Uncovering the self-sustaining cycle in the outer region of turbulent boundary layers

流体在表面(湍流边界层)附近的混乱运动导致了高达70%的船舶阻力和50%的飞机阻力。在湍流边界层中反复观察到某些流体运动，这些运动对表面的阻力有很大影响。以前已经确定了在边界层内部区域维持湍流运动的机制，并启发了减阻控制策略和用于预测的简单模型。这项研究将寻求确定外部区域相干运动的机制，然后可以更好地对其进行建模，以更准确地预测车辆阻力，并可能导致减阻策略。研究生、本科生和高中生将通过研究计划在实验室接受培训和指导，研究生将在伊利诺伊大学厄巴纳-香槟分校的研究型班级接受教育。此外，将在伊利诺伊州中等安全级别的男子州立监狱丹维尔惩教设施实施数学辅导和入门工程教育计划。该项目的目标是评估在湍流边界层外部区域产生和维持连贯运动的假设周期。该项目有三项任务：两项侧重于不同连贯运动相互作用的机制，一项侧重于所涉及的连贯运动的特征。我们将使用时间分辨粒子图像测速技术研究小von Karman涡街与湍流边界层的相互作用，从而深入了解大尺度和小尺度相干运动的相互作用。小尺度相干运动的放大将通过对具有准基流的Navier-Stokes方程进行预解分析来研究，准基流既包括平均运动，也包括单个大尺度相干运动。小尺度相干运动将使用一种新的湍流边界层数据采样方法来表征，该方法利用了目前对湍流边界层中相干运动相互作用的理解。这三项任务将共同识别连贯的运动以及在湍流边界层中维持运动的机制，从而能够预测和控制运输车辆上的阻力。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Chaotic motions of fluid near surfaces (turbulent boundary layers) cause up to 70% of drag on ships and 50% of drag on planes. Certain fluid motions are repeatedly observed in turbulent boundary layers and contribute significantly to drag on surfaces. Mechanisms that sustain turbulent motions in the inner region of the boundary layer were previously identified and have inspired control strategies for drag reduction and simple models for prediction. This research will seek to identify mechanisms for coherent motions in the outer region, which could then be better modelled to more accurately predict vehicle drag and could lead to drag reduction strategies. Graduate, undergraduate, and high school students will be trained and mentored in the laboratory through the research initiative and graduate students will be educated in a research-based class at the University of Illinois at Urbana-Champaign. Additionally, math tutor and introductory engineering educational programs will be implemented at the Danville Correctional Facility, a medium security men’s state prison in Illinois.The goal of the project is to evaluate a hypothesized cycle that generates and sustains coherent motions in the outer region of turbulent boundary layers. The project has three tasks: two that focus on mechanisms through which different coherent motions can interact and one that focuses on the characterization of the coherent motions involved. The interaction of a small von Karman vortex street with a turbulent boundary layer will be studied using time-resolved particle image velocimetry, yielding insights into the interaction of large- and small-scale coherent motions. The amplification of small-scale coherent motions will be investigated by performing a resolvent analysis of the Navier-Stokes equation with a quasi-base flow, which includes both the mean and a single large-scale coherent motion. The small-scale coherent motions will be characterized using a new sampling approach for turbulent boundary layer data that leverages the current understanding of the interaction of coherent motions in turbulent boundary layers. Together, these three tasks will identify the coherent motions and the mechanisms that sustain them in turbulent boundary layers, enabling prediction and control of drag on transportation vehicles.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.