Effects of periodic forcing on mixing in turbulent shear layers and wakes

The effects of a periodic disturbance, applied to the one of the the free streams, on large-scale structure and mixing processes in chemically reacting layers and wakes were investigated over a range of Reynolds numbers above and below the mixing transition range. Two different methods were employed to measure the amount of chemical product and thus the extent of molecular-scale mixing. Absorption by reacted phenolphthalein provided cross-stream average product thickness and laser induced fluorescence intensity provided the product concentration distribution. These methods provided, in addition, effective flow visualization of the large-scale structures and of their response to the periodic forcing. The detailed effects of periodic forcing on distribution of mixing along a free shear layer are complex, but the predominant, overall effect is to increase mixing at low Reynolds number (in respect to the mixing transition) and to decrease it at high Reynolds number.