Effect of free-stream turbulence on large structure in turbulent mixing layers
Effect of free-stream turbulence on large structure in turbulent mixing layers
复制标题
自由流湍流对湍流混合层大结构的影响
DOI：
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发表时间：
1978
影响因子：
3.7
通讯作者：
P. Bradshaw
中科院分区：
文献类型：
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作者：
C. Chandrsuda;R. Mehta;A. D. Weir;P. Bradshaw
流动可视化研究和相关测量表明，Brown & Roshko（1974）的紊流混合层中占主导地位的本质上是二维结构，只有在自由流湍流较低的情况下才会形成。如果自由流扰动很大，就像在大多数实际情况下可能发生的那样，包括从周围夹带“静止空气”的混合层，则三维空间在过渡的早期阶段发展。其他最近的实验有力地表明，如果最初的混合层是湍流的，或者受制于不稳定模式而不是展向涡旋，那么布朗-罗什科结构就不会形成。因此，Brown-Roshko结构在实践中将是罕见的。几个工人发现的混合层中的另一种大结构是强烈的，但完全是三维的，因此不如布朗-罗什科结构有序。证据的平衡表明，如果布朗-罗什科结构确实出现，它最终会放松成另一种完全三维的形式：在一个虚张声势的身体后面的Kármán涡旋街为三维的缓慢发展提供了先例。然而，Brown-Roshko结构，如果形成，很可能松弛得很慢，以至于在实际流的整个长度上都可以识别出来。
Flow-visualization investigations and correlation measurements show that the essentially two-dimensional structures which dominated the turbulent mixing layer of Brown & Roshko (1974) are formed only if the free-stream turbulence is low. If free-stream disturbances are significant, as is likely in most practical cases, including a mixing layer entraining ‘still air’ from the surroundings, three-dimensionality develops at an early stage in transition. Other recent experiments strongly suggest that the Brown-Roshko structure will not form if the initial mixing layer is turbulent or subject to instability modes other than spanwise vortices. Therefore the Brown-Roshko structure will be rare in practice. The alternative large structure in a mixing layer, found by several workers, is intense, but fully three-dimensional and thus less orderly than the Brown-Roshko structure. The balance of evidence suggests that if the Brown-Roshko structure does appear it will eventually relax into the alternative fully three-dimensional form: the Kármán vortex street behind a bluff body provides a precedent for slow development of three-dimensionality. However the Brown-Roshko structure, if formed, may well relax so slowly as to be identifiable for the full length of a practical flow.