The impact of the ground on flow structure and aerodynamic characteristics of a double delta wing

The ground effect (GE) on the formation of the vortical flow over a double delta wing having sweep angles of Λ=70°/40° was examined using the Particle Image Velocimetry (PIV) and dye flow visualization methods in this study. Qualitatively and quantitatively measured results were demonstrated for angles of attack, α=15° and 25° cases in the side and cross-flow planes to reveal the detail of the flow structures. The distance, h between surfaces of the ground and the delta wing normalized with the chord length of the double delta wing, c, was selected as h/c=0.1, 0.25 0.55, and the ground-free case (GFC). The results demonstrated that the magnitudes of the strake and wing vortices at α=15° and the coiled-up vortex at α=25° progressively diminish when the delta wing moves from the free-stream region into the boundary layer region close to the ground surface, h/c=0.1. Also, the rate of the inboard movement of leading edge vortices towards the wing central axis increases on the pressure surface of the wing as the altitude of the double delta wing is gradually lowered towards the ground surface. The lift coefficient, CL of the delta wing enhances due to the development of air stagnation and ram pressure between wing and ground surfaces by lowering the level of the double delta wing from the free-stream region into the boundary layer flow region which is strongly influenced by the presence of the ground surface. Also, the drag coefficient, CD increases more than the lift coefficient, CL resulting in a deterioration of the lift to drag ratio, CL/CD under the ground effect (GE). Finally, an increment in CL/CD is much more prominent at lower angles of attack, α regardless of ground effect.