the boundary layer on the model, and it was observed that the wake relaxes into the final decay law sooner. The unmatched cases, both strong and weak injections, were also investigated; the strong injection case was found to relax into the self-similar weak jet profile and the weaker injection case was observed to approach to the similar wake profile.
Flow visualization was conducted on the plane of symmetry and the entrainment process by the engulfing large eddies was visualized in the pure wake and in the co-flowing jet. A lack of large rotations was observed in the matched injection case and the lack of production of turbulent energy was attributed to the faster decay of the flow properties.
https://thesis.library.caltech.edu/id/eprint/4674A chemically reacting, turbulent shear layer
https://resolver.caltech.edu/CaltechETD:etd-10272006-111937
Authors: {'items': [{'id': 'Breidenthal-R-E', 'name': {'family': 'Breidenthal', 'given': 'Robert Edward'}, 'show_email': 'NO'}]}
Year: 1979
DOI: 10.7907/hj67-9h91
A chemically reacting turbulent shear layer was investigated in a new, blow-down water tunnel. In a diffusion-limited reaction, a pH indicator, phenolphthalein, in one stream mixed and reacted with a base, sodium hydroxide, in the other stream to form a visible reaction product. Using optical densitometry techniques, the amount of product was measured as a function of Reynolds number, at a relatively high Schmidt number of approximately 600. The results were compared with both the previous mixing measurements of Konrad in a gaseous shear layer (Sc = 0.7) and the simple mixing model of Broadwell.
The product was found to be distributed, as expected, in concentrated lumps associated with the large, spanwise-coherent structures of the turbulence. The time averaged amount of product in the layer exhibited a rapid transition at a large-structure Reynolds number of about 5 x 10(3) for a velocity ratio of 0.38. Above the transition, the amount of product within the layer was independent of Reynolds number.
This transition is related to the introduction of small scale, three-dimensional motions into the layer. In the initial region, where the flow was already unsteady and contained large structures but was strictly two-dimensional, very little mixing was observed. Downstream the flow became unstable to three-dimensional perturbations and small scale, three-dimensional motions were introduced into the layer. Across this transition, the aqueous mixing increased by an order of magnitude, indicating the sensitivity of mixing to small scales of the turbulence in a high Schmidt number fluid. At high Reynolds numbers, changing the Schmidt number by three orders of magnitude only altered the molecular mixing by about a factor of two or less. The mixing model of Broadwell, which addresses the effect of Schmidt number, is in satisfactory qualitative agreement with the observations.
The unique flow visualization of the visible reaction product in water permitted a study of the three-dimensional instability and evolution of small scale motions in the layer. Streamwise streaks which had been previously observed in the Brown-Roshko gas apparatus were found to originate from a spanwise-sinuous wiggle which appeared at a large-structure Reynolds number which varied with velocity ratio, indicating an influence of initial conditions on the instability.
https://thesis.library.caltech.edu/id/eprint/4277An Experimental Study of a Turbulent Vortex Ring
https://resolver.caltech.edu/CaltechETD:etd-10092006-080016
Authors: {'items': [{'email': 'ari.glezer@me.gatech.edu', 'id': 'Glezer-Ari', 'name': {'family': 'Glezer', 'given': 'Ari'}, 'show_email': 'YES'}]}
Year: 1981
DOI: 10.7907/RC4P-DA56
<p>An apparatus has been constructed to study the formation and dynamics of vortex rings. The rings are formed in water by a momentary jet discharge downward into a glass tank from an orifice in a submerged plate. The necessary impulse is provided by a pressurized reservoir and is controlled by a fast programmable solenoid valve.</p>
<p>A particular turbulent vortex ring with a relatively thin core was chosen for detailed study. The velocity field of this ring was measured at numerous locations in the tank, using a two-channel laser-Doppler velocimeter with digital tracking electronics. Beamsplitting and frequency-shifting were accomplished by two partially-overlapping radial phase gratings driven by hysteresis-synchronous motors.</p>
<p>The main aim of the research was to verify the similarity properties of the turbulent vortex ring and to determine mean particle paths and mean vorticity contours in the appropriate nonsteady similarity coordinates x/t<sup>¼</sup>, r/t<sup>¼</sup>. Some novel pattern-recognition techniques were developed to overcome the problem of dispersion in the trajectories of individual vortices.</p>https://thesis.library.caltech.edu/id/eprint/3993The Coherent Structure of Turbulent Mixing Layers. I. Similarity of the Primary Vortex Structure. II. Secondary Streamwise Vortex Structure
https://resolver.caltech.edu/CaltechETD:etd-08262005-154143
Authors: {'items': [{'id': 'Bernal-Luis-Paulino', 'name': {'family': 'Bernal', 'given': 'Luis Paulino'}, 'show_email': 'NO'}]}
Year: 1981
DOI: 10.7907/Z52D-SA34
<p>The primary spanwise organized vortex structure and the secondary streamwise vortex structure of turbulent mixing layers have been investigated. Flow visualization motion pictures of a constant density mixing layer were used to measure the properties of the large scale vortices. It was found that after an initial transition region mean properties of the large scale vortices reach the expected linear growth with downstream distance required by similarity. In the self-similar region, the vortex core area and visual thickness increase continuously during its life-span.</p>
<p>A theoretical model of probability distribution function for the large-scale vortex circulation was developed. This distribution is found to be lognormal and to have a standard deviation, normalized with the mean of 0.28. From this model the mean life-span of the vortices could also be obtained and was found to be 0.67 times the mean life-span position.</p>
<p>The streamwise streak pattern observed by Konrad (1976) and Breidenthal (1978) in plan-view pictures of the mixing layer was investigated, using flow visualization and spanwise concentration measurements. It was confirmed that this pattern is the result of a secondary vortex structure dominated by streamwise, counterrotating vortices. A detailed description of its spatial relation to the primary, spanwise vortex structure is presented. From time average flow pictures, the onset position and initial scale of the secondary structures were determined. From concentration measurements, spanwise variations in mean properties, resulting from the secondary structure, were found. This also showed an increase of the spanwise scale with downstream distance and the existence of the streamwise vortices in the fully developed turbulent region. In this region the mean spacing is found approximately equal to the vorticity thickness.</p>https://thesis.library.caltech.edu/id/eprint/3228A Model for the von Kármán Vortex Street
https://resolver.caltech.edu/CaltechETD:etd-05122005-112041
Authors: {'items': [{'email': 'James.Schatzman@futurelabusa.com', 'id': 'Schatzman-James-Carl', 'name': {'family': 'Schatzman', 'given': 'James Carl'}, 'show_email': 'YES'}]}
Year: 1981
DOI: 10.7907/34YN-H995
<p>In the wake of a two-dimensional bluff body placed in a uniform stream, for sufficiently large but not too large flow velocity a distinctive pattern of vorticity is observed. The pattern consists of "vortices" of high vorticity surrounded by nearly irrotational fluid. These vortices are organized in two nearly parallel staggered rows of vortices of opposite direction of rotation. This pattern is called the von Kármán vortex street.</p>
<p>This thesis is a report on the analysis of a model for the von Kármán vortex street. The model is inviscid, incompressible, two-dimensional, and consists of vortices of finite area and uniform vorticity. The first part of this thesis contains a brief survey of the work on this problem, and an explanation of the approach used in the present work; the second part describes calculations of steady solutions of the Euler equations of this kind, and the third part describes an analysis of the stability of these steady solutions to two-dimensional disturbances.</p>
<p>The calculations indicate that the vortex wake can be stabilized by sufficiently large area of the vortices. Data are given which (to some approximation) will permit relating the street to the flow past a body; this is proposed as a suitable study for further work.</p>https://thesis.library.caltech.edu/id/eprint/1751Investigations of Transport in Complex Atmospheric Flow Systems. I. Small Scale Studies of Diffusion through Porous Media, Impact of Fumehood Exhaust Reentry on Indoor Air Quality, and Pollutant Transport Near an Isolated Island. II. Pollutant Transport in Mountain-Valley and Coastal Regions of California
https://resolver.caltech.edu/CaltechETD:etd-12212006-103605
Authors: {'items': [{'email': 'reible@mail.utexas.edu', 'id': 'Reible-Danny-David', 'name': {'family': 'Reible', 'given': 'Danny David'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/X6WE-JD66
<p>This thesis details some applications of tracer techniques from laboratory scale studies of diffusion in porous media to the analysis of the transport and dispersion of pollutants in the mountain-valley and coastal environments that form the majority of the state of California.</p>
<p>Chapter 1 describes a technique for estimating gaseous diffusivities in porous media that is based on the general solution to Fick's second law for diffusion in a tube between two well-mixed volumes. In beds of essentially non-porous particles, the ratio of the measured effective diffusivity to the air diffusivity of a gas was found to be proportional to the bed porosity raised to the 1.43 power, a result in agreement with previous studies on similar materials. High moisture content (>15-20% moisture in sand) was found to significantly reduce the gas diffusivity with respect to that found in dry materials.</p>
<p>Chapter 2 indicates the importance of ventilation system imbalance upon the reentrainment of pollutants exhausted from a building. Tracer was released from a fumehood in a "clean" room at the Jet Propulsion Laboratory. Indoor concentrations as high as 235 PPB/gr-mole tracer released/hr were observed due to infiltration of the exhausted tracer. This concentration is about an order of magnitude higher than has been observed in buildings with more balanced ventilation systems. Predictions of single and multi-compartment stirred-tank models were compared to the dynamics of the tracer infiltration. A simple one-compartment model provided a better description of the infiltration dynamics than a three-compartment model suggested by the design of the ventilation system.</p>
<p>Chapter 3 describes a series of atmospheric tracer studies of the transport and dispersion of pollutants over the ocean and near an isolated island cape. The experiments were designed to determine the impact of local sources on a background air quality sampling program. The horizontal dispersion of the tracer over the ocean surface could be approximated by the Gaussian plume model assuming a neutrally stable atmosphere, in general agreement with the expected atmospheric stability. Tracer releases from the surface of the isolated cape indicated that an essentially well-mixed separated zone existed above and downwind of the cape. The height of this zone extended to 35-40% above the height of the cape, about the same height as the wake downwind of an isolated building. Limited mixing between the separated zone and the freestream resulted in a sharp concentration gradient above this height.</p>
<p>Chapter 5 indicates the difficulties of describing the behavior of pollutants in complex terrain. A series of tracer experiments conducted in the northern and central California Coastal Mountains are described. The Gaussian plume model could be used to describe the dispersion of the tracer during strong, unidirectional winds. During an elevated tracer release, however, wind directional shear with altitude led to plume bifurcation, with the majority being transported through a stable nighttime drainage layer to ground level. The transport through the stable layer occurred at a vertical velocity of about 2 cm/s, surprisingly rapid transport between stably stratified layers of the atmosphere.</p>
<p>Chapter 6 describes the uncertainties associated with mass balance and Gaussian parameter estimates from tracer data. The uncertainty in the calculated final result can be less than the errors (assumed random) associated with any individual experimental measurement, indicating that such calculations can be made with greater accuracy than would initially be expected.</p>
<p>Chapter 7 details the transport of pollutants in the San Joaquin Valley during stable wintertime conditions. The relatively limited net ventilation of the valley indicates that pollutants can remain within the valley for several days subsequent to their release. During one tracer experiment, about 50% of the released tracer was observed to be well-mixed within the southern valley about 72 hours after the beginning of the release. The most significant ventilation mechanism for the valley during the winter was the occasional passage of low pressure frontal systems. Long periods without frontal system passage can lead to significant pollutant buildup.</p>
<p>Chapter 8 describes the transport of pollutants in the San Joaquin Valley during summertime conditions. While much more effectively ventilated than during the winter, the increased solar insolation leads to significant ozone levels within the valley. A strong influx of air at the northern mouth of the valley is balanced during the day by a corresponding efflux at its southern end and by daytime upslope flow on the Sierra Nevada Mountains. At night, an eddy forms in the southern valley due to low level stabilization and terrain blockage of the afternoon efflux over the southern boundary of the valley. This eddy grows as more air is entrained from the influx at the northern mouth of the valley. An accelerated layer of air aloft also develops during the night due to surface layer stabilization and decoupling. These dynamic flow structures are significant factors in the transport and dispersion of pollutants in the valley during the summer.</p>
<p>Chapter 9 details the impact of the San Joaquin Valley on the northern Mojave Desert. The transport of pollutants from the southern valley was linked through both tracer and aerosol data to the rapid nighttime reduction in visibility in the northern Mojave Desert. Unlike winter conditions, most of the pollutants in the southern valley were transported out of the valley within a day after their release.</p>
<p>Chapter 10 describes the impact on the Sierra Nevada Mountains of pollutant sources within the San Joaquin Valley. Tracer released within the valley was efficiently transported upslope, impacting National Park and Forest areas. The maximum concentrations observed upslope could be approximated with the Gaussian plume model, assuming very unstable atmospheric conditions. Nighttime stabilization arrested the upslope movement of the tracer and led to slope and valley impacts throughout the night. The limited nighttime ventilation of the slopes may result in the significant ozone concentrations typically observed at slope sites throughout the night.</p>
<p>Chapter 11 describes the transport characteristics of the Sacramento Valley, the northern half of the California Central Valley. Tracer experiments indicated that San Francisco Bay area pollutants have only a small effect on the air quality in the Sacramento Valley. A midday flow divergence over Sacramento resulted in tracer impacts in both the northern part of the valley and the slopes northeast of the city. A counterclockwise eddy that forms in the southern valley during the morning was a potential mechanism for recirculating aged pollutants within the valley. During one tracer experiment, most of the released tracer was trapped within an elevated layer of air, a potentially important mechanism for multi-day impacts of pollutants.</p>
<p>Chapter 12 evaluates the transport of pollutants in the Santa Barbara Channel off the coast of southern California. Limited vertical mixing combined with diurnal wind reversals resulted in multi-day onshore impacts of the tracer released offshore. Efficient lateral mixing of the tracer during wind reversals led to a widespread coastal impact from a single point source. The existence of many point sources could result in a diluted background concentration (i.e. after wind reversals) that equals or exceeds the concentration directly downwind of a single source.</p>
<p>Chapter 13 develops a two layer model of the atmosphere that semi-quantitatively incorporates much of the basic transport structure observed in the above studies. The method of characteristics and the method of moments were used to examine the implications of the model. The model indicates that the air aloft must be considered in order to accurately predict the impact of a pollutant source, especially when considering the multi-day or long range impact of the source.</p>https://thesis.library.caltech.edu/id/eprint/5109Shock Dynamics in Non-Uniform Media
https://resolver.caltech.edu/CaltechETD:etd-09132006-143337
Authors: {'items': [{'id': 'Catherasoo-Christopher-Jeyaparan', 'name': {'family': 'Catherasoo', 'given': 'Christopher Jeyaparan'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/7knb-5s50
