Experimental Investigation of a Jet Impinging on a Ground Plane in the Presence of a Cross Flow

An experimental investigation has been conducted in a wind tunnel to model the impingement of high velocity jet exhaust flow on the ground, as encountered by vertical or short takeoff and landing (V/STOL) aircraft. A constant jet velocity was maintained while varying the wind tunnel cross flow velocity, upstream boundary layer thickness, and height from the ground to the jet exit plane. The radial wall jet, when interacting with the cross flow, forms an oscillating horseshoe-shaped separation bubble, commonly referred to in the literature as a ground vortex. The streamwise distance of the separation point from the jet impingement point is documented here as a function of the flow parameters and geometry. Flow visualization of the flow field above the ground plane and two-component laser Doppler velocimeter measurements taken through the separation bubble indicate that the separation bubble is highly unsteady and non-symmetric. This unsteadiness may be related to shear layer vortices shed from the lip of the jet.