Article records
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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenThu, 30 Nov 2023 18:27:33 +0000A Flexible Nozzle for a Small Supersonic Wind Tunnel
https://resolver.caltech.edu/CaltechAUTHORS:20141202-143708452
Authors: Dhawan, Satish; Roshko, Anatol
Year: 1951
DOI: 10.2514/8.1922
The design of a small supersonic wind-tunnel test section (4 by 10 in.) incorporating a flexible nozzle is outlined. The flexible nozzle consists of a high-strength stepped steel plate. Two screw jacks provide an easy means of continuously changing the nozzle's shape according to the aerodynamic requirements. The boundary-layer compensation can also be varied during operation. Pressure surveys, together with schlieren and interferometric analysis of the test section, show the flow to be uniform over the operating range (M = 1.1 to 1.5).https://authors.library.caltech.edu/records/erq5z-c7879On the Wake and Drag of Bluff Bodies
https://resolver.caltech.edu/CaltechAUTHORS:20141202-134209863
Authors: Roshko, Anatol
Year: 1955
DOI: 10.2514/8.3286
A modification to Kirchhoff's free streamline introduces the parameter k = √(1- C_ps), which allows arbitrary base pressure and which must depend on the dynamics of the wake. For a cylinder of given cross-sectional shape, the drag, C_D, and the wake width, d', are functions of k only. These functions are used to relate C_D and the dimensionless shedding frequency , S = nd/U_ ∞ to another number, S* = nd' / U_s, which is based on wake parameters. It is found that S* = 0.16 for all cylinders. In another approach, k is evaluated by using Karman's solution for the vortex street.https://authors.library.caltech.edu/records/68wfp-z0v46On the Effect of Air Pressure on Strouhal Number
https://resolver.caltech.edu/CaltechAUTHORS:20141202-140924487
Authors: Roshko, Anatol
Year: 1959
DOI: 10.2514/8.7958
The experimental measurements of reference 1 show an effect of free-stream pressure on Reynolds Number relation for a vortex-shedding cylinder.https://authors.library.caltech.edu/records/9msfc-jgf72On flow duration in low-pressure shock tubes
https://resolver.caltech.edu/CaltechAUTHORS:ROSpof60
Authors: Roshko, Anatol
Year: 1960
DOI: 10.1063/1.1706147
The severe decrease of flow duration in shock tubes operating at low pressures, previously reported by Duff, is confirmed by experiment and by an analysis of the effects of the laminar-boundary layer behind the shock wave. The latter leads to a shock tube similarity length parameter X, which depends on the tube pressure, diameter and shock Mach number, and to a flow duration parameter T. The theoretical relation T = T(X) is determined and compared with experimental results. From the theoretical result Tmax = 1, the maximum possible flow duration τm in a shock tube is determined; it increases linearly with the initial pressure and the square of the tube diameter and decreases strongly with shock Mach number.https://authors.library.caltech.edu/records/fh45t-5w145Experiments on the flow past a circular cylinder at very high Reynolds number
https://resolver.caltech.edu/CaltechAUTHORS:ROSjfm61
Authors: Roshko, Anatol
Year: 1961
DOI: 10.1017/S0022112061000950
Measurements on a large circular cylinder in a pressurized wind tunnel at Reynolds numbers from 10^6 to 10^7 reveal a high Reynolds number transition in which the drag coefficient increases from its low supercritical value to a value 0.7 at R = 3.5 × 10^6 and then becomes constant. Also, for R > 3.5 × 10^6, definite vortex shedding occurs, with Strouhal number 0.27.https://authors.library.caltech.edu/records/m8vtc-33e74A Novel Device for Bursting Shock-Tube Diaphragms
https://resolver.caltech.edu/CaltechAUTHORS:20120907-153109132
Authors: Roshko, A.; Baganoff, D.
Year: 1961
DOI: 10.1063/1.1706238
The new GALCIT 17-in. low-pressure shock tube is equipped with a novel device which was
designed to overcome some of the difficulties encountered in attempting to precisely select and control the bursting pressure of very thin diaphragms. This device is a cutter, shown in Figs. 1 and 2, which consists of two blades set at right angles in a cruciform configuration and positioned in the tube on the low-pressure side of the diaphragm.https://authors.library.caltech.edu/records/pfb7j-dgn38A 17-inch Diameter Shock Tube for Studies in Rarefied Gasdynamics
https://resolver.caltech.edu/CaltechAUTHORS:20130509-141109412
Authors: Liepmann, H. W.; Roshko, Anatol; Coles, Donald; Sturtevant, Bradford
Year: 1962
DOI: 10.1063/1.1746625
A shock tube for studying problems in rarefied gasdynamics is described. The motivation for operating at low density (to increase the length and time scales of certain interesting flows) and the effect of low density on the performance and design of the shock tube are discussed. In order to guarantee uniform and reproducible shock waves of moderate strength, the configuration of the tube is conventional. However, innovations are introduced (for example in the suspension, the pumping system, and the diaphragm loading and rupturing mechanism) to simplify the operation of the large facility. Care in the design of the tube as a vacuum system has resulted in a leak rate of less than 0.01 μ Hg per hour. A series of shakedown runs at relatively high pressures has shown, for example, that the reproducibility of a given shock Mach number is ±0.6%.https://authors.library.caltech.edu/records/z8ejr-5kj55Rarefied gas dynamics
https://resolver.caltech.edu/CaltechAUTHORS:20170731-143548171
Authors: Estermann, I.; Roshko, A.