<p>The theory of shock dynamics in two dimensions is reformulated to treat shock propagation in a non-uniform medium. The analysis yields a system of hyperbolic equations with source terms representing the generation of disturbances on the shock wave as it propagates into the fluid non-uniformities. The theory is applied to problems involving the refraction of a plane shock wave at a free plane gaseous interface. The "slow-fast" interface is investigated in detail, while the "fast-slow" interface is treated only briefly. Intrinsic to the theory is a relationship analogous to Snell's law of refraction at an interface. The theory predicts both regular and irregular (Mach) refraction, and a criterion is developed for the transition from one to the other. Quantitative results for several different shock strengths, angles of incidence and sound speed ratios are presented. An analogy between shock refraction and the motion of a force field in unsteady one-dimensional gasdynamics is pointed out. Also discussed is the limiting case for a shock front to be continuous at the interface. Comparison of results is made with existing experimental data, with transition calculations based on three-shock theory, and with the simple case of normal interaction.</p>https://thesis.library.caltech.edu/id/eprint/3521Cavitation Inception in Separated Flows
https://resolver.caltech.edu/CaltechETD:etd-01252005-084230
Authors: {'items': [{'email': 'katz@jhu.edu', 'id': 'Katz-Joseph', 'name': {'family': 'Katz', 'given': 'Joseph'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/VP9N-K847
<p>The phenomenon of cavitation was studied on four axisymmetric bodies whose boundary layers underwent a laminar separation and subsequent turbulent reattachment. The non-cavitating flow was studied by holography and the Schlieren flow visualization technique. Surface distributions of the mean and the fluctuating pressures were also measured. The conditions for cavitation inception and desinence were determined and several holograms were recorded just prior to and at the onset of cavitation. The population of microbubbles and the nature of the subsequent development of visible cavitation was determined from the reconstructed image.</p>
<p>High rms and peak values of the fluctuating pressure were measured (up to 90 percent of the dynamic head), the negative peaks being larger than the positive ones except for the reattachment zone where large positive peaks existed. The power spectra contained peaks thought to originate within the large eddies of the mixing layer and in one case there were also peaks due to the laminar boundary layer instability waves.</p>
<p>Cavitation inception occurred in the turbulent shear layer downstream of the transition region. When the separation zone was large the inception region was located within the most developed section of the mixing layer but upstream of the reattachment zone. When the separation region was small inception occurred close to the reattachment zone but still detached from the body surface. A comparison between the surface minimum pressure and the cavitation inception indices also indicated that inception could not occur near the surface of the bodies having a large separation region.</p>
<p>The appearance of visible cavities was preceded by the appearance of a cluster of microbubbles only in the cavitation inception region. The nuclei population in the other sections of the flow field remained fairly uniform. This observation supports the assumption that cavitation is initiated from microscopic free stream nuclei. The rate of cavitation events was estimated from the nuclei population and from the dimensions of the separation region. It was shown for one of the bodies that at least one bubble larger than 10 micrometers radius was exposed every second to a pressure peak which was sufficiently large to cause a cavitation event.</p>
https://thesis.library.caltech.edu/id/eprint/331Observations in a Sediment-Laden Flow by Use of Laser-Doppler Velocimetry
https://resolver.caltech.edu/CaltechETD:etd-01192007-155029
Authors: {'items': [{'email': 'vaningen@microsoft.com', 'id': 'van-Ingen-Catharine', 'name': {'family': 'van Ingen', 'given': 'Catharine'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/X24M-0R50
<p>The laser-Doppler velocimetry technique was adapted for use in sediment-laden flows. The developed instrumentation was used to make one-dimensional, instantaneous measurements of both fluid and sediment grain velocities throughout the water column in such a flow. The velocimetry results were obtained in a steady, uniform flow over a natural sediment bed in the high-transport, flat bed regime.</p>
<p>Laser-Doppler velocimetry is particularly attractive for use in sediment-laden flows as no calibration is required and no probe is introduced into the flow field. Measurements of the fluid velocity and the occurrence and velocity of individual sediment grains are possible with the instrumentation developed in this study. The major difficulties encountered are the possible conditional sampling, hence possible biasing, of the fluid velocity data and the failure of the instrumentation to record or resolve individual sediment grains at higher sediment transport rates. The instrumentation employed in this study is still in the developmental stages and suggestions for its improvement are given.</p>
<p>Despite the difficulties encountered, the data obtained in this study give some insights into the mechanics of suspension and entrainment of sediment during transport by water. The longitudinal turbulence intensity does not seem to be significantly affected by the presence of suspended sediment; the turbulence intensities observed in the sediment-laden flow of this study do not differ greatly from the values reported by previous investigators for clear fluid flows. The mean and standard deviation of the sediment grain velocity were observed to be less than those for the fluid velocity in the lower portion of the flow, but respectively greater near the water surface.</p>
<p>The data demonstrate the shortcomings of the continuum approach to the mechanics of the suspension of sediment. The length (or time) scales of the fluid turbulence are smaller than the length (or time) scale of a set of sediment grains required to define suspended sediment concentration. Near the water surface, where the velocimeter acts as a grain counter, the probability density functions of the sediment grain inter-arrival times, the time between the detection of successive sediment grains, were observed to be negative exponentials. The transport of individual sediment grains might be modeled as a Poisson process.</p>
<p>This work is the foundation of an ongoing experimental program of direct measurements of the fine-scale, time-fluctuating characteristics of sediment-laden flows. This study developed and implemented instrumentation capable of making such measurements and established a conceptual framework for the subsequent interpretation of the data obtained. Two-dimensional measurements, with improved instrumentation, will give additional insights into the mechanics of sediment transport.</p>https://thesis.library.caltech.edu/id/eprint/243Acoustic Transmission Imaging for Flow Diagnostics
https://resolver.caltech.edu/CaltechETD:etd-10302003-145444
Authors: {'items': [{'id': 'Trebitz-Bernd-Otto', 'name': {'family': 'Trebitz', 'given': 'Bernd Otto'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/pnsp-rk30
<p>Wave propagation through a given flow field can be utilized to obtain information about the flow. Acoustic waves in particular offer the possibility of measuring velocity fields, because sound waves are convected with the flow, and temperature fields, because the acoustic index of refraction is a strong function of temperature. This work concentrates on situations where the first effect is dominating compared to the latter one.</p>
<p>A sound system capable of "illuminating" a flow field with ultrasonic waves and measuring the amplitude and phase distribution of the transmitted wave as a function of time was constructed. A large area transmitter and a linear receiving array were used as transducers. Parallel signal processing and interleaved data conversion and acquisition result in a maximum frame rate of 10 kHz.</p>
<p>The feasibility of measuring velocity disturbances with ultrasound was demonstrated by transmitting sound through a vortex, which was generated in a duct by an airfoil swirl generator. Assuming an exponential fit for the tangential velocity component, inner core radius and circulation can be determined directly from the phase change of the transmitted wave due to the vortex. A more accurate representation of the radial velocity profile can be found by digital reconstruction via the Abel inversion formula, which allows reconstruction of rotationally symmetric objects from line projections. Even though the flow field under investigation was steady, this is neither a restriction of the technique, nor of the apparatus. However, the repetition rate for consecutive data frames depends on the operating procedure.</p>https://thesis.library.caltech.edu/id/eprint/4320An Empirical Model for Vortex-Induced Vibrations
https://resolver.caltech.edu/CaltechTHESIS:09272018-115401142
Authors: {'items': [{'id': 'Botelho-Dirceu-Luiz-Rodrigues', 'name': {'family': 'Botelho', 'given': 'Dirceu Luiz Rodrigues'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/T3ZS-SH29
<p>Through an analytical-empirical approach, the vortex-excited transverse oscillations of flexibly-mounted circular cylinders in a uniform flow is studied.</p>
<p>A new model is derived, assuming spanwise constant flow velocity within the sub-critical range of Reynolds numbers and using only experimental data obtained from forced cylinders in water.</p>
<p>The steady-state response of flexibly-mounted cylinders is obtained as a function of the structural system and flow parameters and its stability is analyzed. Several characteristics observed experimentally and also present in the model response are discussed.</p>
<p>The resultant model's capability for predicting structural response for a wide range of fluid mediums is illustrated through comparisons between model predictions and results obtained experimentally from flexibly-mounted cylinders in air and in water.</p>
<p>This model developed is expected to yield better results for structures in water, by virtue of being based only on experimental results obtained in water.</p>https://thesis.library.caltech.edu/id/eprint/11205The Effects of a Vortex Field on Flames with Finite Reaction Rates
https://resolver.caltech.edu/CaltechETD:etd-09112006-111139
Authors: {'items': [{'id': 'Norton-Olin-Perry', 'name': {'family': 'Norton', 'given': 'Olin Perry'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/7vn8-4078
<p>A diffusion flame, supported by a one-step chemical reaction, is initiated along the horizontal axis between a fuel occupying the upper half-plane and an oxidizer below. Simultaneously, a vortex of circulation Γ is established at the origin. As time progresses the flame is extended and "wound up" by the vortex flow field. The effect of distortion of the flame is locally described by the time-dependent straining of a one-dimensional flame. The rate of chemical reaction is represented by the characteristic chemical reaction time, <i>t<sub>ch</sub></i>, of the system. The combustion field then consists of a totally reacted core region and an external flame region consisting of a pair of spiral arms extending off at large radii toward their original positions.</p>
<p>The presence of the vortex increases the rate of fuel consumption of the flame. For large values of Γ/D, the augmentation of fuel consumption due to the vortex is proportional to <i>ρ</i>Γ<sup>2/3</sup><i>D</i><sup>1/3</sup>, and is a function of <i>t</i>/<i>t<sub>ch</sub></i> which approaches a constant value as <i>t</i> → ∞. The growth of the fuel consumption rate from zero to its steady value for large times is governed by the time scale <i>t<sub>ch</sub></i>. If the products of combustion occupy more volume than the original reactants, then the spiral flame will appear as an unsteady volume dilitation for times on the order of the chemical time. An unsteady volume dilitation acts as an acoustic source, so the interaction of a vortex and a diffusion flame is shown to result in the generation of a pressure pulse; the peak pressure occurring after a delay proportional to the chemical time, and the strength of the pulse proportional to Γ<sup>2/3</sup><i>D</i><sup>1/3</sup> and inversely proportional to √<i>t<sub>ch</sub></i>.</p>
<p>These results are valid for hypergolic reaction systems in which the reactant temperature does not significantly effect the rates of the chemical reactions. For systems described as having "large activation energy", the rates are strongly temperature dependent and another description is appropriate. For these systems, a vortex established on an already ignited flame exhibits, in addition to the features described above, an extinct core of unburned reactants if the circulation of the vortex is large.</p>
<p>The results provide the fundamental structure for the mechanism of combustion instability proposed by Rogers and Marble in 1956.</p>https://thesis.library.caltech.edu/id/eprint/3457The Effect of Flow Oscillations on Cavity Drag, and a Technique for their Control
https://resolver.caltech.edu/CaltechETD:etd-10292003-133001
Authors: {'items': [{'id': 'Gharib-Morteza', 'name': {'family': 'Gharib', 'given': 'Morteza'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/JJYA-CK31
<p>The phenomenon of cavity flow oscillation is investigated to determine the conditions for onset of periodic oscillations and to understand the relationship between the state of the shear layer and the cavity drag. Experiments have been performed in a water tunnel using a 4" axisymmetric cavity model instrumented with a strip heater on the nose cone and pressure taps in and around the cavity. A complete set of measurements of oscillation phase, amplitude amplification along the flow direction, distribution of shear stress and other momentum flux is obtained by means of a laser Doppler velocimeter. Drag measurements were made by integrating the mean pressure over the solid surfaces of the cavity. Results indicated exponential cavity drag dependence on the length of the cavity. A jump in the cavity drag coefrcient is observed as the cavity flow shows a bluff body wake type behavior. An independent estimate of the drag, which is obtained by integration of shear and mean momentum transfer terms over the peripheral area of the cavity, confirms the exponential dependence of drag on the length of the cavity. Results, also reveal that the drag of the cavity in the non-oscillating mode is less than the case if the cavity were replaced by a solid surface. Natural and forced oscillations of the cavity shear layer spanning the gap are studied. The forced oscillations are introduced by a sinusoidally heated thin-film strip which excites the Tollmein-Schlichting waves in the boundary layer upstream of the gap, For a sufficiently large gap, self-sustained periodic oscillations are observed while for smaller gaps, which do not oscillate naturally, periodic oscillations can be obtained by external forcing through the strip-heater. In the latter case resonance is observed whenever the forcing frequency satisfies the phase criterion φ/(2π) = N, and its amplitude exceeds certain threshold levels, but the phenomenon is non-self-supporting. The drag of the cavity can be increased by one order of magnitude in the non-oscillating case through external forcing. For naturally occurring oscillations, it is possible for two waves to co-exist in the shear layer (natural and forced). Also, it is possible to completely eliminate mode switching by applying external forcing. For the first time a test is performed to cancel or dampen the amplitude of Kelvin-Helmholtz wave in the cavity shear layer. This is done through introducing an external perturbation with the same frequency of the natural component but having a different phase. Reduction by a factor of 2 is obtained in the amplitude of the oscillation.</p>https://thesis.library.caltech.edu/id/eprint/4298Natural Convection Flows and Associated Heat Transfer Processes in Room Fires
https://resolver.caltech.edu/CaltechETD:etd-02052007-132409
Authors: {'items': [{'id': 'Sargent-William-Stapf', 'name': {'family': 'Sargent', 'given': 'William Stapf'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/n7kr-k165
<p>This report presents the results of experimental investigations of natural convection flows and associated heat transfer processes produced by small fires in rooms with a single door or window opening. Calculation procedures have been developed to model the major aspects of these flows.</p>
<p>Two distinct sets of experiments were undertaken.</p>
<p>First, in a roughly 1/4 scale facility, a slightly dense solution of brine was allowed to flow into a tank of fresh water. The resulting density difference produced a flow which simulated a very small fire in a room with adiabatic walls. Second, in an approximately 1/2 scale test room, a nearly stoichioinetric mixture of air and natural gas was burned at floor level to model moderate strength fires. In this latter facility, we directly measured the heat conducted through the walls, in addition to determining the gas temperature and composition throughout the room.</p>
<p>These two facilities complemented each other. The former offered good flow visualization and allowed us to observe the basic flow phenomena in the absence of heat transfer effects. On the other hand, the latter, which involved relatively larger fires, was a more realistic simulation of an actual room fire, and allowed us to calculate the convective heat transfer to the ceiling and walls. In addition, the stronger sources present in these 1/2 scale tests produced significant secondary flows. These secondary flows along with heat transfer effects act to modify the gas temperature or density profiles within the room from those observed in the 1/4 scale experiments.</p>
