Analysis and reduced simulations of laminar/turbulent wake flows.
Item
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Title
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Analysis and reduced simulations of laminar/turbulent wake flows.
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Identifier
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AAI9405508
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identifier
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9405508
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Creator
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Cao, Nian-Zheng.
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Contributor
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Adviser: Nadine Aubry
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Date
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1993
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Language
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English
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Publisher
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City University of New York.
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Subject
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Engineering, Mechanical
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Abstract
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Laminar and turbulent wakes behind bluff bodies are simulated by reduced systems and analyzed through Biorthogonal and Proper Orthogonal Decomposition techniques.;First, a reduced direct numerical simulation is proposed to integrate the Navier-Stokes equations and simulate laminar wake flows, using a spectral expansion based on basis functions satisfying the boundary conditions. More specifically, by removing the body from the computational domain and imposing the spatial mean velocity profile, we derive a reduced representation which successfully reproduces a stable two-dimensional vortex street at Reynolds number Re = 56 (based on the diameter of the cylinder and the speed of the fluid at infinity) and its three-dimensional secondary instability at Reynolds number Re = 230. These results are similar to those obtained at a significantly higher computational effort by direct numerical simulations of the original system including the cylinder.;Second, the spatio-temporal structure of laminar and turbulent wake flows is investigated through a biorthogonal decomposition analysis, particularly in terms of spatio-temporal symmetries. We show for example how the spatio-temporal translation symmetry characterizing traveling waves is broken in a two-dimensional wake flow behind a half-cylinder. As far as turbulence is concerned, if the flow is assumed to satisfy the scaling invariance of the Navier-Stokes equations, then biorthogonal energy spectra should decay exponentially fast. This is indeed observed in the analysis of experimental data in a turbulent wake. We then construct a low-dimensional dynamical system for a laminar wake flow past a half cylinder based on the biorthogonal decomposition. The efficiency of the truncation in the biorthogonal decomposition technique is demonstrated by a successful numerical reproduction of the Karman vortex street.;Third, the structure of a turbulent far-wake behind a circular cylinder (at a downstream distance of 100 cylinder diameters) experimentally measured by Guezennec and Gieseke (1991) is studied via the proper orthogonal decomposition, assuming the homogeneity of the flow in the spanwise and streamwise directions. We show that the first proper orthogonal mode presents the main features of typical coherent patterns which have been discovered by other researchers, including the double-roller structure and spanwise/ streamwise vortices. Finally, a low-dimensional dynamical system based on the first proper orthogonal modes and derived from the Navier-Stokes equations predicts turbulence statistics of approximately the right shape.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
