Unsteady Low Reynolds Number Aerodynamics

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 unsteady
effects 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.