<p>Several calculation procedures have been developed, based on the far field properties of plumes when the density differences are small (the Boussinesq approximation). The simple point source plume solution is used along with hydraulic analysis of flow through an orifice to estimate the temperatures of the hot ceiling layer gas and of the cooler floor zone fluid, as well as the height of the interface between them. A finite source plume model is combined with conservation equations across the interface to compute the evolution of the plume above the interface. This calculation then provides the starting point for an integral analysis of the flow and heat transfer in the turbulent ceiling jet.</p>
<p>The computed results both for the average floor and ceiling zone gas temperatures, and for the connective heat transfer in the ceiling jet agreed reasonably well with our experimental data. This agreement suggests that our computational procedures can be applied to answer practical questions, such as whether the connective heat flux from a given fire in a real room would be sufficient to trigger sprinklers or other detection systems in a given amount of time.</p>https://thesis.library.caltech.edu/id/eprint/502Experiments on Mixing and Combustion with Low Heat Release in a Turbulent Shear Flow
https://resolver.caltech.edu/CaltechETD:etd-09142006-144655
Authors: {'items': [{'email': 'mungal@stanford.edu', 'id': 'Mungal-Mark-Godfrey', 'name': {'family': 'Mungal', 'given': 'Mark Godfrey'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/QZ4F-V692
<p>A new blowdown facility to study mixing and combustion in a turbulent shear layer has been built. The system is capable of 100 m/s for three seconds in a 5 x 20 cm exit area on the high speed side, and 50 m/s in a 10 x 20 cm exit area on the low speed side. Dilute concentrations of hydrogen and fluorine, carried in an inert gas, react when both fluid streams meet at the tip of a splitter plate. The reaction is spontaneous, rapid, and highly exothermic. The resulting temperature field has been studied using a rake of eight fast response thermometers placed across the width of the layer. Runs have been performed for low heat release over a wide range of equivalence (concentration) ratios, at a Reynolds number of 30,800 based on velocity difference and vorticity thickness. The heat release is sufficiently low so that the overall properties of the mixing layer are not significantly changed from the cold case.</p>
<p>The results show the presence of large, hot structures within the flow together with cool, irrotational tongues of freestream fluid that penetrate deep into the layer. Thus, it is possible for the entire width of the layer to be quite hot, owing to the passage of a large structure, or for the layer to be quite cool, owing to the presence of the cool fluid tongues. The mean temperature results from a duty cycle whereby a given point in the flow sees alternating hot and cool fluid which averages into the local mean. The mean temperature profiles do not achieve the adiabatic flame temperature at any location across the layer, with the maximum mean temperature, depending upon the equivalence ratio, varying from 54% to 67% of the adiabatic flame temperature. The location of the maximum mean temperature shifts by about 25% of the visual thickness of the layer for a change of equivalence ratio by a factor of 64. The amount of product formed in the layer is compared to earlier measurements in water, and, it is found that at a speed ratio of 0.40, there exists 20 to 25% more product in gaseous flows, implying that molecular diffusion, or in nondimensional form the Schmidt number, plays a role in mixing at large Reynolds number. The present results compare favorably with the recent theoretical model of Broadwell and Breidenthal for mixing and chemical reaction in a turbulent shear layer. With this model it is possible to bring the results for gases and liquids into quantitative agreement.</p>https://thesis.library.caltech.edu/id/eprint/3541Topics in 2-D Separated Vortex Flows
https://resolver.caltech.edu/CaltechETD:etd-02012007-133412
Authors: {'items': [{'id': 'Tanveer-Saleh-Ahmed', 'name': {'family': 'Tanveer', 'given': 'Saleh Ahmed'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/BR0B-QH56
<p>This thesis is concerned with vortices in steady two dimensional inviscid incompressible flow. In the first three chapters, separated vortex flows are considered in the context of inviscid flow past two dimensional airfoils for which the action of the vortex is to induce large lift. In the fourth and last chapter, we consider vortices in uniform flow in the absence of any physical bodies.</p>
<p>In chapter I, we consider two configurations of vortices for flow past a flat plate with a forward facing flap attached to its rear edge. In the first case, case (a), we consider a potential vortex in the vicinity of the airfoil, while for case (b), we consider a vortex sheet coming off the leading edge of the plate and reattaching at the leading edge of the flap such that the region between the vortex sheet and the airfoil is stagnant. For case (a), the Schwarz-Christoffel transformation is used to find exact solutions to the flow problem. It is found that by suitably placing a potential vortex of appropriate strength it is possible to satisfy the Kutta condition of finite velocity at both the leading edges of the plate and the flap in addition to satisfying it at the trailing edge, provided the plate flap combination satisfies a geometric constraint. The action of the potential vortex is to create a large circulatory region bounded by the airfoil and the streamline that separates smoothly at the leading edge of the plate (due to the Kutta condition) and reattaches smoothly at the leading edge of the flap (from the Kutta condition again). The circulation induced at infinity for such a flow and hence the lift on the airfoil is found to be very large. For case (b), where the vortex sheet location is unknown, a hodograph method is used to find exact solutions. It is found that once a geometric constraint is satisfied, flows exist for which the Kutta condition is satisfied at the trailing edge of the plate-flap combination. As in (a), large values of lift are obtained. However, in both cases (a) and (b), the adverse pressure gradient of top of the flap is recognized as a source of potential difficulty in the experimental realization of the calculated flow.</p>
<p>In chapter II, successive modifications are made to the airfoil considered in chapter I. Exact solutions are once again obtained by a variation of the hodograph method of chapter I. The lift for these airfoils is found to be significantly larger than the one in chapter I. Because the trailing edge is no longer a stagnation point, it is felt that these flows may be easier to realize experimentally.</p>
<p>Chapter III is concerned with the so-called Prandtl-Batchelor flow past the plate-flap geometry of chapter I. The flow consists of an inner region which has a constant vorticity. The region outside of the airfoil and the vortex sheet coming off the leading edge of plate and reattaching at the leading edge of the flap (as in chapter I) is once again irrotational. The common boundary between the exterior flow and the inner flow, i.e. the vortex sheet, is unknown a priori and is determined by continuity of pressure, which translates into a nonlinear boundary condition on an unknown boundary. By extending the function theoretic approach of complex variables to this problem, we reduce the entire problem into one of determining one unknown function of one variable on a fixed domain from which everything else can be calculated. This is then solved numerically. Our calculations provide what we believe to be the first such calculation of a Prandtl-Batchelor flow. The calculations also provide a more realistic model for the vortex sheet flow considered in chapter I.</p>
<p>Chapter IV deals with a steadily translating pair of equal but opposite vortices with uniform cores and vortex sheets on their boundaries, moving without the presence of any physical boundary. The solutions were found for such flows using the function theoretic approach introduced earlier in chapter III for flows where the velocity on the vortex sheet is not a constant. The solutions form a continuum between the hollow vortex case of Pocklington (1898) and those of Deem and Zabusky (1978) and Pierrehumbert (1980) who consider uniform core with no vortex sheet. The iterative scheme for numerical calculation, however, turns out to have severe limitations, as it fails to converge for the cases with no vortex sheet or when the vortex sheet strength is small. In the last section of the chapter, a more traditional approach due to Deem and Zabusky is taken to calculate a pair of touching vortices with uniform core and no vortex sheet on the boundary and an error in Pierrehumbert's (1980) calculations is pointed out.</p>
<p>In appendix I, we point out some errors in Pocklington's paper on the motion of a hollow vortex pair. The errors are corrected and the results are found to be then in agreement with results using the method in chapter IV.</p>https://thesis.library.caltech.edu/id/eprint/442Shock-Wave Properties and High-Pressure Equations of State of Geophysically Important Materials
https://resolver.caltech.edu/CaltechETD:etd-09262002-154053
Authors: {'items': [{'email': 'mbeb@unm.edu', 'id': 'Boslough-Mark-Bruce', 'name': {'family': 'Boslough', 'given': 'Mark Bruce'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/C9GZ-3121
<p>Shock wave (Hugoniot), shock temperature, and release data are presented for several geophysically important, refractory materials. A sensitive multichannel optical pyrometer was developed to measure shock temperatures (2500 to 5600°K at pressures from 48 to 117 GPa) in anorthite (CaAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>) glass. Shock temperatures of 3750 to 6000°K at pressures from 140 to 182 GPa were measured in calcium oxide (CaO). Temperature data were used to constrain the energetics of the B1-B2 phase transition at 70 GPa in CaO, and to construct a finite-strain equation of state for CaO consistent with previous Hugoniot data.</p>
<p>The new CaO equation of state was used with equation of state parameters of other oxides to construct a theoretical mixed oxide Hugoniot of anorthite, which is in agreement with new Hugoniot data above about 50 GPa, determined using new experimental techniques developed in this study. The mixed oxide model, however, overestimates the shock temperatures, and does not accurately predict measured release paths. Both shock temperature and release data for anorthite indicate that several high pressure phase regions of stability exist above 50 GPa. A similar mixed oxide Hugoniot was constructed for lunar gabbroic anorthosite, and agrees with two new Hugoniot points at 120 GPa. Release data from lunar gabbroic anorthosite shocked to 120 GPa give evidence for shock vaporization.</p>
<p>Because the densities and bulk properties of CaO and the high pressure phase or phases of anorthite are so close to those determined seismologically for the lower mantle, the amount of these materials present in the lower mantle is not well constrained. The possibility of significant enrichment of the lower mantle in these refractory materials, as predicted by inhomogeneous accretion models, is still open.</p>
<p>A simple model is developed to explain the measured time dependences of radiated light in the shock temperature experiments, and constrain the absorption coefficient of the shocked material. The absorption coefficient is found to be an increasing function of shock pressure in shocked anorthite glass.</p>
<p>Hugoniot and release paths were determined using electromagnetic particle velocity gauges for San Gabriel anorthosite and San Marcos Gabbro shocked to peak stresses between 5 and 11 GPa. The data indicate a loss of shear strength in both rocks, and a partial phase transition of the anorthosite to a denser phase. This implies that estimates of shock wave attenuation in these materials based on elastic-plastic models are too high, and previously calculated amounts of internal energy gained by surface materials from impact or explosion events have been underestimated.</p>https://thesis.library.caltech.edu/id/eprint/3777Existence and Stability of Vortices and Vortex Arrays
https://resolver.caltech.edu/CaltechETD:etd-12212006-111951
Authors: {'items': [{'id': 'Robinson-Allen-Conrad', 'name': {'family': 'Robinson', 'given': 'Allen Conrad'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/0db7-mr48
<p>The stability to three-dimensional disturbances of three classical steady vortex configurations in an incompressible inviscid fluid is studied in the limit of small vortex cross-sectional area and long axial disturbance wavelength. The configurations examined are the single infinite vortex row, the Karman vortex street of staggered vortices and the symmetric vortex street. It is shown that the single row is most unstable to a two-dimensional disturbance, while the Karman vortex street is most unstable to a three-dimensional disturbance over a significant range of street spacing ratios. The symmetric vortex street is found to be most unstable to three-dimensional or two-dimensional symmetric disturbances depending on the spacing ratio of the street. Short remarks are made concerning the relevance of the calculations to the observed instabilities in free shear layer, wake and boundary layer type flows.</p>
<p>The three-dimensional linear stability of a steady rectilinear vortex of elliptical cross-section existing in an irrotational straining field is studied numerically in the case of finite strain. It is shown that the instability predicted for weak strain persists for finite strain and that the weak strain results continue to be quantitatively valid for finite strain. Parametric dependence of the growth rates of the unstable modes on the strain and the axial disturbance wavelength is discussed. It is also shown that a three-dimensional instability is always more unstable than a two-dimensional instability in the range of parameters of most interest.</p>
<p>The radially symmetric Burgers' vortex is an example of a solution to the Navier-Stokes equations in which the intensification of vorticity due to vortex stretching is balanced by the diffusion of vorticity through viscosity. We present analytical solutions obtained from a perturbation analysis as well as numerical computations of non-symmetric Burgers' vortices in which the radial flow field in a plane perpendicular to the vorticity is non-symmetric. We also demonstrate the linear stability of the symmetric Burgers' vortex to a restricted class of two-dimensional perturbations.</p>https://thesis.library.caltech.edu/id/eprint/5111Large Structure in the Far Wakes of Two-Dimensional Bluff Bodies
https://resolver.caltech.edu/CaltechETD:etd-09132005-131650
Authors: {'items': [{'email': 'jmc6@psu.edu', 'id': 'Cimbala-John-Michael', 'name': {'family': 'Cimbala', 'given': 'John Michael'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/KZ05-YC02
<p>Smoke-wire flow visualization and hot-wire anemometry have been used to study near and far wakes of two-dimensional bluff bodies. For the case of a circular cylinder at 70 < Re < 2000, a very rapid (exponential) decay of velocity fluctuations at the Karman vortex street frequency is observed. Beyond this region of decay, larger-scale (lower wave-number) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the center of the band shifting to lower frequencies as downstream distance is increased.</p>
<p>The far-wake structure does not depend directly on the scale or frequency of the original Karman vortices; the growth of this structure is due to hydro-dynamic instability of the developing mean wake profile; it is not caused by amalgamation of the Karman vortices. Under certain conditions amalgamation can take place, but is purely incidental, and is not the driving mechanism responsible for the growth of larger-scale structure. Similar large structure is observed downstream of porous flat plates (Re ≈ 6000), which do not initially shed Karman-type vortices into the wake.</p>
<p>Hot-wire measurements show that two-dimensional locally-parallel inviscid linear stability theory is adequate to explain the growth of downstream structure. Namely, measured prominent frequencies in the cylinder wake are in close agreement with those predicted by the theory, when streamwise growth of wake width is taken into account.</p>
<p>Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed.</p>https://thesis.library.caltech.edu/id/eprint/3515Experiments on Entrainment, Mixing and Chemical Reactions in Turbulent Jets at Large Schmidt Number
https://resolver.caltech.edu/CaltechETD:etd-12062006-104125
Authors: {'items': [{'id': 'Dahm-Werner-Johann-Anton', 'name': {'family': 'Dahm', 'given': 'Werner Johann Anton'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/k93g-jh12
<p>Entrainment, mixing and chemical reactions are investigated in the far field of steady, axisymmetric, momentum-driven, turbulent jets issuing into an unconfined, quiescent medium in the large Schmidt number (liquid-phase) regime. Visualization experiments using both passive and chemically sensitive planar laser induced fluorescence (LIF) techniques show the importance of large scale transport in the jet far field, and suggest that entrainment, mixing and chemical reactions in the far field are dominated by a large scale organization of the flow. Successive instantaneous profiles of the jet fluid concentration along the axial and radial directions in the jet far field are measured by combining these LIF techniques with direct, high-resolution, linear photodiode array imaging and high-speed digital data acquisition. These imaging measurements have revealed an axial similarity concentration variable for which probability density functions (PDFs) in the jet far field are self-similar along rays. A chemical reaction method is presented which allows the self-similar form of these PDFs to be measured with full resolution at all scales of transport and mixing. Furthermore, these imaging measurements have shown that instantaneous radial profiles of the jet fluid concentration do not resemble the mean concentration profile. Specifically, unmixed ambient fluid is found deep within the jet and the composition of molecularly mixed fluid within large regions in the jet is approximately uniform. The results from these experiments are interpreted in the context of a simple conceptual model for large scale organization of entrainment, mixing and chemical reactions in the far field of turbulent jets.</p>https://thesis.library.caltech.edu/id/eprint/4815Structural Analysis of Imperfect Three-Legged Truss Columns for Large Space Structures Applications
