Effect of Local Periodic Disturbance on Mixing Layer at Exit of Two-Dimensional Jet

The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. A mixing layer was formed between the jet from the nozzle and the surrounding quiescent fluid. The plates oscillated vertically in relation to the mean flow. The oscillation frequency was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation. Mean and fluctuating velocity components in the streamwise and normal directions were measured by hot-wire anemometers. The oscillation was found to be effective in enhancing the mixing, though the amplitude was the same order as the momentum thickness of the boundary layer at the nozzle exit. The disturbance traveled downstream as the convective instability, though it was damped only far downstream. The downstream development rate of fluctuating velocity in the normal component was larger than that in the streamwise one. Thus, the need for linear instability analysis of non-parallel flow was suggested. Streamwise variations were examined in the fluctuating velocity and perturbation energy production and convection rates, which contribute to the velocity. The streamwise variation in the streamwise component did not correspond to that of the normal component.