Low Reynolds number tests of NACA 64-210, NACA 0012, and Wortmann FX67-K170 airfoils in rain

Wind-tunnel experiments were conducted on Wortmann FX67-K170, NACA 0012, and NACA 64-210 airfoils at a simulated rain rate of 1000 mm/h and Reynolds number of 3.1 X105 to compare the aerodynamic performance degradation of the airfoils in heavy rain conditions and to identify the various mechanisms that affect airfoil performance in rain conditions. Lift and drag were measured in both dry and wet conditions, and a variety of flow-visualiz ation techniques were employed. At low angles of attack, the lift degradation in wet conditions varied significantly between the airfoils. The Wortmann section had the greatest lift degradation (-25%) and the NACA 64-210 airfoil had the least (-5%). At high angles of attack, the NACA 64-210 and NACA 0012 airfoils were observed to have improved aerodynamic performance in rain conditions due to a reduction of boundary-layer separation. Performance degradation in heavy rain for all three airfoils at low angles of attack could be emulated by forced boundary-layer transition near the leading edge. Time-resolved measurements indicate two primary mechanisms are responsible for the observed performance degradation. The initial effect of rain is to cause premature boundary-layer transition at the leading edge. The second effect occurs at time scales consistent with top surface water runback (1-10 s). The runback layer is thought to alter the airfoil geometry effectively, but this effect is most likely exaggerated in these tests due to the small scale. The severity of the performance degradation for the airfoils varied. The relative differences appeared to be related to the susceptibility of each airfoil to premature boundary-layer transition.