https://resolver.caltech.edu/CaltechETD:etd-11182004-161353
Authors: {'items': [{'email': 'dovel@rafael.co.il', 'id': 'Elyada-Dov', 'name': {'family': 'Elyada', 'given': 'Dov'}, 'show_email': 'YES'}]}
Year: 1985
DOI: 10.7907/YXNE-YE65
<p>Three-legged truss columns are basic structural components of many envisioned large outer-space structures. They constitute three longerons ('legs') forming, in the column cross-section, the vertices of an equiliateral triangle. Their longerons are held together by uniformly spaced battens while a shear web, usually made of diagonals, restrains shear deformation.</p>
<p>This work deals with configurations characterized by having relatively stiff battens, longerons which are pinned to the battens and prestressed string diagonals. Considered are only simply-supported slender columns having slender longeron segments and relatively thin and lightly preloaded diagonals. The columns are allowed to have global (overall) as well as local (longeron segment) geometrical imperfections - not necessarily small ones.</p>
<p>Investigated is the static structural behavior of such columns when loaded by purely axial compressive concentrated forces acting at the supports. Addressed are the topics of global and local buckling, post-buckling, imperfection sensitivity, global-local mode interaction, complete non-linear response, limit loads and diagonals slackening and post-slackening.</p>
<p>The approach is a theoretical one; a system of non-linear, ordinary differential equations is set up which represents the column, and results, mostly in closed form, are obtained by solving that system for a variety of cases of varying generality.</p>
<p>First, a highly idealized case is studied in detail, in which the diagonals are removed and infinite shear rigidity is postulated instead. The results exhibit most of the essential features of the more complicated cases. Next, the case of the undeflected or only-slightly deflected column is considered. Results include the prebuckling behavior, slackening and local buckling loads, global buckling load, initial post-buckling behavior and imperfection sensitivities. Diagonals slackening in a deflecting column is studied next. This is done by means of slackening loci constructed in the load-deflection plane. Solutions are obtained for some special cases of a deflecting column. These include a complete analysis of the locally-perfect case and the cases of small load and high defection. Also obtained is an engineering-oriented load-deflection working relation valid for the most general case but based on generalization rather than on rigorous solution. A torsion-compression mode, dominant in post-slackening, is also analyzed. The work is concluded by investigating the error committed in treating continuous longerons as if they were discontinuous-pinned.</p>
https://thesis.library.caltech.edu/id/eprint/4597Laser Doppler Velocity and Vorticity Measurements in Turbulent Shear Layers
https://resolver.caltech.edu/CaltechETD:etd-08312005-112325
Authors: {'items': [{'email': 'dbl@tyrvos.caltech.edu', 'id': 'Lang-Daniel-Bernard', 'name': {'family': 'Lang', 'given': 'Daniel Bernard'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/YFR4-VE59
<p>A Laser Doppler Velocimeter (LDV) system was developed to measure the instantaneous spanwise vorticity, -w<sub>z</sub>, in a turbulent shear layer. It was necessary to design and fabricate the LDV optics and processing electronics, as no commercially available LDV systems met the specifications of measuring the velocity at four closely spaced points to the requisite accuracy. Measurements were also made of the instantaneous u, v, u', v', and -u'v'. The instantaneous vorticity was processed to obtain an estimate of its probability density function, from which the mean and rms values were estimated. It was also possible to separate the irrotational fraction of the flow (-w<sub>z</sub> ≈ 0) from the rotational (intermittent) fraction of the flow (-w<sub>z</sub> ≠ 0). The development of the intermittency profiles, based on vorticity, as a function of the downstream distance from the splitter plate was studied. A notable feature is that the vorticity is found to have values opposite the mean sense of rotation, i.e., -w<sub>z</sub>(t) < 0, a significant fraction of the time. Additionally, a detailed study was performed to evaluate the approximation of -∂v/∂x, in terms of various local temporal derivatives ∂v/u(y)∂t. The optimum choice for u(y) can be found and is influenced by the relative local convection velocities of the small and large scale structures.</p>
https://thesis.library.caltech.edu/id/eprint/3291Edgetones and Acoustic Resonances in a Duct
https://resolver.caltech.edu/CaltechETD:etd-01222007-092606
Authors: {'items': [{'id': 'Aaron-Kim-Maynard', 'name': {'family': 'Aaron', 'given': 'Kim Maynard'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/bfy8-1995
<p>Undesirable sound generation in the combustion chamber of segmented solid propellant rocket motors has been attributed to vortex shedding from obstructions that are uncovered as the propellant burns back. This phenomenon has been investigated experimentally and the mechanism explained.</p>
<p>A pair of aluminum baffles within a lucite duct through which air is drawn models the important aspects which enable the sound generation mechanism to operate. The baffles form an edgetone system which interacts with the longitudinal acoustic modes of the chamber. Acoustic tones occur spontaneously, at frequencies determined by the acoustic resonances, when the spacing between the baffles satisfies certain criteria.</p>
<p>Flow visualization using smoke and a strobe light triggered by the pressure oscillations indicate that vortex shedding occurs at the first baffle in phase with the acoustic velocity oscillations there. The interaction of these vortices with the downstream baffle drives the acoustic resonance which, in turn, triggers the formation of new vortices at the upstream separation point.</p>
<p>The phase relations for this feedback to operate require that there be close to an integral number of wavelengths, or vortices, from the separation point to the impingement point.</p>
<p>A model has been developed which predicts the experimentally observed behaviour well. Pressure amplitudes are predicted within an order of magnitude. Mean flow rates and baffle spacings yielding maximum response are determined correctly by the model.</p>https://thesis.library.caltech.edu/id/eprint/274Effects of a Periodic Disturbance on Structure and Mixing in Turbulent Shear Layers and Wakes
https://resolver.caltech.edu/CaltechETD:etd-03252008-144801
Authors: {'items': [{'id': 'Roberts-Fredrick-Allen', 'name': {'family': 'Roberts', 'given': 'Fredrick Allen'}, 'show_email': 'NO'}]}
Year: 1985
DOI: 10.7907/syy5-a334
<p>Large scale structure and mixing processes are investigated in chemically reacting wakes and shear layers to which a periodic disturbance is applied. The experiments employ a diffusion-limited acid-base reaction to directly measure the extent of mixing. Optical diagnostics used include laser absorption and laser induced fluorescence. Absorption of laser light by reacted product provides a measure of cross-stream average product. Fluorescence was measured by a self-scanning linear photodiode array using high speed computer data acquisition to obtain the product distribution across the layer.</p>
<p>Previous results showing that forcing alters the structure and growth rate of shear layers are confirmed. Forcing artificially extends the lifetime of vortices whose size is consistent with the disturbance wavelength. Amalgamation of smaller vortices is enhanced over that in the natural layer until the frequency locked scale is achieved. At high Reynolds number product measurements show reduction of product with forcing. At moderate Reynolds numbers, on the other hand, there is an increase in product when forced. In one case a five fold increase in product was observed. The differences are related to the different effects of forcing on entrainment, composition ratio and secondary structure.</p>
<p>A dramatic, order of magnitude increase in mixing was discovered for certain forced wake flows. This effect is strongly associated with an interaction between the spanwise organized wake vortices and the test-section side walls.</p>
https://thesis.library.caltech.edu/id/eprint/1123The Structure and Control of a Turbulent Reattaching Flow
https://resolver.caltech.edu/CaltechETD:etd-03102008-104655
Authors: {'items': [{'id': 'Sigurdson-Lorenz-Willard', 'name': {'family': 'Sigurdson', 'given': 'Lorenz Willard'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/prfm-x404
<p>An unsteady and three-dimensional large-scale structure is proposed for the reattachment region of a separation bubble, based on a visualization study of the flow over a plate with a square leading edge and its axisymmetric counterpart, a flat-faced circular cylinder aligned coaxially with the free-stream. The initial free shear layer structures are primarily two-dimensional but evolve into boundary layer type structures as they near reattachment and interact with the wall. Some segments form "loops" which convect away from the wall and downstream, while spanwise adjacent parts convect toward the wall and upstream. The loops are sometimes clearly arranged in a staggered pattern. Their legs form a series of counter-rotating streamwise vortex pairs which bridge the reattachment zone. These observations reconcile apparently contradictory propositions concerning the fate of the structures as they encounter reattachment. The interaction between successive vortices at alternating spanwise locations is fundamental to several flows. The structure of turbulent wakes is also discussed.</p>
<p>An experimental study was made of the effect of a periodic velocity perturbation on the separation bubble downstream of the sharp- edged blunt face of a circular cylinder aligned coaxially with the free stream. Velocity fluctuations were produced with an acoustic driver located within the cylinder and a small circumferential gap located immediately downstream of the fixed separation line to allow communication with the external flow. The flow could be considerably modified when forced at frequencies lower than the initial Kelvin-Helmholtz frequencies of the free shear layer, and with associated vortex wavelengths comparable to the bubble height. Reattachment length, bubble height, pressure at separation, and average pressure on the face were all reduced. The effects on the large-scale structures were studied on flow photographs obtained by the smoke-wire technique. The forcing increased the entrainment near the leading edge.</p>
<p>In both forced and unforced cases it was concluded that the final vortex of the shear layer before reattachment is an important element of the flow structure. There are two different instabilities involved, the Kelvin-Helmholtz instability of the free shear layer and the "shedding" type instability of the entire bubble. The latter results from an interaction with the image vortices due to the presence of the wall. A method of frequency scaling is proposed that correlates data for a variety of bubbles and supports an analogy with Karman vortex shedding.</p>
<p>New methods for approximating axisymmetric flows are presented. Transition of shear-layers and separation bubbles is also discussed.</p>https://thesis.library.caltech.edu/id/eprint/919Study of the stall-spin phenomena using analysis and interactive 3-D graphics
https://resolver.caltech.edu/CaltechETD:etd-03242008-113141
Authors: {'items': [{'email': 'pdubois@onera.fr', 'id': 'Dubois-P-C', 'name': {'family': 'Dubois', 'given': 'Pascale C.'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/SFMF-Z114
<p>The purpose of this study is to gain a better understanding of the nonlinear stall-spin phenomenon through numerical analysis and interactive 3-D graphics.</p>
<p>The linear aerodynamic range was thoroughly examined for the NAVION, a light aviation aircraft. Nonlinear aerodynamic behavior was modeled by adding nonlinearities to the lift, pitching and rolling moments. The results of this analysis are promising; however, a more sophisticated model is needed to fully simulate the stall-spin phenomenon.</p>
<p>A graphic tool is described which allows the user to interact with the simulation process. This gives the user a "feel" for the dynamics of aircraft and effectively displays the characteristic features of the dynamic model.</p>
https://thesis.library.caltech.edu/id/eprint/1102Longitudinal Mode Combustion Instabilities in Air Breathing Engines
https://resolver.caltech.edu/CaltechETD:etd-07022004-165628
Authors: {'items': [{'email': 'Jim_Sterling@kgi.edu', 'id': 'Sterling-James-David', 'name': {'family': 'Sterling', 'given': 'James David'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/2WKT-AY26
<p>An investigation of the self-excitation of low frequency (100-600 Hz), longitudinal acoustic modes of a rearward-facing step combustor has been performed. As in combustion instabilities of air breathing propulsion systems, the pressure oscillations are excited by a fluctuating heat release from a flame that is stabilized in a recirculation zone. Flow visualization results and flame radiation intensity data reveal that large vortex structures are responsible for this fluctuating heat release. The vortices are shed at frequencies corresponding to longitudinal acoustic modes of the system or to the first subharmonic of one of the modes.</p>
<p>A series of parametric studies were performed to determine the dependence of the vortex shedding frequency upon the step height, mean flow speed, fuel type, and equivalence ratio. It was discovered that the vortex shedding frequency can shift between modes as a result of changes in the chemical reaction time of the reactants or as a result of changes in the mixing process of the cold reactants with the hot products.</p>
<p>Detailed investigations into the mechanism of sustenance of the oscillations during instability were performed for several operating conditions. The distribution of the combustion associated with vortex shedding was investigated by measuring the radiation intensity from the flame region. These results were used in Rayleigh's Criterion to determine regions of driving and damping of the oscillations.</p>
<p>A one-dimensional linearized acoustic model is used to predict the natural modes of the system and a fluctuating volumetric source is used to model the oscillatory heat release. First, independent driving is applied to determine the system response to driving at different frequencies. One important result is that, the phase difference between the pressure oscillations and the velocity oscillations at the flameholder is very sensitive to the frequency of driving near natural modes of the system.</p>
<p>Finally, a velocity-sensitive volumetric source with a time delay is included as feedback to determine the linear stability characteristics of the system. Various mechanisms leading to nonlinear limit cycle behavior are then discussed and compared to experimental data obtained during transition to instability.</p>https://thesis.library.caltech.edu/id/eprint/2804Cavitation Inception Scale Effects: I. Nuclei Distributions in Natural Waters. II. Cavitation Inception in a Turbulent Shear Flow
https://resolver.caltech.edu/CaltechETD:etd-04022004-094117
Authors: {'items': [{'id': "O'Hern-Timothy-John", 'name': {'family': "O'Hern", 'given': 'Timothy John'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/G8TY-K105
<p>Cavitation scale effects can be grouped into two major categories: susceptibility of the water to cavitation, i.e., the amount, size, and type of microbubbles or microparticulates in the water acting as inception nuclei, and flow field effects due to such factors as velocity and pressure distributions, body size and shape, viscous effects, and turbulent phenomena. Experimental investigations into these two aspects of scale effects were performed in the present study.</p>
<p>Field investigations of marine nuclei populations were made using underwater holography to observe microbubbles and particulates, including microplankton in oceanic waters of Los Angeles Harbor, San Pedro Channel and near Santa Catalina Island. Holographic detection was shown to be a reliable method of measuring the nuclei number concentration density distributions. Overall, very high concentrations of the various types of potential cavitation nuclei were observed at all of the test sites and depths examined, although the statistical significance of these results is strong only in the smaller size ranges (less than 50 µm), where a significant number of counts were made. Relatively high bubble concentrations during calm sea conditions, and their population inversion below the thermocline where organism activity was high, indicate a possible biological source of bubble production rather than the usual surface mechanisms of breaking waves and whitecaps. The measured population of particulates is somewhat higher than comparable data in the ocean or in cavitation test facilities, and the number density distribution of particulates decreases approximately as the fourth power of the particle size, as often reported in the literature. An increase in particle concentration near the bottom of the thermocline in clear coastal waters is observed. The total concentration of particles and bubbles in a liquid provides an upper bound on the number of potentially active cavitation nuclei. The measured bubble sizes can be used to indicate that the average tensile strength of the ocean waters examined in this study should be on the order of a few thousand Pascals, with a minimum expected value of about one hundred Pascals. The present results support the recommendation of Billet (1985), that a concentration of at least 3 bubbles per cm<sup>3</sup> in the 5 to 20 µm radius range is needed in test facility water in order to model marine conditions.</p>
