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A Caltech Library Repository Feedhttp://www.rssboard.org/rss-specificationpython-feedgenenTue, 16 Apr 2024 14:56:20 +0000Integral Theory for Turbulent Base Flows at Subsonic and Supersonic Speeds
https://resolver.caltech.edu/CaltechETD:etd-10302003-141417
Authors: {'items': [{'id': 'Alber-Irwin-Emanuel', 'name': {'family': 'Alber', 'given': 'Irwin Emanuel'}, 'show_email': 'NO'}]}
Year: 1967
DOI: 10.7907/0YEZ-H062
<p>The integral near wake analysis of Reeves and Lees developed for supersonic laminar base flows is extended to the case of fully turbulent separated adiabatic flow behind a rearward facing step at both subsonic and supersonic speeds. A turbulent eddy viscosity model is formulated for the shear stress scaling of the dissipation integral in the mechanical energy equation. It is shown that the eddy viscosity can be described simply by one incompressible constant (valid for both shear layers and wakes) and one reference density ρ<sub>r</sub>. Using a compressibility transformation, theoretical solutions for the spreading rates of free shear layers are found to agree with experiment when the reference density is chosen to be the centerline density for the wake flow.</p>
<p>Two alternate methods are presented for joining the wake flow solution to the body first, through a turbulent free shear layer mixing solution, and then through the use of a two parameter family of velocity profiles valid near the body. A simple conservation model is presented to relate the viscous sublayer after expansion to the initial boundary layer ahead of the step.</p>
<p>For free stream Mach numbers M<sub>1</sub> ≤ 2.3, the integral theory is found to give good estimates for the length scales and centerline pressure variations measured experimentally for both wake flows and step flows (where reattachment is to a solid surface).</p>
<p>An iterative method of solution for the incompressible wake flow problem is presented as an extension of the work of Green. The calculation proposes the proper criteria for obtaining a convergent solution. The base pressure coefficient is found to be equal to the difference between the momentum thicknesses in the far wake and at the base.</p>
https://thesis.library.caltech.edu/id/eprint/4317The Effect of a Circular Hole on the Buckling of Cylindrical Shells
https://resolver.caltech.edu/CaltechETD:etd-04012009-110903
Authors: {'items': [{'id': 'Starnes-James-Herbert', 'name': {'family': 'Starnes', 'given': 'James Herbert'}, 'show_email': 'NO'}]}
Year: 1970
DOI: 10.7907/VGHN-EQ52
<p>An experimental and theoretical investigation of the effect of a circular hole on the buckling of thin cylindrical shells under axial compression was carried out. The experimental program consisted of tests performed on seamless electroformed copper shells and Mylar shells with a lap joint seam. The copper shells were tested in a controlled displacement testing machine equipped with a noncontacting surface displacement measuring device. Three-dimensional surface plots obtained in this manner showed the changes in the displacement field over the entire shell, including the hole region, as the applied load was increased. The Mylar shells were tested in a controlled load testing machine and demonstrated the effect of increasing the hole radius on the buckling loads of the cylinder.</p>
<p>The theoretical solution was based on a Rayleigh-Ritz approximation. The solution provided an upper bound for the buckling stresses of the cylinders tested for hole radii less than ten per cent of the shell radii. The theoretical solution also identified the governing parameter of the problem as being related to the hole radius, the shell radius, and the shell thickness.</p>
<p>The theoretical part of the investigation showed that even a small hole should significantly reduce the buckling stresses of circular cylinders. Experimentally, it was found that the effect of a small hole is masked by the effects of initial deformations but, at larger hole radii, the reduction in buckling stress took the form predicted by the theory. The experimental results also showed that the character of the shell buckling was dependent on the hole size. For very small holes the shell buckled into the general collapse configuration and there was no apparent effect of the hole on the buckling mode of the shell. For slightly larger holes the shell still buckled into the general collapse configuration, but the buckling stresses of the shell were sharply reduced as the hole size increased. For still larger holes the buckling stresses did not decrease as sharply as the hole size increased and the shell buckled into a stable local buckling configuration.</p>
https://thesis.library.caltech.edu/id/eprint/1244An energy approach to the dynamic buckling of spherical caps
https://resolver.caltech.edu/CaltechETD:etd-03312009-153153
Authors: {'items': [{'id': 'Liu-T', 'name': {'family': 'Liu', 'given': 'Ting-Lung'}, 'show_email': 'NO'}]}
Year: 1971
DOI: 10.7907/8JD0-W843
An experimental study to determine the dynamic buckling load of a spherical cap under impulsive loading was carried out. Impulsive loading was realized experimentally by use of a spray deposited explosive (Silver Acetylide-Silver Nitrate). The experimental dynamic buckling loads were compared to the dynamic buckling loads as calculated by using an energy criterion. The critical load from the energy criterion was determined by the conducting static pressure volume tests on the spherical caps. It was found that experimental results were consistently below the dynamic buckling load as predicted by the energy criterion. It was thought that this inconsistency resulted from the fact that transition state found in the static experiment was not the same as the dynamic transition state.
https://thesis.library.caltech.edu/id/eprint/1235The effect of imperfections on the buckling of cylindrical shells
https://resolver.caltech.edu/CaltechETD:etd-05012008-155801
Authors: {'items': [{'id': 'Imbert-J', 'name': {'family': 'Imbert', 'given': 'Jean-Francois'}, 'show_email': 'NO'}]}
Year: 1971
DOI: 10.7907/RHXA-BJ33
A theoretical investigation of the effect of general imperfections on the buckling of a cylindrical shell under axial compression was carried out. A limit point analysis was performed to determine the buckling loads using a simplified imperfection and displacement model consisting of one axisymmetric and two asymmetric components with the same circumferential wave number.
The wave number dependence of imperfections for a class of shells obtained by the same manufacturing processes was characterized by using an imperfection model to fit the experimental imperfection coefficients available. Buckling load calculations were performed using both experimental and fitted data as imperfection coefficients.
For the experimental data available the three-mode solution was found to have only a small additional effect with respect to the two-mode solution. In addition, by extrapolating imperfection coefficients for high wave numbers by means of the imperfection model, it was found that a strong interaction effect would exist between a low wave number axisymmetric mode and two classical asymmetric modes.
