Experimental Study of Secondary Vortex Structures in a Rotor Wake

Abstract

The aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake. Rotor wake simulations using modern computational methods have the potential to capture a high level of detail, which recently triggered discussions of secondary vortex braids occurring in the shear layer stretching between the primary tip vortices. Despite being aerodynamically meaningful, these structures are highly dependent on the numerical settings and need validation. The current work experimentally investigates the wake of a subscale rotor in ground effect using the "Shake-The-Box" technique, which enables a time-resolved identification of three-dimensional volumetric structures by means of gradient-based criteria. The current data clearly verifies the existence of the secondary vortex structures and is able to capture their "S"-shaped layout and their sense of rotation.