A simulation on bypass transition and its key mechanism

The scenario of bypass transition is generally described theoretically as follows: low-frequency disturbances in the free-stream would generate long stream-wise streaks in the boundary layer, which later would induce secondary instability, leading to rapid increase of high-frequency disturbances, then possibly turbulent spots would emerge, and through their merging, fully developed turbulence appears. This description, however, is insufficient in the sense that it does not provide the explanation on why during the breakdown stage, 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 leading to this phenomenon is found to be the change of the stability characteristics of mean flow profile, which has a positive feedback effect on the change of the profile itself. 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, the disturbance waves will change to another branch of inviscid modes, and the latter play the key role in bypass transition.