A Full-scale Rotor-wake Investigation of a Free-flying Helicopter in Ground Effect using BOS and PIV

Measurements in the wake of a free-flying full-scale helicopter in ground effect were performed for both quasi-steady and unsteady maneuvering flights using stereoscopic particle image velocimetry (PIV), a time-resolved background- oriented Schlieren (BOS) setup, and an optical marker tracking technique. The systems were used in a complimentary way to both visualize blade tip vortices in a large portion of the rotor wake and to capture spatially resolved velocity data close to the ground. The high sensitivity of the BOS system enabled the detection of vortices up to an age of Ψ = 630° . Different instability mechanisms as long-wave, short-wave, and pairing instabilities were observed with varying intensity for different flight conditions. A quantitative analysis of vortex locations showed a periodic variation resulting from interactions between consecutive vortices that led to vortex pairing. Characteristics of the wake outwash close to the ground were investigated by means of averaged velocity fields. Different patterns such as wall jet, recirculation, and ground vortex flow were quantitatively analyzed and found to be in good agreement with previous model helicopter experiments. The instantaneous velocity data was used to detect individual blade tip vortices with ages above 450° close to the ground and to extract vortex parameters. For a takeoff maneuver, both concentrated vortices and the formation of larger vortex structures due to bundling of several vortices were observed.