<p>Experimental studies were also made on the inception processes in a large turbulent free shear layer generated by a sharp edged plate in a water tunnel at Reynolds numbers up to 2 x 10<sup>6</sup>. Two distinct types of vortex motion were evident in the shear layer, the primary spanwise and the secondary longitudinal vortices. Cavitation inception occurs consistently in the secondary shear layer vortices and more fully developed cavitation is visible in both structures, with the streamwise cavities primarily confined to the braid regions between adjacent spanwise vortices. A Rankine vortex model indicates that the secondary vortex strength is always less than 10% of that of the primary structure. Measurements of fluctuating pressures in the turbulent shear layer are made by holographically monitoring the size of air bubbles injected into the non-cavitating flow, showing that pressure fluctuations were much stronger than previously reported, with positive and negative pressure peaks as high as 3 times the freestream dynamic pressure, sufficient to explain the occurrence of cavitation inception at high values of the inception index. Cavitation inception indices display a strong dependence on the dissolved air content and thus on the availability of freestream bubble cavitation nuclei. The present inception data do not display a clear dependence on freestream velocity (or Reynolds number) but do fall into the overall range of data of previous bluff body investigations. The occurrence of inception in the secondary vortices of the shear layer, and previous reports of velocity dependence of these cores (Bernal 1981) may provide the key to explaining the commonly observed Reynolds number scaling of the inception index in shear flows.</p>https://thesis.library.caltech.edu/id/eprint/1246Transient Gas Jets into Liquid
https://resolver.caltech.edu/CaltechETD:etd-04262004-113723
Authors: {'items': [{'id': 'Lin-Jane-Ming-Chin', 'name': {'family': 'Lin', 'given': 'Jane Ming-Chin'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/4qp0-4x37
<p>An experimental investigation of the development of high velocity, impulsively initiated gas jets into liquid was conducted in an effort to understand some of the physical processes that occur for a jet of very light fluid into a dense ambient atmosphere. Four gases, refrigerants 12 and 22, nitrogen, and helium were injected into water at nozzle exit Mach numbers from 1.0 to 2.2.</p>
<p>The study showed that a gas jet into water develops in at least three stages: startup, transition, and global steady state. The startup is characterized by bubble growth; the growth rate is well predicted by classical bubble-growth theory. Jet transition is marked by axially directed flow, which penetrates through the startup bubble and which forms a cylindrical protrusion along the axis of symmetry. A combination of strong recirculating flow and liquid entrainment causes the startup bubble to deflate and to lift off and move downstream. In the steady state, instantaneous photographs show small-scale fluctuations of the jet boundary, but time-averaged photographs show the expected conical spreading of the steady jet; the measured spreading angles range from 18-25 degrees.</p>
<p>However, the most significant finding of this study is that under some conditions, the gas jet into liquid never reaches the global steady state. Instead, the jet boundary exhibits chugging: large nonlinear oscillations which lead to irregular collapses of the gas column followed by explosive outward bursts of gas. The unsteadiness observed is much more violent than the familiar fluctuations typical of constant-density jets. The length scale of the motion is generally on the order of several jet diameters; the time scale is on the order of the period for bubble collapse.</p>
<p>It was found that the amplitude and frequency of chugging are strongly dependent on the ratio of the liquid density to the gas density, the jet Mach number, and the operating pressure ratio. The conditions under which unsteadiness occurs were determined experimentally. In particular, a quantitative measure of jet susceptibility to unsteadiness has been established. Steady jets can be achieved in two ways: by being discharged from deLaval nozzles (increasing the exit Mach number) or by being overpressured.</p>
<p>The unsteady behavior is modeled as the collapse of a bubble in liquid; comparisons of collapse times show good agreement. A mechanism for the unsteadiness is discussed. It is proposed that the chugging is the response of the jet boundary to a pressure difference between the jet and surrounding liquid, which arises as the result of the rapid expansion of a light fluid into a dense ambient atmosphere. The flow is shown to be similar to the discharge of a gas from a nozzle into a channel of larger cross section. An upper limit to the pressure difference is determined based on estimates of the minimum base pressure for such channel flows; a lower limit is established for the collapse time. All experimental values are within the bounds. The derived values indicate that the pressure differences between the jet and liquid may be more than 90 percent of the ambient pressure.</p>https://thesis.library.caltech.edu/id/eprint/1504Experimental Investigation of Heterogeneous Compressible Shear Layers
https://resolver.caltech.edu/CaltechETD:etd-12192007-085328
Authors: {'items': [{'id': 'Papamoschou-Dimitri', 'name': {'family': 'Papamoschou', 'given': 'Dimitri'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/M7PQ-Y494
<p>The compressible, two-dimensional shear layer is investigated experimentally in a novel facility. In this facility, it is possible to flow similar or, dissimilar gases of different densities and to select different Mach numbers for each stream over a wide range of Reynolds numbers. In the current experiments, ten combinations of gases and Mach numbers are studied in which the freestream Mach numbers range from 0.2 to 4, the density ratio varies from 0.2 to 9.2, and the velocity ratio varies from 0.13 to 1. The growth of the turbulent region of the layer is measured by means of pitot pressure profiles obtained at several streamwise locations. The resulting growth rate is estimated to be about 80% of the visual growth rate. The transition from laminar to turbulent flow, as well as the structure of the turbulent layer, are observed with Schlieren photographs of 20 nanosecond duration. Streamwise pressure distribution and total pressures are measured by means of a Scanivalve-pressure transducer system.</p>
<p>An underlying objective of this investigation was the definition of a compressibility-effect parameter that correlates and consolidates the experimental results, especially the turbulent growth rates. A brief analytical investigation of the vortex sheet suggests that such a parameter is the Mach number in a frame of reference moving with the phase speed of the disturbance, called here the convective Mach number. In a similar manner, the convective Mach number of a turbulent shear layer is defined as the one seen by an observer moving with the convective velocity of the dominant waves and structures. It happens to have about the same value for each stream. In the current experiments, it ranges from 0 to 1.9.</p>
<p>The correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases gradually with increasing convective Mach number, reaching an asymptotic value of about 0.25 for supersonic convective Mach numbers. The above behavior is in qualitative agreement with results of linear stability theory as well as with those of previous, one-stream experiments.</p>
<p>Large-scale structures, resembling those observed in subsonic shear layers, are evident in the Schlieren photographs. It is estimated that the mean structure spacing, normalized by the local thickness, is reduced to about half its incompressible value as the convective Mach number becomes supersonic.</p>
<p>An estimate of the transition Reynolds number has been obtained from the photographs of two shear layers having quite different convective Mach numbers, one low subsonic and the other sonic. In both cases, it is about 2 x 10<sup>5</sup>, based on distance to transition and properties of the high unit Reynolds number stream, thus suggesting that, in this experiment, transition is dominated by instabilities of the wake, rather than of the shear layer.</p>
https://thesis.library.caltech.edu/id/eprint/5067On the Start Up of Supersonic Underexpanded Jets
https://resolver.caltech.edu/CaltechETD:etd-03042008-081340
Authors: {'items': [{'email': 'nehemias@alumni.caltech.edu', 'id': 'Lacerda-Nehemias-Lima', 'name': {'family': 'Lacerda', 'given': 'Nehemias Lima'}, 'show_email': 'YES'}]}
Year: 1987
DOI: 10.7907/W3CX-2Z48
<p>An impulsively started jet can be formed by a gas confined in a high pressure reservoir that escapes suddenly through an exit orifice, into a controlled atmosphere. Supersonic gas jets of this type are unsteady and differ from the steady jet that develops later by the presence of a bow shock, a jet head and a nonstationary Mach disk. The effects of the pressure ratio between the high pressure gas inside the reservoir and the lower pressure atmospheric gas, as well as the gas combination used, are studied experimentally. The gases used for the jet and the atmosphere were selected from helium, nitrogen and sulfur hexafluoride.</p>
<p>The data acquisition consisted of: high resolution flash photography to obtain detail from the pictures; high-speed movie pictures to obtain the time development of selected features; and fast-response pressure transducers located at the reservoir end plate, the tank end plate and the jet exit. The initial development of the jet is highly time dependent. During this phase, the shape that the jet assumes varies with pressure ratio and with the choice of gas. In particular an extremely light gas exhausting into a heavy atmosphere, exhibits an uncommon shape. It develops as a bubble wrapped by the bow shock, that increases its volume with flow time and pressure ratio. As the pressure ratio increases, it becomes more tightly wrapped by the bow shock. At later times the jet assumes conventional linear growth.</p>
<p>After the jet starts, a Mach disk is observed close to the jet exit which moves downstream as the exit pressure builds up. The monotonic increase in exit pressure is caused by the slow breaking of the diaphragm. The position of the Mach disk is furthest from the jet exit when the exit pressure is a maximum. After that it oscillates around the location predicted by the steady theory of Ashkenas and Sherman (1966) at a frequency close to one of the resonant frequencies of the reservoir. The features observed for the inner structure of the jet were verified to agree with those obtained for impulsive flow generated by a muzzle blast.</p>
<p>The frontal part of the jet forms the jet head, whose shape changes with the flow conditions. The initial evolution of the jet head is linear but after propagating a distance of around ten exit diameters, it reaches asymptotic behavior with an evolution that is approximately proportional to square root of time. The head creates a bow shock ahead of it that propagates downstream and increases the pressure of the atmospheric gas. This bow shock was found to be less attenuated than in spherically symmetric explosions. The asymptotic behavior of the bow shock was reached after about eight exit diameters.</p>https://thesis.library.caltech.edu/id/eprint/865On the Combustion of Bituminous Coal Chars
https://resolver.caltech.edu/CaltechETD:etd-11132007-081003
Authors: {'items': [{'id': 'Sahu-Ranajit', 'name': {'family': 'Sahu', 'given': 'Ranajit'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/zrnz-qc25
<p>The combustion of chars from pulverized bituminous coals was experimentally and theoretically investigated. The chars were made by pyrolyzing size-graded PSOC 1451 coal particles in nitrogen at temperatures of 1000-1600K. Sized char particles were then used for further experiments. Low temperature reactivities of such cenospheric chars were measured at 800K on a thermogravimetric analyzer. The effects of initial coal size, char size, pyrolysis temperature, and oxygen concentration were investigated. Single particle combustion experiments were done in both air and 50% oxygen ambients at wall temperatures of 1000-1500K in a drop tube laminar flow furnace. Particle temperatures were measured during the entire course of combustion. From the complete temperature-time histories of such burning particles, the apparent activation energy and pre-exponential factors were inferred, using numerical models and statistical modelling techniques. Questions of particle-particle variability were addressed. The ignition transients of single burning particles were studied and a model that predicted delay times observed experimentally was developed. Char samples were also partially oxidized at temperatures in the range 1200-1500K (particle temperatures) and physically characterized. Methods of characterization included optical and electron microscopy, gas adsorption methods for specific surface area and pore volume distributions, and mercury porosimetry for pore volume distribution measurements. The results of these characterizations were compared with those done on chars oxidized at 500°C.</p>
<p>The combustion of single char particles was numerically modelled. A continuum model for asymptotic shrinking-core combustion was developed using apparent reaction rates and temperature-dependent properties. Simplified assumptions were made regarding the gas-phase combustion. Parametric sensitivity of this model yielded significant insight into the combustion process. A more general continuum model was then developed. This model treated the internal pore structure more realistically, as inferred from experiments. The steady state diffusion equation was solved inside the particle to determine its theoretical temperature-time history. Good agreement with experiments was found. The model was extended to include the effects of some nonlinear kinetic reaction rate expressions. A discrete model for a cenospheric particle was also developed. This model consists of spherical voids randomly placed in a spherical particle. It simulates the combustion by taking into account the connectivity of the internal pore structure. This connectivity influences the access of reactant to the interior of the particle and, therefore, the extent of internal reaction. The changes in the internal connectivity led to a percolation type behavior in most particles.</p>https://thesis.library.caltech.edu/id/eprint/4535Effects of Controlled Three-Dimensional Perturbations on Boundary Layer Transition
https://resolver.caltech.edu/CaltechETD:etd-02142007-094214
Authors: {'items': [{'email': 'steves@purdue.edu', 'id': 'Schneider-Steven-Philip', 'name': {'family': 'Schneider', 'given': 'Steven Philip'}, 'show_email': 'YES'}]}
Year: 1989
DOI: 10.7907/ZJ0M-GG83
<p>The laminar-turbulent transition in a flat plate boundary layer was studied experimentally using a spanwise array of computer-controlled surface heating elements. The elements were used to introduce disturbances at a point just downstream of the critical Reynolds number. When sinusoidal heating at an unstable frequency is carried out, instability waves develop and grow as they travel downstream. Measurements were made using flush-mounted hot-film wall shear sensors, and the later stages of transition were visualized using dye injection. Oblique Tollmien-Schlichting waves were successfully introduced, and their downstream development into the turbulent regime was studied. Exploratory studies of other types of 3D forcing are also reported.</p>
<p>Measurements of oblique waves in the linear region yielded phase speeds and wave angles that were consistent with the linear theory. Subharmonics of the oblique-wave wall shear were seen downstream, in the nonlinear region. Surprisingly, the amplitude of these subharmonic waves decreased abruptly with increasing oblique-wave angle, so that an oblique wave of about 10 degrees had a subharmonic amplitude which was an order of magnitude below that for a 2D wave. Waves of larger oblique angles did not produce detectable subharmonics. A simple explanation of this behavior is given, in terms of the wave-interaction theory.</p>
<p>The intermittency, defined as the fraction of time in which the wall shear is turbulent, was measured to determine the relative location of transition. These measurements, carried out further downstream, show that the introduction of a 2D wave is most effective in moving the transition point upstream, for a given power input. This upstream movement of transition slowly decreases as the oblique wave angle is increased. The fact that there is no abrupt movement of transition corresponding to the abrupt disappearance of the subharmonic nonlinear breakdown mechanism suggests there should be a simpler explanation for the nonlinear breakdown.</p>https://thesis.library.caltech.edu/id/eprint/635An investigation of the inviscid spatial instability of compressible mixing layers
https://resolver.caltech.edu/CaltechETD:etd-11132007-094001
Authors: {'items': [{'id': 'Zhuang-M', 'name': {'family': 'Zhuang', 'given': 'Mei'}, 'show_email': 'NO'}]}
Year: 1990
DOI: 10.7907/650s-9t96
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
The behavior of both unbounded and bounded compressible plane mixing layers with respect to two- and three-dimensional, spatially growing wave disturbances is investigated using linear stability analysis. The mixing layer is formed by two parallel streams with different gases and the flow is assumed to be inviscid and non-reacting.
