Towards drag reduction strategies for high Reynolds number wall-turbulence

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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.