Year: 1964
DOI: 10.1016/0376-0421(64)90007-7
The program of this symposium, which was held in Paris 26–29 June, 1962, included papers on new developments in the theory of flow of neutral and partly ionized gases at low densities, interactions between gas molecules and solid surfaces, high-speed and thermal molecular beams, measurements of drag and heat transfer in the transition and free molecule flow regime, and new experimental techniques. About 25 of the 60 papers presented are discussed in the following pages; the complete program is reproduced as Appendix I.https://authors.library.caltech.edu/records/xspgc-c0523Measurements of test time in the GALCIT 17-inch shock tube
https://resolver.caltech.edu/CaltechAUTHORS:20141202-141432548
Authors: Roshko, Anatol; Smith, Jerome A.
Year: 1964
DOI: 10.2514/3.2272
Experimental measurements of test time were obtained in the GALCIT 17-in. shock tube using both air and argon for driven gases. One series of tests was conducted using a constant driver pressure (pure helium) for various initial pressures of the driven gases. Another series was conducted using air for the driven gas at various initial pressures holding the shock Mach number constant. The data are presented and compared to theoretical predictions computed from the theory in two recent papers by Mirels for the case of a laminar and turbulent wall boundary layer.https://authors.library.caltech.edu/records/hjfef-77f02Observations of turbulent reattachment behind an axisymmetric downstream-facing step in supersonic flow
https://resolver.caltech.edu/CaltechAUTHORS:20141202-142049273
Authors: Roshko, Anatol; Thomke, Gerald J.
Year: 1966
DOI: 10.2514/3.3590
Supersonic flow over a downstream-facing step on the circumference of a large, ducted, axisymmetric body was used to study flow reattachment. Step heights h were 0.25, 1.00, and 1.68 in., compared to a body radius of 6 in. Freestream Mach numbers were in the range 2 to 4.5. Theturbulent boundary-layer thickness just ahead of the step varied from 0.14 to 0.19 in. (momentum thicknesses of about 0.01 in.).
Surface pressure distributions throughout the region of separation and reattachment were measured, and points of reattachment were determined. Comparison of the shapes of the pressure distributions for various step heights shows that the initial (steepest) parts of the reattachment pressure rise, up to the point of reattachment, tend to become superimposed when plotted against x/h. Downstream reattachment the curves branch out, exhibiting a dependence on geometry and probably on initial shear layer profile. In the region of the initial pressure rise (near the end of the "dead air" region) dynamic pressures are low; the pressure rise there apparently is balanced by turbulent shear stress.https://authors.library.caltech.edu/records/gc4eb-8bn08Effect of a shoulder modification on turbulent supersonic base flow
https://resolver.caltech.edu/CaltechAUTHORS:20141202-143708545
Authors: Roshko, Anatol; Thomke, Gerald J.
Year: 1967
DOI: 10.2514/3.4084
It has been observed experimentally by Hama and discussed theoretically by Weinbaum that effects of the fast expansion and consequent lip shock at the shoulder of a supersonic base or downstream-facing step can be quite appreciable at high Mach number. Hama found that the lip shock can be much stronger than has been assumed. He also drew attention to characteristic humps or peaks in the pressure distribution on the reattachment surface; these, he showed, could be attributed to secondary waves directed toward the surface from the point of interaction of the lip shock with the main recompression shock. Scherberg and Smith have also drawn attention to the possible strong effects connected with a lip shock. In this note, we report some further observations of the occurrence of this phenomenon, and its elimination by a small modification at the shoulder to alleviate the fast expansion there.https://authors.library.caltech.edu/records/xrxqt-gkr11Transition in incompressible near-wakes
https://resolver.caltech.edu/CaltechAUTHORS:ROSpof67
Authors: Roshko, Anatol
Year: 1967
DOI: 10.1063/1.1762441
An experimental study of the near-wakes of a family of flat-based cylinders of varying chord-thickness ratio, L/d, shows that the phenomena of vortex shedding and transition are similar to those for a cylinder of circular section. The Reynolds number for appearance of turbulence in the wake is correlated with L/d.https://authors.library.caltech.edu/records/c8k0y-bf930On Real Fluid Flow Over Yawed Circular Cylinders
https://resolver.caltech.edu/CaltechAUTHORS:20141210-143927153
Authors: Chiu, W. S.; Lienhard, J. H.; Dalton, C.; Roshko, A.; Trefethen, L.
