Geometry Tip Cap Effects on Formation and Near Wake Evolution of the Rotor Tip Vortices

Abstract

A combined numerical and experimental investigation is performed on the fundamental aspects of tip vortex formation and convection mechanisms for a two bladed rotor. The paper is focused on identifying similarities and differences in the near wake fields caused by blade tip geometry modification . Two different configurations are investigated. The first one corresponds to a square blade tip while the second one corresponds to a rounded blade tip cap. PIV measurements of the crossflow velocities around the forming tip vortex were performed for a forward flight regime characterized by a 0.1 advance ratio. RANS simulations of the rotor flow were conducted for the same rotor configurations as used in the experimental tests but considering only the hover regime. Both experimental and numerical tests confirm that there is a qualitative difference in the tip vortex formation and evolution between the square blade tip and rounded blade tip configurations. This qualitative change is shown in the velocity distributions measured and computed around and downstream of the tip. It is explained by the differences in the flow separation identified for the two configurations.