Studies on By-Pass Transition of Boundary Layr

在本研究中，所谓的旁路边界层过渡所造成的高强度扰动进行了实验研究。例如，我们对这种高强度扰动的边界层响应的了解，对于理解后掠机翼上的附着线污染是至关重要的。此外，本文的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。整个实验是在开放式射流风洞中进行的。在开放试验段内设置边界层板。在亚临界雷诺数（线性不稳定性）下，通过一个小的壁孔在Blasius边界层中引入了良好控制的发夹涡。实验结果表明，由发夹涡产生的局部湍流斑只有在雷诺数<rheta>大于130（基于动量厚度）时才能发展。在这种低雷诺数转捩中， 关于我们 湍流斑块的侧向生长非常弱，即，横向扩展角小于2 °。随着雷诺数的增加，横向扩展角迅速增加，在高雷诺数下接近典型值（10度）。实验清楚地表明了湍流斑的横向扩展角与R_1的关系<rheta>。在亚临界边界层转捩过程中，湍流斑的发展与初始发夹涡的强度密切相关，壁面剪切层的连续发展是湍流斑发展的关键（在低速条纹上抬起），流向涡变为发夹涡，这与平面Poiseille流中带状诱导转捩后期观察到的近壁现象几乎相同。在临界位置（R_<rheta>=130）以下，即使高度扰动，一旦产生带有流向涡的壁面条纹，它们最终也会向下游衰减。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