Eppink Jenna L, Wlezien Richard W, King Rudolph A, Choudhari Meelan
NASA Langley Research Center, Hampton, Virginia 23681.
Iowa State University, Ames, Iowa 50011.
AIAA J. 2018 Feb;56(2):497-509. doi: 10.2514/1.J056267. Epub 2017 Sep 19.
A swept flat plate model with an imposed pressure gradient was experimentally investigated in a low-speed flow to determine the effect of a backward-facing step on transition in a stationary crossflow-dominated flow. Detailed hotwire measurements of boundary-layer flow were performed to investigate the upstream shift in transition due to a step height of 49% of the local unperturbed boundary-layer thickness. Increasing the initial stationary crossflow amplitude caused an upstream movement of the transition front for the backward-facing step case. The step caused a local increase in the growth of the stationary crossflow instabilities, but the stationary crossflow amplitude at transition was sufficiently low (<0.04 ) so that stationary crossflow was not solely responsible for transition. The unsteady velocity spectra downstream of the step were rich with unsteady disturbances in the 80- to 1500-Hz range. Three distinct families of disturbances were identified based on phase speed and wave angle, namely, a highly oblique disturbance (possibly traveling-crossflow-like), a Tollmien-Schlichting-wave-like disturbance, and a shear-layer instability. The stationary crossflow disturbances caused a modulation of the unsteady disturbances, resulting in spatially concentrated peaks in unsteady disturbance amplitude. This modulation of the unsteady disturbances is believed to be the reason for the upstream movement of the transition front with increasing stationary crossflow amplitude.
在低速流中对带有外加压力梯度的后掠平板模型进行了实验研究，以确定后向台阶对定常横流主导流动中转变的影响。进行了边界层流动的详细热线测量，以研究因台阶高度为当地未受扰动边界层厚度的49%而导致的转变上游偏移。对于后向台阶情况，增加初始定常横流振幅会使转变前沿向上游移动。台阶导致定常横流不稳定性的增长局部增加，但转变时的定常横流振幅足够低（<0.04），因此定常横流并非转变的唯一原因。台阶下游的非定常速度谱在80至1500赫兹范围内充满了非定常扰动。基于相速度和波角识别出了三个不同的扰动族，即高度倾斜的扰动（可能类似于行波横流）、类托利米恩 - 施利希廷波的扰动以及剪切层不稳定性。定常横流扰动引起了非定常扰动的调制，导致非定常扰动振幅在空间上出现集中峰值。这种非定常扰动的调制被认为是转变前沿随着定常横流振幅增加而向上游移动的原因。
