Control of Complex Turbulent Flow

能够操纵流场以将其改变为所需形式的能力是流体动力学的基本主题之一。虽然流动控制在不同的工程装置中都可以看到，但它对空气动力学的影响更为显著，特别是在低雷诺数流区。在低速流动中，任何扰动都可能使流型发生剧烈的变化，导致阻力突然增大，升力和推力损失突然增大。因此，为了获得更好的气动效率，需要在低雷诺数下采用合适的流动控制技术。低雷诺数下的非定常空气动力学是在各种实际工程应用中常见的现象。典型的例子是风力涡轮机、飞行无人机、微型飞行器(MAV)、小型无人机(UAV)，即在低密度大气中运行的飞行物体，而不是地球上的小范围升力表面，如昆虫和鸟类的翅膀。小尺度和低速度的结合导致了低雷诺数工况，粘性效应将导致非定常流动分离。低雷诺数空气动力学的基本性质具有非常丰富的流动物理，这使得它成为一个令人兴奋的研究课题。在该流态下，当流量变化较大时，被动流量控制不能改善气动性能。因此，在复杂的流动条件下，需要主动的流动控制来修正流型。在不同的主动流动控制方法中，周期驱动边界层方法表现出较好的控制效果。智能材料，如宏纤维复合材料执行器，可以产生改善空气动力学性能所需的周期性运动。在开发智能材料方面，随着科学技术的进步，有可能在表面上产生不同的形状(表面变形)来控制流动模式。在这个研究项目中，将从水上游泳者的启发出发，研究行波形式的表面变形对控制水流分离的影响。对壁面附近复杂的流动行为进行了研究。文中还将探讨不同的表面行波通过边界层的动量交换机制。此外，还将建立一个分析模型，以预测各种工作条件下的最佳表面运动。该模型可以在以后的闭环控制系统中用来调整控制器。这种流动控制将在风力机叶片上进行数值测试，以评估其性能，以改善非定常气动载荷。这项计划提高了我们目前对表面变形在流动动力学中的作用的认识，这将转移到开发一种控制流体流动的新方法的创新技术的开发上。从设计智能风力涡轮机到侦察昆虫，它对设计新工具和新技术具有重大而广泛的影响。

The ability to manipulate a flow field to change it to the desired form is one of the essential topics in fluid dynamics. Although employing flow control can be seen in different engineering devices, its impact is more significant in the aerodynamics, especially at the low Reynolds number flow regime. The flow pattern may dramatically be reformed by any disturbance in low speed flow and causes a sudden increase of drag and loss of lift and thrust. Therefore, in order to achieve better aerodynamic efficiency, suitable flow control techniques are required in low Reynolds number. Unsteady aerodynamics at a low Reynolds number is a common phenomenon observed in a variety of real-world engineering applications. Typical examples are wind turbines, flying drones, micro aerial vehicles (MAV), small unmanned aerial vehicles (UAV), a flying object operating at low-density atmosphere other than Earth's small-scale lifting surfaces such as insect and bird wings. The combination of small length scale and low velocities results in low Reynolds operating conditions where the viscous effect will cause the unsteady flow separation. The fundamental nature of low Reynolds aerodynamics has enormously rich flow physics, making it an exciting research topic. In this flow regime, the passive flow control cannot improve the aerodynamic performance when the flow changes dramatically. Therefore, active flow control is required to modify the flow pattern in the complex flow conditions. Among different active flow control methods, periodic actuation of the boundary layer has shown a better performance. Smart materials such as Macro Fiber Composites actuators can produce the periodic motion required for aerodynamic performance improvement. Improving science and technology in developing smart materials makes it possible to generate different shapes on the surface (surface morphing) to control the flow pattern. In this research program, the effects of the surface morphing in the form of a traveling wave on controlling the flow separation will be studied by inspiring from aquatic swimmers. The complex flow behavior near the wall region will be investigated. Different mechanisms of exchanging momentum across the boundary layer due to the traveling wave on the surface will be explored. Also, an analytical model will be developed that predicts the optimum surface motion in various working conditions. This model can be used later in the closed-loop control system to adjust the controller. This flow control will be tested numerically on a wind turbine blade to assess its performance to improve the unsteady aerodynamic loads. This program advances our current knowledge in the role of surface morphing on the flow dynamics, which transfers to the development of innovative techniques in developing a new method for controlling the fluid flow. It has significant and broad implications for engineering new tools and technologies from designing smart wind turbines to spy insects.