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

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.

This research will benefit the transportation and the energy supply sector in the UK and around the world. This project is relevant to various applications in these sectors, particularly, the aerospace and maritime shipping industries and the oil and natural-gas production industries. In addition to the commercial private sector (Economy) and the associated environmental impacts (Society), this research will deliver benefits via the scientific advances (Knowledge) and training (People).

The aircraft industry has committed to reducing NOx emissions by 80% and halving carbon emissions by 2020. Skin-friction drag reduction is not likely to feature in the efforts towards achieving these targets. Similarly, the maritime or oil/gas transportation sectors do not indulge in skin-friction reduction strategies. This is primarily because skin-friction drag reduction strategies up to this point are limited to laboratory scale experiments and their feasibility when scaled-up to practical applications remains an open question. This proposed project attempts to open a new window of opportunity to overcome the feasibility questions and go beyond the current commitments in the future.

The proposed project is at a low Technology Readiness Level (TRL) and therefore is perfectly suited for funding from EPSRC. Understanding the non-equilibrium effects, in the way proposed here, is new and is still in its infancy and progress through this project will allow us to raise awareness on the importance of these non-equilibrium effects on drag reduction strategies. Throughout the course of this project, we will invite members from different companies relevant to the transport and energy-supply sectors to the University and brief them on the progress of this project and its importance to strategies that they may be already pursuing. Ultimately, this project aims to be game changer in terms of options available to us for reducing drag in high Reynolds number flows.

Apart from contributing to an essential component (i.e. drag reduction) of our efforts to address climate change, the additional benefits of the current project to the general public is two-fold. First, the fact that this is first study to systematically examine non-equilibrium effects in high Reynolds number flows will contribute to establishing UK as a leader in this area of research. Second, the training of a researchers in this critical research area will prevent the erosion of expertise in the area of experimental fluid mechanics in the UK, which is an area of priority for EPSRC.