LARGE EDDY SIMULATION OF FLOW AROUND A FINITE SQUARE CYLINDER
Date
2012-06-05
Authors
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Journal ISSN
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Publisher
ORCID
Type
Degree Level
Doctoral
Abstract
The main objective of this research is to develop, document and study numerically the flow around finite-height square cylinders mounted on a ground plane, particularly in the near-wake region, under various geometrical conditions. Both the time-averaged and instantaneous flow fields are studied. This thesis consists of three main parts: a comprehensive study of flow over an aspect ratio AR = 5 square cylinder, the effect of sub-grid scale (SGS) models on the numerical simulation and the effect of aspect ratio on the flow structure.
The first part of the thesis presents the time-averaged and instantaneous flow fields for flow over a wall-mounted finite-height square cylinder of aspect ratio of AR = 5 at a Reynolds number of Re = 500. The time-averaged flow field results are shown to be in good agreement with experiments. Comparison of the time-averaged results with the velocity field for a square cylinder immersed in a thicker boundary layer, suggests that the boundary layer thickness especially affects the upwash flow (Wang et al., 2009). The instantaneous velocity fields provide an in-depth view of the unsteady nature of the flow field. For the flow over a square cylinder of AR = 5, the instantaneous velocity fields are symmetric near the free end. However, antisymmetric patterns observed downstream may be an indication of the presence of periodic von-Karman type vortices.
Since the wake regions are characterized by large-scale unsteady motions, turbulent flow over bluff bodies is well suited to large eddy simulation in which the large energy-containing scales of motion, which are responsible for most of the momentum transport, are resolved whereas the small-scale turbulent fluctuations are modeled. In the second part of the thesis, the performance of the three SGS models, the Smagorinsky model (SM), dynamic Smagorinsky model (DSM) and dynamic non-linear model (DNM) are studied for two grid sets of lower and higher resolution. The results indicated that in case of the DSM insufficient grid resolution leads to erroneous predictions, whereas the DNM is a major improvement as the predictions are similar on both the coarse and fine grids.
In the third and final part of the thesis, the effect of aspect ratio on the flow over a wall-mounted finite-height square cylinder is numerically investigated. The wake of a finite square cylinder is studied for three aspect ratios of AR = 3, 5 and 7. The time-averaged vorticity was shown to vary with aspect ratio, e.g. as the aspect ratio increases, the vortex structures in a horizontal plane at mid-height became shorter and rounder in shape. The flow field of the finite cylinder is known to be strongly affected by the aspect ratio (Adaramola et al., 2006). For cylinders with relatively small aspect ratios, the two ends affect the flow patterns and significantly alter the flow structure.
Description
Keywords
LARGE EDDY SIMULATION (LES) FINITE SQUARE CYLINDER, BLUFF BODY
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering