NUMERICAL RESEARCH ON THE COHERENT STRUCTURES IN A MIXING LAYER WITH CROSS-SHEAR

The evolution of a time-developing mixing layer with cross-shear is simulated numerically using a pseudo-spectral method. The results indicate that stretching by the rollers is responsible for the formation of the streamwise vortices in a mixing layer with cross-shear. When the cross-shear is relatively strong (such as θ=20°), the co-rotating streamwise vortices related to the early spanwise Kelvin–Helmholtz instability are intensified rapidly by stretching and collapse into rib-shaped vortices, which are very similar to the ribs in a plane mixing layer. Atθ =20°, the vortex corresponding to the “quadrupole” in a plane mixing layer is also observed in the core region, and a set of streamwise vortices with signs opposite to those of the vortices containing the ribs lie at the spanwise braid region. The counterparts of the ribs, however, are of flat shape and much weaker. When θ is up to 30°, the ribs are so strong that their counterparts cannot develop. When θ is down to 10°, the symmetry of the streamwise vortices is more obvious, but the ribs do not form. Additionally, it is revealed that the introduction of the strong cross-shear results in enhanced mixing compared to a two-dimensional mixing layer.