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

流体在表面（湍流边界层）附近的混沌运动造成了高达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，并对表面的阻力起重要作用。维持边界层内部区域湍流运动的机制先前已经确定，并启发了减少阻力的控制策略和简单的预测模型。这项研究将试图确定外部区域的相干运动机制，然后可以更好地建模，更准确地预测车辆阻力，并可能导致阻力减少策略。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-香槟分校的研究型课程中接受教育。此外，数学导师和入门工程教育课程将在丹维尔惩教设施实施，这是伊利诺斯州的一所中等安全级别的男子监狱。该项目的目标是评估一个假设的循环，该循环产生并维持湍流边界层外部区域的相干运动。该项目有三个任务：两个侧重于不同的相干运动可以相互作用的机制，另一个侧重于所涉及的相干运动的表征。一个小的冯·卡门涡旋街与湍流边界层的相互作用将使用时间分辨粒子图像测速法进行研究，从而深入了解大型和小型相干运动的相互作用。通过对包含平均和单一大尺度相干运动的准基流的Navier-Stokes方程进行解析分析，研究了小尺度相干运动的放大。利用当前对湍流边界层中相干运动相互作用的理解，采用一种新的湍流边界层数据采样方法来表征小尺度相干运动。这三项任务将共同确定湍流边界层中的相干运动和维持它们的机制，从而实现对运输车辆阻力的预测和控制。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。

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.