Flat plate drag reduction by turbulence manipulation
Flat plate drag reduction by turbulence manipulation
复制标题
通过湍流操纵减少平板阻力
DOI：
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发表时间：
1988
期刊：
影响因子：
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通讯作者：
K. Sreenivasan
中科院分区：
文献类型：
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作者：
R. Narasimha;K. Sreenivasan
本文的主要目的是描述通过插入被动物体操纵的湍流边界层可能导致比未操纵的流动中的总阻力更小的条件。文中指出，几乎任何插入边界层的装置都会产生下表面摩擦阻力。实验证据支持这一结论，这是最容易被认为是“墙尾流”的特征。然而，当机械手的流向范围不小时，施加在机械手边界上的防滑条件改变了通过这里所描述的“叶片”效应的流动。这种效应的存在可以从实验数据中推断出来，这些实验数据明确地表明，具有边界层厚度δ量级的平板与具有较高尾迹动量厚度ϑw的圆柱杆产生相同量级的表面摩擦减阻。由于通常叶片效应和尾迹效应是同时运行的，因此在由两个变量c/δ和ϑw/δ形成的平面上分析了现有的数据。这一分析表明，净减阻即使发生，也是相当小的，而且它的实现需要大于约40ϑw的叶片弦(在零迎角时)。最后指出，如果如通常所说的那样，这样的操纵器使边界层动量厚度永久减小，那么在边界层有望恢复平衡的下游足够远的地方，由于动量厚度雷诺数较低，任何站点的局部表面摩擦系数都必须高于同一站点的未操纵流动。因此，由于机械手引起的任何可能的阻力减少只能在机械手后面的某些有限的下游长度内实现。
The major objective of the present paper is to delineate the conditions under which a turbulent boundary layer manipulated by the insertion of a passive object may lead to a lower overall drag than in the unmanipulated flow. It is pointed out that almost any device inserted in the boundary layer will lead to a lowerskin friction drag. Experimental evidence is presented to support this conclusion, which is most easily thought of as characterizing “wall wakes”. However when the streamwise extent of the manipulator is not small, the no-slip condition forced on the manipulator boundary modifies the flow through what is here described as the “blade” effect. The presence of this effect may be inferred from experimental data which unambiguously show that a flat plate with a chordc of the order of the boundary layer thicknessδ produces the same order of skin friction reduction as a cylindrical rod with a higher wake momentum thicknessϑw. As in general both blade and wake effects are operating simultaneously, available data are analysed in the plane formed by the two variablesc/δ andϑw/δ. This analysis shows that the net drag reduction, if it occurs at all, is quite small, and that its realization demands a blade chord (at zero incidence) larger than about 40ϑw.Finally, it is pointed out that if, as often claimed, such manipulators effect a permanent decrease in boundary layer momentum thickness, then sufficiently far downstream where the boundary layer may be expected to have returned to equilibrium, the local skin friction coefficient at any station must be higher than in the unmanipulated flow at the same station because of the lower momentum thickness Reynolds number. It therefore follows that any possible reduction in drag due to the manipulator can only be achieved for certain limited downstream lengths behind the manipulator.