Unsteady Low Reynolds Number Aerodynamics

研究流体流动的基本原理一直是一个一致的研究领域，在1903年莱特兄弟的第一次动力飞行中达到高潮，努力满足非地面移动的雄心。虽然非定常空气动力学现象在自然界中大量存在，但通常有可能制定必要的模型来描述假定的稳定状态下的飞机运动。因此，在稳定空气动力学领域有一个坚实的现有知识基础，随着计算流体动力学的发展，知识和空气动力学设计的实质性改进已经成为可能。非定常气动流动，由于其高度的复杂性，研究和理解却少得多。尽管如此，早在20世纪30年代，Kussner， Theodorsen和Wagner等人就开发了分析模型，描述了机翼遇到非定常扰动时的力响应，灵感来自于对自然发生的湍流引起的颤振和建模载荷的研究。从那时起，这些模型形成了许多设计规范的基础，并已成功地纳入低阶模型，例如用于描述直升机飞行中的动态失速。从那时起，随着这类问题在航空和非航空应用中出现，扩大对非定常空气动力学理解的必要性急剧上升。自然发生的大气湍流或从跑道附近的建筑物或从航空母舰上的船舶结构脱落的流动结构可能导致全尺寸飞机进入阵风，导致潜在的危险事件。此外，阵风引起的非定常效应对卡车构成了威胁，因为它们的侧表面积很大，而一级方程式大奖赛期间发生的硬断机动导致汽车前倾，或者减阻系统在打开和关闭尾翼襟翼时产生非定常现象，这些都是稳态假设导致不完整或错误答案的进一步例子。此外，近年来微型飞行器（MAV）的快速发展使得微型飞行器在军事和民用行动中的使用越来越频繁。由于必须在空气动力学的“肮脏”环境中飞行，阵风和气流干扰很常见，因此它们的任务轮廓受到了损害。在这里，由于大气湍流引起的阵风已经被证明会导致机翼很容易超过25度的攻角，从而产生巨大的升力峰值。如果阵风的长度尺度在翼幅的数量级内，这一点尤为重要。因此，降低阵风敏感性是扩展MAV飞行包线的关键优化参数，但如果没有对非定常空气动力学的深刻理解，则极具挑战性。由Kussner， Theodorsen和Wagner建立的分析模型描述了这些非稳态效应，假设无粘性和不可压缩流动以及小攻角，其中库塔条件被强制执行，使得流动平滑地离开后缘。然而，大迎角和前后涡的形成使得这些假设不适用于MAV飞行。最近对这些假设理论进行了更多的研究，但重点领域往往是在比MAV的适当雷诺数大一个数量级的情况下，或针对旋翼飞机或固定翼飞机遇到的非定常效应。本项目的目的是在基础水平上对适用于MAV飞行的非定常原理进行实验研究。

Researching the fundamental principles of fluid flows has been a consistent area of research climaxing in the first powered flight in 1903 by the Wright brothers, striving to satisfy the ambition of non-ground-bound mobility. Although unsteady aerodynamic phenomena are largely found in nature, it is often possible to formulate the necessary models required to describe aircraft motion in an assumed steady state. Consequently, there is a strong base of existing knowledge in the field of steady aerodynamics and with the development of computational fluid dynamics, substantial improvements in knowledge and aerodynamic design have been possible. Unsteady aerodynamic flows, due to their heightened complexity are conversely much less researched and understood. Nonetheless, as early as the 1930's Kussner, Theodorsen and Wagner, amongst others, developed analytical models describing the force response of wings encountering unsteady disturbances, inspired by investigating flutter and modelling loads caused by naturally occurring turbulence. These models have since then formed the basis of many design codes and have been successfully incorporated into low-order models used for example to describe dynamic stall in helicopter flight.The necessity to broaden the understanding of unsteady aerodynamics has since then risen dramatically, with such problems arising in aeronautic as well as in non-aeronautic applications. Naturally occurring atmospheric turbulence or flow structures shed from buildings near runways or from ship structures from aircraft carriers can cause full-scale aircraft to enter gusts, leading to potential dangerous incidents. Further to this, unsteady effects caused by gusts pose threats to trucks, owed to their large side surface area, whilst hard breaking maneuverers occurring during Formula One Grand Prix races causing the car to pitch forward or the drag reduction system creating unsteady phenomena upon the opening and closing of the rear wing flap present further examples of where steady state assumptions lead to incomplete or wrong answers. Further to this, the rapid development of Micro Air Vehicles (MAV) in the recent years has caused the use of MAV's to become increasingly frequent in military and civil operations. Their mission profile is compromised by having to fly in aerodynamic 'dirty' environments where gust and flow disturbances are common. Here, gusts due to atmospheric turbulence have been shown to cause the wings to readily exceed angles of attack of 25 degrees, consequently creating large spikes in lift. This is especially critical if the length scale of the gust is within on order of magnitude of the wing span. Decreasing gust sensitivity is therefore, a crucial optimisation parameter to extend the flight envelope of MAV's, yet extremely challenging without a strong understanding of unsteady aerodynamics.The analytical models derived by Kussner, Theodorsen and Wagner describing these unsteadyeffects assume inviscid and incompressible flow as well as small angles of attack, where the Kutta condition is enforced such that the flow leaves the trailing edge smoothly. The large angles of attack and the formation of leading and trailing edge vortices, however, make these assumptions not applicable to MAV flight. More recent studies regarding these postulated theories have been conducted but the area of focus is, more often than not, in Reynolds number regimes one order of magnitude larger than appropriate for MAV's or aimed at the unsteady effects encountered by rotorcraft or fixed wing aircraft.It is the aim of this project to experimentally investigate the unsteady principles applicable to MAV flight on a fundamental level.