The Effect of Coherent Modes on the Evolution of a Turbulent Mixing Layer

Abstract

We have performed direct numerical simulations of a temporally growing mixing layer, focusing on the interaction between the various unstable modes during transition to turbulence. Even at very low levels of initial excitation, the two-dimensional modes play a critical role in the subsequent evolution of the flow. There is a very strong secondary instability present in the flow that is characterized by the growth of counterrotating, streamwise vortices in the braids between the large spanwise vortices. The growth rate of the three-dimensional, secondary instabilities is a function of the amplitude of the spanwise, two-dimensional modes. It appears that the degree and nature of mixing in a reacting mixing layer can be significantly modified by appropriate low-amplitude forcing.