Towards drag reduction strategies for high Reynolds number wall-turbulence

Reduction of skin-friction drag in various applications in the transportation and energy generation sectors would translate directly to reductions in fuel consumption and emissions. Consequently, there is flurry of activity around the world aimed at developing control strategies to reduce skin-friction drag. Almost all of these strategies focus on controlling the near-wall ``streaks" by using a range of exciting control methodologies. However, all these techniques are developed in low Reynolds number computations and experiments and their applicability in high Reynolds number flows is an open question. Moreover, recent studies have unravelled new physics at higher Reynolds numbers, i.e. the existence of very large scale motions (VLSM). These VLSMs make a significant contribution to kinetic energy production in high Reynolds number flows and influence the near-wall cycle thereby making a significant contribution to skin-friction drag. Therefore, effective control of VLSMs could directly lead to a decrease in skin-friction drag. In the current project, a novel physics-based control strategy for high Reynolds number wall-turbulence is proposed in which the goal is to control the impact of VLSMs. In the first part of the project, a detailed exploration of the physical mechanism of VLSMs and its relationship to skin-friction drag will be carried out. In the second part, an active control strategy will be developed to manipulate these VLSMs with the ultimate goal of reducing skin-friction drag.

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. Control of larger structures in the outer region is a new topic that is in its infancy and progress through this project will allow us to raise awareness on the importance of these larger structures for 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 explore control of large-scale motions in high Reynolds number flows will contribute to establishing UK as a leader in this area of research. Second, the training of a researcher and a student 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.