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

由于与这种减阻相关联的高经济和能量效益，减小与相对于物体运动的粘性流体接触的物体上的阻力的问题是非常重要的。当相对速度（相应地，流动的雷诺数）增加时，已知阻力增加，而已知许多常见的减阻技术随着雷诺数的增加而失去有效性。该项目将调查这种有效性丧失的原因，并提出可能克服这种限制的新策略。特别地，将考虑在流过管道的流体中的减阻机制，该减阻机制向管道的壁赋予一定的运动。通过在雷诺数范围内的高保真计算模拟，将研究这样的壁运动的特定参数（如频率和波长）显着影响减阻的假设。该项目还将通过加强课程设置，让本科生参与研究，该项目将调查以下假设，即随着雷诺数的增加，一些常见的减阻机制的性能损失是这与大尺度的影响的增加有关，而大尺度的影响可能不能被传统的减阻机构有效地控制。特别是，传统的内尺度驱动和较少研究的外尺度驱动机制将进行比较和分析的湍流管流的横向壁速度的流向行波作为减阻机制的例子。根据行波的频率和波长，湍流中不同尺度的运动将被驱动，并且它们在取决于雷诺数的减阻中的作用将被量化。此外，一种新的方法，同时针对小规模和大规模的流动，称为多尺度致动，将进行调查。这种新方法有望克服以前探索的方法的局限性，特别是在高雷诺数流动。在雷诺数增加的情况下提高降低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NSF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.