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

Abstract

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. 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 variation of the fluctuating velocity and fluctuating energy production and convection rates, which contribute to the velocity, were examined. The streamwise variation in the streamwise component did not correspond to that of the normal component.

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

“…The experimental apparatus is the same one used in a previous paper (5) . The mixing layer is formed just downstream of a two-dimensional nozzle with an aspect ratio of 31 (310 mm in width and 10 mm in height, h).…”

“…The fence-type flat plates oscillated and protruded vertically in relation to the jet with low frequency, which is two orders of magnitude lower than the fundamental frequency. In the previous paper, protrusion of the plate was so small that a disturbance by the protrusion itself was not created (5) . The streamwise velocity component and development of the mixing layer were measured.…”

“…In many studies concerning the transition in the mixing layer and the jet before an establishment of self-similarity, the effects of forced disturbance have been investigated. Representative studies are mentioned in the previous paper (5) . As one example of a forced disturbance, a periodic disturbance has been introduced, mainly as a frequency equal to the fundamental frequency of velocity fluctuation in the linear region of the natural transition process.…”

Randomness Representation in Turbulent Flows with Kolmogorov Complexity (In Mixing Layer)

“…The mixing layer is formed at the beginning of the jet ejected into a quiescent fluid from a rectangular nozzle. We measured the distributions of the mean and fluctuating velocities, spreading of the jet, production, convection and dissipation rates of the fluctuating energy (49) . We also revealed the progression of the laminar-turbulent transition process in the flow-normal direction, and obtained the randomness factor (1) which was proposed by Sato (2) as an indicator of the transition progress, then examined its validity in the transition process.…”

“…Such upstream regularity is due to the smallness of the velocity fluctuation. Details of the transition progress including the fact that the flow just behind the nozzle exit is still laminar were given in the previous report (1) (49) .…”