Subgrid scale stresses and their modeling in the turbulent plane wake

J. O'Neil

Ph.D. Thesis, The Johns Hopkins University
October 1996, Baltimore MD

ABSTRACT: Turbulence velocity measurements are performed in the plane wake of a cylinder at several downstream locations, at high Reynolds numbers. The measurements employ two subminiature x-wires and a single subminiature hot wire. The data is used in a study of the subgrid-scale (SGS) stresses and their models, which are needed to close the filtered Navier-Stokes equations used for Large Eddy Simulation (LES) of turbulent flows. The present experimental study expands on previous low Reynolds number and simple geometry Direct Numerical Simulation studies. Comparisons of various globally time-averaged quantities involving the measured and modeled SGS stress are made, with special emphasis on the SGS energy dissipation rate. Within the constraints of the experimental data, the globally averaged results show that all models considered, namely the Smagorinsky and similarity models, as well as their dynamic counterparts, reproduce profiles of the real SGS dissipation. Some discrepancies near the outer edge of the wake are observed. Because of the very limited information that is contained in a few low order-moments such as mean dissipation, a more focussed comparison is made by conditional averaging based on particular physical phenomena: (i) the outer-intermittency of the wake, and (ii) large-scale coherent structures of the turbulent wake. Thus, the interaction of the subgrid-scales with the resolved flow and model viability can be individually tested in regions where isolated mechanisms such as outer-intermittency, vortex stretching, rotation, turbulence production, etc., are dominant. Conditioning on outer-intermittency did not help to clarify observed features of the measurements. Alternatively, the large-scale organized structures are found to have a strong impact upon the distribution of SGS energy dissipation even at filter scales well inside the inertial range where the SGS stress is nearly isotropic. The similarity model is able to capture this result, while the Smagorinsky model gives a more uniform (i.e. unrealistic) distribution. Both dynamic models reproduce realistic distributions, but only if all filter levels are well contained inside the inertial range.