周期的振動撹乱が二次元混合層の乱流遷移に及ぼす影響

抄録

The laminar-turbulent transition of a mixing layer induced by oscillating flat plates at an exit of a two-dimensional nozzle was experimentally investigated at Re = 3000. 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, 5 Hz, was two orders of magnitude smaller than the fundamental frequency of the velocity fluctuation in the natural transition process. Three types of plate oscillation were tested: stationary, symmetrical and anti-symmetrical oscillation states. The amplitude of the plates was 1.5 mm and 3.0 mm. Various statistics, such as fluctuating velocity and that with plate oscillating frequency in the streamwise component were measured by hot-wire anemometers. The role of the oscillation modes and amplitudes on the turbulent transition was also examined by smoke visualization. The turbulent transition in the mixing layer was promoted in the order of anti-symmetrical oscillating, symmetrical oscillating, and stationary states. The contribution of fluctuating velocity promoted extensively the anti-symmetrical oscillation but locally the symmetrical oscillation. The periodical oscillation in the mixing layer prevailed more extensively in the anti-symmetrical than in the symmetrical oscillation modes.