The influence of velocity ratio on the evolution of streamwise vortices in the simulated plane mixing layer

In this paper we perform Large Eddy Simulations of the plane turbulent mixing layer, to assess the effect of the velocity ratio parameter on the evolution of the streamwise vortex structure in the flow. The mixing layers originate from a physically-correlated inflow condition. A simulation performed against reference experimental flow conditions produces reliable flow statistics, and the velocity ratio parameter is subsequently varied by changing the low-speed freestream velocity of the flow. It is found that the residual streamwise vorticity in the flow upstream of the splitter plate is amplified, and subsequently realigns into a single row of spatially stationary streamwise vortices. The distance required for this realignment process decreases with increasing values of the velocity ratio parameter. Once formed, the average spacing of the spatially stationary streamwise vortices evolves in a stepwise fashion, with the overall spanwise wavelength of the streamwise structure scaling linearly with the local vorticity thickness. This scaling is independent of the velocity ratio parameter for all values tested here.