Studies on By-Pass Transition of Boundary Layr

在本研究中，对高强度扰动引起的所谓旁通边界层转变进行了实验研究。例如，我们对边界层对这种高强度扰动的响应的了解对于理解后掠翼上的附线污染是至关重要的。此外，实验结果还可以为壁面湍流的产生机理和相关的临界流动条件提供更多的信息。整个实验在开式喷射式风洞中进行。开放测试段内设置有界层板。在亚临界雷诺数(线性不稳定性条件下)的Blasius边界层中，通过一个小的壁面孔口引入了良好控制的发夹涡。结果表明，只有当雷诺数大于R_&lt；Rheta&gt；=130(基于动量厚度)时，由小孔产生的发夹涡形成的局部湍流斑块才能发展。在这样的低雷诺数转变中，L…湍流斑块的侧向增长很弱，即侧向扩展角小于2°。随着雷诺数的增加，侧向展开角在高雷诺数时迅速增大，接近典型值(10°)。实验清楚地证明了湍流斑块的横向展开角与R_t；值得注意的是，在这种亚临界边界层转变中，湍流斑块的增长高度依赖于初始发夹涡的强度。观察到的一个关键事件是壁面剪切层(在低速条带上抬升)与流向涡向发夹涡的连续增长，这与平面Poiseille流带状诱导转变后期观察到的近壁现象几乎相同。在临界站(R_&lt；Rheta&>t；=130)以下，即使受到高度扰动，一旦形成沿水流方向的涡流条纹，它们最终也会在下游衰减。较少

In the present study, the so-called by-pass boundary-layr transition caused by high-intensity disturbances is examined experimentally. Our knowledge of the boundary-layr response to such high-intensity disturbances is crucial, for instance, for understanding the attachment-line contamination on a swept wing. In addition, it is expected that the results give much information on the mechanism of wall turbulence generation and the related critical flow condition.The whole experiment is carried out in a wind tunnel with open jet type. A boundary-layr plate is set in the open test section. Well-controlled hairpin eddies is introduced in Blasius boundary layr at subcritical Reynolds numbers (for linear instability) through a small wall orifice. The results obtained are summarized below.Local turbulent patch created by the orifice-generated hairpin vortices can develop only at Reynolds numbers above R_<rheta>=130 (based on the momentum thickness). In such low-Reynolds number transition, the l … More ateral growth of turbulent patch is very weak, i.e., the lateral spreading angle is less than 2 deg. With increasing the Reynolds number, the lateral spreading angle is rapidly increased towards the typical value (10 deg) at high Reynolds numbers. The dependency of the lateral spreading angle of turbulent patch on R_<rheta> is clearly demonstrated in the present experiment. It is also worth noting that the growth of turbulent patch is highly dependent on the intensity of initial hairpin vortices in this subcritical boundary-layr transition.A key event observed is found to be successive growth of wall shear layrs (lifted up on low-speed streaks) with streamwise vortices into hairpin vortices, which is almost the same as the near-wall phenomena observed in the late stage of the ribbon-induced transition in plane Poiseille flow. Below the critical station (R_<rheta>=130), even if highly disturbed and once the wall streaks with streamwise vortices are created, they eventually decay downstream. Less