https://thesis.library.caltech.edu/id/eprint/1569I. Development of a Cryogenic Shock Tube. II. Experimental Investigation of the Interaction of a Shock Wave with Liquid Helium and I and II
https://resolver.caltech.edu/CaltechTHESIS:10072010-095321580
Authors: {'items': [{'email': 'jccummi@comcast.net', 'id': 'Cummings-John-Chester', 'name': {'family': 'Cummings', 'given': 'John Chester'}, 'show_email': 'YES'}]}
Year: 1973
DOI: 10.7907/AM0F-DB64
<p>I. Development of a cryogenic shock tube</p>
<p>A cryogenic shock tube has been developed as a tool for research in fluid mechanics and low temperature physics. The shock tube is designed to operate with the test section immersed in a cryogenic liquid. A unique diaphragm changing mechanism makes this shock tube an economical and practical device.</p>
<p>There are several advantages in operating a shock tube at cryogenic temperatures. Shock waves of very large Mach number can be produced. The flow field can be accurately calculated using ideal shock tube - perfect gas theory. Boundary layer effects are decreased so that long test times are possible.</p>
<p>The cases which have been studied are test gas temperatures of 300, 77, 4.2, and 2.3°K. Helium is used as both test and driver gas. The largest Mach numbers which have been observed range from 2.4 at 300°K to 32 at 2.3°K (several runs at 1.46°K have produced Mach 40 shocks). As the test gas temperature is decreased the observed Mach numbers approach those calculated using the ideal shock tube equation. The observed test times can be interpreted using laminar or turbulent boundary layer theory if the effects of shock formation distance and wall temperature rise are taken into account.</p>
<p>As a laboratory tool the cryogenic shock tube may be applied in many areas and modified for use in even more. Large Mach number shocks and large Reynolds number flows can be produced with this device. The rapid increase in temperature and pressure across a shock wave is useful for studies of sublimation, evaporation, or chemical reactions. Quantum mechanical effects in cryogenic materials, superconductors, or superfluid helium can also be investigated.</p>
<p>II. Experimental investigation of the interaction of a shock wave with liquid helium I and II</p>
<p>The flow field produced by a shock wave reflecting from a helium gas -liquid interface has been investigated using a new cryogenic shock tube. Incident and reflected shock waves have been observed in the gas; transmitted first and second sound shocks have been observed in the liquid. Wave diagrams have been constructed to compare the data to theoretical wave trajectories. Qualitative agreement between data and theory has been shown. Quantitative differences between data and theory indicate a need for further analysis of both the gas-liquid interface and the propagation of nonlinear waves in liquid helium.</p>
<p>This work is essentially a first step in the experimental investigation of a very complex nonequilibrium state. The well controlled jump in temperature and pressure across the incident shock wave provides unique initial conditions for the study of dynamic phenomena in superfluid helium.</p>
<p>The results clearly demonstrate the usefulness of the cryogenic shock tube as a research tool.</p>https://thesis.library.caltech.edu/id/eprint/6115Flow 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/3835The effects of elliptic and rectangular cutouts on the buckling of cylindrical shells loaded by axial compression
https://resolver.caltech.edu/CaltechETD:etd-12042006-090542
Authors: {'items': [{'id': 'Toda-S', 'name': {'family': 'Toda', 'given': 'Susumu'}, 'show_email': 'NO'}]}
Year: 1975
DOI: 10.7907/473V-C743
The results of experimental investigations of the effect of elliptic cutouts on the buckling of thin cylindrical shells under axial compression are presented. The experiments were performed on Mylar shells with a radius to thickness ratio of 400 and with two diametrically opposed circular, elliptic or rectangular holes. The results show that, for a given shell geometry, the area of a cutout determined the shell buckling behavior, but that the configuration of the cutout had little influence on the buckling loads.
A simplified analytical study based on Van Dyke's stress analysis and a strictly empirical design formula which gives a lower bound for the existing experimental data are also presented.https://thesis.library.caltech.edu/id/eprint/4776Dynamic Analyses of Suspension Bridge Structures and Some Related Topics
https://resolver.caltech.edu/CaltechTHESIS:03012018-135434228
Authors: {'items': [{'id': 'Abdel-Ghaffar-Ahmed-Mansour', 'name': {'family': 'Abdel-Ghaffar', 'given': 'Ahmed Mansour'}}]}
Year: 1976
DOI: 10.7907/Y1XR-WG52
<p>The thesis is divided into two parts. The first part develops a method of dynamic analysis for vertical, torsional and lateral free vibrations of suspension bridges, based on linearized theory and the finite-element approach. The method involves two distinct steps: (1) specification of the potential and kinetic energies of the vibrating members of the continuous structure, leading to derivation of the equations of motion by Hamilton's Principle, (2) use of the finite-element technique to: (a) discretize the structure into equivalent systems of finite elements, (b) select the displacement model most closely approximating the real case, (c) derive element and assemblage stiffness and inertia properties, and finally (d) form the matrix equations of motion and the resulting eigenvalue problems. The stiffness and inertia properties are evaluated by expressing the potential and kinetic energies of the element (or the assemblage) in terms of nodal displacements. Detailed numerical examples are presented to illustrate the applicability and effectiveness of the analysis and to investigate the dynamic characteristics of suspension bridges with widely different properties. This method eliminates the need to solve transcendental frequency equations, simplifies the determination of the energy stored in different members of the bridge, and represents a simple, fast and accurate tool for calculating the natural frequencies and modes of vibration by means of a digital computer. The method is illustrated by calculating the modes and frequencies of a bridge and comparing them with the measured frequencies.</p>
<p>The second part contains two studies on the effect of differential motions of two foundations upon the response of the superstructure of a bridge. The first study deals with the dynamic response of a "long beam" model of a bridge to both steady-state and random excitations applied at the supports. The second study develops a method to analyze the dynamic soil-bridge interaction of a simple bridge model erected on an elastic half-space, and the input motion is in the form of incident plane SH-waves. The dynamic response of the bridge and the effect of the radiative damping in the half-space on the interaction of the bridge are also studied.</p>https://thesis.library.caltech.edu/id/eprint/10741Self-similar elastodynamic solutions for the plane wedge
https://resolver.caltech.edu/CaltechETD:etd-11132006-080224
Authors: {'items': [{'id': 'Wojcik-G-L', 'name': {'family': 'Wojcik', 'given': 'Gregory Lynn'}, 'show_email': 'NO'}]}
Year: 1977
DOI: 10.7907/E1D3-0T11
Wave propagation in a two-dimensional elastic wedge is fundamental to a large class of problems in elastodynamic theory, however until now analytical solutions to all but certain degenerate cases were unknown. In this thesis a general elastodynamic solution is derived for the wedge in a state of plane strain. Surface tractions are, restricted to uniform normal and shear loads spreading from the wedge vertex at constant velocity. The geometry and loading then allow self-similar solutions of the governing differential equations and boundary conditions in hyperbolic and elliptic domains. Hyperbolic solutions are found in terms of the elliptic solutions by the method of characteristics, while elliptic solutions are reduced using analytic function theory to two independent Fredholm integral equations of the second kind in one dimension. Although numerical solutions are beyond the scope of the investigation, the integral equations are solvable by standard techniques. Such solutions can be used to solve a number of plane elastodynamic problems involving an edge.https://thesis.library.caltech.edu/id/eprint/4533Determining Models of Structures from Earthquake Records
https://resolver.caltech.edu/CaltechETD:etd-11212003-110242
Authors: {'items': [{'email': 'jimbeck@caltech.edu', 'id': 'Beck-James-Leslie', 'name': {'family': 'Beck', 'given': 'James Leslie'}, 'show_email': 'NO'}]}
Year: 1979
DOI: 10.7907/JP7R-KF57
