An experimental study of a diffuser in ground effect

A study of a three-dimensional generic diffuser in moving ground effect was performed. Measurements were conducted with model tests in a low speed wind tunnel equipped with a moving ground facility. Techniques employed were force balance, pressure tappings, and surface flow visualization. The diffuser flow in moving ground effect was found to be characterized by vortex shedding and flow separation. Three types of force behavior were observed: (a) moderate down-force at high ride heights characterized by a symmetric surface flow, (b) high down-force at moderate ride heights characterized by a symmetric surface flow and separation on diffuser ramp, and (c) down-force reduction at low ride heights characterized by an asymmetric surface flow and flow separation. The down-force reduction near the ground is attributed to the three-dimensional flow separation at the diffuser inlet and subsequent loss of suction in the flrst half of the diffuser.