Control of near-wall turbulence via slip and transpiration

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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.