Effect of Reynolds number on drag reduction: from near-wall cycle to large-scale motions.

减少与相对于它们运动的粘性流体接触的物体的阻力问题是非常重要的，因为这种减少阻力带来了很高的经济和能源效益。众所周知，当相对速度（相应的，流动的雷诺数）增加时，阻力会增加，而已知许多常见的减阻技术随着雷诺数的增加而失效。该项目将调查这种有效性丧失的原因，并提出可能克服这一限制的新策略。特别地，流体流过管道时，使管壁产生一定运动的减阻机制将被考虑。通过在雷诺数范围内的高保真计算模拟，研究了壁面运动的特定参数（如频率和波长）对减阻的显著影响的假设。该项目还将通过加强课程设置、让本科生参与研究以及向凤凰城地区代表性不足的学生开展外展活动，寻求提高工科学生基础数学和物理学科的教育质量。该项目将研究一个假设，即一些常见减阻机制的性能随着雷诺数的增加而下降，这与传统减阻机制可能无法有效控制的大尺度影响的增加有关。特别地，本文将对传统的内尺度驱动和较少研究的外尺度驱动机制进行比较和分析，并以紊流管道流动为例，以横壁速度的顺行波作为减阻机制。根据行波的频率和波长，湍流中不同尺度的运动将被激发，并根据雷诺数对其减阻作用进行量化。此外，将研究一种同时针对小尺度和大尺度流量的新方法，称为多尺度驱动。这种新方法有望克服以前探索方法的局限性，特别是在高雷诺数流动中。在雷诺数增加的情况下，增强减少表面摩擦阻力的能力，接近实际应用的目标，将对世界经济、全球能源使用和减排产生重大影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The problem of reducing drag on the objects in contact with viscous fluids moving relative to them is of great importance due to high economic and energy benefits associated with such drag reduction. The drag is known to increase when the relative velocity (correspondingly, the Reynolds number of the flow) increases, while many of the common drag reduction techniques are known to lose effectiveness with the increase in Reynolds number. This project will investigate the reasons for this loss of effectiveness and propose new strategies that may potentially overcome this limitation. In particular, a drag reduction mechanism in fluids flowing across a pipe that imparts a certain motion to the walls of the pipe will be considered. A hypothesis that specific parameters (such as frequency and wavelength) of such wall motions significantly influence the drag reduction will be investigated, via high-fidelity computational simulations across the range of Reynolds numbers. The project will also seek to improve the quality of education of fundamental mathematical and physical disciplines for the engineering students, via enhancing the course curriculum, engaging undergraduate students in research, and conducting outreach activities to underrepresented students across the metropolitan Phoenix area.The project will investigate the hypothesis that the loss of performance of some common drag reduction mechanisms with increase in Reynolds number is associated with the increasing influence of large scales which may not be efficiently controlled by conventional drag reduction mechanisms. In particular, the traditional inner-scaled actuation and a less investigated outer-scaled actuation mechanisms will be compared and analyzed on an example of a turbulent pipe flow with streamwise traveling waves of transverse wall velocity as a drag reduction mechanism. Depending on the frequency and the wavelength of a traveling wave, different scales of motions in a turbulent flow will be actuated, and their role in drag reduction depending on the Reynolds number will be quantified. Additionally, a novel approach that targets small and large scales of flow simultaneously, termed a multi-scaled actuation, will be investigated. This new approach promises to overcome the limitations of previously explored methods, especially at high Reynolds number flows. Enhancing the capabilities to reduce skin friction drag at increased Reynolds numbers, approaching the target for realistic applications, will have a strong impact on the world economy, global energy usage, and emission reductions.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.