For unbounded mixing layers, the effects of the free stream Mach number, velocity ratio, temperature ratio, gas constant (molecular weight) ratio and the ratios of specific heats on the linear spatial instability characteristics of a mixing layer are determined. A nearly universal dependence of the normalized maximum amplification rate on the convective Mach number is found for two-dimensional spatially growing disturbances. The effects of the mean flow profiles on the instability behavior of the mixing layers are also studied. It is shown that decreasing the thickness of the total temperature profile relative to the mean velocity profile, or adding a wake component in the mean velocity profile can make the normalized amplification rate decrease slower as the convective Mach number increases for both subsonic and supersonic convective Mach numbers.
For an unbounded mixing layer with subsonic convective Mach numbers, there is only one unstable mode propagating with a phase velocity [...] approximately equal to the isentropically estimated convective velocity of the large scale structures [...]. As the convective Mach number approaches or exceeds unity, there are always two unstable spatial modes. One is with a phase velocity [...] (slow mode and the other is with a phase velocity [...] (fast mode). For the low supersonic convective Mach numbers, the fast mode is more unstable than the slow mode when the heavy gas is on the low speed side and the slow mode is dominant when the heavy gas is on the high speed side.
The effect of parallel flow guide walls on a spatially growing mixing layer is also investigated. It is shown that, in this case, if the convective Mach number exceeds a critical value of approximately unity, there are many supersonic unstable modes. The maximum amplification rates of mixing layers approach an asymptotic value and this maximum amplification rate increases to a maximum value and decreases again as the distance between the walls decreases. For a mixing layer inside parallel flow guide walls, the growth rate of three-dimensional modes is larger than the corresponding two-dimensional mode at high convective Mach numbers. But the growth rate of two-dimensional supersonic instability waves has a larger value than their three-dimensional counterparts for a mixing layer inside a rectangular duct (Tam & Hu [1988], [1989]). Contour plots of the pressure perturbation fields for both unbounded and bounded mixing layers indicate that there are waves propagating outward from the mixing layer along the Mach angle, and that the walls provide a feedback mechanism between the growing mixing layer and this compression/expansion wave system. The bounded mixing layers are more unstable than the corresponding free mixing layers for supersonic convective Mach numbers. The streaklines of the flow confirm that the spreading rate of the mixing layer is unusually small for supersonic disturbances.
https://thesis.library.caltech.edu/id/eprint/4540Structure in the Near Field of the Transverse Jet
https://resolver.caltech.edu/CaltechETD:etd-02232007-075829
Authors: {'items': [{'id': 'Fric-Thomas-Frank', 'name': {'family': 'Fric', 'given': 'Thomas Frank'}, 'show_email': 'NO'}]}
Year: 1990
DOI: 10.7907/JVHG-E582
<p>Photographs of an axisymmetric turbulent jet issuing from a wall into a crossflow display the four types of vortical structures which exist in the near field: the jet shear layer vortices, the nascent far field vortex pair, the near wall horseshoe vortices, and a system of vortices in the wake of the jet.</p>
<p>Additionally, results of hot-wire measurements in the wake of the transverse jet are presented. Among these results are characteristic wake Strouhal frequencies, which vary with the jet to crossflow velocity ratio, and wake velocity profiles.</p>
<p>It is found that the wake vorticity is not "shed" from the jet but is formed from vorticity which originated in the wall boundary layer. Therefore, analogies between the wakes of transverse jets and the wakes of solid cylinders are incorrect. Since the jet is not a solid obstacle to the crossflow, as a cylinder is, new vorticity is not generated at the interface between the jet and the crossflow. Instead, the boundary layer on the wall from which the jet issues separates near the downstream side of the jet because it cannot negotiate the adverse pressure gradient imposed on it by the flow around the jet, which is not "separated" as it is for a cylinder. The wake vortices subsequently formed are found to be most coherent near a jet to crossflow velocity ratio of four.</p>
<p>The near field development of the counterrotating vortex pair, which is the dominant structure of the far field jet, is also addressed. It is argued that the source of vorticity for the vortex pair is the vorticity from the boundary layer within the jet nozzle. Estimates for the strength of these vortices are obtained by considering the flux of vorticity emanating from the nozzle.</p>
<p>Possible implications for mixing are briefly discussed.</p>https://thesis.library.caltech.edu/id/eprint/719Active control of the flow past a cylinder executing rotary motions
https://resolver.caltech.edu/CaltechETD:etd-04132004-141454
Authors: {'items': [{'id': 'Tokumaru-P-T', 'name': {'family': 'Tokumaru', 'given': 'Phillip Takeo'}, 'show_email': 'NO'}]}
Year: 1991
DOI: 10.7907/v7p7-d977
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Exploratory experiments have been performed on circular cylinders executing forced rotary motions in a steady uniform flow. These motions include harmonic oscillations, steady rotation, and combinations of the two. Flow visualization and laser-Doppler velocimetry measurements were used to characterize the wake structure, and to estimate the convection speed, spacing, and strength of the vortical structures. Laser-Doppler velocimetry measurements were also made to estimate the cylinder drag coefficient and wake displacement thickness. In addition, the periodic flow close the cylinder and in the near wake region was mapped for a particular forced case. The data show that a considerable amount of control can be exerted over the flow by such means. In particular, a large increase, or decrease, in the resulting displacement thickness, estimated cylinder drag, and associated mixing with the free stream can be achieved, depending on the frequency and amplitude of oscillation. In order to assess the effects of oscillatory forcing on a cylinder with a net (mean) rotation rate, a novel method for estimating the steady lift forces was employed. Using this method, it was also found that the addition of forced rotary oscillations to the steady rotation of the cylinder helped to increase [...] in the cases where the wake would normally be separated in the steadily rotating case, and decrease it otherwise. Results obtained for a steadily rotating cylinder (no forced oscillations) compare favorably with similar data published in the literature.https://thesis.library.caltech.edu/id/eprint/1368Study of spherical couette flow via 3-D spectral simulations: large and narrow-gap flows and their transitions
https://resolver.caltech.edu/CaltechETD:etd-04162004-103555
Authors: {'items': [{'email': 'gdumas@gmc.ulaval.ca', 'id': 'Dumas-G', 'name': {'family': 'Dumas', 'given': 'Guy'}, 'show_email': 'YES'}]}
Year: 1991
DOI: 10.7907/Z6DW-4T51
Incompressible, viscous flows in the spherical gap between a rotating inner-sphere and a stationary outer-shell, Spherical Couette Flows (SCF), are studied via direct numerical simulations. The investigation covers both "small-gap" and "large-gap" geometries, and is concerned primarily with the first occurrence of transition in those flows. Strong emphasis is put on the physical understanding of the basic flows and their transition mechanisms.
An alias-free spectral method, based on divergence-free vector expansions for the 3-D velocity field in spherical coordinates, is developed. The vector expansions are constructed with Chebyshev polynomials in the radial direction and Vector Spherical Harmonics for the two angular directions. Accuracy and spectral convergence of the resulting initial-value code are thoroughly tested. Three-dimensional transitional flows in both narrow-gaps and large-gaps as well as axisymmetric transitions in moderate-gaps are simulated.
For small-gap SCF's, this study shows that the formation of Taylor-vortices at transition is a deterministic process and not the result of the instability of initial perturbations. The formation process involves the sub-critical appearance of a saddle-stagnation point within the meridional circulation cell in each hemisphere. A minimum length-scale ratio is shown necessary, and for a given inner-sphere radius, this leads to a theoretical prediction of the largest gap-width in which Taylor-vortices may form.
This investigation confirms that the first transition in large-gap SCF's is caused by a 3-D instability of a linear nature. It is found that the process is characterized by very small growth-rates of the disturbance and by the absence of a "jump" in the friction torque. The supercritical flow is a complex-structured, laminar, time-periodic flow that exhibits traveling azimuthal-waves. The physical mechanism responsible for the large-gap transition is shown to be related to a shear instability of the "radial-azimuthal jet" that develops at the equator of the basic flow. A physical model is proposed in which that jet is viewed as a sequence of adjacent "fan-spreading quasi-2-D plane jets". Predictions from the model are presented and verified from the computed unstable disturbance field. Extension of the model to the transition toward waviness in the Taylor-Couette flow, the Gortler-vortex flow and the Dean-vortex flow is proposed.https://thesis.library.caltech.edu/id/eprint/1397Rotordynamic forces generated by discharge-to-suction leakage flows in centrifugal pumps
https://resolver.caltech.edu/CaltechETD:etd-03272007-091813
Authors: {'items': [{'id': 'Guinzburg-A', 'name': {'family': 'Guinzburg', 'given': 'Adiel'}, 'show_email': 'NO'}]}
Year: 1992
DOI: 10.7907/dv7v-hy83
In recent years, increasing attention has been given to fluid-structure interaction problems in turbomachines. The present research focuses on just one such fluid-structure interaction problem, namely the role played by fluid forces in determining the rotordynamic stability and characteristics of a centrifugal pump. While the geometry of the impeller shroud/pump casing annulus varies considerably, previous studies indicate that the contributions from the leakage flow can be of the same order as the contributions from the nonuniform pressure acting on the impeller discharge. Thus, the emphasis of this study is to investigate the contributions to the rotordynamic forces from the discharge-to-suction leakage flows between the front shroud of the rotating impeller and the stationary pump casing. An experiment was designed to measure the rotordynamic shroud forces due to simulated leakage flows for different parameters such as flow rate, shroud clearance, face-seal clearance and eccentricity. The data demonstrates substantial rotordynamic effects and a destabilizing tangential force for small positive whirl ratios; this force decreased with increasing flow rate. The rotordynamic forces appear to be inversely proportional to the clearance and change significantly with the flow rate. Two sets of data taken at different eccentricities yielded quite similar nondimensional rotordynamic forces indicating that the experiments lie within the linear regime of eccentricity.
Like earlier measurements of the total fluid induced rotordynamic forces on impellers [Chamieh et al. (1985), Jery et al. (1985), Adkins et al. (1988)], the forces measured in these experiments scaled with the square of the rotor speed. The functional dependence on the ratio of whirl frequency to rotating frequency (termed the whirl ratio) is very similar to that measured in experiments and similar to that predicted by the theoretical work of Childs. Childs' bulk flow model yielded some unusual results including peaks in the rotordynamic forces at particular positive whirl ratios, a phenomenon which Childs tentatively described as a "resonance" of the leakage flow. This unexpected phenomenon developed at small positive whirl ratios when the inlet swirl velocity ratio exceeded about 0.5. Childs points out that a typical swirl velocity ratio at inlet (pump discharge) would be about 0.5 and may not therefore be large enough for the resonance to be manifest. To explore whether this effect occurs, an inlet guide vane was constructed which introduced a known amount of swirl into the flow upstream of the leakage flow inlet. A detailed comparison of model predictions with the present experimental program is presented. The experimental results showed no evidence of the "resonances", even at much larger swirl inlet velocities than explored by Childs.
https://thesis.library.caltech.edu/id/eprint/1170Planar Rayleigh Scattering Measurements of Shock Enhanced Mixing
https://resolver.caltech.edu/CaltechETD:etd-07202007-130628
Authors: {'items': [{'id': 'Budzinski-John-Michael', 'name': {'family': 'Budzinski', 'given': 'John Michael'}, 'show_email': 'NO'}]}
Year: 1992
DOI: 10.7907/pwj0-h036
This investigation was concerned with the mixing which occurs after the unsteady interaction of a shock wave with a laminar jet of helium. The jet of helium was injected normal to the direction of the propagation of the shock. The vorticity created at the boundaries of the jet during the shock interaction generates a stream of air which divides the initial circular cross section of the jet into two lobes. Each lobe is further divided into a tail region and a vortex core. The vortex cores from each lobe form a vortex pair that pulls ahead of the tail regions. In the present investigation the primary diagnostic, planar Rayleigh scattering, had sufficient spatial and temporal resolution to resolve the smallest diffusion scales present and allowed helium mole fractions to be measured in two-dimensional planes normal to the original jet flow direction. The amount of molecular mixing was evaluated with a mass distribution function at increasing times after the shock interaction. The total masses of helium contained in regions where the molar concentration of helium was at least 30% and 50% were also calculated. The shock Mach number was varied, and the effect of a reflected shock was studied. The velocity and spacing of the vortex pairs was measured. It was found that shock interactions can significantly increase the mixing between the air and helium. As the Mach number increases, better mixing occurs as the stream of air divides the jet. However, less mixing occurs at the later times when the vortex pairs are moving ahead of the tails. A rough collapse of the mixing data occurs when time is normalized with the change in velocity of the air behind the shock. The measured velocities and estimated values of the circulation agree very well with previous computational results. An increase in the enhancement of mixing occurred after the interaction with the reflected shock.https://thesis.library.caltech.edu/id/eprint/2954Transmission of thin light beams through turbulent mixing layers
https://resolver.caltech.edu/CaltechETD:etd-08162007-133929
Authors: {'items': [{'id': 'Wissler-J-B', 'name': {'family': 'Wissler', 'given': 'John B.'}, 'show_email': 'NO'}]}
Year: 1992
DOI: 10.7907/pw33-qc68
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Light transmission through incompressible gaseous turbulent mixing layers is investigated with the objective of understanding the effects of large-scale coherent structures and mixing transition on the optical quality of the mixing layer. Experiments are done in a vertically flowing mixing layer which is enclosed inside a pressure tank and fed by two banks of high-pressure gas bottles. The study considers both the unequal density (high-speed [...] and low-speed [...]) and equal density (high-speed [...] and low-speed [...]) cases; the mixing of dissimilar gases is the source of the optical aberrations. Large-scale Reynolds numbers range between 3500 and 80000 over pressures from 2 to 6 bar. Light transmission characteristics are first studied qualitatively using a network of thin sheets of short-exposure ([...]) white light which are aberrated by the mixing layer and then image directly onto photographic film. Light transmission characteristics are then studied quantitatively using a lateral effect detector to dynamically track a thin He-Ne laser beam as it wanders under the influence of the passing coherent structures.
