(311e) Role of Streamwise Vortices On Mixing Process In Highly Excited Confined Mixing Layer

The rapid mixing process in confined mixing layer in a pipe is investigated under different Reynolds numbers, velocity ratios and forcing intensities recently. The roles of spanwise and streamwise vortex structures are identified.

  From the side view, it can be found the large scale spanwise vortex is intensively enhanced at some specific forcing frequencies, 3.5 Hz and 5.3 Hz respectively. However, the mixing effect at 3.5 Hz is obviously weaker than that at 5.3 Hz. The investigations indicate that, the peak magnitude of mean velocity of the secondary flow in vertical direction is much higher at 5.3 Hz, than at any other frequencies. The high velocity of the secondary flow leads to a large spreading rate of the mixing layer. The vertical mean velocity also stretches the spanwise vortices, and causes their earlier breakdown and the succedent rapid mixing.

  The cause of the vertical mean velocity can be attributed to the important streamwise vortex structures. Experiments indicate that both the mean streamwise vorticity and fluctuated streamwise vorticity, are very sensitive to the forcing frequency of 5.3 Hz. At the frequency, the asymmetric counter-rotation vortex pairs are strongly intensified which generate vertical mean velocity. Meanwhile, the velocity profile is obviously distorted quasi periodically in spanwise. This further enhances the mixing process in spanwise direction. At the inlet section of mixing chamber, the streamwise vortices are dominant by corner vortices whose vorticity decays slowly along streamwise distance. However, after the region, the streamwise vorticity (estimated by enstrophy) increases with the streamwise position due to some unknown mechanisms. Further investigations will be conducted to study the mechanism.