Three-dimensional vortical structures of transition in plane channel flow

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Abstract
Three‐dimensional visualization of flow field structures in transitional plane channel flow obtained from a numerical simulation is presented at two Reynolds numbers. At the one‐spike stage, independent of Reynolds number, it is seen that the flow is characterized by a multilayer vortex system. In the upper layer, total vorticity vector plots indicate a high‐shear layer dominated by spanwise vorticity, whereas the middle layer (corresponding approximately to the critical layer) forms a vortex loop (λ vortex) consisting of strong streamwise and spanwise vorticity components. At the three‐spike stage prior to breakdown, the vortical structure becomes concave in the flow direction. This process is accompanied by intense vortex liftup activity near the wall, which seems to originate at the legs of the vortex loop trailing the high‐shear layer. Finally, at the five‐spike stage, it is suggested that high‐vorticity regions (vortical structures) develop into horseshoe eddies in planes inclined to the main flow direction.