Visual study on effect of forcing oscillation on coherent vortex in mixing layer of parallel coflows.

The effect of periodic two-dimensional excitation on the development of a turbulent mixing region in open-channel parallel coflows was investigated experimentally by making use of visual study and hot-film anemometers. Controlled oscillations of variable frequency were applied at the initiation of mixing between two parallel water flows. When the frequency of the forced oscillations was lower than the fundamental frequency (in the present case it was 1 Hz) which might describe the interaction of vortices, i.e. pairing, the spreading rate of flow was increased by enhancing the coalescence of neighbouring eddies. On the other hand, at the frequency higher than the fundamental one, the controlled and resonant vortices were generated in a single array of large vortices, which did not interact with each other. Consequently, the spreading rate of flow was depressed, and also the Reynolds stress became negative.