The Influence of Turbulence on the Flow Structures and Loads on Airfoils and Wings at Low Reynolds Numbers

微型飞行器（MAV）通常很小，（< 0.5米）和低速飞行的遥控或自主飞行器（< 10米/秒），并设想执行一些重要的任务，包括监视，协助搜索和救援，作为通信中继等，在这些任务期间，微型飞行器将在大气层的最低区域内度过较长的时间，被称为大气边界层（ABL），预计它们将保持稳定的飞行。虽然微型飞行器的优点和用途已经得到了很好的证实，但由于缺乏对其飞行领域相关的流动动力学的了解，其设计、开发和部署受到了阻碍。ABL内的流场可能非常复杂和不稳定。气流与地球表面的结构和障碍物之间的相互作用以及其他热效应使气流变得非常湍流。ABL内存在的自由流湍流可以使用湍流强度和积分长度尺度来表征。在ABL内进行的流动测量中观察到的湍流强度高达30%，积分长度范围从小于1米到几十米不等。飞行中的微型飞行器将受到低雷诺数效应（由于其尺寸和飞行速度）和高水平自由流湍流（ABL内通常存在）的影响。因此，为了使微型飞行器能够在多变的天气条件下飞行，有真实的必要了解在相关雷诺数下大尺度自由流湍流对翼型和机翼的影响，将采用最先进的测量技术来突出各种湍流条件下自由流湍流和翼型/机翼上流动之间的瞬时和统计相互作用。在每种湍流条件下，对于翼型，将使用PIV同时测量表面压力和速度分布，以突出湍流对截面载荷和力矩的影响。随后，将对小展弦比机翼进行力和速度测量，以评估机翼性能，并进一步阐明紊流对综合力和力矩的影响。压力/力和PIV测量值之间的详细比较将用于将瞬时扰动与翼型所经历的流动结构和载荷联系起来。从将要进行的研究中获得的结果将详细地了解在不同湍流条件下翼型和机翼上的流动物理学，因为当微型飞行器在ABL中工作时，它们实际上会发生。MAV控制系统的发展，以及主动流量控制装置，可以在未来应用，以提高在这种条件下的MAV的性能的根本利益。

Micro Air Vehicles (MAVs) are typically small (< 0.5 m) and remote controlled or autonomous aircrafts that fly at low speeds (< 10 m/s) and are envisaged to carry out a number of vital missions including surveillance, assist in search and rescue, act as communication relay, etc. During these missions, MAVs will spend extended durations within the lowest region of the atmosphere, known as the Atmospheric Boundary Layer (ABL), and they will be expected to maintain stable flight. Though the advantages and uses of MAVs are well established, their design, development and deployment has been hindered mainly due to the lack of understanding of the flow dynamics associated with their flight domain.The flow field within the ABL can be extremely complex and unsteady. The interaction between the airflow and structures and obstacles present on the Earth's surface and other thermal effects rendered the flow to become very turbulent. The free-stream turbulence present within the ABL can be characterized using the turbulence intensity and integral length scale. Turbulence intensities up to 30% and integral length scales ranging less than a meter to many tens of meters have been observed in flow measurements taken within the ABL. MAVs in flight would be exposed to both low Reynolds number effects (by virtue of their size and flight speed) and high levels of free-stream turbulence (commonly present within the ABL). Therefore, to make MAVs flyable in variable weather conditions, there is a real necessity to understand the influence of large scale free-stream turbulence on airfoils and wings at relevant Reynolds numbers.State-of-the-art measurement techniques will be applied to highlight the instantaneous as well as statistical interactions between free-stream turbulence and the flow over airfoils/wings under various turbulence conditions. Under each turbulence condition, for an airfoil simultaneous measurements of surface pressure and velocity profiles using PIV will be made to highlight the influence of turbulence on the sectional loads and moments. Subsequently, force and velocity measurements will be taken over a low aspect ratio wing to assess wing performance and shed further light on the influence of turbulence on the integrated forces and moments. Detailed comparisons between the pressure/force and PIV measurements will be used to relate instantaneous disturbances to the resulting flow structure and loads experienced by the airfoil/wing.The results obtained from the investigations to be performed will give a detailed insight into the flow physics over airfoils and wings under different turbulent conditions as they occur in reality when MAVs operate in ABL. The understanding of these processes is of fundamental interest for development of MAV control systems as well as active flow control devices which may be applied in the future to increase the performance of MAVs under such conditions.