Politecnico di Milano, Dipartimento di Ingegneria Aerospaziale, Campus Bovisa, 20156 Milano, Italy.
Philos Trans A Math Phys Eng Sci. 2011 Apr 13;369(1940):1428-42. doi: 10.1098/rsta.2010.0366.
Drag-reduction techniques capable of reducing the level of turbulent friction through wall-parallel movement of the wall are described, with special emphasis placed on spanwise movement. The discussion is confined to active open-loop control strategies, although feedback control is briefly mentioned with regard to peculiarities of spanwise sensing and/or actuation. Theoretical considerations are first given to explain why spanwise motion is expected to be particularly effective in skin-friction drag reduction. A review of the spanwise oscillating-wall technique is given next, with discussion of recent results and prospects. Last, waves of spanwise velocity are addressed, either spanwise- or streamwise-travelling. The latter include the oscillating wall as a special case. The generalized Stokes layer--i.e. the laminar, transverse oscillating boundary layer that develops under the action of the streamwise-travelling waves--is described, and its importance in determining turbulent drag reduction discussed. Finally, open issues like energetic efficiency and its dependence on Reynolds number are addressed.
描述了通过壁面平行运动来减少壁面湍流摩擦水平的减阻技术，特别强调了展向运动。讨论仅限于主动开环控制策略，尽管反馈控制也简要提及了展向感应和/或激励的特点。首先给出了理论考虑，以解释为什么展向运动预计在减少表面摩擦阻力方面特别有效。接下来回顾了展向振荡壁技术，并讨论了最近的结果和前景。最后，讨论了展向速度波，包括展向或流向传播的速度波。后者包括作为特殊情况的振荡壁。描述了广义斯托克斯层，即横向振荡边界层，它在流向传播波的作用下发展，并讨论了它在确定湍流减阻方面的重要性。最后，还讨论了能量效率及其对雷诺数的依赖性等未解决的问题。
