School of Aeronautics, Universidad Politécnica de Madrid, 28040 Madrid, Spain.
Philos Trans A Math Phys Eng Sci. 2011 Apr 13;369(1940):1412-27. doi: 10.1098/rsta.2010.0359.
The interaction of the overlying turbulent flow with riblets, and its impact on their drag reduction properties are analysed. In the so-called viscous regime of vanishing riblet spacing, the drag reduction is proportional to the riblet size, but for larger riblets the proportionality breaks down, and the drag reduction eventually becomes an increase. It is found that the groove cross section A(g)(+) is a better characterization of this breakdown than the riblet spacing, with an optimum A(g)(+1/2) ≈ 11. It is also found that the breakdown is not associated with the lodging of quasi-streamwise vortices inside the riblet grooves, or with the inapplicability of the Stokes hypothesis to the flow along the grooves, but with the appearance of quasi-two-dimensional spanwise vortices below y(+) ≈ 30, with typical streamwise wavelengths λ(x)(+) ≈ 150. They are connected with a Kelvin-Helmholtz-like instability of the mean velocity profile, also found in flows over plant canopies and other surfaces with transpiration. A simplified stability model for the ribbed surface approximately accounts for the scaling of the viscous breakdown with A(g)(+).
分析了湍流与肋纹的相互作用及其对减阻性能的影响。在肋纹间距趋于零的所谓粘性区，减阻与肋纹尺寸成正比，但对于较大的肋纹，这种比例关系不再成立，减阻最终会增加。研究发现，槽截面 A(g)(+)比肋纹间距更能描述这种破坏，最佳 A(g)(+1/2) ≈ 11。还发现，这种破坏与准流向涡旋在肋纹槽内的阻塞无关，也与沿槽流动不适用 Stokes 假设无关，而是与 y(+) ≈ 30 以下出现准二维展向涡旋有关，其典型流向波长 λ(x)(+) ≈ 150。这些涡旋与植物冠层和其他具有蒸腾作用的表面上的平均速度剖面的 Kelvin-Helmholtz 型不稳定性有关。一个简化的肋状表面稳定性模型可以近似描述粘性破坏与 A(g)(+)的标度关系。
