Studies on Turbulent Contamination

本文对高强度发卡涡触发的边界层转捩进行了实验研究。我们对这种高强度扰动的边界层响应的了解对于了解后掠翼上的附着线污染至关重要。此外，本文还对壁面湍流的产生机理及相应的临界流动条件进行了研究。首先，对边界层平板前缘产生的发卡涡引发的亚临界边界层转捩进行了声学研究。结果清楚地表明，当近壁流动受到能量发夹涡扰动时，只有在雷诺数R_（基于动量厚度）=130- 150之后，才能从抬升的近壁剪切层中连续增长发夹涡<rheta>。通过在上述临界站下方和周围的站处使用壁孔来控制发夹形涡流。在这种（亚临界）转捩中，壁湍流仅沿沿着从壁孔向下游延伸的窄条发展，也就是说，在如此低的雷诺数下，横向湍流污染非常弱。实际上，在x-雷诺数R_x=1*10^5以下的过渡区中，湍流斑的横向扩展角（半角）仅约为2度，这比湍流斑或楔横向增长的典型值（约10度）小得多。随着雷诺数的略微增加，横向扩展角大大增加，当R_x=1.1-1.7*10^5时，横向扩展角约为4 °，当R_x = 2.0-2.8*10^5时，横向扩展角约为6.5 °。研究还发现，在横向增长（污染）过程中，发夹涡的展向间距约为100（按局部壁面单位）。

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.