(129c) Nonlinear Response of a Confined Mixing Layer to Active Forcing

In 2003, Wang found a rapid mixing phenomenon in forced mixing layer. As the completely different exhibition of the response of flow under forcing from the previous investigations (such as subharmonic mixing mechanism), the essence becomes very attractive. He attributed that to the existence of corner vortex.

  The dynamical process is in details investigated using PIV recently. Due to the tight relation between passive mixing and velocity fluctuations of flow, the response of flow under different Reynolds number and velocity ratios are studied. Several important results are found.

  First, in the nozzle section at the same forcing intensity, the velocity fluctuation under a unique frequency—5.3 Hz is apparently higher than under other frequencies. The velocity fluctuation decreases at first along streamwise position in the mixing chamber, and it then increases further downstream. Secondly, the velocity fluctuation at the inlet section increases with forcing intensity first, and then descends sharply. As the forcing intensity is further increased, the velocity fluctuations resume the trend of increasing with the forcing intensity with a inflection point, regardless of the forcing frequency. Thirdly, at 5.3 Hz of the forcing frequency, the velocity fluctuations are very high and a strong mean flow in vertical direction is observed, due to the highly intensified streamwise mean vorticity. It’s believed the mean vorticity also play important roles in the mixing process, especially at large scale. The experimental results indicate that corner vortices are dominant in generating the streamwise vortices in the near field of the shear flow. However, it’s also shown that the statistical enstrophy of streamwise vorticity decreases along the streamwise position first, and then increases. This implies the existence of other mechanisms in the evolution of streamwise vortex structures at far field.  Further investigations will be pursed in future.