A numerical study of mixing control in spatially evolving shear flows

We present results from three-dimensional finite-difference simulations of the transition region of a subsonic, spatially evolving mixing layer. The numerical model solves the compressible, time-dependent, conservation equations for mass, momentum and energy using the Flux-Corrected Transport algorithm and direction and timestep splitting. A passive scalar is used to examine the flow entrainment. The mixing layer is very sensitive to spanwise perturbations. In response to the span wise excitation and as a result of the vortex stretching of the initial streamwise vorticity by the spanwise rolls, streamwise vortices (ribs) appear in the braids. Flow visualization of the computed fields is used to examine the dynamics of the ribs during a vortex-roll pairing. We evaluate the entrainment rate based on the vortical content of the fluid. Since entrainment is considerably augmented by the presence of the ribs, spanwise excitation has the potential of providing a useful mechanism for mixing enhancement.