<p>The problem of determining linear models of structures from seismic response data is studied using ideas from the theory of system identification. The investigation employs a general formulation called the output-error approach, in which optimal estimates of the model parameters are obtained by minimizing a selected measure-of-fit between the responses of the structure and the model. The question of whether the parameters can be determined uniquely and reliably in this way is studied for a general class of linear structural models. Because earthquake records are normally available from only a small number of locations in a structure, and because of measurement noise, it is shown that it is necessary in practice to estimate parameters of the dominant modes in the records, rather than the stiffness and damping matrices.</p>
<p>Two output-error techniques are investigated. Tests of the first, an optimal filter method, show that its advantages are offset by weaknesses which make it unsatisfactory for application to seismic response. A new technique, called the modal minimization method, is developed to overcome these difficulties. It is a reliable and efficient method to determine the optimal estimates of modal parameters for linear structural models.</p>
<p>The modal minimization method is applied to two multi-story buildings that experienced the 1971 San Fernando earthquake. New information is obtained concerning the properties of the higher modes of the taller building and more reliable estimates of the properties of the fundamental modes of both structures are found. The time-varying character of the equivalent linear parameters is also studied for both buildings. It is shown for the two buildings examined that the optimal, time-invariant, linear models with a small number of modes can reproduce the strong-motion records much better than had been supposed from previous work using less systematic techniques.</p>https://thesis.library.caltech.edu/id/eprint/4611Leading edge flutter of a supercavitating hydrofoil
https://resolver.caltech.edu/CaltechETD:etd-11032006-091936
Authors: {'items': [{'email': 'kiam.t.oey@yahoo.com', 'id': 'Oey-K-T', 'name': {'family': 'Oey', 'given': 'Kiam Thian'}, 'show_email': 'YES'}]}
Year: 1979
DOI: 10.7907/9DGB-DW93
<p>Leading edge flutter is a problem that is unique to a super-cavitating hydrofoil. At high speed, the leading edge portion has been observed to oscillate while the trailing edge remains motionless.</p>
<p>In this study, several flat plate hydrofoils were tested. The experimental results indicate that the phenomenon is a complex function of speed, angle of attack, cavitation number and mass ratio. Leading edge flutter was also observed to cause cavity pinching. A theoretical study was also conducted. Two mathematical models are presented here. The first one models the flexible chord foil as a rigid chord foil hinged at the trailing edge; the second model treats the fluid-structure interaction problem of a flexible chord foil cantilevered at the trailing edge. Both models resemble leading edge flutter near zero cavitation number in some respects. At short and moderate cavity lengths, leading edge flutter phenomenon is influenced by the cavity closure condition.</p>
https://thesis.library.caltech.edu/id/eprint/4394Failure of Liquid Storage Tanks Due to Earthquake Excitation
https://resolver.caltech.edu/CaltechTHESIS:04132018-090928397
Authors: {'items': [{'id': 'Shih-Choon-Foo', 'name': {'family': 'Shih', 'given': 'Choon-Foo'}, 'show_email': 'NO'}]}
Year: 1981
DOI: 10.7907/m0v8-hs31
<p>Above ground liquid storage tanks have suffered serious damage during earthquakes. The damage of tanks can vary from local yielding or buckling of the tank wall, to loss of contents, or to collapse which leads to an unrepairable tank. Considerable work has been carried out on this problem with varying degree of success. However, the results are largely directed toward response rather than failure prediction. The information on failure mechanisms is very limited. The present work consists of scale model testing, correlation with existing analysis and failure prediction with laboratory verification. The scale model testing incorporates dynamic similarity of the fluid/structure interaction problem. The model study shows that small plastic models can be useful in studying the dynamics and buckling of liquid-filled tanks under ground excitation even though the model does not display complete similitude. The buckling criterion proposed in this study is based upon static considerations and the complex stress field in the shell wall is supplanted by a simple field for which analytical/experimental results are available. Harmonic buckling tests demonstrate that the static buckling criterion is satisfactory even though the prebuckling stress field is time dependent. The harmonic buckling tests, when correlated with the stresses from a response analysis, also indicate that the buckling is largely dependent upon the n = 1 response, Transient buckling tests are also carried out and the results show that the linear analysis together with the static buckling criterion gives a good prediction of the failure of a full fluid-filled tank. The test parameters in these buckling tests include water depth, title angle, thickness of tank wall, top end condition, ground excitation pattern, etc. In addition, buckling tests of unanchored tanks are conducted to study the influence of changing the anchorage of the tank base. An analytical model is suggested to predict the response of an unanchored tank due to overturning moment. The current design criterion of an unanchored tank is also assessed in this study. The results of this investigation, in addition to those carried out previously, provide a better understanding of the forced vibration problem, failure criterion and appropriate design procedure for a liquid storage tank.</p>
https://thesis.library.caltech.edu/id/eprint/10805The Growth of Impact Damage in Compressively Loaded Laminates
https://resolver.caltech.edu/CaltechETD:etd-04132004-084629
Authors: {'items': [{'id': 'Chai-Herzl', 'name': {'family': 'Chai', 'given': 'Herzl'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/58D3-ZZ41
<p>The thesis is divided into two chapters. The first chapter describes an experimental program carried out to determine the phenomenological aspects of composite panel failure (graphite/epoxy laminate) under simultaneous in-plane compression and low velocity transverse impact (0-250 ft/sec). High-speed photography and the shadow moire technique were used to record a full-field surface deformation history of the impacted plate. The information gained from these records, supplemented by plate sectioning and observation for interior damage, has shown that the predominant failure mechanism is the coupled effect of delamination and buckling.</p>
<p>In chapter 2, this process of failure is modelled by delamination buckling wherein the local delamination stability, growth or arrest are governed by an energy release rate criterion. Various degrees of problem simplifications are employed, starting with a one-dimensional, delaminated-beam model. In the most sophisticated treatment, it is assumed that the damaged area has an elliptical shape. It was found that this model is capable of describing the growth conditions and the growth behavior of the damaged area. It was also found that the predominant parameter controlling delamination growth or arrest is the fracture energy associated with delamination.</p>
<p>In the appendix at the end of this work, the fracture energy of a composite laminate is considered. A modified cleavage analysis is developed, and is applied to evaluate this quantity for two composite materials of current interest. The test results are then examined with reference to impact damage tolerance data available for these materials. A viscoelastic characterization of the two resins used to fabricate the two composites mentioned above is also provided. The results of these measurements are then examined with reference to long-term applicability of the matrix material.</p>https://thesis.library.caltech.edu/id/eprint/1366Experimental Observations of the Effect of Foundation Embedment on Structural Response
https://resolver.caltech.edu/CaltechTHESIS:05162018-100416743
Authors: {'items': [{'id': 'Lin-Albert-Niu', 'name': {'family': 'Lin', 'given': 'Albert Niu'}, 'show_email': 'NO'}]}
Year: 1982
DOI: 10.7907/ksr8-pf30
<p>Ambient, ring-down, and forced vibration tests were used to determine the effect of foundation embedment on the response of a one-story model structure 10 ft square in plan and 11.4 ft high. The tests, conducted at the full-, half- and unembedded foundation conditions, led to the identification of the fundamental translatory mode in the primary (east-west) and secondary (north-south) directions, and two torsional modes. The forced vibration consisted of horizontally incident SH-waves generated at an excitation structure located 47.5 ft (center-to-center) away. During these tests, detailed measurements of the near-field ground motion and modal displacement ratios were obtained at the fundamental mode in the primary direction. The displacement ratios were used to calculate the structural and foundation-soil stiffnesses and damping coefficients for comparison to theoretical results. Foundation embedment increased the model frequencies and decreased the contribution of the foundation motion to the overall displacement of the superstructure. For the fundamental mode response, which consisted of translatory and rocking motions, the resonant frequency predicted by lumped parameter analysis was higher than that measured experimentally by 25% for the unembedded case. While the experimental and theoretical fundamental mode shapes were in close agreement, the calculated effect of embedment on the response was less than that measured. These results were consistent with the comparison of the impedances and embedment factors. Serious discrepancies between analytical and experimental results were found for the case of torsion; a simple two-degree-of-freedom model was consistent only with the first of the two measured resonant frequencies.</p>https://thesis.library.caltech.edu/id/eprint/10896An Experimental Investigation into the Mechanics of Dynamic Fracture
