Subcritical Transition in Boundary Layers

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The present study is concerned with the laminar-turbulent transition in boundary layer flows at subcritical Reynolds numbers. The subcritical transition can occur along cascade blades in various turbo-machinaries because of strong disturbance conditions, and it has a great effect on their performance. However, our understanding of the subcritical transition in boundary layer flows is quite poor as it involves complex non-linear phenomena. In the present mainly experimental study on the transition in a flat-plate boundary layer, we try to realize the flow-stages, similar to the so-called spike-stages observed by the present investigators in the ribbon-induced transition in plane Poiseuille flow, at the leading edge of the plate by means of acoustic excitation.We found that the flow can develop wall turbulence structures below the displacement thickness Reynolds number R = 500 when the forcing is strong enough. In this case, the flow separates at the leading edge to form the so-called bubble with intense thin shear layer. The bubble shear layer rapidly develops into three-dimensional hairpin-shaped eddies so that we can observe spike-stages such as 1-spike and 2-spike stages noted above. In the case of 2-spike flow, wall shear layer appear in the low-speed region associated with the leading-edge generated hairpin eddies and it eventually develops into hairpin eddies, which we call the wall hairpin eddies. With the passage of such wall hairpin eddies, the low-speed wall streaks continue to appear and so do the assocated wall shear layers, which evolve into new wall hairpin eddied. The process of regeneration continues downstream to lead to the subcritical transition.Thus, it is concluded that the successive generation or regeneration of wall hairpin eddies is the key event which sustains the wall turbulence.