Year: 1967
The equations for both the boundary layer and the outer potential flow over a yawed cylinder can be resolved into equations for the crosswise and spanwise velocity components. These components of the boundary layer are evaluated using Sears' method, and the separation point is found to be uninfluenced by the yaw angle. The potential-flow solutions for the spanwise and crosswise flows are added together to determine vortex patterns behind the cylinder. The approximate direct dependence of the Strouhal number upon the cosine of the yaw angle and/or the drag coefficient upon the square of the cosine, are verified. Experimental determinations of the Strouhal number and visualization of the flow pattern are consistent with the analysis.https://authors.library.caltech.edu/records/v8q72-pf939Shock Tubes in Rarefied Gas Flow Research
https://resolver.caltech.edu/CaltechAUTHORS:20130510-081002532
Authors: Coles, D.; Liepmann, H. W.; Roshko, A.; Sturtevant, B.
Year: 1968
The flow
within a shock wave is governed by the relaxation times of
the molecular degrees of freedom. Advances in shock tube
design and instrumentation have made it possible in recent
years to resolve all the relaxation times including the shortest,
corresponding to the translational degree of freedom.
The shock tube thus becomes an important tool for critical
experiments in the study of the range of applicability of the
Navier-Stokes equations and similar approximations and of
the character of solutions of the Boltzmann equation. Significant
progress has been made recently in the understanding
of the most obvious such problem, the flow within a shock in
a monatomic gas. Theory and experiment are now in substantial
agreement and the over-all process of energy exchange
is understood. Problems connected with shock wave
reflection from real walls have made progress but a host of
problems remain to be studied including surface interaction
effects. The extension of this type of shock tube research to
more complicated systems, reacting gases, gas mixtures,
and the like has begun and some progress can be reported.
Recent experimental progress is illustrated by a number of
measurements made at GALCIT in the 17- and 6-in. shock
tubes. Sophistications in shock tube design and instrumentation
will be discussed.https://authors.library.caltech.edu/records/yxtj9-f8y97Shock Tubes in Rarefied Gas Flow Research
https://resolver.caltech.edu/CaltechAUTHORS:20130509-142125916
Authors: Coles, D.; Liepmann, H. W.; Roshko, A.; Sturtevant, B.
Year: 1969
DOI: 10.1063/1.1692612
The flow within a shock wave is governed by the relaxation times of the molecular degrees of freedom.
Advances in shock-tube design and instrumentation in recent years have made it possible to resolve all the
relaxation times including the shortest, corresponding to the translational degrees of freedom. The shock
tube thus becomes an important tool for critical experiments in the study of the range of applicability of
the Navier-Stokes equations and similar approximations and of the character of solutions of the Boltzmann
equation. Significant progress has recently been made in the understanding of the most obvious such problem,
the flow within a shock in a monatomic gas. Theory and experiment are now in substantial agreement and
the over-all process of energy exchange is understood. Progress has been made in problems connected with
shock wave reflection from real walls, but a host of others remain to be studied including surface interaction
effects. The extension of this type of shock-tube research to more complicated systems, reacting gases, gas
mixtures, and the like has begun and some progress can be reported. Recent experimental progress is illustrated
by a number of measurements made in the 6- and 17-in. shock tubes at the California Institute of
Technology.https://authors.library.caltech.edu/records/fd0ez-w3x85Near wake of a hypersonic blunt body with mass addition
https://resolver.caltech.edu/CaltechAUTHORS:20141201-162815216
Authors: Collins, Donald J.; Lees, Lester; Roshko, Anatol
Year: 1970
DOI: 10.2514/3.5775
An experimental investigation of the steady, laminar near-wake flowfield of a two-dimensional, adiabatic, circular cylinder ·with surface mass transfer has been made at a freestream Mach number of 6.0. The pressure and mass- concentration fields associated with the transfer of argon, nitrogen, or helium into the near wake were studied for mass transfer from the forward stagnation region, and from the base. For sufficiently low mass transfer rates from the base, for which a recirculating zone exists, the entire near-wake flowfield correlates with the momentum flux, not the mass flux, of the injectant, and the mass-concentration field is determined by counter-current diffusion into the reversed flow. For mass addition from the forward stagnation region, the pressure field is undisturbed and the mass- concentration field is nearly uniform in the region of reversed flow. The axial decay of argon mass concentration in the intermediate wake, downstream of the neck, is explained with the aid of an integral solution in the incompressible plane, from which the location of the virtual origin for the asymptotic far-wake solution has been derived as one result.https://authors.library.caltech.edu/records/k9t1y-dnm29The effect of a density difference on shear-layer instability
https://resolver.caltech.edu/CaltechAUTHORS:DAVjfm72
Authors: Davey, Robert F.; Roshko, Anatol
Year: 1972
DOI: 10.1017/S0022112072000291
Measurements of mass flow rate and mean density have been made in separated laminar boundary layers with large transverse density gradients. Two-dimensional shear layers were formed by exhausting a half-jet of one gas into a reservoir of another gas with a different molecular weight. Two freons with a density ratio of 1-98 and unusual properties which permitted the measurement of the mass flow rate with a single hot wire were used. A n analysis of the mass flow rate fluctuations showed that a negative density gradient (i.e. light gas flowing into heavy) increases the amplification rate of the instability oscillations and reduces the frequency and wave number. Opposite trends were observed when the density gradient was positive. These findings are in agreement with recent theoretical predictions.https://authors.library.caltech.edu/records/618bv-cf546On density effects and large structure in turbulent mixing layers
https://resolver.caltech.edu/CaltechAUTHORS:BROjfm74
Authors: Brown, Garry L.; Roshko, Anatol
Year: 1974
DOI: 10.1017/S002211207400190X
Plane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus. Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures. High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones. The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates. Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed.https://authors.library.caltech.edu/records/m46cp-n2977Flare-Induced Interaction Lengths in Supersonic, Turbulent Boundary Layers
https://resolver.caltech.edu/CaltechAUTHORS:20141201-160815389
Authors: Roshko, A.; Thomke, G. J.
