Properties of the laminar-turbulent transition in a mixing layer by the low-frequency disturbance (Effect of the anti-symmetrical disturbance)

The laminar-turbulent transition of a mixing layer excited by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet issued from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. Upper and lower flat plates oscillated anti-symmetrically. The oscillation frequency, 5 Hz, was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation in the natural transition process. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. The results were compared with the previous case in which the plates oscillated symmetrically. Anti-symmetrical oscillation promoted the expansion of the mixing layer and promoted the disappearance of the potential core more than symmetrical oscillation. The periodic motion of the flow promotes the fluctuation from the time-averaged velocity. The fluctuations from time and phase averages in the anti-symmetrical oscillation were larger than those in the symmetrical oscillation. The contribution of periodic fluctuation disappeared downstream in the symmetrical oscillation but persisted longer in the anti-symmetrical oscillation. In the linear region of the transition, the irregular fluctuation grew exponentially. There were not any differences in the spatial growth rate between the streamwise and normal directions and among three oscillating states. In the nonlinear region, factors other than the temporal fluctuation energy convection and production rates may contribute to the increase in velocity temporal fluctuation in the streamwise component. On the other hand, the convection and production rates may contribute to the increase in velocity fluctuation in the normal component there.