Understanding and exploiting non-equilibrium effects on turbulent boundary layers: Towards realisable drag reduction strategies

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The reduction of skin-friction drag even by a few percent in the transportation and energy generation sectors translates directly to reductions in fuel consumption and emissions, the need for which is now almost universally accepted. Consequently, in the last two decades, a whole range of Drag Reduction (DR) strategies has been proposed, but many of these have been tested and validated in fully-developed internal flows (i.e. pipe or channel flows) where there is no development of the flow along the streamwise direction. Therefore, the flow reaches an equilibrium with the wall condition and the potential for large drag reductions has been reported. However, almost all practical applications involve external flows where a turbulent boundary layer (TBL) will grow along the streamwise direction, such that it exhibits non-equilibrium behaviour as it continuously adjusts to the prescribed wall condition. More importantly, there could be significant potential benefits in exploiting non-equilibrium behaviour where only parts of the developing flow are affected. Therefore, it is of fundamental importance to examine non-equilibrium effects not only to understand the limitations of implementing a drag reduction strategy but also to exploit any practical benefits.In this collaborative project, we explore the fundamental problem of non-equilibrium effects on wall-turbulence by examining the effects of two different types of non-equilibrium wall condition: (1) change in surface roughness and (2) change in the characteristics of a harmonically-varying in-plane surface wave. The surface roughness is a passive boundary condition and may locally increase surface shear stress while the in-plane surface wave is an active boundary condition. Both sets of experiments introduce non-equilibrium effects that will alter the development of skin-friction behaviour depending on the nature of the surface change. Crucially, both can reduce the local surface shear stress under specific conditions. By examining the two in parallel and comparing the response of a turbulent boundary layer to these different boundary conditions, we will provide new insights on the scaling and the adjustment of the boundary layers to these non-equilibrium effects. This is very much within the spirit of Clauser's "black box" analogy - the perturbation of an unknown system and the assessment of the response, here, with the added motivation of identifying the drag-altering behaviour.