Subcritical Transition in Boundary Layers

本文研究了亚临界雷诺数下边界层流动的层流-湍流转变。各种透平机械的叶栅在强扰动条件下都会发生亚临界转变，对其性能有很大影响。然而，由于边界层流动涉及复杂的非线性现象，我们对边界层流动的亚临界转折现象的理解很差。在平板边界层转变的主要实验研究中，我们试图通过声激励来实现平板前缘的流动阶段，类似于目前研究者在平面Poiseuille流带状诱导转变中观察到的所谓尖峰阶段。我们发现，当强迫足够强时，流动可以在位移厚度雷诺数R=500以下发展壁面湍流结构。在这种情况下，流动在前沿分离，形成具有强烈薄剪切层的所谓气泡。气泡剪切层迅速发展为三维发夹型涡旋，因此我们可以观察到上述的1-尖峰和2-尖峰等尖峰阶段。在双峰流的情况下，壁面剪切层出现在与前缘产生的发夹涡相关的低速区，并最终发展为发夹涡，我们称之为壁发夹涡。随着这种墙发夹涡的通过，低速墙体条纹继续出现，伴生的墙体剪切层也不断出现，并演化为新的墙体发夹涡流。再生过程在下游继续进行，导致亚临界转变，因此，壁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.