https://resolver.caltech.edu/CaltechETD:etd-10302003-104624
Authors: {'items': [{'email': 'kravi@mail.utexas.edu', 'id': 'Ravi-Chandar-K', 'name': {'family': 'Ravi-Chandar', 'given': 'K.'}, 'show_email': 'YES'}]}
Year: 1982
DOI: 10.7907/ZSX5-Q128
<p>Current theories of dynamic fracture are based on elastodynamic analyses of mathematically sharp plane cracks and as such do not explain the observed terminal velocities or the phenomenon of crack branching satisfactorily. The present investigation addresses the above problems by using both microscopic and macroscopic interpretations. The experimental scheme that is used in this investigation is the configuration of a pressure loaded semi-infinite crack in an infinite medium. The loading is achieved through an electromagnetic device which provides highly repeatable loading. The method of caustics is used in conjunction with a high speed camera to obtain the time histories of the crack tip stress intensity factor and the crack position.</p>
<p>The problems of crack initiation and crack arrest are explored. The stress intensity factor at initiation is found to be independent of the rate of applied loading when the latter is below about 10<sup>4</sup>MPA/sec, but the initiation stress intensity factor increases considerably when the loading rate is increased further. Crack arrest is obtained in large specimen by using very low energy loading pulses. It was found that the stress intensity factor at crack arrest was constant and also that, within the time resolution of the high speed camera (5 μsec), the crack comes to a stop abruptly.</p>
<p>The crack propagation and branching aspects were investigated first using post-mortem analysis of the fracture surfaces and high speed photomicrography to get an idea of the microscopic processes that occur in the fracure process. From this investigation, it was found that crack propagation involving high stress intensity factor and high velocity situations takes place by the growth and interaction of microcracks, due to the voids present in the material. A surprising result of this investigation was that cracks propagated at a constant velocity, although the stress intensity factor varied. Current theories of dynamic fracture cannot explain such behaviour. The crack branching process was found to be a continuous process arising out of propagation along a straight line. High speed photomicrographs of the branching process indicated the presence of a number of part-through attempted branches that interact with one another and finally the successful emergence of a few full fledged branches.</p>
<p>The microscopic observations on the crack propagation and branching process leads to a new interpretation of dynamic fracture that attempts to qualitatively explain the constancy of the velocity of propagation, the terminal velocity and crack branching. The crack branching mechanism is a logical continuation of the mechanism for crack propagation.</p>
https://thesis.library.caltech.edu/id/eprint/4316Analytical Models for the Dynamics of Buildings
https://resolver.caltech.edu/CaltechTHESIS:08162019-113014700
Authors: {'items': [{'id': 'Jain-Sudhir-Kumar', 'name': {'family': 'Jain', 'given': 'Sudhir Kumar'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/4k04-ca84
<p>This thesis investigates the significance of in-plane floor flexibility on the dynamics of buildings, and develops analytical models for structures that have flexible floor diaphragms. Experience with past earthquakes demonstrates that this feature is particularly important for long, narrow buildings and buildings with stiff end walls. In the method developed in this study, the equations of motion and appropriate boundary conditions for various elements of the structure are written in a single coordinate system and then are solved exactly.</p>
<p>One- and two-story buildings with end walls are analyzed by treating their floors and walls as bending and shear beams, respectively. The resulting equations of motion and the boundary conditions are solved to obtain the dynamic properties of the structure. The expected low torsional stiffness of the end walls or frames is confirmed by analysis of a single-story example structure. Study of a similar two-story building showed that the first two modes, dominated by the floor and the roof vibrations, make the largest contributions to the total base shear in the structure.</p>
<p>Floors of multistory buildings with end walls (or frames) are idealized as equivalent, distributed beams while the walls or frames are treated as bending or shear beams. Analysis of a nine-story building showed that the structure possesses several lower modes in which floors vibrate essentially as pinned-pinned beams.</p>
<p>Buildings with large numbers of uniform stories and frames (or walls) are treated as vertically-oriented anisotropic plates. It is concluded that the floors in such buildings can be assumed rigid for seismic analysis, since the modes involving floor deformations are not excited by uniform ground motion.</p>
<p>The approach can be generalized further to study more complex structures. An example is the Imperial County Services Building, which has two end walls in the upper stories and several walls in the ground story. The analytical model of this building predicts several important features of the complex dynamic behavior of the structure.</p>
https://thesis.library.caltech.edu/id/eprint/11761An 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/11205An Engineering Analysis of Polymer Film Adhesion to Rigid Substrates
https://resolver.caltech.edu/CaltechETD:etd-05152007-111322
Authors: {'items': [{'id': 'Heymans-Luc-J', 'name': {'family': 'Heymans', 'given': 'Luc J.'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/3GQW-YP87
<p>An important source of interface fracture contributing to adhesive failure in a bimaterial sandwich, consisting of a rigid substrate and a viscoelastic encapsulant material, arises from residual stresses. The encapsulant is often deposited on the substrate above its glass transition temperature region but used below this temperature range. In order to determine the magnitude of the residual stresses a viscoelastic stress analysis of a bimaterial sandwich is carried out, taking into account the time-dependent material properties of the polymeric layer and the environmental "loading" conditions. The theoretical analysis is paralleled by an experimental examination of the time-dependent out-of-plane deformation of thin, circular sandwiches.</p>
<p>Polyvinyl acetate was chosen as a model material exhibiting significant viscoelastic effects under room test conditions. Therefore the pertinent physical and mechanical properties of PYA<sub>c</sub> are determined; these include the thermal coefficient of expansion, the shear creep compliance and the relaxation modulus. In the experimental work BK-7 glass is taken as the "rigid" substrate. The measurements connected to the stress analysis are monitored with laser interferometry (Newton's rings). A comparison between theory and experiment completes the viscoelastic stress analysis.</p>
<p>In the second part of this study time dependent adhesive failure of rubbery materials is investigated. Polymeric materials are being used increasingly for a wide variety of applications. Some of these materials are applied as protective layers to isolate their substrates from a hostile environment. Others achieve remarkable structural bond strengths thereby displacing the traditional mechanical fasteners like bolts and rivets. If one wants to investigate the long time integrity of a layer assembly the time dependence of the material properties of the adhesives needs to be carefully analyzed. This time dependence is also reflected in the energy required to create new surfaces as interfacial debonding proceeds the adhesive fracture energy is one of the dominant parameters in time dependent adhesive failure. In our investigation it is characterized through peel testing.</p>
