Direct numerical simulations of separated and separated-reattaching flows on massively parallel processing computers
Najjar, Fady Michel
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https://hdl.handle.net/2142/21667
Description
Title
Direct numerical simulations of separated and separated-reattaching flows on massively parallel processing computers
Author(s)
Najjar, Fady Michel
Issue Date
1994
Doctoral Committee Chair(s)
Vanka, Surya Pratap
Department of Study
Mechanical Science and Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Language
eng
Abstract
Direct numerical simulations of separated-reattaching and separated flows have been performed on massively parallel processing computers. Two basic geometrical configurations have been studied: the separated-reattaching flow past a normal flat plate with an attached downstream splitter plate and the separated flow past a flat plate held normal to a uniform stream. A high-order finite-difference formulation on collocated grids has been developed to perform unsteady fluid flow simulations in rectangular geometries. The numerical procedure is based on a fifth-order upwind-biased scheme for the convective terms and a fourth-order accurate stencil for the diffusive terms. A direct solver based on eigenvalue decomposition has been developed for the pressure-Poisson equation. A mixed Fourier-spectral/finite-difference formulation is used for the spanwise discretization, and a data-parallel algorithm has been developed for the CM-5. The performance of the algorithm has been evaluated on various grid sizes in model flow problems and for different partition sizes.
The characteristics of the separated-reattaching flow have been investigated through two-dimensional simulations in the steady and unsteady regimes. The shedding mechanism is characterized by two major modes at Re = 250 and a single mode at Re = 375 and 500. Further, the instability of the separated shear layer is found to be consistent with inviscid theory. For the two-dimensional study of the separated flow past a normal flat plate, the time-mean flow quantities are observed to be over-estimated compared to the experiments. The time-mean drag coefficient is also over-predicted by a factor of up to 2. This is attributed to the high coherence of the vortices predicted by the two-dimensional simulations. Two interaction regions have been identified in the far wake for Re = 500 and 1000 and are observed to occur pseudo-periodically.
Large-scale computations of the three-dimensional separated flow have also been carried out. The flow is seen to break down into small-scale structures and the spanwise development of these structures has been studied. Streamwise ribs and spanwise rolls are identified in the near wake through visualization techniques. The presence of these coherent structures corroborates the experimental observations and numerical calculations for mixing layers and wakes of circular cylinders. The time-mean velocities and Reynolds stresses are found to agree well with the experiments conducted at higher Reynolds numbers.
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