The rotor wake in ground effect and its investigation in a wind tunnel

摘要

Rotorcraft operating in ground effect have been known to experience beneficial performance effects as well as operational difficulties during low speed forward flight. While these have been attributed to the formation of the ground induced wake, limited experimental investigations of the wake have been conducted. Difficulties in re-creating representative forward flight ground boundary conditions in wind tunnels have meant a lack of quantitative information on the ground effect wake. This research is based on experimentally investigating the ground effect wakes of rotors in forward flight, to detail the fluid mechanics associated with the ground effect wake and to identify the influence of rotor parameters and ground boundary conditions on the formation of the ground effect wake. Particle Image Velocimetry (PIV) tests were conducted on wide regions of the ground effect wake produced by a rotor model in a wind tunnel fitted with a moving ground. Results from this research showed the ground effect wake to consist of a flow separation boundary and a region of recirculation, formed by the rotor tip vortex system trailing along the ground plane. Unsteadiness of the wake was seen to result in a constantly evolving wake, and this was observed to affect the classification of the ground effect flow regimes. Flow visualisation experiments conducted on the brownout phenomenon showed the ground effect wake features to influence the dust pick-up and transport mechanisms involved in the dust cloud formation. Experimental testing showed rotor parameters such as collective angles, rotor trim settings, rotor ground distance and root cut-out ratios to cause insignificant changes to the fluid mechanics of the ground effect wake, affecting only the location of the wake features. Effects of ground boundary conditions on the ground effect wake fluid mechanics were observed to be most significant, affecting both the location and detailed structure of the wake features. Results from this research show that ground effect experimental testing can be conducted in the wind tunnel environment and highlight the importance of accurately representing the ground boundary conditions during ground effect experimental testing.