<p>With the knowledge of the pertinent material properties as well as of the adhesive fracture energy, we then proceed to formulate a criterion for continuing interfacial crack propagation. The analysis is carried out for elastic solids, with the effect of viscoelastic behavior incorporated later on. Debond tests provide a way to check how well the theoretical predictions correspond to experimental debond results.</p>https://thesis.library.caltech.edu/id/eprint/1821The Propagation and Arrest of an Edge Crack in an Elastic Half-Space Under Conditions of Anti-Plane Shear: Analytical and Numerical Results
https://resolver.caltech.edu/CaltechETD:etd-09052006-082841
Authors: {'items': [{'id': "O'Sullivan-Timothy-Christopher", 'name': {'family': "O'Sullivan", 'given': 'Timothy Christopher'}, 'show_email': 'NO'}]}
Year: 1983
DOI: 10.7907/jb3j-5460
<p>The motion of an edge crack extending non-uniformly in an elastic half-space under conditions of anti-plane shear is analyzed. An expression for the stress intensity factor at the crack tip is obtained, and an energy balance crack propagation criterion is used to find the equation of motion of the tip. On solving this equation numerically, it is found that crack arrest occurs before the second reflected wave from the boundary reaches the tip.</p>
<p>In the second half of this investigation, a numerical procedure for studying anti-plane shear crack propagation problems using finite differences is developed. To approximate the elastodynamic field as accurately as possible near the moving crack tip, where singular stresses occur, the local asymptotic displacement field near the tip is incorporated into the finite difference scheme. The numerical procedure is applied to the edge crack problem analyzed in the first part of this study, and the numerical and exact results are compared.</p>https://thesis.library.caltech.edu/id/eprint/3333Interaction of Weak Shock Waves and Discrete Gas Inhomogeneities
https://resolver.caltech.edu/CaltechETD:etd-06232005-110318
Authors: {'items': [{'id': 'Haas-Jean-François-Luc', 'name': {'family': 'Haas', 'given': 'Jean-François Luc'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/T37C-X215
<p>An experimental investigation of the interaction of shock waves with discrete gas inhomogeneities is conducted in the GALCIT 15 cm diameter shock tube. The gas volumes are cylindrical refraction cells of 5 cm diameter with a 0.5 µm thick membrane separating the test gas (helium or Freon 22) from the ambient air and large spherical soap bubbles containing the same gases. The incident wave Mach numbers are nominally 1.09 and 1.22. The wave pattern and the deformation of the gas volumes are documented by shadowgraphs. The transmitted and diffracted wave pressure profiles are recorded by pressure transducers at various distances behind the cylinders. The basic phenomena of acoustic wave refraction, reflection and diffraction by cylindrical acoustic lenses, with indices of refraction appropriate to the gases used in the experiments, are illustrated with computer-generated ray and wave-front diagrams.</p>
<p>In the case of a Freon 22-filled cylinder, the wave diffracted externally around the body precedes the wave transmitted from the interior which goes through a focus just behind the cylinder, while in the case of the helium-filled cylinder the expanding transmitted wave runs ahead of the diffracted wave. Both sets of waves merge a few cylinder diameters downstream. The wave patterns inside the cylinder, showing initially the refracted waves and later the same waves reflected internally, present some interesting phenomena.</p>
<p>The mechanisms by which the gas volumes are transformed into vertical structures by the shock motion are observed. The unique effect of shock acceleration and Rayleigh-Taylor instability on the spherical volume of helium leads to the formation of a strong vortex ring which rapidly separates from the main volume of helium. Measurements of the wave and gas-interface velocities are compared to values calculated for one-dimensional interactions and for a simple model of shock-induced Taylor instability. The behavior of thin liquid membranes accelerated by shocks under varying conditions is documented by high speed photography.</p>
<p>In a related experiment, shock waves of Mach number between 1.005 and 1.36 interact with a dense random array of 2 mm diameter helium filled soap bubbles. Experimental results (based on shadowgraphs and pressure measurements) show that very weak shock waves (M<sub>s</sub> ≤ 1.01) are strongly scattered by the array, which is left undisturbed by the shock, and that stronger shock waves, only locally disturbed by each bubble, maintain undisturbed pressure profiles because of nonlinear effects, while the array undergoes shock-induced mixing. A simple criterion for multiple scattering shows that the combined effect of many bubbles is necessary in order to produce important modifications on the shock wave pressure profile.</p>https://thesis.library.caltech.edu/id/eprint/2701Analysis of the Observed Earthquake Response of a Multiple Span Bridge
https://resolver.caltech.edu/CaltechTHESIS:11262018-124513833
Authors: {'items': [{'id': 'Wilson-John-Charles', 'name': {'family': 'Wilson', 'given': 'John Charles'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/5321-6p57
<p>Accelerograms obtained during the 1979 Coyote Lake, California earthquake are used to examine the response of a multiple-span, steel girder bridge to strong earthquake loading. The structure studied, the San Juan Bautista 156/101 Separation Bridge, is typical of many highway bridges in seismic regions of the United States. Although the bridge was not damaged, the strong-motion records are of significant engineering interest as they are the first to be recorded on such a structure.</p>
<p>An engineering seismology study suggests that long-period ground displacements at the bridge site were caused by Rayleigh waves. A three-second period, pseudostatic response of the superstructure is attributed to small amounts of differential support motion induced by the surface waves.</p>
<p>A time-domain technique of system identification is used to determine linear models which can closely replicate the observed bridge response. Using time-invariant models, two structural modes at 3.50 and 6.33 Hz, are identified in the horizontal direction. Each mode, having approximately ten-percent damping, involves coupled longitudinal and transverse motions of the superstructure. Time-variations of frequency and damping in the horizontal response are also identified using a moving-window analysis.</p>
<p>A three-dimensional finite element model which includes soil-structure interaction predicts several important features of the dynamic response of the bridge. The first two computed horizontal frequencies are found to be in excellent agreement with the observed responses provided the model's expansion joints are locked, preventing relative translational motions from occurring across the joints. Locking is confirmed by the observed deformations of the structure in the fundamental mode. Fundamental vertical frequencies of the individual spans, predicted by the finite element model, are in very good agreement with ambient vibration test data. Results of the strong-motion data analysis and the finite element modeling are used to recommend a plan for expansion of the strong-motion instrumentation array on the bridge.</p>https://thesis.library.caltech.edu/id/eprint/11283The Effects of Curvature on Turbulent Mixing Layers
https://resolver.caltech.edu/CaltechETD:etd-01222007-142548
Authors: {'items': [{'id': 'Wang-Chiun', 'name': {'family': 'Wang', 'given': 'Chiun'}, 'show_email': 'NO'}]}
Year: 1984
DOI: 10.7907/BC48-BE98
<p>Experimental studies have been conducted in a curved mixing layer in which both the velocity ratio and the density ratio were variable. Flow visualization studies and profile measurements covered a wide range of experimental conditions. The structures observed experimentally were examined in the light of three different instability mechanisms which can exist in the same mean flow.</p>
<p>For the case of mixing layers with uniform density, it was found that the normal large spanwise vortex structures can be weakened or inhibited by Taylor-Gortler instability if the inner stream is faster than the outer stream. For the case of mixing layers with different densities, three-dimensionality is greatly enhanced by Rayleigh-Taylor instability if the inner stream is heavier than the outer stream, and especially if the inner stream is also faster. In the former case the growth rate of the mixing layer was found to be insensitive to changes in the velocity ratio.</p>
<p>The effects of curvature on the structure of the curved turbulent mixing layer were explored in terms of length scales and celerity for the large spanwise structures where these structures could be observed. Other things being equal, the celerity of the large structures was found to depend on density ratio and velocity ratio but not on the sense of the mean streamline curvature.</p>https://thesis.library.caltech.edu/id/eprint/275Structural 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/4597Flow-Induced Vibration of Long Structures
https://resolver.caltech.edu/CaltechETD:etd-11072006-091025
