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The scenario of bypass transition is generally described theoretically as follows: the low-frequency disturbances in the free-stream generate long stream-wise streaks in the boundary layer, which later would trigger secondary instability, leading to rapid increase of high-frequency disturbances, then possibly turbulent spots emerge, and through their merging, fully developed turbulence appears. This description, however, is insufficient in the sense that it does not provide the inherent mechanism of transition that during the breakdown stage, why a large number of waves with different frequencies and wave numbers would appear almost simultaneously, leading to a swift change of the mean flow profile. In this paper, the mechanism is found to be the swift change of the stability characteristics of mean flow, which has a positive feedback effect on the change of mean flow profile. And another interesting finding is that, during the transition, the unstable disturbance waves which appear first belong to a branch of inviscid modes, while following the change of the stability characteristics of the mean flow profile, disturbance waves belong to another branch of inviscid modes, which play the key role in bypass transition.
Author(s):
Ming Dong
Tianjin University
China