RESPONSE OF A TURBULENT BOUNDARY-LAYER TO SINUSOIDAL FREE-STREAM UNSTEADINESS
RESPONSE OF A TURBULENT BOUNDARY-LAYER TO SINUSOIDAL FREE-STREAM UNSTEADINESS
复制标题
DOI：
10.1017/s0022112090003512
复制
发表时间：
1990-12-01
影响因子：
3.7
通讯作者：
JAYARAMAN, R
中科院分区：
文献类型：
--
作者：
BRERETON, GJ;REYNOLDS, WC;JAYARAMAN, R
在本文中，报告了关于强制自由流不稳定对湍流边界层的影响的详细实验研究的选定结果。受迫不稳定是正弦曲线，局部叠加在原本稳定的主流上，超出了在恒压条件下形成的湍流边界层。在引起自由流不稳定的区域内，压力梯度的正弦变化介于零极值和正值之间，平均水平为正。通过同时测量速度场的 u 和 v 分量，研究了边界层对这些自由流效应的局部响应。尽管对非定常、湍流、完全发展的管道和渠道流进行了广泛的研究，但发展中的湍流边界层问题及其对强制自由流不定常的响应受到的关注相对较少。本研究旨在纠正这种不平衡，与非定常湍流边界层的其他研究相比，其独特之处在于：（i）它在大量强迫不稳定频率下具有明显的主流调制幅度，（ii）其测量既详细又具有高空间分辨率，因此可以辨别流动的近壁行为，并且（iii）它允许局部调制 湍流边界层之外的主流，该边界层是在众所周知的稳定、二维、恒压流条件下形成的。报告的结果允许在相同的平均外部条件下比较边界层的行为，但其自由流速度具有不同的时间依赖性。这些时间依赖性对应于：（i）稳定流，（ii）准稳定变化流，以及（iii）不同主流不稳定频率下的不稳定流。实验结果侧重于流量的时间平均性质；它们表明，湍流边界层的平均结构足够稳健，即使在流动的瞬时条件基本上偏离其准稳态的频率下，施加自由流不稳定性也只会导致相对于稳定流的平均特征的微小差异。湍流产生的平均水平同样不受自由流不稳定性的影响，并且湍流的时间产生似乎仅是由于作为时间平均测量而有助于湍流产生的运动的调制而产生的。
In this paper, selected findings of a detailed experimental investigation are reported concerning the effects of forced free-stream unsteadiness on a turbulent boundary layer. The forced unsteadiness was sinusoidal and was superimposed locally on an otherwise-steady mainstream, beyond a turbulent boundary layer which had developed under constant-pressure conditions. Within the region over which free-stream unsteadiness was induced, the sinusoidal variation in pressure gradient was between extremes of zero and a positive value, with a positive average level. The local response of the boundary layer to these free-stream effects was studied through simultaneous measurements of the u- and v-components of the velocity field.Although extensive studies of unsteady, turbulent, fully-developed pipe and channel flow have been carried out, the problem of a developing turbulent boundary layer and its response to forced free-stream unsteadiness has received comparatively little attention. The present study is intended to redress this imbalance and, when contrasted with other studies of unsteady turbulent boundary layers, is unique in that: (i) it features an appreciable amplitude of mainstream modulation at a large number of frequencies of forced unsteadiness, (ii) its measurements are both detailed and of high spatial resolution, so that the near-wall behaviour of the flow can be discerned, and (iii) it allows local modulation of the mainstream beyond a turbulent boundary layer which has developed under the well-known conditions of steady, two-dimensional, constant-pressure flow.Results are reported which allow comparison of the behaviour of boundary layers under the same mean external conditions, but with different time dependence in their free-stream velocities. These time dependences correspond to: (i) steady flow, (ii) quasi-steadily varying flow, and (iii) unsteady flow at different frequencies of mainstream unsteadiness. Experimental results focus upon the time-averaged nature of the flow; they indicate that the mean structure of the turbulent boundary layer is sufficiently robust that the imposition of free-stream unsteadiness results only in minor differences relative to the mean character of the steady flow, even at frequencies for which the momentary condition of the flow departs substantially from its quasi-steady state. Mean levels of turbulence production are likewise unaffected by free-stream unsteadiness and temporal production of turbulence appears to result only from modulation of the motions which contribute to turbulence production as a time-averaged measure.