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Unsteady Low Reynolds Number Aerodynamics

非定常低雷诺数空气动力学

基本信息

批准号：
1947089
负责人：
金额：
--
依托单位：
University of Cambridge
依托单位国家：
英国
项目类别：
Studentship
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=studentship-1947089
关键词：
Unsteady Low Reynolds Number Aerodynamics

项目摘要

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.

研究流体流动的基本原理一直是研究的一个一致领域，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野心。虽然非定常气动力现象在自然界中大量存在，但通常可以用公式表示所需的模型，以描述假定的定常状态下的飞机运动。因此，在定常空气动力学领域有着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。非定常空气动力学流动由于其高度的复杂性而相反地少得多地被研究和理解。尽管如此，早在20世纪30年代，Kussner、Theodorsen和瓦格纳等人就开发了描述机翼在遇到非定常扰动时的力响应的分析模型，其灵感来自于研究自然发生的紊流引起的颤振和模拟载荷。从那时起，这些模型就成为许多设计规范的基础，并已成功地纳入低阶模型中，例如用于描述直升机飞行中的动态失速。从那时起，扩大对非定常空气动力学的理解的必要性急剧增加，这类问题在航空和非航空应用中都出现了。自然发生的大气湍流或从跑道附近的建筑物或航空母舰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。除此之外，阵风引起的不稳定效应对卡车构成威胁，这是由于卡车的侧表面积很大，而在一级方程式大奖赛期间发生的硬制动故障会导致汽车向前倾斜，或者减阻系统在打开和关闭尾翼襟翼时产生不稳定现象，这些都是稳态假设导致不完整或错误答案的进一步例子。除此之外，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。它们的使命轮廓由于必须在空气动力学“肮脏”的环境中飞行而受到损害，在这种环境中阵风和气流扰动是常见的。在这里，由于大气湍流引起的阵风已被证明会导致机翼很容易超过25度的迎角，从而产生大的升力峰值。如果阵风的长度尺度在翼展的数量级之内，这一点尤其重要。因此，减小阵风灵敏度是扩展MAV飞行包线的一个关键优化参数，但在不深入了解非定常气动力的情况下，这是一个极具挑战性的问题。Kussner、Theodorsen和瓦格纳推导的描述这些非定常效应的分析模型假设了无粘不可压缩流以及小迎角，其中库塔条件被强制执行，使得流平稳地离开后缘。然而，大迎角和前缘和后缘涡流的形成使这些假设不适用于MAV飞行。关于这些假设理论的最近研究已经进行，但重点领域往往是，在雷诺数制度的一个数量级大于适当的MAV的或针对旋翼机或固定翼飞机遇到的非定常效应。这是本项目的目的是实验研究适用于MAV飞行的非定常原理的基本水平。