Year: 1976
DOI: 10.2514/3.61429
Experimental results are presented for the effects of Mach number, Reynolds number, and corner angle on flare-induced separation of a supersonic, turbulent boundary layer. Measurements were obtained for upstream interaction distance ℓ_0 from the flare to the beginning of the interaction for Mach numbers 2≤M≤4.5, boundary-layer thickness Reynolds numbers 10^5 < R_δ < 10^6, and adiabatic wall conditions. Flares of angle α≤40° were attached to a hollow-cylinder model of 12 in. diam at either x_c= 14 or 18 in. downstream from the sharp leading edge. It was found that ℓ_0/δ_0 decreases with increasing Mach number and Reynolds number and increases with flare angle. For constant α, when ℓ_0/δ_0 is plotted vs the local skin-friction coefficient, C_(f0), the Mach number dependence disappears. From this observation, a simple correlation formula was obtained and used to compare results from other investigations, and also to correlate incipient separation data.https://authors.library.caltech.edu/records/1kj67-05y95Structure of Turbulent Shear Flows: A New Look
https://resolver.caltech.edu/CaltechAUTHORS:20141201-155051947
Authors: Roshko, Anatol
Year: 1976
DOI: 10.2514/3.61477
The problem of turbulent now continues to be an outstanding
one in technology and in physics. Of the nine
Dryden research lectures so far, four have been on some
aspect of the turbulence problem. At meetings such as this one
the turbulence problem is always the subject of some sessions
and lurks in the background of many others; for example,
separated now, combustion, jet noise, chemical lasers, atmospheric
problems, etc. It is continually the subject of conferences,
workshops and reviews. In his time Hugh Dryden
wrote several reviews of turbulent now. In reading some of
them again, one statement particularly relevant to the
present lecture caught my attention: "-it is necessary to
separate the random processes from the nonrandom
processes. It is not yet fully clear what the random elements
are in turbulent now." Neither is it fully clear what the
nonrandom, orderly elements are, but some of them are
beginning to be recognized and described.
Generally the picture one has had of turbulence is of chaos
and disorder, implicit in the name. Although it was known
that organized motion could exist, superimposed on the
background of "turbulence," for example, vortex shedding
from a circular cylinder up to Reynolds numbers of 10^7, such
examples were regarded as special cases closely tied to their
particular geometric origins and not characteristic of "well-developed"
turbulence. It was known that large structures are
important in the development of turbulent shear flows and
that these ought to possess some definable features. But even
when the concept of a characteristic "big eddy" was explored,
it was usually in the context of a statistical quantity. The
earliest and most decisive attempts to define the form of such
large eddies were made by Townsend and his students. In
recent years it has become increasingly evident that turbulent
shear flows do contain structures or eddies whose description
is more deterministic than had been thought, possessing identifiable
characteristics, existing for significant lifetimes,
and producing recognizable and important events. More accurate
descriptions of their properties, how they fit into the
complete description of a turbulent flow, to what extent are
they central to its development, and how they can be reconciled
with the apparent chaos and disorder, are problems
which are becoming of interest to an increasing number of
researchers. It is the purpose of this lecture to describe some
of these new developments. The discussion will draw largely
on experiences from our own laboratory; it is not intended to
be a complete survey. Other discussions of these ideas can be
found in various recent publications.https://authors.library.caltech.edu/records/aj81m-pgq19An experimental study of geometrical effects on the drag and flow field of two bluff bodies separated by a gap
https://resolver.caltech.edu/CaltechAUTHORS:KOEjfm85
Authors: Koenig, Keith; Roshko, Anatol
Year: 1985
DOI: 10.1017/S002211208500204X
This paper describes an experimental investigation of the shielding effects of various disks placed coaxially upstream of an axisymmetric, flat-faced cylinder. Remarkable decrease of the drag of such a system was observed for certain combinations of the basic geometric parameters, namely the diameter and gap ratios. For such optimum shielding the stream surface which separates from the disk reattaches smoothly onto the front edge of the cylinder, in what is close to a 'free-streamline' flow; alternatively, the flow may be viewed as a cavity flow. For the optimum as well as other geometries, flow pictures, pressure distributions and some LDV measurements were also obtained. From these, several flow regimes depending on the gap/diameter parameters were identified. Variations on the axisymmetric disk–cylinder configuration included a hemispherical frontbody, rounding of the front edge of the cylinder and a change from circular to square cross-section.https://authors.library.caltech.edu/records/7bxvw-9ar84Streamwise vortex structure in plane mixing layers
https://resolver.caltech.edu/CaltechAUTHORS:BERjfm86
Authors: Bernal, L. P.; Roshko, A.
