Experimental Study of Secondary Vortex Structures in a Rotor Wake

Abstract

The operation and aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake, in particular regarding vortex-vortex interactions of hovering rotors. Rotor simulations using modern computational methods have the potential to capture high levels of detail, which recently triggered discussions of secondary vortex braids entangling the primary tip vortices. These structures are highly dependent on the numerical settings and need experimental validation. The current work investigates the wake of a subscale rotor in ground effect by time-resolved and volumetric flow-field measurements using the "Shake-The-Box" technique. Both the Lagrangian tracks of the flow tracers and the derived gradient-based vortex criteria clearly verify the existence of secondary vortices. A post-processing scheme is applied to isolate these vortices in larger data sets. No distinct spatial organization of the structures was observed, but a slightly preferred sense of rotation which agrees to the shear of the wake swirl. The secondary structures were created shortly downstream of the rotor blades, starting at wake ages of approximately 75 degree.