Effect of Local Periodic Disturbance on Mixing Layer Downstream of Two-Dimensional Jet:(Spatial Structure and Quantitative Representation of Laminar-Turbulent Transition Process)

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The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated. The mixing layer was formed between the jet, which issued 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. In the oscillating state, the same phenomenon as in the natural transition process appeared more upstream. In the early stage within the nonlinear region, the growth of fluctuating velocity weakened and the Reynolds shear stress component decreased. The decrease in their production rate due to the expansion of the mixing layer contributed to the weakening and decrease. The probability of the streamwise and normal fluctuating velocity components taking the same sign increased or decreased in accordance with the increase and decrease of the Reynolds shear stress component. The randomness factor, which had been proposed by Sato, appeared to be a reasonable indicator of the present transition process, especially in the process in which the periodic velocity fluctuation became irregular. However, this factor certainly indicated the same value at two streamwise positions.