Influence of turbulence intensity on the flow structure over a thin airfoil at lower reynolds numbers

Large-scale freestream turbulence commonly present within the atmosphere poses significant challenges to the flight of Micro Air Vehicles. In order to examine the influence of turbulence intensity on the loads occurring over an airfoil, two turbulence conditions of nominally the same longitudinal integral length scale (Lxx/c = 1) but with significantly different intensities (Ti = 7.2% & 12.3%) were generated within a wind tunnel and time-varying surface pressure measurements and smoke flow visualization were made on a thin flat-plate airfoil at Reynolds number relevant to MAV flight. At lower angles of attack, the rapid changes in oncoming flow velocity and direction rendered a large variation in instantaneous flow structure over the airfoil, consequently leading to a significant increase in surface pressure fluctuations. At higher angles of attack, flowfield over the airfoil was noted to be considerably different whereby enhanced rollup of the leading edge separated shear layer occurred. This resulted in the formation of large Leading Edge Vortices which had a significant influence on the aerodynamic loads experienced by the airfoil. The rate of LEV formation was dependent on the angle of attack until 10 degrees and it was independent of the turbulence properties tested.