DNS Study on Role of Linearly Unstable Modes in Flow Transition

Abstract

Flow transition from laminar to turbulent flow is widely considered as caused by linear unstable modes through absolute or convective instability. However, our DNS study shows that is not the case. In our previous AIAA paper, the nature of flow transition is described as an inherent property of fluid flow that fluid cannot tolerate shear layer and shear must transfer to rotation when the Reynolds number is large, which will lead to flow transition. In the current DNS study, two inflow disturbances, 2D T-S waves and 2D+3D T-S waves, are tested separately and carefully compared. The development of disturbances and late flow transition are investigated by DNS. It is found that the late flow transition are all caused by shear layer instability including the vortex ring formation, multilevel shear layers, multiple level sweeps and ejections. The role of all unstable modes are same which is to trigger the vorticity rollup and change the base flow profile to have inflection points. Since all disturbances, like dust, gust, noise, mosquito, fly, sands, etc., can trigger the vorticity rollup, the idea to use control of linear modes to control flow transition is hard to get success and the key issue is to avoid the vorticity rollup, like use of suctions.