Authors: {'items': [{'id': 'Jones-Nicholas-Patrick', 'name': {'family': 'Jones', 'given': 'Nicholas Patrick'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/ZXWC-ZZ44
<p>When a body is exposed to a flowing fluid, oscillations can occur due to one or more of several different mechanisms. The resulting large amplitudes of motion and fatigue are potential sources of structural failure. Furthermore, the drag force on a structure can be increased due to the effectively larger cross-sectional area presented to the flow from the oscillation. A complete understanding of the nature of such vibration is essential in the design of many civil and mechanical engineering systems.</p>
<p>Previous solutions to the vortex-induced vibration problem were primarily based on modal analysis, using a one- or two-mode approximation. Use of modal analysis implies a "locked-in" condition: the vortex shedding frequency and a natural frequency of the system are coincident. Observations made on long cable systems indicate that the amplitude of response is smaller than is predicted by a conventional modal analysis. The drag forces on such structures are therefore overestimated by current design approaches.</p>
<p>In very long structures, typical of those found in ocean applications, modes are closely spaced, and it is not reasonable to assume total spanwise correlation in the fluid forces or response. The approach used herein attempts to avoid the limitations associated with the modal solution of such problems by implementing a solution based on traveling waves. The technique draws on earlier theoretical and empirical models for the complex vortex-shedding phenomenon, and incorporates these into a new method for analyzing the structural response problem.</p>
<p>The traveling wave approach can be used to model effectively spanwise variable flow environments by summing the calculated responses of adjacent active sections of cable. Until this method was developed, there was no suitable method available for modeling flow characteristics of this type. Modal analysis is effectively limited to systems with uniform flow over all or part of the system.</p>https://thesis.library.caltech.edu/id/eprint/4441Analysis of Unanchored Liquid Storage Tanks under Seismic Loads
https://resolver.caltech.edu/CaltechTHESIS:10232019-110854163
Authors: {'items': [{'id': 'Peek-Ralf', 'name': {'family': 'Peek', 'given': 'Ralf'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/w43k-hj54
<p>Because of cost, cylindrical, ground supported liquid storage tanks are often not fixed to their foundation, even in seismic areas. For such an unanchored tank made of steel, the weight of the cylindrical shell is mostly insufficient to prevent local uplift due to seismic overturning moments. Although, for properly designed connecting pipes, uplift itself is not a problem, it results in larger vertical compressive stresses in the tank wall at the base, opposite to where the uplift occurs. These compressive stresses have often caused buckling, even in earthquakes which did not cause much damage to other structures.</p>
<p>Various investigators have studied the behavior of unanchored tanks experimentally, but, due to the complexity of the problem, so far very little theoretical work has been done. Two methods of analysis for static lateral loads are presented: An approximate one in which the restraining action of the base plate is modeled by nonlinear Winkler springs, and a more comprehensive one in which the two dimensional nonlinear contact problem is solved by the finite difference energy method. The theoretical results are compared with existing experimental results and with the approach from current U.S. design standards. The theoretical peak compressive stresses are in good agreement with the experimental results, but in some cases exceed those calculated by the code method by more than 100%.</p>
<p>Finally, a new design concept, by which the tank wall is preuplifted all around its circumference by inserting a ring filler is described. It will be shown theoretically and experimentally that this preuplift method substantially improves the lateral load capacity.</p>https://thesis.library.caltech.edu/id/eprint/11848The Nature of Oblique Instability Waves in Boundary Layer Transition
https://resolver.caltech.edu/CaltechETD:etd-05242007-150746
Authors: {'items': [{'id': 'Robey-Harry-Francis-III', 'name': {'family': 'Robey', 'given': 'Harry Francis, III'}, 'show_email': 'NO'}]}
Year: 1986
DOI: 10.7907/7VG8-Y513
<p>An experimental study of both the weakly non-linear as well as the three-dimensional nature of boundary layer transition is conducted using the active surface heating technique of Liepmann et al. In the present study, this technique is extended to provide a means for controllably and repeatably introducing three-dimensional disturbances into a laminar boundary layer. A review of the surface heating technique is presented along with a discussion of some peculiarities encountered in extending this technique to three-dimensional geometries. A thorough description of the design and operation of a programmable 32-element heater array and the supporting instrumentation are given as well.</p>
<p>The heater array is first used to study the effect of weak nonlinearity on boundary layer transition. By keeping the forced disturbances as two-dimensional as possible, it is shown that the effects of weak non-linearity are relatively benign. The growth rates are seen to follow the linear theory up to perturbation amplitudes (τ'<sub>w</sub>τ̅<sub>w</sub>) of nearly twelve percent. The only deviation from the linear theory arises in the form of non-linearly generated harmonics phase-locked to the fundamental. It is concluded that although these non-linearly generated harmonics do alter the wave behavior to some extent, they are by themselves not sufficient to explain the transition from small linear oscillations to the large amplitude, broad-band, three-dimensional oscillations characteristic of a fully turbulent boundary layer.</p>
<p>The effect of three-dimensionality on boundary layer transition is then investigated through an analytical and experimental study of single oblique instability waves. This subject has remained largely unexplored, as such disturbances were generally thought to be more stable and therefore less dangerous than their two-dimensional counterparts. Through a series of experiments, however, it is shown that certain conditions exist for which oblique waves are observed to be more unstable than any two-dimensional wave. It is shown that oblique waves exhibit a non-stationary period-doubling behavior that is not seen in two-dimensional disturbances. A vortex pairing mechanism is proposed to explain this behavior, and is shown to occur in a manner consistent with the Biot-Savart law for the induced velocity field.</p>https://thesis.library.caltech.edu/id/eprint/2023Steady-State and Transient Methods for Modeling Chemical Reactions on Supported Catalysts
https://resolver.caltech.edu/CaltechETD:etd-06142006-131434
Authors: {'items': [{'id': 'Prairie-Michael-Roland', 'name': {'family': 'Prairie', 'given': 'Michael Roland'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/x3zp-aa54
<p>A systematic experimental strategy based on fluid-phase measurements is proposed for modeling dynamic behavior of heterogeneous catalytic reactions. The strategy utilizes steady-state rate, step-response, cycled-feedstream, and feedback-induced bifurcation techniques. Ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub> was studied using this strategy. In addition, transmission infrared spectroscopy is applied to investigate support effects which accompany ethylene hydrogenation on Pt/Al<sub>2</sub>O<sub>3</sub>, and to the detailed study of CO adsorption, desorption and oxidation on Rh/Al<sub>2</sub>O<sub>3</sub>. The proposed experimental strategy combined with surface infrared spectroscopy provides a very powerful means for identification and validation of dynamic kinetic models.</p>
<p>Observed bifurcation behavior can be accurately attributed to a model for the catalytic reaction only if each dynamic element in the closed-loop experimental hardware is properly accounted for. Accordingly, time delay and feedback gain were the manipulated parameters in a feedback-induced bifurcation scheme aimed at validating a dynamic model for an experimental gas-phase reactor flow system without reaction. The apparatus consists of an isothermal, stirred, fixed-bed reactor, mass flow controllers, an infrared gas analysis system, and a computerized data acquisition and control system. Experimental bifurcations to sustained oscillations show that the stability of the reactor system is strongly influenced by delay. The relationships of time delay to Hopf bifurcation gains and frequencies provide a very sensitive basis for model comparisons.</p>
