Abstract:

The evolution of a time developing mixing layer with cross shear is simulated numerically using the 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 (the cross angle of two streams is equal to 40 degrees, for example), the co rotating streamwise vortices related to the early spanwise Kelvin Helmholtz instability grow rapidly by stretching and collapse into rib shaped vortices, which are very similar to the ribs in a plane mixing layer. When the cross angle is 40 degrees, 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 in the spanwise braid region.The counterparts of the ribs, however, are in flat shape and much weaker. When the cross angle is up to 60 degrees, the ribs are so strong that their counterparts can not develop. While the cross angle is down to 20 degrees, 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 and that increases of initial intensity and wavenumber of the spanwise disturbance are also conducive to mixing.

Key words: mixing layers, cross-shear, coherent structures, pseudo-spectral method, numerical simulation