Ground effect on the aerodynamics of a two-dimensional oscillating airfoil

This paper reports results of an experimental investigation into ground effect on the aerodynamics of a two-dimensional elliptic airfoil undergoing simple harmonic translation and rotational motion. Ground clearance (D) ranging from 1c to 5c (where c is the airfoil chord length) was investigated for three rotational amplitudes (α _m) of 30°, 45° and 60° (which respectively translate to mid-stroke angle of attack of 60°, 45° and 30°). For the lowest rotational amplitude of 30°, results show that an airfoil approaching a ground plane experiences a gradual decrease in cycle-averaged lift and drag coefficients until it reaches D ≈ 2.0c, below which they increase rapidly. Corresponding DPIV measurement indicates that the initial force reduction is associated with the formation of a weaker leading edge vortex and the subsequent force increase below D ≈ 2.0c may be attributed to stronger wake capture effect. Furthermore, an airfoil oscillating at higher amplitude lessens the initial force reduction when approaching the ground and this subsequently leads to lift distribution that bears striking resemblance to the ground effect on a conventional fixed wing in steady translation.