Hovering rotor aerodynamics in extreme ground effect

This paper presents an experimental and numerical study of the aerodynamics of a moderate-scale rotor hovering in the Extreme Ground Effect(EGE)where rotor height-offground is below half the rotor radius.The tip vortex field was visualized by using the PIV technique.The aerodynamic performance,tip vortex trajectory,wall jet characteristics,surface pressure and velocity fields were measured and analyzed.To explore more deeply the flow mechanisms of the extreme ground effect,Detached Eddy Simulation(DES)was conducted on completely structured meshes.The results showed significant deviations of the rotor performance in EGE from that in Regular Ground Effect(RGE)with the rotor heights of more than half the rotor radius.Moreover,the flow structures of the rotor in EGE are considerably complex,such as the wall jet and groundwash flow separation.The rotor wake flow and tip vortices impact the ground more frequently,resulting in distinctive characteristics of the surface pressure and velocity fields in EGE.