Control of near-wall turbulence via slip and transpiration

壁面附近的湍流运动主要是由两个关键的相干结构：准流向涡和条纹之间的相互作用所驱动的准循环的自持过程。这一过程已被观察到对湍流表面摩阻阻力有很大贡献，这表明湍流控制技术将通过寻求破坏它而受益。一种这样的技术是以这样的方式操纵流动，从而导致不同速度分量的湍流统计在壁法线方向上相对于彼此移动。这可以通过复杂的几何结构(如肋骨)或在数值模拟中使用非常规边界条件(如表面滑移)来实现。另一种被称为蒸腾作用的技术，通过壁面本身的流动喷射和吸力来抵消壁面附近的速度波动，也可能导致湍流统计数据的相对变化。本项目的主要目的是利用直接数值模拟方法来模拟、理解和模拟地表滑移和蒸腾对近壁湍流的综合影响。这样就有可能确定用于减阻的流向滑移、跨距滑移和正常壁面蒸腾的最佳组合，这些组合稍后可用于辅助设计新的控制技术。

Turbulent motions near the wall are dominated by a quasi-cyclic, self-sustaining process driven by the interaction between two key coherent structures: quasi-streamwise vortices and streaks. This process has been observed to contribute significantly to turbulent skin friction drag, suggesting that turbulent flow control techniques would benefit by seeking to disrupt it. One such technique is to manipulate the flow in such a way that results in the turbulence statistics of the different velocity components being shifted relative to each other in the wall normal direction. This may be achieved with complex geometrical structures, such as riblets, or with the use of non-conventional boundary conditions in numerical simulations, such as surface slip. Another technique, know as transpiration, counteracts velocity fluctuations near the wall by flow injection and suction at the wall itself, and can also result in a relative shift in the turbulence statistics. The primary aim of this project will be to simulate, understand and model the combined effect of surface slip and transpiration on near-wall turbulence by using direct numerical simulations (DNS). It might then be possible to identify an optimum combination of streamwise slip, spanwise slip and wall-normal transpiration for drag reduction that could later be used to aid in the design of novel control techniques.