The study finds that the spanwise coherent structures generate systematic deflections of the light beam in the streamwise direction; the greatest deflections occur near the trailing edges of the structures at a formation called the cusp, where the high-speed fluid and low-speed fluid are entrained into the vortex core. The streamwise coherent structures, which form later in the mixing layer's development than the spanwise structures, generate substantial beam deflections in the span-wise direction which are closely associated with the streamwise streaks in plan-view shadowgraphs. The rms fluctuations of the streamwise and spanwise deflection angles increase rapidly during mixing transition, peaking at 380 high-speed-side momentum thicknesses downstream from the splitter plate, then decrease far down-stream to asymptotic values of 0.6 to 0.8 as scaled by the static pressure and the Gladstone-Dale constant shift across the mixing layer. The data suggest that a possible mechanism for the deflections is the interaction of the beam with the thin interfaces which bound relatively uniform bodies of fluid inside the structures.https://thesis.library.caltech.edu/id/eprint/3147An adaptive Lagrangian method for computing 1-D reacting flows, and, The theory of Riemann invariant manifolds for the compressible Euler equations
https://resolver.caltech.edu/CaltechETD:etd-09192007-130342
Authors: {'items': [{'id': 'Lappas-T', 'name': {'family': 'Lappas', 'given': 'Tasso'}, 'show_email': 'NO'}]}
Year: 1993
DOI: 10.7907/bw0e-2789
In the first part of this thesis, a method for computing one-dimensional, unsteady compressible flows, with and without chemical reactions, is presented. This work has focused on accurate computation of the discontinuous waves that arise in such flows. The main feature of the method is the use of an adaptive Lagrangian grid. This allows the computation of discontinuous waves and their interactions with the accuracy of front-tracking algorithms. This is done without the use of additional grid points representing shocks, in contrast to conventional, front-tracking schemes. The Lagrangian character of the present scheme also allows contact discontinuities to be captured easily. The algorithm avoids interpolation across discontinuities in a natural and efficient way. The method has been used on a variety of reacting and non-reacting flows in order to test its ability to compute complicated wave interactions accurately and in a robust way.
In the second part of this thesis, a new approach is presented for computing multidimensional flows of an inviscid gas. The goal is to use the knowledge of the one-dimensional, characteristic problem for gas dynamics to compute genuinely multidimensional flows in a mathematically consistent way. A family of spacetime manifolds is found on which an equivalent 1-D problem holds. These manifolds are referred to as Riemann Invariant Manifolds. Their geometry depends on the local, spatial gradients of the flow, and they provide locally a convenient system of coordinate surfaces for spacetime. In the case of zero entropy gradients, functions analogous to the Riemann invariants of 1-D gas dynamics can be introduced. These generalized Riemann Invariants are constant on the Riemann Invariant Manifolds. The equations of motion are integrable on these manifolds, and the problem of computing the solution becomes that of determining the geometry of these manifolds locally in spacetime.
The geometry of these manifolds is examined, and in particular, their relation to the characteristic surfaces. It turns out that they can be space-like or time-like, depending on the flow gradients. An important parameter is introduced, which plays the role of a Mach number for the wave fronts that these manifolds represent. Finally, the issue of determining the solution at points in spacetime, using information that propagates along space-like surfaces is discussed. The question of whether it is possible to use information outside the domain of dependence of a point in spacetime to determine the solution is discussed in relation to the existence and uniqueness theorems, which introduce the concept of domain of dependence.
This theory can be viewed as an extension of the method of characteristics to multidimensional, unsteady flows. There are many ways of using the theory to develop practical, numerical schemes. It is shown how it is possible to correct a conventional, second-order Godunov scheme for multidimensional effects, using this theory. A family of second-order, conservative Godunov schemes is derived, using the theory of Riemann Invariant Manifolds, for the case of two-dimensional flow. The extension to three dimensions is straightforward. One of these schemes is used to compute two standard test cases and a two-dimensional, inviscid, shear layer.
https://thesis.library.caltech.edu/id/eprint/3644Studies of mixing and combustion in hypervelocity flows with hot hydrogen injection
https://resolver.caltech.edu/CaltechETD:etd-08222007-092852
Authors: {'items': [{'email': 'jacquesbelanger@comcast.net', 'id': 'Belanger-Jacques', 'name': {'family': 'Belanger', 'given': 'Jacques'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/012K-GE91
<p>The ability to build an air-breathing single-stage-to-orbit propulsion system requires examination of key elements such as turbulent mixing rates, especially at the "zero shear" fuel-air mixing condition, and combustion efficiency. The required data can only be obtained in experiments which simultaneously match the flight total pressure and total enthalpy as well as the fuel conditions. GALCIT, with its new free piston shock tunnel T5, has the capability to do some of these combustion experiments. But prior to these tests, it was felt that there was a need to simulate the gas dynamical processes in the free piston shock tunnel and also in a new combustion driven shock tunnel built for these experiments so that both systems could be used as efficiently as possible. The numerical code helped explain the piston motion in the free piston shock tunnel. The code was also very useful for the design of the combustion driven shock tunnel.</p>
<p>Because hydrogen has to be injected into the combustion chamber of the propulsion system after being used as a cooling fluid, a combustion driven shock tunnel was built to reproduce this "hot" hydrogen fuel. The system has been used successfully to supply hydrogen at up to 1500 K for the experiments. To reduce the complexity of the problem, a very basic configuration for the hydrogen injection system was tested. This was first done with an injection system mounted flush with the surface of a flat plate in the test section of T5. Different test conditions as well as Mach 2 and 5 nozzle injectors at angles of 15° or 30° were tested to determine criteria for significant combustion. Lower limits in pressure and enthalpy were found where hydrogen combustion becomes very limited using this "hot" hydrogen fuel. The second set of experiments still used an injection system mounted flush with the surface but involved a small combustor model previously tested in the hypervelocity HYPULSE facility. Low pressure experiments were performed to reproduce some of the HYPULSE tests and excellent agreement was found. Experiments at high pressure were also performed to better match the real flight total pressure and some hydrogen combustion was detected in these tests.</p>https://thesis.library.caltech.edu/id/eprint/3197Direct numerical simulations of unsteady separated flows using vortex methods
https://resolver.caltech.edu/CaltechETD:etd-11122003-082957
Authors: {'items': [{'email': 'petros@inf.ethz.ch', 'id': 'Koumoutsakos-P-D', 'name': {'family': 'Koumoutsakos', 'given': 'Petros D.'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/TCQ9-9C86
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Numerical simulations are presented for viscous incompressible flows with and without solid wall boundaries. Our numerical method is based on vortex methods. The classical inviscid scheme is enhanced to account for viscous effects via the method of particle strength exchange. The method is extended to account for the enforcement of the no-slip boundary condition as well by appropriately modifying the strength of the particles. Computations are possible for extended times by periodically remeshing the vorticity field.
The particles are advanced using the Blot-Savart law for the evaluation of the velocity. Computations are made using up to [...](10[superscript 6]) vortex particles by efficiently implementing the method of multipole expansions for vector computer architectures to obtain an [...](N) algorithm.
The method is used to simulate the inviscid evolution of an elliptical vortex in an unbounded fluid as well as unsteady separated flows around circular cylinders for a wide range of Reynolds numbers (40 - 9500). Direct comparisons are made of the results of the present method with those from a variety of theoretical, computational and experimental studies. The results exhibit the robustness and validity of the present method and allow to gain physical insight as to vorticity formation and its relation to the forces experienced by the body.https://thesis.library.caltech.edu/id/eprint/4516Flow past a circular cylinder with a permeable wake splitter plate
https://resolver.caltech.edu/CaltechETD:etd-04012005-092116
Authors: {'items': [{'email': 'greg.cardell@jpl.nasa.gov', 'id': 'Cardell-G-S', 'name': {'family': 'Cardell', 'given': 'Gregory Scott'}, 'show_email': 'NO'}]}
Year: 1993
DOI: 10.7907/25C5-1150
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Measurements in the near wake region of a circular cylinder in a uniform flow in the Reynolds number range [...] with permeable splitter plates spanning the wake center plane are presented. Permeability is defined by the pressure drop across the plates, and the relationship between permeability and plate solidity is determined for a set of plates constructed from woven wire mesh, permitting unambiguous characterization of the splitter plates by the solidity. The effects of different solidities on the flow in the near wake are investigated using smoke wire flow visualization, hot-wire anemometry, and measurements of the mean pressure at the cylinder surface, and the results are related to cylinder flow without a splitter plate.
Flow visualization results demonstrate that the introduction of low solidity splitter plates does not change the basic near wake structure, and that sufficiently high solidity uncouples the large-scale wake instability from the body, with the primary vortex formation occurring downstream of the separation bubble due to instability of the wake profile. Hotwire and surface pressure measurements confirm and quantify the flow visualization results, showing that the permeable splitter plates reduce the drag and modify the primary wake frequency. When the solidity is high enough that the wake is convectively unstable, the base pressure is independent of the Reynolds number and solidity. For a wide range of solidities, the same asymptotic value of the Strouhal number is reached at high Reynolds numbers. The relationship between the Strouhal number and the base pressure is discussed.
Detailed measurements in the separating shear layers with splitter plates moderating the primary vortex formation show that in the mean the development of the separating shear layers is similar to that of the plane mixing layer. The presence of the splitter plates enhances shear layer development, and it is found that for all solidities the instability amplifies a broad frequency band without discrete spectral features. Turbulent transition in the shear layer and its role in the pronounced Reynolds number dependency of the flow in this Reynolds number range is discussed. Acoustic excitation of the separated shear layers confirms the broad band frequency response of the shear layer instability, and the effect of the driven shear layers on the near wake is investigated.https://thesis.library.caltech.edu/id/eprint/1240Nominally 2-Dimensional Flow About a Normal Flat Plate
https://resolver.caltech.edu/CaltechETD:etd-04042005-105646
Authors: {'items': [{'email': 'lisoski@aerovironment.com', 'id': 'Lisoski-Derek-Lee-Ashton', 'name': {'family': 'Lisoski', 'given': 'Derek Lee Ashton'}, 'show_email': 'NO'}]}
Year: 1993
DOI: 10.7907/AZEG-2T16
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Towing tank and water channel experiments and a two-dimensional vortex element numerical model were used to study the forces experienced by a bluff flat plate set normal to a nominally two-dimensional flow. Intrinsic (small scale) and extrinsic (large scale) three-dimensional motions in the experimental flow were isolated and their separate and combined effects on forces and overall wake development were studied. Transient flow development starting from rest, as well as steady flow conditions, were investigated.
A force balance was used to measure the unsteady lift and drag of vertically oriented models projecting through a free surface with various lower end conditions; simultaneous LIF flow visualizations imaged the structure of the vortices in the wake. Plate aspect ratio, lower end condition and angle of attack were varied to effect changes in large scale three-dimensional motions, while changes in Reynolds number and Richardson number (flow stratification) modified the small scale three dimensionality intrinsic to the flow.
Towing tank experiments indicated that normal plates required sixty to one hundred chord lengths of travel to establish steady vortex shedding. An initial drag peak during acceleration was followed by a drag minimum of [...] reached while the wake was confined to a symmetric vortex bubble. Subsequent to the breakdown of this bubble, a region of symmetric flow with [...] and no vortex shedding was apparent for twenty to thirty chord lengths, followed by the final onset of vortex shedding which occurred exponentially. During this onset forces overshot their final steady-state values [...]. Flows with less large scale extrinsic three dimensionality (higher aspect ratio, "more two-dimensional" end conditions, and stratified flow) had longer development times and higher subsequent overshoot levels.
Small geometric asymmetries (angle of attack variations) increased the minimum drag level seen after the acceleration and resulted in an earlier breakdown of the closed wake, followed by an immediate transition to steady shedding. The breakdown of the initial bubble in this case was more coherent spanwise and did not result in a long-lasting symmetric nonshedding flow.
During "steady-state" shedding, modulation in the vortex shedding amplitude at a time scale of five to ten Strouhal periods resulted in a twenty percent fluctuation in mean drag level, with a corresponding increase in rms lift. This modulation accompanied a slow oscillation in the formation distance of the shed vorticity, the period of which was Reynolds number independent but decreased with increasing aspect ratio, reaching a minimum value of six Strouhal periods for aspect ratios greater than ten.
Agreement between three-dimensional experimental and two-dimensional numerical-model results was good at early times, indicating the experiments were two-dimensional until the breakdown of the closed wake bubble following the initial acceleration. Prior to this breakdown the numerical model of a normal plate gave a drag coefficient [...], similar to that given by the Kirchhoff-Rayleigh free-streamline prediction but lower than experiments. Small asymmetries of the 2d model resulted in an increase in the minimum drag level to [...] . Subsequent to the closed wake breakdown, drag levels of [...] are 65% higher than steady-state experimental values.
