(126a) Nonlinear Mechanism for Mixing Enhancement in Confined Mixing Layer

A drastic fluid mixing phenomenon in confined mixing layer and wake in a pipe by periodic forcing has been discovered by Wang (2003, 2006). To understand the mechanism, the corresponding flow is studied by both flow visualization and PIV system. Compared with results in the free mixing layer, where the linear instability such as Kelvin-Helmholtz instability or von Karman vortex street is dominant and the mixing enhancement is tightly related with vortex pairing process, the mixing enhancement in confined mixing layer becomes more nonlinear and 3-dimensional. Several different kinds of vortex structures are found by visualization and vorticity distribution measurement. One is the spanwise ?mushroom-like? vortex, compared with the similar streamwise ?mushroom-like? structures in free mixing layer which is the result of spanwise instability, or elliptical instability. Other two kinds of vortex structures are found in streamwise. One vortex pair is just at the downstream of the corners between trailing edge of splitter plate and nozzle. The cause is still under investigation. Either corner vortex instability or the fluctuating flow around the trailing edge can induce it. Another kind vortex structure does not apparently vary with the forcing frequency. All the flow structures are under further investigations. The statistics about the flow field under forcing are measured. It is shown, under the forcing level in our experiment (about 11%), the mean flow is evidently changed. Especially, the vertical velocity becomes comparable to the velocity of the bulk flow. The velocity fluctuations become more 3-dimensional. Compared with the forcing under other frequencies, the flow has better receptivity under 5.3 Hz. As resonance is not the dominant mechanism, the feedback mechanism can not attribute to the pressure fluctuations. It could be due to some kind of absolute instability. Because the phenomenon becomes apparent only at high forcing level, it should be a kind of nonlinear instability. More studies will be carried out in future.