Airfoils at Low Reynolds Numbers

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

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.

 

The project is funded  by  the  German Research Foundation (DFG) under grant number HA 6926/2-1.

 

Applicant: Dr.-Ing. R. Hain

 

Research assistant: NN