Although no region corresponding to the post-acceleration non-vortex-shedding seen experimentally was found in the basic numerical results, the addition of circulation decay to the numerical-model resulted in a region which appeared qualitatively similar. This circulation decay also decreased mean drag levels [...] and gave an exponential shedding onset with subsequent long period shedding modulation.
Stabilizing spanwise stratification of the experimental flow had little effect for Richardson numbers [...] (based on chord). For [...] and [...] a longer lasting post-acceleration closed wake was followed by strong initial shedding and a large drag overshoot, with a subsequent decrease in shedding amplitude and increase in formation distance to the levels seen in the unstratified [...] case, which exhibited considerable Reynolds number dependence. For plates at [...] angle of attack the symmetric nonvortex shedding region was reduced in duration and subsequent "steady-state" drag levels were increased ten to fifteen percent [...] from the unstratified case.https://thesis.library.caltech.edu/id/eprint/1265Tip effects on the vortex wake of an axisymmetric body at angle of attack
https://resolver.caltech.edu/CaltechETD:etd-08232007-081713
Authors: {'items': [{'email': 'david.bridges@tamucc.edu', 'id': 'Bridges-D-H', 'name': {'family': 'Bridges', 'given': 'David Henry'}, 'show_email': 'YES'}]}
Year: 1993
DOI: 10.7907/DZ4H-M010
Experiments on the effects of conditions at the tip of an axisymmetric body on the vortex wake of the body at large angle of attack in incompressible flow have been performed. The separation patterns that occur at the nose of a cone with a hemispherical tip have been documented for ranges of Reynolds number based on tip radius and of relative angle of attack. Tips with cross sections that vary smoothly from elliptic to circular have been designed and tested on a cone. The effectiveness of the tips in varying the cone side force coefficient as the tip is rotated has been demonstrated. The relation between tip roll angle and side force coefficient has been shown to be essentially independent of Reynolds number, for the range of Reynolds number tested, as well as cone base conditions and roll direction. The relation between tip roll angle and side force coefficient has been shown to be a strong function of angle of attack. The reasons for nonsmooth variation of side force coefficient with tip roll angle at higher angles of attack have been determined by examinations of the vortex wake geometry. Peaks in the magnitude of the side force coefficient after zero crossings have been shown to be caused by a rapid shift in the wake vortices away from a symmetric configuration as the tip rotates away from a symmetry condition. Reductions in magnitude of the side force coefficient between magnitude peaks have been demonstrated to be a result of the near vortex crossing over the cone centerline. This vortex crossover has been shown to occur near and after the breakaway from the cone of the far vortex. The ability of one of the elliptic cross section tips to produce smooth variations of side force coefficient with roll angle ahead of vortex breakaway has been demonstrated, suggesting that the tip might be an effective yaw control device for aircraft at high angle of attack.https://thesis.library.caltech.edu/id/eprint/3205Laser-Induced Thermal Ccoustics
https://resolver.caltech.edu/CaltechETD:etd-09182007-085047
Authors: {'items': [{'id': 'Cummings-Eric-Bryant', 'name': {'family': 'Cummings', 'given': 'Eric Bryant'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/p7mb-d967
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
Laser-induced thermal acoustics (LITA) is a new technique for remote nonintrusive measurement of thermophysical gas properties. LITA involves forming, via opto-acoustic effects, grating-shaped perturbations of gas properties using intersecting beams from a short-pulse laser. A third beam scatters coherently into a signal beam off the perturbation grating via acousto-optical effects. The evolution of the gas perturbations modulates the scattered signal beam. Accurate values of the sound speed, transport properties, and composition of the gas can be extracted by analyzing the signal beam.
An analytical expression for the spectrum, absolute magnitude, and time history of the LITA signal is derived. The optoacoustic effects of thermalization and electrostriction are treated. Finite beam-diameter, beam-duration, and thermalization-rate effects are included in the analysis. The expression accurately models experimental signals over a wide range of gas conditions.
Experimental tests using LITA have been conducted on pure and [...]-seeded air and helium at pressures ranging from ~0.1 kPa-14 MPa. Carbon dioxide has been explored near its liquid-vapor critical point. Accuracies of 0.1% in sound speed measurements have been achieved in these tests. Accuracies of ~1% have been achieved in measurements of thermal diffusivity, although beam misalignment effects have typically degraded this accuracy by a factor of ~10-20. Using LITA, susceptibility spectra have been taken of approximately a femtogram of [...]. The effects of fluid motion and turbulence have been explored. LITA velocimetry has been demonstrated, in which the Doppler shift of light scattered from a flowing fluid is measured. LITA velocimetry requires no particle seeding, has a coherent signal beam, and can be applied to pulsed flows. LITA has also been applied to measure single-shot [...] or "Rayleigh scattering" spectra of a gas using a technique of wavelength-division multiplexing, called multiplex LITA. The LITA apparatus used in these tests costs about one-tenth that of many conventional laser diagnostics. Narrowband LITA measurements of the sound speed and transport properties and multiplex LITA measurements of the spectral properties of gases may be taken in a single laser shot.
https://thesis.library.caltech.edu/id/eprint/3627Transport, stirring and mixing in two-dimensional vortex flows
https://resolver.caltech.edu/CaltechETD:etd-10182007-132306
Authors: {'items': [{'id': 'Min-I-A', 'name': {'family': 'Min', 'given': 'Inki A.'}, 'show_email': 'NO'}]}
Year: 1995
DOI: 10.7907/8y2c-8203
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
To understand the basic contribution of vortex motion in the transport and mixing of passive fluid, we study a system of N discrete vortices. With variation of N and [...] (a vorticity distribution parameter), we are able to experiment with a range of vortex dynamics sufficient to capture many of the features of two-dimensional turbulence in their elementary form - such as vortex merging (inverse cascade of energy), filamentation (enstrophy cascade), etc. With this model the mixing of the fluid is numerically studied via stretch statistics and the spatial distribution of a non-diffusive scalar interface. The spectrum of spatial distribution of scalars as a result of the stirring motion of the N vortices is particularly important in view of the recent (as well as historical) interest in the characterization of the scalar distribution in turbulence. We also examine the velocity field statistics and the Lagrangian motion of fluid particles. It is also instructive to look at the kinematic causes behind the types of statistics that are obtained for the velocity structure functions. A 'building block' approach to understanding these effects in turbulence may lie in building up from a collection of discrete vortices, as done in this thesis, to adding vortices of different scales and the three-dimensional effects. It is in the context of these wider issues that we study the N-vortex problem.
In the final part of this thesis we investigate the two-dimensional mixing produced by large scale vortical structures during the evolution of a spatially developing mixing layer. Although the advent of three-dimensionality and fully developed turbulence are essential features of mixing layers, it is still dominated by the large scale two-dimensional structures and its effect on the mixing is illustrated here.
https://thesis.library.caltech.edu/id/eprint/4169Bluff Body Flows in the Presence of a Free Surface
https://resolver.caltech.edu/CaltechETD:etd-06082007-075443
Authors: {'items': [{'id': 'Valluri-Siddhartha', 'name': {'family': 'Valluri', 'given': 'Siddhartha'}, 'show_email': 'NO'}]}
Year: 1996
DOI: 10.7907/2wbk-9k81
An experimental study is performed in a water tunnel (Re = 40,000 to Re = 60,000) to study the interaction between the wake of a circular disk and the free surface. The deformation of the free surface is correlated with the behavior of the wake by utilizing surface pictures, wake flow visualization, drag measurement and Digital Particle Image Velocimetry techniques. It is observed that the wake can exist in two modes with different stabilities. The flow can switch between these two modes and the switching process exhibits hysteresis. The topological differences between these modes and their relation to the observed surface patterns are discussed. The changes in the wake are reflected by an increase in Cd which reaches a maximum value when the upper edge of the disk is 0.125 diameters from the surface. Comparison is also made with a disk approaching a solid boundary.https://thesis.library.caltech.edu/id/eprint/2516On the Evaluation of Time-Dependent Fluid-Dynamic Forces on Bluff Bodies
https://resolver.caltech.edu/CaltechETD:etd-08102005-132659
Authors: {'items': [{'email': 'flavio@its.caltech.edu', 'id': 'Noca-Flavio', 'name': {'family': 'Noca', 'given': 'Flavio'}, 'show_email': 'NO'}]}
Year: 1997
DOI: 10.7907/K2Z0-9016
NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.
We present some exact expressions for the evaluation of time-dependent forces on a body in an incompressible and viscous cross-flow which only require the knowledge of the velocity field (and its derivatives) in a finite and arbitrarily chosen region enclosing the body.
Given a control volume V with external surface S which encloses an arbitrary body, the fluid-dynamic force F on the body can be evaluated from one of the following three expressions (in abbreviated form):
F = [...] + body motion terms,
F = [...] + body motion terms,
F = no volume integral terms + [...] + body motion terms,
where N is the space dimension, u is the flow velocity, [...] is the vorticity, x is the position vector, and the tensors [...], [...], [...] depend only on the velocity field u and its (spatial and temporal) derivatives.
The first equation is already known for either simply connected domains or inviscid flows. We re-derive it here for viscous flows in doubly connected domains (i.e. domains which include a body). We then obtain the second and third equation through a simple algebraic manipulation of the first equation.
These expressions are particularly useful for experimental techniques like Digital particle Image Velocimetry (DPIV) which provide time sequences of 2D velocity fields but not pressure fields.
They are tested experimentally with DPIV on two-dimensional, low Reynolds number circular cylinder flows. Both steady and unsteady motions are studied.https://thesis.library.caltech.edu/id/eprint/3081High-enthalpy shock/boundary-layer interaction on a double wedge
https://resolver.caltech.edu/CaltechETD:etd-02272008-125333
Authors: {'items': [{'email': 'jpdavis@sandia.gov', 'id': 'Davis-J', 'name': {'family': 'Davis', 'given': 'Jean-Paul'}, 'show_email': 'NO'}]}
Year: 1999
DOI: 10.7907/4C98-MN23
Interaction between a shock wave and a boundary layer at a compression corner can produce a region of separated flow. The length of separation is important in determining aerodynamic forces, and the heat transfer at reattachment is important for the design of thermal protection systems. The effects of high-enthalpy flow on these phenomenon, particularly separation length, are not well known. Experiments to measure separation length and reattachment heating are performed in the T5 Hypervelocity Shock Tunnel using nitrogen test gas and a double-wedge geometry which allows greater control over local flow conditions at separation and, at high incidence angle, may produce real-gas effects due to dissociation behind the leading shock. Local external flow conditions were found by computational reconstruction of the inviscid nonequilibrium flow field.
Application of results from asymptotic theory to a simple model for separation leads to a new scaling parameter which approximately accounts for wall temperature effects on separation length for a laminar nonreacting boundary layer and extends previous results to arbitrary viscosity law. A. classification is introduced which divides mechanisms for real-gas effects into those acting internal and external to viscous regions of the flow, with internal mechanisms further subdivided into those arising upstream and downstream of separation. Application of the ideal dissociating gas model to a scaling law based on local external flow parameters and a nonreacting boundary layer shows that external mechanisms due to dissociation decrease separation length at low incidence but depend on the free-stream dissociation at high incidence, and have only a small effect on peak heating. A limited numerical study of reacting boundary layers shows that internal mechanisms due to recombination in the upstream boundary layer cause a slight decrease in separation length and a large increase in heat flux relative to a nonreacting boundary layer with the same external conditions.
Correlations are presented of experimentally measured separation length using local external flow parameters computed for reacting flow, which scales out external mechanisms but not internal mechanisms. These show the importance of the new scaling parameter in high-enthalpy flows, a linear relationship between separation length and reattachment pressure ratio as found previously for supersonic interactions, and a Reynolds-number effect for transitional interactions. A significant increase in scaled separation length is observed for high-enthalpy data in the laminar regime, and this is attributed to an internal recombination mechanism occurring in the separated shear layer. Experimental data for reattachment heat flux are found to agree roughly with existing correlations and to exhibit an increase due to an internal recombination mechanism, but cannot provide further insight due to large scatter.https://thesis.library.caltech.edu/id/eprint/788Effects of Damping and Reynolds Number on Vortex-Induced Vibrations
https://resolver.caltech.edu/CaltechETD:etd-11292006-120631
Authors: {'items': [{'email': 'joseph.klamo@gmail.com', 'id': 'Klamo-Joseph-Thomas', 'name': {'family': 'Klamo', 'given': 'Joseph Thomas'}, 'orcid': '0000-0003-1458-0379', 'show_email': 'YES'}]}
Year: 2007
DOI: 10.7907/S27Z-J533
<p>Vortex-induced vibrations have been studied experimentally with emphasis on damping and Reynolds number effects. Our system was an elastically-mounted rigid circular cylinder, free to oscillate only transverse to the flow direction, with very low inherent damping. We were able to prescribe the mass, damping, and elasticity of the system over a wide range of values, with the damping controlled by a custom-made variable magnetic eddy-current damping system.</p>
<p>Special emphasis is put on a nontraditional parameter formulation. The advantages of this formulation are explained, and an important new parameter, effective stiffness, is introduced. Using this new formulation, the amplitude and frequency responses are only a function of damping, Reynolds number, and effective stiffness. We show the effects that damping and Reynolds number each have on the amplitude and frequency response profiles and make the interesting observation that changes in damping or Reynolds number have similar effects.</p>
<p>The maximum amplitudes of our systems are studied in detail. We theoretically show that they should be functions of both damping and Reynolds number. This allows us to create constant-Reynolds-number curves of maximum amplitude over a large range of damping values, which we call a "generalized" Griffin plot. We also define maximum amplitudes in the case of zero damping as limiting amplitudes, and show that they are only a function of Reynolds number. We experimentally determine our limiting amplitude dependence on Reynolds number over the range 200 < Reynolds number < 5050.</p>
<p>Discontinuities in the amplitude response profile are also investigated. The discontinuity between the initial branch and the large-amplitude, upper branch is studied in two ways. First, the time-averaged behavior is examined to understand what controls the discontinuity and look for damping and Reynolds number effects. Second, we track the cycle-by-cycle transient response through this discontinuous amplitude change, induced either by changes in the tunnel velocity or system damping. Finally, we also find a new discontinuity hysteresis region between the lower branch and the desynchronized region, which appears to be a low Reynolds number effect and is only seen in systems with Reynolds number < 1000.</p>https://thesis.library.caltech.edu/id/eprint/4684