<p>Steady-state, step-response, feedback-induced Hopf bifurcation and forced concentration cycling experiments were applied to study ethylene hydrogenation over 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C. Step-response experiments indicate a time scale of 5000 s which is associated with chemisorbed hydrogen. Conversely, feedback-induced Hopf bifurcation data indicate this time scale to be on the order of 1 s in magnitude. In the overall strategy of dynamic modeling, the two techniques are complementary since each inherently focuses on an opposite region in the spectrum of time scales for the reactor system. Cycling the feedstream composition resulted in improvement of the time-average reaction rate for the ethylene hydrogenation reaction compared to steady-state reactor operation.</p>
<p>Steady-state, step-response and Hopf bifurcation data are also presented for 0.5% Pt/Al<sub>2</sub>O<sub>3</sub> at 30°C and compared with results for the 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst. A single value of 2.5 s for the surface time constant associated with chemisorbed hydrogen is sufficient for modeling behavior on 0.5% Pt/Al<sub>2</sub>O<sub>3</sub>, whereas the lower-loaded 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> catalyst requires two very different values. In addition, the 0.5% catalyst was used to demonstrate the general result that small discrepancies between the actual and chosen reference steady state give rise to imperfect, cusp-like bifurcations. Steady-state bifurcation data are also shown to be useful for discriminating among rival kinetic models.</p>
<p>Ethylene hydrogenation on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Transmission infrared spectroscopy was used to study hydrogen spillover dynamics on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub> at 80°C via hydroxyl/deuteroxyl exchange. Ethylene in the gas-phase markedly slows the rate of spillover. The presence of ethylene likely reduces the concentration of platinum-adsorbed hydrogen adatoms, the precursors of hydrogen spilled onto alumina, due to catalytic hydrogenation on the platinum. Surface transport of hydrogen atoms on spillover-activated alumina is proposed as an explanation for the very slow transient behavior observed for ethylene hydrogenation on 0.05% Pt/Al<sub>2</sub>O<sub>3</sub>. Infrared spectra exhibit characteristics of both hydroxyl and deuteroxyl groups for reactor feed containing only D<sub>2</sub> and C<sub>2</sub>H<sub>4</sub>. This observation confirms the existence of a dissociative ethylene adsorption process.</p>
<p>A section of the thesis unrelated to ethylene hydrogenation investigates modeling applications of transmission infrared spectoscopy (TIR) by applying it to study adsorbed CO on Rh/Al<sub>2</sub>O<sub>3</sub> during CO chemisorption, steady-state, step-response, and forced-cycling oxidation experiments at 900 torr. At 300°C, the catalyst initially supported primarily a dicarbonyl CO species, but after use exhibited spectra characteristic of a surface mostly covered by linearly bound CO. A model that describes transient, diffusion-influenced CO adsorption and desorption for the supported catalyst is presented. It suggests that the CO desorption energy depends linearly on coverage, and that the magnitude of this dependence is a function of temperature. Observed rate dependence on bulk CO concentration for O<sub>2</sub> effluent levels of 0.5% and 0.25% is interpreted considering the effects of internal and external mass transport at 300°C. Step-response and forced-cycling oxidation experiments across stoichiometric conditions exhibit oxygen and CO storage effects characteristic of CO oxidation catalysts. Data indicating autonomous oscillation of CO coverage and CO<sub>2</sub> production are also presented.</p>https://thesis.library.caltech.edu/id/eprint/2590Experimental Study of Unsteady Hydrodynamic Force Matrices on Whirling Centrifugal Pump Impellers
https://resolver.caltech.edu/CaltechETD:etd-03262007-130547
Authors: {'items': [{'id': 'Jery-Belgacem', 'name': {'family': 'Jery', 'given': 'Belgacem'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/cmn0-qk37
<p>An experimental facility was constructed and instrumented. A study was conducted on a set of centrifugal flow pumps whose impellers were made to follow a controlled circular whirl motion. The aim was to characterize the steady and unsteady fluid forces measured on the impeller under various pump operating conditions. The postulation was that the unsteady lateral forces result from interactions between the impeller and the surrounding diffuser and or volute (via the working fluid), and that under certain flow regimes these forces can drive unstable lateral motions of the pump rotor.</p>
<p>The lateral hydrodynamic forces were decomposed into their steady and unsteady parts, the latter being further expressed in terms of a generalized fluid stiffness matrix. A study of this matrix as a function of the whirl to pump speed ratio supported the following chief conclusions:</p>
<p>i) the common assumption of matrix skew-symmetry is justified;</p>
<p>ii) the magnitudes and signs of the matrix elements are such that rotor whirl can indeed be caused by the hydrodynamic forces, in pumps operated well above their first critical speed,</p>
<p>iii) as expected, the matrix is very sensitive to the value of the flow coefficient, especially at flow rates below the design;</p>
<p>iv) the commonly postulated quadratic variation of the matrix elements with the reduced whirl frequency, resulting in the so-called rotordynamic coefficients (stiffness, damping and inertia) is not justified for flow coefficients significantly below design; and</p>
<p>v) surprisingly, it was discovered that the presence, number and orientation of diffuser guide vanes have little effect on the forces.</p>
<p>Conclusions regarding the effect of impeller geometry could not be reached given the similarity of the tested designs. However, other results on phenomena such as skin friction and leakage flow are presented. Some of the findings are compared to experimental and theoretical data from other sources. Finally, the rotordynamic consequences of the results are discussed as the present data were applied by another author to the case of the Space Shuttle Main Engine's (SSME) High Pressure Oxidizer Turbopump (HPOTP).</p>https://thesis.library.caltech.edu/id/eprint/1144Response and Failure of Fluid-Filled Tanks under Base Excitation
https://resolver.caltech.edu/CaltechETD:etd-11062007-092509
Authors: {'items': [{'id': 'Natsiavas-Sotirios', 'name': {'family': 'Natsiavas', 'given': 'Sotirios'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/CNWT-V417
<p>An analytical model is developed that leads to better understanding of the response of fluid-filled tanks whose bottom may separate from and lift off the foundation, during base excitation. First, the hydrodynamic problem is solved in closed form, for the most general motion of the structure. This eliminates the fluid response unknowns and therefore only the structural degrees-of-freedom need to be considered. Then, application of Hamilton's Principle in the structural domain sets up the system equations of motion. During this procedure, the uplifting behavior is modeled by an appropriate rotational spring, placed between the foundation and the bottom of the tank. Equivalent springs are also used for modeling the ground/structure interaction. Moreover, shell flexibility and liquid sloshing effects are also incorporated and investigated.</p>
<p>Using this model, results are obtained and compared with experimental data. This comparison reveals some interesting effects of the base uplift on the system response. Ground flexibility is found to reduce the effective beam-type stiffness of the structure, but this reduction is much smaller than the substantial stiffness reduction induced by the possibility of uplifting. For the cases examined, the stiffness reduction due to the base uplift changes dramatically the dynamics of the system, which in turn alters the developed hydrodynamic loads, through the fluid/structure coupling process. Also, the shell flexibility effects - which can be important for the anchored tank case - are found to be negligible for an unanchored tank. Knowledge of the structural response leads to direct calculation of the hydrodynamic loads and consequently to prediction of failure. Buckling phenomena observed experimentally at the top and the bottom of scale model tanks are studied and explained.</p>
https://thesis.library.caltech.edu/id/eprint/4425An Experimental Investigation of Vibration Suppression in Large Space Structures Using Positive Position Feedback
https://resolver.caltech.edu/CaltechETD:etd-08112005-104730
Authors: {'items': [{'email': 'james.l.fanson@jpl.nasa.gov', 'id': 'Fanson-James-L', 'name': {'family': 'Fanson', 'given': 'James L.'}, 'show_email': 'NO'}]}
Year: 1987
DOI: 10.7907/0SA8-HW86
<p>A new technique for vibration suppression in Large Space Structures is demonstrated in laboratory experiments on a thin cantilever beam, resulting in substantially reduced dynamic response. This technique, called <i>Positive Position Feedback</i>, makes use of generalized displacement measurements to accomplish vibration suppression.</p>
