Studies on Turbulent Contamination

本文通过实验研究了高强度发夹涡流引发的边界层跃迁。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着线污染是至关重要的。此外，结果还提供了壁面湍流产生机理和相关临界流动条件的信息。首先研究了在边界层板前缘产生的发夹涡流在声学上引发的亚临界边界层跃迁。结果清楚地表明，当近壁流动受到高能发夹涡流的扰动时，发夹涡流只能在雷诺数（基于动量厚度）R_<rheta>=130 ~ 150以上才会从上升的近壁剪切层中连续生长。接下来研究的是通过在上面提到的临界站的下面和周围使用壁孔引入控制良好的发夹涡流来增加扰动。在这个（亚临界）转变中，壁面湍流只沿着从壁面孔口向下游延伸的窄带发展，也就是说，在如此低的雷诺数下，侧向湍流污染非常弱。实际上，在x-雷诺数R_x=1*10^5以下的过渡区，湍流斑块的横向扩展角（半角）仅为2度左右；这比湍流斑或楔的横向增长的典型值（约10度）要小得多。随着雷诺数的轻微增加，横向扩展角大大增加，R_x=1.1 ~ 1.7*10^5时扩展角约为4°，2.0 ~ 2.8*10^5时扩展角约为6.5°。在横向生长（污染）过程中，发夹涡的展向间距在局部壁面单位上约为100。

In the Present study, boundary-layr transition trggered by high-intensity hairpin eddies is examined experimentally. Our knowledge of the boundary-layr response to such high-intensity disturbances is crucial for understanding the attachment-line contamination on a swept wing. In addition, the results give much information on the mechanism of wall turbulence generation and the related critical flow condition.First examined is the subcritical boundary-layr transition triggered by the hairpin eddies generated at the leading-edge of a boundary-layr plate acoustically. The results clearly showed that when the near-wall flow is disturbed by energetic hairpin eddies, successive growth of hairpin eddies from the lifted-up near-wall shear-layr can occur only beyond the Reynolds number (based on the momentum thickness) R_<rheta>=130-150.Next examined is the disturbance growth through introducing well-controlled hairpin eddies by using a wall orifice at the station below and around the critical station cited above. In this (subcritical) transition, wall turbulence develops only along a narrow strip extending downstream from the wall orifice, that is, the lateral turbulent contamination is very weak at such low Reynolds numbers. Indeed, the lateral spreading angle (half angle) of turbulent patch is only about 2 deg in the transition region below the x-Reynolds number R_x=1*10^5 ; which is much smaller than the typical value (about 10 deg) for the lateral growth of turbulent spot or wedge. With slightly increasing the Reynolds number, the lateral spreading angle is much increased, about 4 deg for R_x=1.1-1.7*10^5 and about 6.5 deg for 2.0-2.8*10^5. It is also found that in the process of the lateral growth (contamination), the spanwise spacing of hairpin eddies is about 100 in terms of the local wall unit.