Mode interaction models in near-wall turbulence.
Item
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Title
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Mode interaction models in near-wall turbulence.
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Identifier
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AAI9218266
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identifier
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9218266
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Creator
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Sanghi, Sanjeev.
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Contributor
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Adviser: Nadine Aubry
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Date
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1992
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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 | Physics, Fluid and Plasma
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Abstract
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The dynamics of near wall turbulence is characterized by intermittent bursts during which fine scale turbulence is produced. Bursting events have been observed in low dimensional dynamical models built from empirical eigenfunctions of the proper orthogonal decomposition (POD). Severely truncated models, which did not account for streamwise variations in an explicit way, have been derived and studied by Aubry et al. (1988). In the present work, the persistency of intermittent behavior in higher dimensional models is investigated. First, the new truncations account for streamwise variations. Intermittent behavior persists in all the models, but exhibits additional complexities. In particular, the non zero streamwise modes become excited during the bursts so that the infinitely long streamwise rolls burst in the streamwise direction into smaller scales. In all cases, intermittency seems to be due to homoclinic cycles connecting hyperbolic fixed points or complicated limit sets. The triggering of the higher order streamwise modes by nonlinear interactions is also analyzed. The influence of another proper orthogonal mode (i.e. smaller structures in the normal direction) is also investigated: bursts still occur in the same way, the second mode participating along with the first. A coherent structure study is performed in these models in order to connect the findings of this investigation to those of the other researchers. Most experimentally observed structures, i.e. streaks, streamwise rolls, horseshoe vortical structures and shear layers, are clearly identified and it is proposed that they are a part of the same structure.;The POD technique is used to expand the vorticity field into 'coherent vortical structures'. To calculate the POD modes for any vector, its autocorrelation tensor is needed. It is, however, shown in this work that the POD modes and eigenvalues of the vorticity vector can be calculated from the POD modes and the eigenvalues of the velocity vector. This method is applied to the turbulent wall layer and the spectrum of eigenvalues of the vorticity modes is compared with the velocity modes. As expected, the peak shifts towards the right for the vorticity spectrum. As a trial case the first two vortical structures are evaluated for one realization.;Finally a dynamic eddy viscosity model based on the application of the approximate inertial manifold technique to the POD modes is proposed.
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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.
