Trailing edge effect on fast mixing in forced confined mixing layers

It was believed that due to nonlinear effect and saturation, the spreading rate in forced 2D mixing layers can only reach about two times of that in the unforced one. The limited enhancement restricted the related technique in practical application. We found recently that confined mixing layer can overcome the saturation under a specific and narrow frequency band to achieve ultra fast mixing. Here, we report the spanwise vortices are extremely sensitive to the sharpness of the trailing edge. Without trailing edge, there are no spanwise vortices. In a blunt trailing edge, there can be spanwise wortices, but there is no spanwise counter-rotation vortice. With sharp trailing edge, there are large spanwise vortices and at high forcing level counter-rotation vortices, which can cause initially fast mixing. The influence of trailing edge sharpness on this spanwise counter-rotation shredding vortices could be explained by the relation between acoustic particle displacement (APD) and the curvature radius of trailing edge. When the APD becomes larger or equal to the radius of curvature, the acoustically induced shredding vortices emerge due to the friction of wall and nonlinear flow. That could be why sharper trailing edge can induce strong vortices and fast mixing.