Aerodynamics of a transonic airfoil in ground effect

The Wing in Ground Effect (WIG) aircraft operates with larger lift to drag ratio compared to a conventional aircraft at low subsonic Mach numbers. To increase the traffic volume of the WIG aircraft, one possible way is to increase the flight speed, which can result in transonic flow. Currently the studies on transonic flight in ground effect are very few. The goal of this paper is to study the aerodynamics and flow physics of a typical transonic RAE2822 airfoil at angles of attack (AOA) from 0 to 12 deg. and Mach numbers from 0.5 to 0.8 in ground effect by varying the ground clearance above the ground. The compressible Reynolds-Averaged Navier-Stokes equations with Spalart-Allmaras (SA) turbulence model are solved using the commercial CFD solver ANSYS FLUENT. For flight near the flat ground surface, some interesting shock formations and flow phenomenon are obtained due to transonic flow. For the unsteady shock buffet phenomenon on the upper surface, the buffet boundary in the Angle of Attack (AOA) – Mach number (M) plane shrinks with the decreasing ground clearance. Compared to the unbounded flow field, there exists a steady shock on the lower surface of the airfoil in ground effect for low AOAs because the channel between the lower surface of the supercritical airfoil and the ground is of typical converging-diverging shape, resulting in decrease in lift and increase in drag. For extreme conditions of very small ground clearance at small AOA and high Mach numbers, a new coupling between the shock buffets on the lower and the upper surface of the airfoil is observed.