Subcritical Transition in Boundary Layers

本文研究了亚临界雷诺数下边界层流动中的层流-湍流过渡问题。在各种涡轮机械中，由于强扰动条件，叶栅沿叶栅会发生亚临界转捩，对机械性能有很大影响。然而，由于涉及到复杂的非线性现象，我们对边界层流动中的亚临界转捩的认识还很差。在本研究中，我们主要是对平板边界层中的过渡进行实验研究，我们试图通过声激励来实现在平板前缘的流动阶段，类似于目前研究人员在平面泊泽维尔流的带状诱导过渡中观察到的所谓尖峰阶段。研究发现，当压力足够大时，在位移厚度雷诺数R = 500以下，流动可以形成壁面湍流结构。在这种情况下，流动在前缘分离，形成具有强烈的薄剪切层的所谓气泡。气泡剪切层迅速发展成三维发夹形状的涡流，因此我们可以观察到上面提到的尖峰阶段，如1尖峰和2尖峰阶段。在2尖峰流动情况下，低速区出现壁面剪切层，与前缘产生的发夹涡流相结合，最终发展为发夹涡流，我们称之为壁面发夹涡流。随着这种壁面发夹涡的通过，低速壁面条纹继续出现，相应的壁面剪切层也继续出现，形成新的壁面发夹涡。再生过程继续下游，导致亚临界转变。因此，壁面发夹涡的连续产生或再生是维持壁面湍流的关键事件。

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.