Lifting surfaces in low Reynolds number flows: bridging the gap between laboratory research and practice

从现代涡扇发动机到风力涡轮机再到无人驾驶航空/潜水器，许多新兴技术的空气动力学性能与经典空气动力学的预测有很大不同。相关系统工作在低雷诺数空气动力学领域，层流分离会导致系统性能下降。翼型的几何形状优化和流动控制可用于延迟分离和/或最小化分离的流动区域的尺寸，从而增强升力和减少阻力。然而，实施这些方法需要深入了解流动物理，包括固有的复杂的层流到湍流的转变和非定常流动动力学。这一领域以前的大部分工作都集中在稳定来流中的二维翼型布局上。相比之下，所有相关的实际应用都涉及暴露在非稳定来流(例如，阵风、上游物体的尾流等)中的有限机翼/叶片。该计划旨在弥合这一研究差距，并将当前最先进的低雷诺数空气动力学发展到实际相关的有限跨度升力面结构，在稳定和非稳定流动条件下运行，从而通过设计优化和流动控制来提高系统性能。该程序的主要目标是研究来流参数的三维几何形状和时间变化对有限翼上流动发展的影响及其对性能的影响。这将通过在风洞中利用最先进的速度和力测量对二维和有限跨度模型进行新颖的实验研究来实现。采用先进激光工具的时间分辨和相位平均速度测量将与直接力测量相结合，以提供对流动发展和相关负载变化的独特见解。这将得到速度数据中新的压力和分段载荷估计的补充，以改进对跨向载荷分布和动力学的了解。除了它们的新颖性和对基础流体力学的意义外，拟议的研究结果将对广泛的现代和新兴应用产生强大的影响。它们将能够为风力涡轮机、小型螺旋桨、飞机发动机、高空平台、无人驾驶空中和水下航行器设计更高效的升力面，以及设计和实施有效的流动控制战略，以进一步提高性能。该方案还将促进对研究生的培训，这些研究生将支持相关的知识转让和进一步的研究和开发活动。

The aerodynamic performance of many new and emerging technologies, ranging from modern turbofan engines to wind turbines to unmanned aerial/submersible vehicles, differs substantially from the predictions of classical aerodynamics. The relevant systems operate in the domain of low Reynolds number aerodynamics, where laminar flow separation leads to a degradation in system performance. Airfoil geometry optimization and flow control can be used to delay separation and/or to minimize the size of the separated flow region, thereby enhancing lift and decreasing drag. However, implementing these methods requires in-depth knowledge of the flow physics involving inherently complex laminar-to-turbulent transition and unsteady flow dynamics. Most prior work in this domain focused on two-dimensional airfoil configurations in steady incoming flows. In contrast, all relevant practical applications involve finite wings/blades exposed to unsteady incoming flows (e.g., wind gusts, wakes from upstream objects, etc.). The proposed program aims to bridge this research gap and advance the current state-of-the-art in low Reynolds number aerodynamics to practically relevant, finite-span lifting surface configurations operating in both steady and unsteady flow conditions, thereby enabling enhancement of system performance through design optimization and flow control. The main objectives of the proposed program focus on the effects of the three-dimensional geometry and temporal variations in the incoming flow parameters on the flow development over finite wings and the associated impact on their performance. This will be achieved by conducting novel experimental studies on both two-dimensional and finite-span models in a wind tunnel utilizing simultaneous state-of-the-art velocity and force measurements. Time-resolved and phase-averaged velocity measurements with advanced laser-based tools will be coupled with direct force measurements to provide unique insights into the flow development and the associated changes in loading. This will be complemented by novel pressure and sectional load estimations from the velocity data for improved insight into spanwise load distributions and dynamics. In addition to their novelty and significance for fundamental fluid mechanics, the results of the proposed research will have a strong impact on a wide range of modern and emerging applications. They will enable the design of more efficient lifting surfaces for wind turbines, small-scale propellers, aircraft engines, high-altitude platforms, unmanned aerial and underwater vehicles, as well as designing and implementing effective flow control strategies for further performance improvements. The program will also facilitate training of graduate students who will support the associated knowledge transfer and further research and development activities.