Division of Engineering and Applied Science

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	Quelques documents de Division of Engineering and Applied Science
	A theoretical and computational study on active wake control 1993 Auteurs : Luca Cortelezzi Masquer le résumé In the first part of this dissertation a two-dimensional unsteady separated flow past a semi-infinite plate with transverse motion is considered. The flow is assumed incompressible and at high Reynolds number. The rolling-up of the separated shear-layer is modelled by a point vortex whose time dependent circulation is predicted by an unsteady Kutta condition. A power-law starting flow is assumed along with a power-law for the transverse motion. The effects of the motion of the plate on the starting vortex circulation and trajectory are presented. A suitable vortex shedding mechanism is introduced and a class of flows involving several vortices is presented. Subsequently, a control strategy able to maintain constant circulation when a vortex is present is derived. An exact solution for the non-linear controller is then obtained. Dynamical system analysis is used to explore the performance of the controlled system. Finally, the control strategy is applied to a class of flows and the results are discussed.In the second part of this dissertation the previous results are extended to the case of a two-dimensional unsteady separated flow past a plate of variable length. Again the rolling-up of the separated shear-layer is modelled by a vortex pair whose time dependent circulation is predicted by an unsteady Kutta condition. A power-law starting flow is assumed while the plate length is kept constant. The results of the simulations are presented and the model validated. A time-dependent scaling which unveils the universality of the phenomenon is discussed. The previous vortex shedding mechanism is implemented and a vortex merging scheme is tested in a class of flows involving several vortices and is shown to be highly accurate. Subsequently, a control strategy able to maintain constant circulation when a vortex pair is present is derived. An exact solution for the non-linear controller is obtained in the form of an ordinary differential equation. Dynamical system analysis is used to explore the performance of the controlled system and the existence of a controllability region is discussed. Finally, the control strategy is applied to two classes of flows and the results are presented. Source : Caltech THESIS   Internal solitons generated by moving disturbances 1986 Auteurs : Jinlin Zhu Masquer le résumé A new phenomenon of the generation of internal solitons is investigated in this thesis by applying theoretical models and is observed in both numerical and experimental results. By imposing an external disturbance, such as a top surface pressure or a bottom bump, that moves with a constant velocity within a trans critical range after an impulsive start from rest, upon a two-layered or a continuously stratified fluid system, a series of solitons are generated, one after another periodically, each surging ahead of the disturbance in turn. Two theoretical models, belonging to the generalized Boussinesq class, are developed to investigate the generation of weakly nonlinear and weakly dispersive long waves and their evolution in an inviscid, immiscible, and incompressible stratified fluid system under the forcing of the external disturbances. The top surface may be either free or covered by a rigid horizontal plate. For the generalized Boussinesq class for two-layered fluid systems, we have derived the FOUR-equation model for the free top-surface case and the THREE-equation model for the rigid horizontal top-surface case; these are extensions of the one-layer homogeneous fluid system previously considered by Wu (1979). For primarily unidirectional motions a forced KdV equation is obtained which represents each normal mode of a two-layer system or a continuously stratified fluid system. Numerical schemes have been successfully developed to solve these equations. Experiments were performed to investigate this phenomenon,henomenon using fresh water to form the upper layer and brine the lower layer. The relationship between the main properties (the amplitude and the period of generation) of the generated solitons and the forcing function configurations is discussed along with comparisons of theoretical, numerical and experimental results. Qualitatively all the results are consistent in exhibiting the salient features of the resulting motion. Quantitatively the numerical results based on the continuously stratified fluid model seem to be more satisfactory than those given by the two-layered fluid model in comparison with the present experiments. The discrepancy between the theory and experiment is supposedly due to the viscous effects, which will be left for future work. Source : Caltech THESIS   Frequency dependent electromagnetic fields : models appropriate for the brain 1986 Auteurs : Harrison Mon Fook Leong Masquer le résumé NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.This dissertation addresses the problem of modeling electromagnetic fields in and about the brain-skull-scalp system that are generated by active neural populations. Specifically, frequency dependence of Maxwell's fields is explored for the case of a dipole-like current source embedded in a spherical conductor surrounded by a vacuum. Frequency dependence was found to be small. Loosely, the difference between frequency dependent and frequency independent fields reached approximately 1% at 10[...]Hz and reached up to 16% at 10[...]Hz. Frequency dependence was found to be highly dependent on conductivity, the size of the conductor, and on the phase of generated fields. These findings indicate that the degree to which the magnetic field is coupled to the electric field depends on interference patterns occurring within the conductor. Several highly distinguishable exceptions to general trends in the data were found to be consistent with this view. Source : Caltech THESIS   Frequency dependent electromagnetic fields : models appropriate for the brain 1986 Auteurs : Harrison Mon Fook Leong Masquer le résumé NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.This dissertation addresses the problem of modeling electromagnetic fields in and about the brain-skull-scalp system that are generated by active neural populations. Specifically, frequency dependence of Maxwell's fields is explored for the case of a dipole-like current source embedded in a spherical conductor surrounded by a vacuum. Frequency dependence was found to be small. Loosely, the difference between frequency dependent and frequency independent fields reached approximately 1% at 10[...]Hz and reached up to 16% at 10[...]Hz. Frequency dependence was found to be highly dependent on conductivity, the size of the conductor, and on the phase of generated fields. These findings indicate that the degree to which the magnetic field is coupled to the electric field depends on interference patterns occurring within the conductor. Several highly distinguishable exceptions to general trends in the data were found to be consistent with this view. Source : Caltech THESIS   The atomic structure of some metallic glasses and the influence of the preparation technique on the amorphous state 1985 Auteurs : Madhav Mehra Masquer le résumé NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.In this work the structure of some new transition metal-metalloid glasses is discussed. Based on the Ru-Zr-B and the Rh-Si-B alloy series, these glasses are unique in that they contain up to 52 atomic percent metalloids. Available dense random packing models are not successful in explaining the structure of these glasses. The presence of a peak in the reduced radial distribution functions, G(r), at a distance of [...] times the nearest-neighbour distance suggests the presence of octahedral coordination. This coordination, absent in the dense random packed models is explained by invoking a packing of trigonal prisms. With this proposed short range order the main features in the experimentally obtained G(r) have been reproduced.Also presented is a study that compares the amorphous state obtained in the same alloy by two different techniques. Films of [...] obtained by sputtering are found to be more disordered than their liquid quenched counterparts. This increased disorder is evidenced through measurements of the G (r ) and superconducting properties. The films have a lower atomic density than liquid quenched films of the same composition. This lower density reflects the presence of more entrapped free volume.Finally, the results of a study on wear resistant amorphous coatings is reported. It is found that, by coating a surface of 52100 bearing steel with a thin film of [...], the wear resistance of the steel is increased by three orders of magnitude. This encouraging result suggests that the technological hopes for amorphous materials may indeed be realised. Source : Caltech THESIS   The atomic structure of some metallic glasses and the influence of the preparation technique on the amorphous state 1985 Auteurs : Madhav Mehra Masquer le résumé NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.In this work the structure of some new transition metal-metalloid glasses is discussed. Based on the Ru-Zr-B and the Rh-Si-B alloy series, these glasses are unique in that they contain up to 52 atomic percent metalloids. Available dense random packing models are not successful in explaining the structure of these glasses. The presence of a peak in the reduced radial distribution functions, G(r), at a distance of [...] times the nearest-neighbour distance suggests the presence of octahedral coordination. This coordination, absent in the dense random packed models is explained by invoking a packing of trigonal prisms. With this proposed short range order the main features in the experimentally obtained G(r) have been reproduced.Also presented is a study that compares the amorphous state obtained in the same alloy by two different techniques. Films of [...] obtained by sputtering are found to be more disordered than their liquid quenched counterparts. This increased disorder is evidenced through measurements of the G (r ) and superconducting properties. The films have a lower atomic density than liquid quenched films of the same composition. This lower density reflects the presence of more entrapped free volume.Finally, the results of a study on wear resistant amorphous coatings is reported. It is found that, by coating a surface of 52100 bearing steel with a thin film of [...], the wear resistance of the steel is increased by three orders of magnitude. This encouraging result suggests that the technological hopes for amorphous materials may indeed be realised. Source : Caltech THESIS   A model for the Von Karman vortex street 1981 Auteurs : James Carl Schatzman Masquer le résumé In the wake of a two-dimensional bluff body placed in a uniform stream, for sufficiently large but not too large flow velocity a distinctive pattern of vorticity is observed. The pattern consists of vortices of high vorticity surrounded by nearly irrotational fluid. These vortices are organized in two nearly parallel staggered rows of vortices of opposite direction of rotation. This pattern is called the von Karman vortex street.This thesis is a report on the analysis of a model for the von Karman vortex street. The model is inviscid, incompressible, two-dimensional, and consists of vortices of finite area and uniform vorticity. The first part of this thesis contains a brief survey of the work on this problem, and an explanation of the approach used in the present work; the second part describes calculations of steady solutions of the Euler equations of this kind, and the third part describes an analysis of the stability of these steady solutions to two-dimensional disturbances.The calculations indicate that the vortex wake can be stabilized by sufficiently large area of the vortices. Data are given which (to some approximation) will permit relating the street to the flow past a body; this is proposed as a suitable study for further work. Source : Caltech THESIS