Year: 1986
DOI: 10.1017/S002211208600099X
The development of three-dimensional motions in a plane mixing layer was investigated experimentally. It is shown that superimposed on the primary, spanwise vortex structure there is a secondary, steamwise vortex structure. Three aspects of this secondary structure were studied. First, the spanwise vortex instability that generates the secondary structure was characterized by measurements of the critical Reynolds number and the spanwise wavelength at several flow conditions. While the critical Reynolds number was found to depend on the velocity ratio, density ratio and initial shear-layer-profile shape, the mean normalized wavelength is independent of these parameters. Secondly, flow visualization in water was used to obtain cross-sectional views of the secondary structure associated with the streamwise counter-rotating vortices. A model is proposed in which those vortices are part of a single vortex line winding back and forth between the high-speed side of a primary vortex and the low-speed side of the following one. Finally, the effect of the secondary structure on the spanwise concentration field was measured in a helium-nitrogen mixing layer. The spatial organization of the secondary structure produces a well-defined spanwise entrainment pattern in which fluid from each stream is preferentially entrained at different spanwise locations. These measurements show that the spanwise scale of the secondary structure increases with downstream distance.https://authors.library.caltech.edu/records/b849g-3ew33The effect of flow oscillations on cavity drag
https://resolver.caltech.edu/CaltechAUTHORS:GHAjfm87
Authors: Gharib, M.; Roshko, A.
Year: 1987
DOI: 10.1017/S002211208700106X
An experimental investigation of flow over an axisymmetric cavity shows that self-sustained, periodic oscillations of the cavity shear layer are associated with low cavity drag. In this low-drag mode the flow regulates itself to fix the mean-shear-layer stagnation point at the downstream corner. Above a critical value of the cavity width-to-depth ratio there is an abrupt and large increase of drag due to the onset of the 'wake mode' of instability. It is also shown by measurement of the momentum balance how the drag of the cavity is related to the state of the shear layer, as defined by the mean momentum transport $\rho\overline{u}\overline{v}$ and the Reynolds stress $\rho\overline{u^{\prime}v^{\prime}}$, and how these are related to the amplifying oscillations in the shear layer. The cavity shear layer is found to be different, in several respects, from a free shear layer.https://authors.library.caltech.edu/records/1pdp3-c5n63Observations of supersonic free shear layers
https://resolver.caltech.edu/CaltechAUTHORS:20141210-135606485
Authors: Papamoschou, D.; Roshko, A.
Year: 1988
DOI: 10.1007/BF02745657
Visual spreading rates of turbulent shear layers with at least one stream supersonic were measured using Schlieren photography. The experiments were done at a variety of Mach number-gas combinations. The spreading rates are correlated with a compressibility-effect parameter called the convective Mach number. It is found that for supersonic values of the convective Mach number, the spreading rate is about one quarter that of an incompressible layer at the same velocity and density ratio. The results are compared with other experimental and theoretical results.https://authors.library.caltech.edu/records/evwwr-4fj03Large structure in the far wakes of two-dimensional bluff bodies
https://resolver.caltech.edu/CaltechAUTHORS:CIMjfm88
Authors: Cimbala, John M.; Nagib, Hassan M.; Roshko, Anatol
Year: 1988
DOI: 10.1017/S0022112088001314
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 Kármán-vortex-street frequency is observed. Beyond this region of decay, larger-scale (lower wavenumber) structure can be seen. In the far wake (beyond one hundred diameters) a broad band of frequencies is selectively amplified and then damped, the centre of the band shifting to lower frequencies as downstream distance is increased.
The far-wake structure does not depend directly on the scale or frequency of Kármán vortices shed from the cylinder; i.e. it does not result from amalgamation of shed vortices. The growth of this structure is due to hydrodynamic instability of the developing mean wake profile. 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 [approximate] 6000), which do not initially shed Kármán-type vortices into the wake.
Measured prominent frequencies in the far cylinder wake are in good agreement with those estimated by two-dimensional locally parallel inviscid linear stability theory, when streamwise growth of wake width is taken into account. Finally, three-dimensionality in the far wake of a circular cylinder is briefly discussed and a mechanism for its development is suggested based on a secondary parametric instability of the subharmonic type.https://authors.library.caltech.edu/records/tywrh-ydt20Vortex formation in the wake of an oscillating cylinder
https://resolver.caltech.edu/CaltechAUTHORS:20170726-124346409
Authors: Williamson, C. H. K.; Roshko, A.
Year: 1988
DOI: 10.1016/S0889-9746(88)90058-8
When a body oscillates laterally (cross-flow) in a free stream, it can synchronize the vortex formation frequency with the body motion frequency. This fundamental "lock-in" regions is but one in a whole series of synchronization regions, which have been found in the present paper, in an amplitude-wavelength plane (defining the body trajectory) up to amplitudes of five diameters. In the fundamental region, it is shown that the acceleration of the cylinder each half cycle induces the roll-up of the two shear layers close to the body, and thereby the formation of four regions of vorticity each cycle. Below a critical wavelength, each half cycle sees the coalescence of a pair of like-sign vortices and the development of a Karman-type wake. However, beyond this wavelength the like-sign vortices convect away from each other, and each of them pairs with an opposite-sign vortex. The resulting wake comprises a system of vortex pairs which can convect away from the wake centerline. The process of pairing causes the transition between these modes to be sudden, and this explains the sharp change in the character of the cylinder forces observed by Bishop and Hassan, and also the jump in the phase of the lift force relative to body displacement. At precisely the critical wavelength, only two regions of vorticity are formed, and the resulting shed vorticity is more concentrated than at other wavelengths. We interpret this particular case as a condition of "resonant synchronization", and it corresponds with the peak in the body forces observed in Bishop and Hassan's work.https://authors.library.caltech.edu/records/0nbns-0xt75The compressible turbulent shear layer: an experimental study
https://resolver.caltech.edu/CaltechAUTHORS:PAPjfm88
Authors: Papamoschou, Dimitri; Roshko, Anatol
Year: 1988
DOI: 10.1017/S0022112088003325
The growth rate and turbulent structure of the compressible, plane shear layer are 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. Ten combinations of gases and Mach numbers are studied in which the free-stream Mach numbers range from 0.2 to 4. Schlieren photography of 20-ns exposure time reveals very low spreading rates and large-scale structures. The growth of the turbulent region is defined by means of Pitot-pressure profiles measured at several streamwise locations. A compressibility-effect parameter is defined that correlates and unifies the experimental results. It is the Mach number in a coordinate system convecting with the velocity of the dominant waves and structures of the shear layer, called here the convective Mach number. It happens to have nearly the same value for each stream. In the current experiments, it ranges from 0 to 1.9. 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 rapidly with increasing convective Mach number, reaching an asymptotic value of about 0.2 for supersonic convective Mach numbers.https://authors.library.caltech.edu/records/ktwm0-v0j30Measurements of base pressure in the wake of a cylinder at low Reynolds numbers
https://resolver.caltech.edu/CaltechAUTHORS:20141201-152316304
Authors: Williamson, C. H. K.; Roshko, A.
Year: 1990
Measurements have been made of the base pressure
coefficients of long circular cylinders at low Reynolds numbers
from 35 to 1100 in a cross flow. The variation of "base suction"
(negative base pressure coefficient or - C_(pb)) with Reynolds number
shows a remarkably good correspondence with the variation
of the Strouhal number. It is also possible to relate the variation
in suction with the physical modes of wake formation found in
previous studies. It is shown that the level of base suction is
affected in only a minor fashion by whether parallel or oblique
shedding is present, and it is found that the data are independent
of the base pressure hole sizes and cylinder diameters that were
used. The base suction is essentially constant along the span of
a cylinder (outside of regions extending about 20 diameters in
from each end), and the results arc independent of cylinder
aspect ratio (L/D) provided that one exceeds a critical value of
L/D.https://authors.library.caltech.edu/records/6h03s-1s451Maximum values of gas-dynamic flux densities
https://resolver.caltech.edu/CaltechAUTHORS:20120308-110759673
Authors: Roshko, A.
Year: 1993
DOI: 10.1063/1.858624
A general result valid for any compressible fluid is noted. It gives the maximum values of the flux densities of mass, momentum, and kinetic energy in steady and unsteady flows which are expanding isentropically from a reservoir.https://authors.library.caltech.edu/records/rc7x0-9sj63Perspectives on bluff body aerodynamics
https://resolver.caltech.edu/CaltechAUTHORS:20141201-131826320
Authors: Roshko, A.
Year: 1993
DOI: 10.1016/0167-6105(93)90007-B
The variations of drag and base suction of circular cylinders and bluff plates over the range of Reynolds number from 10 to 10^7 is discussed, with emphasis on the importance of the separated shear layers. A model for flows with a wake splitter plate is proposed. Effects of "three dimensionality" are discussed.https://authors.library.caltech.edu/records/tk83s-rzk94Experiments on flow past rough circular cylinders at large Reynolds numbers
https://resolver.caltech.edu/CaltechAUTHORS:20130509-075105354
Authors: Shih, W. C. L.; Wang, C.; Coles, D.; Roshko, A.
Year: 1993
DOI: 10.1016/0167-6105(93)90030-R
Data have been obtained in the 12-foot pressurized wind tunnel at NASA Ames Research Center on flow past smooth and rough circular cylinders at high Reynolds numbers. Steady and unsteady surface pressures were measured around and along the cylinders. Analysis of the results provides lift and drag coefficients and Strouhal numbers for cylinders of several roughness at Reynolds numbers up to 8 × 10^6.https://authors.library.caltech.edu/records/y6ac5-38y40Vortical structure in the wake of a transverse jet
https://resolver.caltech.edu/CaltechAUTHORS:FRIjfm94
Authors: Fric, T. F.; Roshko, A.
