Towards drag reduction strategies for high Reynolds number wall-turbulence

在运输和能源生产部门的各种应用中，减少表面摩擦阻力将直接转化为燃料消耗和排放的减少。因此，全世界都在开展旨在发展控制策略以减少表面摩擦阻力的活动。几乎所有这些策略都集中在通过使用一系列令人兴奋的控制方法来控制近壁“条纹”。然而，所有这些技术都是在低雷诺数计算和实验中发展起来的，它们在高雷诺数流动中的适用性是一个悬而未决的问题。此外，最近的研究揭示了更高雷诺数下的新物理，即存在超大尺度运动（VLSM）。这些VLSM在高雷诺数流动中对动能产生有重要贡献，并影响近壁循环，从而对表面摩擦阻力有重要贡献。因此，对VLSM的有效控制可以直接导致表面摩擦阻力的减小。在目前的项目中，提出了一种新的基于物理的控制策略，高雷诺数壁湍流的目标是控制VLSM的影响。在项目的第一部分中，将详细探索VLSM的物理机制及其与表面摩擦阻力的关系。在第二部分中，将开发一种主动控制策略来操纵这些VLSM，最终目标是减少表面摩擦阻力。

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.