LARGE EDDY SIMULATION OF THE FLOW AROUND A FINITE SQUARE PRISM MOUNTED ON A GROUND PLANE

Abstract

Dynamic subgrid-scale models for large eddy simulation (LES) offer the promise of being able to dynamically calibrate the residual stress field to the local flow conditions. The first part of thesis reports on the application of two different dynamic subgrid-scale (SGS) models to predict the turbulent wake of a finite-height square prism mounted vertically on a ground plane. The prism aspect ratio was AR = 3, and the Reynolds number, based on the prism width and freestream velocity, was Re = 500. The approach flow was laminar with a thin boundary layer; the thickness at the location of the prism was approximately 0.2D. The flow over the top of the prism interacts with the flow along the ground plane and the vertical shear layers from the sidewalls to create a complicated wake structure. Both a linear dynamic Smagorinsky model (DSM) and dynamic nonlinear model (DNM) were implemented and tested for their ability to resolve the complex wake structure. Investigation of the dissipation of turbulence kinetic energy reveals that the DSM has a much larger SGS dissipation, whereas the DNM has a greater resolved-scale dissipation. The backscatter associated with the DSM is much more pervasive than that predicted by the DNM, which accounts for the numerical instability of the DSM. Overall, specific differences are observed in the wake predicted by the two SGS models, including some features of the mean velocity field. The second part of the thesis explores the phase-averaged structure of the wake based on the prediction of the DNM. Phase-averaging based on the Strouhal number reveals a wake structure with quasi-periodic features that is much different from the mean vorticity field, which is characterised by two pairs of counter-rotating streamwise vortex tubes. The phase-averaged near-wake structure is dominated by vertical vortex cores formed by the shear layers being shed from the two sides of the prism. These tubes re-orient and interact as they detach from the prism and move downstream, giving evidence of the half-loop structures documented in previous studies. Each half-loop consists of a short vertical core near the ground plane, and a connector strand that tilts back upstream toward the prism. The shedding process is almost symmetric just downstream of the prism, but develops an asymmetric pattern farther downstream, characterised by the alternate development of half-loop structures on opposite sides of the wake. Further downstream in the wake, the phase-averaged flow is dominated by approximately streamwise vortex tubes associated with the connector strands.