<p>The concept of a piezoelectric <i>active-member</i> is developed in relation to controlling space-truss type structures. The active-member functions dually as a structural member and a control actuator. Piezoelectric ceramic material is adhered to a thin cantilever beam and simulates the use of an active-member. This <i>space-realizable</i> control scheme makes use of strain measurements, a preferred measurement quantity for vibration suppression, and internal control forces which completely decouple the rigid-body motion from the elastic motion.</p>
<p>A simple necessary and sufficient condition for stability with Positive Position Feedback is presented. This condition is non-dynamic and is in general easily satisfied. As a result, Positive Position Feedback is demonstrated to have superior robust stability properties. It is also demonstrated that with Positive Position Feedback, all control and observation spillover is stabilizing. Five experiments are described in which the first six modes of vibration of the cantilever beam are controlled.</p>
https://thesis.library.caltech.edu/id/eprint/3094Transient 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/1504Compression Failure of Fibrous Laminated Composites in the Presence of Stress Gradients: Experiment and Analysis
https://resolver.caltech.edu/CaltechETD:etd-11062003-092741
Authors: {'items': [{'email': 'dcw@umich.edu', 'id': 'Waas-Anthony-Marius', 'name': {'family': 'Waas', 'given': 'Anthony Marius'}, 'orcid': '0000-0002-5258-2749', 'show_email': 'YES'}]}
Year: 1988
DOI: 10.7907/PP86-PE50
<p>A series of experiments were performed to determine the mechanism of failure in compressively loaded laminated plates in the presence of stress gradients generated by a circular cutout. Real time holographic interferometry and in-situ photomicrography of the hole surface, were used to observe the progression of failure.</p>
<p>The test specimens are multi-layered composite flat plates, which are loaded in compression. The plates are made of two material systems, T300/BP907 and IM7/8551-7. Two different lay-ups of T300/BP907 and four different lay-ups of IM7/8551-7 are investigated.</p>
<p>The load on the specimen is slowly increased and a series of interferograms are produced during the load cycle. These interferograms are video-recorded. The results obtained from the interferograms and photo-micrographs are substantiated by sectioning studies and ultrasonic C-scanning of some specimens which are unloaded prior to catastrophic failure, but beyond failure initiation. This is made possible by the servo-controlled loading mechanism that regulates the load application and offers the flexibility of unloading a specimen at any given instance in the loadtime history.</p>
<p>An underlying objective of the present investigation is the identification of the physics of the failure initiation process. This required testing specimens with different stacking sequences, for a fixed hole diameter, so that consistent trends in the failure process could be identified.</p>
<p>It is revealed that the failure is initiated as a localized instability in the 0° plies at the hole surface, approximately at right angles to the loading direction. This instability emanating at the hole edge and propagating into the interior of the specimen within the 0° plies is found to be fiber microbuckling. The microbuckling is found to occur at a local strain level of ≃ 8600 µstrain at the hole edge for the IM material system. This initial failure renders a narrow zone of fibers within the 0° plies to loose structural integrity. Subsequent to the 0°-ply failure, extensive delamination cracking is observed with increasing load. The through thickness location of these delaminations is found to depend on the position of the 0° plies.</p>
<p>The delaminated portions spread to the undamaged areas of the laminate by a combination of delamination buckling and growth, the buckling further enhancing the growth. When the delaminated area reaches a critical size, about 75-100% of the hole radius in extent, an accelerated growth rate of the delaminated portions is observed. The culmination of this last event is the complete loss of flexural stiffness of each of the delaminated portions leading to catastrophic failure of the plate. The levels of applied load and the rate at which these events occur depend on the plate stacking sequence.</p>
<p>A simple mechanical model is presented for the microbuckling problem. This model addresses the buckling instability of a semi-infinte layered half-plane alternatingly stacked with fibers and matrix, loaded parallel to the surface of the half-plane. The fibers are modelled using Bernoulli-Navier beam theory, and the matrix is assumed to be a linearly elastic foundation. The predicted buckling strains are found to overestimate the experimental result. However, the dependence of the buckling strain on parameters such as the fiber volume fraction, ratio of Youngs moduli of the constituents and Poisson's ratio of the matrix are obtained from the analysis. It is seen that a high fiber volume fraction, increased matrix stiffness, and perfect bonding between fiber and matrix are desirable properties for increasing the compressive strength.</p>https://thesis.library.caltech.edu/id/eprint/4415On the Collapse of Long Thick-Walled Circular Tubes under Biaxial Loading
https://resolver.caltech.edu/CaltechETD:etd-11062007-080516
Authors: {'items': [{'email': 'madhavan1@fuchinobe.oilfield.slb.com', 'id': 'Madhavan-Raghu', 'name': {'family': 'Madhavan', 'given': 'Raghu'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/VPAC-QV51
<p>The collapse phenomenon of long, thick-walled tubes subjected to axial tension and external pressure is investigated. A combined experimental and analytic approach is adopted. The diameter to thickness ratio (DA) of the tubes studied is in the range 10-40.</p>
<p>A series of collapse tests are conducted using thick-walled, small diameter tubes of two different materials. Careful measurements of geometrical and material parameters are carried out before each collapse test. Tension-Pressure collapse envelopes are obtained for tubes of different D/t and loading paths. Collapse tests involving initially ovalized tubes are also carried out. The results show that collapse strength is strongly influenced by initial ovality.</p>
<p>A two-dimensional model is used for predicting the collapse strength. The limit point behavior of a long tube with initial geometric imperfections has been modeled. The tube is assumed to be under generalized plane strain conditions and the possible variations of material and geometric parameters along the length are not considered. Hill's anisotropic plasticity theory involving a quadratic yield function is used to model the anisotropies in yield shown by drawn tubes. A power law creep model is employed to assess the effect of primary creep on collapse strength.</p>
<p>The interaction between collapse pressure and tension is found to depend on both material and geometric parameters. The yield behavior of the tube material strongly affects the collapse phenomenon. Initial ovality of the tube is shown to be a very important geometric parameter that influences collapse strength. The effect of primary creep on collapse is shown to be not very significant, for the type of materials used (304 stainless steel and 6061-O aluminum).</p>
https://thesis.library.caltech.edu/id/eprint/4421Plastic Buckling of Cylinders Under Biaxial Loading
https://resolver.caltech.edu/CaltechETD:etd-11212007-083506
Authors: {'items': [{'id': 'Giezen-Jurgen-Johannes', 'name': {'family': 'Giezen', 'given': 'Jurgen Johannes'}, 'show_email': 'NO'}]}
Year: 1988
DOI: 10.7907/ycv3-kp17
<p>An experimental investigation is carried out to study the effects of nonproportional loading in the plastic range on a buckling load. The discrepancy between experimental and theoretical results points to some principal shortcoming in the analysis. The problem has been simplified by applying axial tensile load and external press to simple cylindrical shell specimen and observing the buckling load for various nonproportional load-paths. Results are compared to numerical predictions (BOSOR5) using classical type plasticity models such as J₂ deformation and J₂ incremental theory. Significant discrepancy was found an attributed to inadequate modeling of the nonlinear material behavior. The effects of geometrical imperfections and large deflections were found to insignificant, thereby leading to an idea <u>how much</u> of the discrepancy between test and theory is due to a use of inadequate plastic model. The introduction of the Southwell plot into elastic shell buckling problem reduced the already minor effects of geometric imperfections.</p>
<p>The Christoffersen-Hutchinson corner theory model was introduced into BOSAR5 in its simplest form as presented by Poh-Sang Lam. Results obtained with this model, which allows corners to form on an initially smooth yield surface, displayed better agreement with experimental data. However, increased computational time and problems related to abrupt changes in load-path at the corner are a major concern at this present time.</p>
https://thesis.library.caltech.edu/id/eprint/4628