An investigation on the aerodynamics of a symmetrical airfoil in ground effect

Abstract

The flow characteristics over a symmetrical airfoil––NACA 0015––are studied experimentally in a low speed wind tunnel. The pressure distribution on the airfoil surface was obtained, lift and drag forces were measured and mean velocity profiles were obtained over the surface. The wake region was also explored in detail and measurements of mean velocity and turbulence intensities were performed at two stations downstream of the trailing edge. Experiments were carried out by varying the angle of attack, α, from 0° to 10° and ground clearance of the trailing edge from the minimum possible value to one chord length. It was found that high values of pressure coefficient are obtained on the lower surface when the airfoil is close to the ground. This region of high pressure extended almost over the entire lower surface for higher angles of attack. As a result, higher values of lift coefficient are obtained when the airfoil is close to the ground. The flow accelerates over the airfoil due to flow diversion from the lower side, and a higher mean velocity is observed near the suction peak location. The pressure distribution on the upper surface did not change significantly with ground clearance for higher angles of attack. The upper surface suction causes an adverse pressure gradient especially for higher angles of attack, resulting in rapid decay of kinetic energy over the upper surface, leading to a thicker wake and higher turbulence level and hence a higher drag. The lift was found to drop at lower angles of attack at some values of ground clearance due to suction effect on the lower surface as the result of formation of a convergent–divergent passage between the airfoil and the ground plate. For the angle of attack of 12.5°, a very thick wake region was observed and higher values of turbulence intensity were recorded.