The effect of turbulence on micro air vehicle airfoils

Micro air vehicles (MAVs) are small remotely piloted aircraft designed to fly at low altitudes and speeds and were developed for specific mission roles of low altitude reconnaissance and sensing. The MAV’s low-speed flight close to the ground exposes the MAV to the low-altitude region of the atmospheric boundary layer that results in relatively high turbulence levels, long integral length scales and low Reynolds numbers. In this research work, turbulence intensities comparable to those experienced by MAVs flying at low speeds and altitudes are replicated in two wind tunnel facilities using mesh grids. Three airfoils in two and three-dimensional configurations were exposed to grid turbulence of varying turbulence intensities and integral length scales at low Reynolds numbers. However, the integral length scales were considered shorter than those found in the atmospheric boundary layer. The pressure based method was used to calculate the time-averaged aerodynamic performance of each of the airfoils at several turbulence intensities, integral length scales and Reynolds numbers. The wind-tunnel tests of the three airfoils in smooth flow and in the presence of the grid turbulence showed that the freestream turbulence intensity and longitudinal integral length scale does significantly influence the time-averaged aerodynamic behavior of the three airfoils.<br><br>The effect of the increase in the freestream turbulence intensity and longitudinal integral length scale on the flat plate, circular arc and Clark-Y airfoils were only beneficial to the time-averaged aerodynamic performance of these airfoils if the freestream turbulence intensity and longitudinal integral length scale were increased to certain values. This phenomenon was attributed to the receptivity effect that is commonly found in boundary layer research.<br><br>As the independent variation of turbulence intensity and integral length scale was not possible, it is difficult to draw definite conclusions about the influence of each parameter. However, whilst the aerodynamic performance generally improved on the turbulence intensity and integral length scale increase, this was shown to be not always the case. Since the changes in the aerodynamic performance for a thin symmetrical flat plate, a cambered version of the same flat plate and a commonly used thicker cambered airfoil (Clark-Y airfoil) were significant, it is recommended that the effects of turbulence intensity, length scale and Reynolds number be further studied and focused on increasing the replicated length scales to those found in the atmospheric boundary layer.<br>