Effect of Turbulence and Sinusoidal Pitching on Low-Reynolds-Number Lift

Abstract

Small fixed-wing unmanned aerial vehicles (UAVs) are becoming increasingly popular in civil and military use. However, their ability to fly in adverse weather and turbulent flows is hindered by their low-Reynolds-number wings. It is shown that turbulence produces a significant pitching flow, as observed by the wing, which raises the possibility of intermittent dynamic stall comparable to a pitching airfoil situation. Two airfoils (one symmetrical and the other cambered) were tested in the wind tunnel at Reynolds numbers relevant to small UAVs: between 50,000 and 200,000. Two turbulence intensities of 5 and 15% were compared to sinusoidal pitching of the airfoils at equivalent amplitudes and three frequencies. It is shown that an increase in either the pitch amplitude or the onset turbulence intensity produces comparable changes in the time-averaged lift coefficient, and both experience forms of dynamic stall. The transient behavior is, however, significantly different; and the airfoil form has greater influence on the response to pitching motion than to turbulent flow.