Numerical Simulations of Spatially Developing Mixing Layers
Turbulent mixing layers have been researched for many years. Currently, research is focused on studying compressible mixing layers because of their widespread applications in high-speed flight systems. While the effect of compressibility on the shear layer growth rate is well established, there is a lack of consensus over its effect on the turbulent stresses and hence warrants additional research in this area. Computational studies on compressible shear layers could provide a deep cognizance of the dynamics of fluid structures present in these flow fields which in turn would be viable for understanding the effects of compressibility on such flows. However, performing a Direct Numerical Simulation (DNS) of a highly compressible shear layer with experimental flow conditions is extremely expensive, especially when resolving the boundary layers that lead into the mixing section. The attractive alternative is to use Large Eddy Simulation (LES), as it possesses the potential to resolve the flow physics at a reasonable computational cost. Therefore the current work deals with developing a methodology to perform LES of a compressible mixing layer with experimental flow conditions, with resolving the boundary layers that lead into the mixing section through a wall model. The wall model approach, as opposed to a wall resolved simulation, greatly reduces the computational cost associated with the boundary layer regions, especially when using an explicit time-stepping scheme. An in house LES solver which has been used previously for performing simulations of jets, has been chosen for this purpose. The solver is first verified and validated for mixing layer flows by performing simulations of laminar and incompressible turbulent mixing layer flows and comparing the results with the literature. Following this, LES of a compressible mixing layer at a convective Mach number of 0.53 is performed. The inflow profiles for the LES are taken from a precursor RANS solution based on the k-ε and RSM turbulence models. The results of the LES present good agreement with the reference experiment for the upstream boundary layer properties, the mean velocity profile of the shear layer and the shear layer growth rate. The turbulent stresses, however, have been found to be underpredicted. The anisotropy of the normal Reynolds stresses have been found to be in good agreement with the literature. Based on the present results, suggestions for future work are also discussed.