Year: 1994
DOI: 10.1017/S0022112094003800
Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events' in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.https://authors.library.caltech.edu/records/aygbw-wpx87Temporal Behavior of Lifted Turbulent Jet Flames
https://resolver.caltech.edu/CaltechAUTHORS:20141201-130213554
Authors: Hammer, Jay A.; Roshko, Anatol
Year: 2000
DOI: 10.1080/00102200008947285
Measurements of the fluctuating liftoff height of turbulent jet flames were made using a linear photo-diode array. A wide range of experimental conditions was investigated, including varying the fuel, nozzle diameter and liftoff height.
The amplitudes of the fluctuations in h were found to be of the order of the local large scale of the jet. There is a slight increase in normalized fluctuation level h′/h¯ with the mean liftoff height h¯ and there is some variation of h′/h¯ with fuel type. The dominant time scales of the fluctuations of h were found to be considerably longer than the local large-scale time of the turbulence defined as the local jet width divided by the centerline velocity, τ_δ ≡δ/U_(cl). By using fuels of different chemical times to vary τ_δ. the measured correlation time τ_(1/2) normalized by τ_δ was found to collapse with Richardson number ξ_h However, the Richardson numbers achieved are dependent largely on the chemical rates of the fuels. Also, experiments in which the nozzles were oriented horizontally showed no change in τ_(1/2).https://authors.library.caltech.edu/records/2v2c3-wd783Small is good
https://resolver.caltech.edu/CaltechAUTHORS:20141201-124222548
Authors: Roshko, Anatol
Year: 2000
Dave Belden's letter announcing the award was really a surprise, almost a shock. At first l wondered whether it was another example of a story which you may have heard and which, I believe, originated in the FSU. Two friends are at a grand reception sipping cocktails when one notices a man with his chest almost completely covered with medals. Says one to the other, "Do you have any idea what those medals are for?" and the other replies ... Well, you see that one at the top left? That one was a mistake: and the others followed automatically ... I humored myself out of that thought but not out of a feeling of guilt. You see, I suddenly felt terrible that I was not a member of the ASME. There had been opportunities but somehow l had let them go by. One reason is that I was concerned about another onslaught of communications, information and other paper that always results and require attention. Fortunately, ASME lost no time in relieving my guilt. In a few weeks I received a nice invitation and forms to fill out, and am I am Member No. 6143358. And sure enough, information has begun to roll in: a beautiful, glossy magazine, notice of various meetings, etc.https://authors.library.caltech.edu/records/7ws9k-xx110On the problem of turbulence
https://resolver.caltech.edu/CaltechAUTHORS:ROScs00
Authors: Roshko, Anatol
Year: 2000
A central theme in the history of the turbulence problem is about the method of 'closure' in the models and 'theories' which have been proposed. Closure has invariably been by empirical calibration with experimental data. In this note we draw attention to a paper by Morris, Giridharan and Lilley, in which for the first time empiricism is obviated. For the turbulent mixing layer, this is accomplished by including in its description the mechanism for production of turbulent shear stress (i.e. turbulent momentum transfer), by large-scale instability waves. Some implications for the theory of turbulent shear flows are discussed.https://authors.library.caltech.edu/records/fprzf-bp252Flow-induced vibration of a circular cylinder at limiting structural parameters
https://resolver.caltech.edu/CaltechAUTHORS:20141201-121315237
Authors: Shiels, D.; Leonard, A.; Roshko, A.
Year: 2001
DOI: 10.1006/jfls.2000.0330
Transverse oscillation of a dynamically supported circular cylinder in a flow at Re=100 has been numerically simulated using a high-resolution viscous-vortex method, for a range of dynamical parameters. At the limiting case with zero values of mass, damping and elastic force, the cylinder oscillates sinusoidally at amplitude A/D=0·47 and frequency fD/U_∞=0·156. For zero damping, the effects of mass and elasticity are combined into a new, "effective" dynamic parameter, which is different from the classic "reduced velocity". Over a range of this parameter, the response exhibits oscillations at amplitudes up to 0·6 and frequencies between 0·15 and 0·2. From this response function, the classic response in terms of reduced velocity can be obtained for fixed values of the cylinder/fluid ratio m*. It displays "lock-in" at very high values of m*.https://authors.library.caltech.edu/records/q5sbg-vj314Aspects of flow-induced vibration
https://resolver.caltech.edu/CaltechAUTHORS:20141201-122214519
Authors: Leonard, A.; Roshko, A.
Year: 2001
DOI: 10.1006/jfls.2000.0360
Phenomena associated with flow-induced transverse oscillation of an elastically mounted body are considered. The use of a recently introduced parameter that combines the effect of mass and elasticity—effective elasticity—is exploited to demonstrate the predictive value of the new approach and to provide insights into solution branching, the maximum amplitude of vibration, and modeling.https://authors.library.caltech.edu/records/mbe2a-j3v76On the maximum amplitude for a freely vibrating cylinder in cross-flow
https://resolver.caltech.edu/CaltechAUTHORS:20141201-122718180
Authors: Klamo, J. T.; Leonard, A.; Roshko, A.
Year: 2005
DOI: 10.1016/j.jfluidstructs.2005.07.010
The response of a freely oscillating circular cylinder ("free vibration") in cross-flow has been studied experimentally using controlled magnetic eddy current to provide variable damping. In general, the nondimensional response amplitude, A*, and dominant frequency, ω*, depend on the Reynolds number, Re, and the nondimensional mass, m*, damping, b*, and elasticity, k*, of the system. The main objective of this study is to characterize the maximum amplitude that is achieved for a given system as cross-flow velocity is varied. We find that this maximum amplitude, A*_(max), occurs within a small range of values of k*_(eff) = ω*^2m* + k*. For values of Reynolds number in the range 525https://authors.library.caltech.edu/records/qtkkr-z1538The effects of damping on the amplitude and frequency
response of a freely vibrating cylinder in cross-flow
https://resolver.caltech.edu/CaltechAUTHORS:20110630-142700268
Authors: Klamo, J. T.; Leonard, A.; Roshko, A.
Year: 2006
DOI: 10.1016/j.jfluidstructs.2006.04.009
We have studied the effects of controlled damping on the amplitude and frequency response profiles of an elastically mounted cylinder in cross-flow. The dimensionless damping parameter, b^*=2b/ρLDU, which is closely related to the traditional "mass-damping" parameter, m^*ζ, was varied over a wide range of values through the use of a variable magnetic eddy current damping system. For low damping and sufficiently high Reynolds number we observe the previously described large-amplitude, three-branch (initial, upper, lower) response profile, and for high damping or low Reynolds number we observe the small-amplitude, two-branch (initial and lower) response profile. However we find that, because of the influence of Reynolds number, the traditional labels of "high mass-damping" and "low mass-damping" are incomplete with regard to predicting a large or small-amplitude response profile. In our experiments, as damping is systematically increased, we observe a transition between these two profiles characterized by a gradual "erosion" and eventual disappearance of the large-amplitude section (upper branch) and the scaling down of the lower branch region. We find that jumps from the upper to the initial branch originate on the 2S/2P boundary in the Williamson–Roshko plane. Another new finding is a hysteresis between the lower branch and the desynchronized region, which only appears at low Reynolds numbers. We also explore changes in the frequency response profile, which are connected with the changes in the amplitude profile, for our upper branch cases. We observe that analogous to the three amplitude branches, there are three distinct branches for the frequency response.https://authors.library.caltech.edu/records/dfb6y-3ft71In memoriam Prof. Hans W. Liepmann (1914–2009)
https://resolver.caltech.edu/CaltechAUTHORS:20141201-123635773
Authors: Hornung, H. G.; Roshko, A.
Year: 2009
DOI: 10.1007/s00193-009-0223-9
Hans Wolfgang Liepmann, a pioneering researcher and passionate
educator in fluid mechanics, passed away at the age
of 94. Liepmann, the Theodore von Kármán Professor of
Aeronautics, Emeritus, at the California Institute of Technology
(Caltech), passed away June 24 at his home in La
Cañada Flintridge. Widely honored for his contributions to
aeronautics, Liepmann came to Caltech in 1939 and was the
third director of Caltech's Graduate Aeronautical Laboratories
(GALCIT), from 1972 to 1985.https://authors.library.caltech.edu/records/j9bvq-9vn02Turbulent shear layers and wakes
https://resolver.caltech.edu/CaltechAUTHORS:20130301-151223359
Authors: Brown, Garry L.; Roshko, Anatol
Year: 2012
DOI: 10.1080/14685248.2012.723805
This paper considers the lectures on turbulent shear layers and wakes presented and discussed at the Marseille meeting in 1961 and provides our perspective on progress in understanding the mechanics of these flows since that time. The initial discussion is based on the understanding in 1961 gained from prior work. Particular emphasis is then placed on the subsequent experimental revelation of the large-scale vortical structure (coherent structure) found to be essential to understanding the mechanics of the turbulent shear layer. Critical insight into the mechanics that determines the growth rate (the shear stress), for example, is provided by the Biot–Savart relationship.
Conclusions are drawn from the experiments and some unresolved questions posed. This is followed by a discussion of plane wakes. Four regions of the plane wake are identified and experimental results on the large-scale structure are discussed. Again emphasis is placed on the vorticity and the vorticity fluxes that contribute directly to the derivative of the principal Reynolds stress. Results from numerical calculations offer new insights into the mechanics, especially through the vorticity and vorticity fluxes that could not previously be measured. For this case too, conclusions are drawn and outstanding questions posed.https://authors.library.caltech.edu/records/b7jhy-0yz60Hans W. Liepmann, 1914-2009
https://resolver.caltech.edu/CaltechAUTHORS:20130502-133421110
Authors: Narasimha, Roddam; Roshko, Anatol; Gharib, Morteza
Year: 2013
DOI: 10.1146/annurev-fluid-120710-101108
This article presents a brief account of the life and work of Hans W.
Liepmann, a distinguished fluid dynamicist, an outstanding teacher and
leader, and the third Director of the Graduate Aeronautical Laboratories,
California Institute of Technology.https://authors.library.caltech.edu/records/4k1qj-zdj58