论文标题

湍流在改变拖动的表面上的平滑壁样行为:统一的虚拟原素框架

The smooth-wall-like behaviour of turbulence over drag-altering surfaces: a unifying virtual-origin framework

论文作者

Ibrahim, Joseph I., Gómez-de-Segura, Garazi, Chung, Daniel, García-Mayoral, Ricardo

论文摘要

我们对湍流通道进行直接模拟,使用罗宾,类似滑动的边界条件为所有三个速度施加了不同的虚拟起源,以研究置换不同流量成分所感知的起源的效果,这是一种对小质量纹理表面共有的机制。我们还使用反对控制探讨了这种效果。对于RIBLET,Luchini等人。 (1991年)提出,它们的作用可以降低为流向和跨度的起源之间的偏移,后者是湍流所感知的起源。后来对超疏水表面的结果表明,明显的壁正常起源也可能起作用。我们的结果支持相关参数是平均流量和湍流所感知的起源之间的偏移。前者由流向滑动长度确定,但后者取决于壁正常和跨度滑动的长度。反过来,流向速度波动上的滑动至少对湍流的虚拟起源有忽略不计的影响,至少在拖放状态下。这表明,诱导表面的横流速度所感知的来源是确定湍流的虚拟起源的关键,而近壁条纹所感知的,这与流向速度波动相关的是次要作用。在此框架中,流量控制文献中通常报告的湍流数量的变化仅是起源选择的结果,当引用湍流所经历的起源时,不存在。除了这种起源的转变外,我们还表明湍流基本上仍然是光滑的壁状,并且简单的表达方式可以预测湍流的虚拟起源。可以通过将虚拟起源引入光滑的墙壁涡流框架中,从而将对阻力的效果复制。

We conduct direct simulations of turbulent channels imposing different virtual origins for all three velocities using Robin, slip-like boundary conditions to study the effect of displacing the origins perceived by different flow components, a mechanism common to small-textured surfaces. We also explore this effect using opposition control. For riblets, Luchini et al. (1991) proposed that their effect could be reduced to the offset between the streamwise- and spanwise-velocity origins, the latter being the origin perceived by turbulence. Later results on superhydrophobic surfaces suggest that the apparent wall-normal-velocity origin could also play a role. Our results support that the relevant parameter is the offset between the origins perceived by the mean flow and by turbulence. The former is determined by the streamwise slip length, but the latter depends on both the wall-normal and spanwise slip lengths. The slip on the streamwise velocity fluctuations, in turn, has a negligible effect on the virtual origin for turbulence, at least in the drag-reduction regime. This suggests that the origin perceived by the quasi-streamwise vortices, which induce the cross velocities at the surface, is key in determining the virtual origin for turbulence, while that perceived by the near-wall streaks, which are associated with the streamwise velocity fluctuations, plays a secondary role. In this framework, the changes in turbulent quantities typically reported in the flow-control literature are shown to be only a result of the choice of origin and are absent when referenced to the origin experienced by turbulence. Other than this shift in origin, we show that turbulence remains essentially smooth-wall-like, and a simple expression can predict the virtual origin for turbulence. The effect on drag can be reproduced a priori by introducing the virtual origins into a smooth-wall eddy-viscosity framework.

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