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Subcritical Transition in Boundary Layers

边界层中的亚临界转变

基本信息

批准号：
62550049
负责人：
NISHIOKA Michio
金额：
$ 1.28万
依托单位：
University of Osaka Prefecture
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (C)
财政年份：
1987
资助国家：
日本
起止时间：
1987 至 1988
项目状态：
已结题

项目摘要

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.

本研究涉及亚临界雷诺数下边界层流的层流-湍流转变。各种透平机械的叶栅叶片在强扰动条件下都会发生亚临界转变，对其性能影响很大。然而，我们对边界层流亚临界转变的理解还很差，因为它涉及复杂的非线性现象。在目前对平板边界层转变的主要实验研究中，我们试图通过声激励在板的前缘实现流动阶段，类似于目前研究人员在平面泊肃叶流中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以下形成壁湍流结构。在这种情况下，流动在前缘分离，形成具有强烈薄剪切层的所谓气泡。气泡剪切层迅速发展成三维发夹形涡流，因此我们可以观察到尖峰阶段，例如上述的1-尖峰和2-尖峰阶段。在2尖峰流的情况下，壁面剪切层出现在与前缘产生的发夹涡相关的低速区域，并最终发展成发夹涡，我们称之为壁面发夹涡。随着壁面发夹涡的通过，低速壁面条纹不断出现，相关的壁面剪切层也不断出现，并演变成新的壁面发夹涡。再生过程继续向下游延伸，导致亚临界转变。由此得出结论，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。