The influence of turbulence on a flat plate aerofoil at Reynolds numbers relevant to MAVs

Micro Air Vehicles (MAVs) are small remotely controlled or autonomous aircraft that fly relatively slowly and very close to the Earth’s surface. MAVs typically fly in the Reynolds number range of 50,000 to 200,000 where the flow usually remains laminar over a large portion of the airfoil. Previous workers have found that the performance of airfoils reduces significantly in this flow regime primarily due to the influences of laminar separation. The influence of the various other flow structures that develop over the airfoil in this Reynolds number range is also relatively poorly understood.

As well as low Reynolds number effects, MAVs are exposed to the high levels of turbulence present within the Atmospheric Boundary Layer (ABL). A significant gap in knowledge has been identified in this area, where little is known about the influence of both turbulence intensity and integral length scale on the flow over the airfoil and on airfoil performance. The work presented in this thesis aims to increase understanding on the interactions between turbulence and airfoils at Reynolds numbers relevant to MAVs. This was accomplished by measuring the time-varying surface pressures that occurred over a thin flat-plate airfoil with an elliptical Leading Edge (LE) and a tapered Trailing Edge (TE), when exposed to various turbulence conditions including nominally smooth flow. Turbulence intensity was varied whilst keeping the integral length scale nominally constant and vice versa, to uncover the individual influences of both. Surface pressures were simultaneously measured over a number of spanwise and chordwise locations to identify the pressure and consequently the force and moment fluctuations experienced when exposed to the different turbulence conditions. Velocity measurements and smoke flow visualization were also conducted to augment the pressure measurements.

The pressure measurements provided insights on a number of flow features that formed over the airfoil surface including LSBs, large-scale vortices on reattachment and bluff-body-like vortex shedding at higher AOAs. Various transient properties of the LSB, such as the rate of formation of instabilities in the separated shear layer, shear layer flapping, and reattachment point oscillation length were also identified In the range of AOAs where LSBs formed.

In elevated levels of turbulence (Ti=7% and 12%), enhanced roll up of the shear layer on separation from the LE was observed. This led to the formation of strong vortex cores which advected downstream imparting large variations in surface pressure and velocity. In comparison to smooth flow, a significant delay in stall along with an increase in the maximum lift generated was noticed at higher levels of turbulence. Increasing the turbulence intensity and length scale led to an increase in the maximum lift generated. However, they had an opposing influence on the lift-curve-slope and the moment coefficient of the airfoil. It was also identified that an increase in turbulence intensity and integral length scale resulted in an increased magnitude of pressure and consequently lift fluctuations. Rolling moments however were more influenced by the integral length scale than by the turbulence intensity.