Studies on Turbulent Contamination

在本研究中，我们对高强度发夹涡对边界层过渡过程进行了实验研究。我们对这种高强度扰动的边界层响应的了解对于理解后掠翼上的附线污染是至关重要的。此外，这些结果还为壁面湍流的产生机制和相关的临界流动条件提供了大量的信息。首先，研究了由边界层板前缘产生的发夹涡在声学上触发的亚临界边界层转变。结果表明，当近壁流动受到高能发夹涡的扰动时，只有在雷诺数(动量厚度)R_&lt；Rheta&gt；=130-150范围内，才能从提离的近壁剪切层中产生发卡涡的连续增长。在这种(亚临界)转变过程中，壁面湍流只沿着从壁面孔口向下延伸的狭窄条带发展，即在如此低的雷诺数下，横向湍流污染非常弱。在x-雷诺数R_x=1*10^5以下的过渡区，湍流斑块的横向展开角(半角)仅为2°左右，远小于湍斑或湍流楔体横向增长的典型值(约10°)。Rx=1.1~1.7×10~(-5)时，横向展开角增大约4°；R_x=2.0~2.8×10~(^5)时，横向展开角增大约6.5°。此外，在横向发展(污染)过程中，发夹型涡旋的横向间距约为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.