Towards Tip Vortex Measurements on Rotors in the High Pressure Wind Tunnel Göttingen (HDG) of the DLR

Abstract

The wake of a helicopter rotor features vortices originating from the blade tips. The vortices are relevant for the rotor noise and rotor performance, hence, the analysis of their propagation, growth and decay is of particular interest. It is widely believed that the vortex Reynolds number, defined as the ratio of the vortex circulation to the fluid viscosity, is an important factor in this process. Being related to turbulence and instability mechanisms, the Reynolds number effect on the vortex evolution is difficult to capture in numerical simulations. Therefore, experiments with a variation of the Reynolds number are required. Atmospheric subscale test facilities cannot reproduce the Reynolds numbers encountered on full-size helicopter rotors due to size limitations, whereas field measurements on free-flying helicopters are affected by the engine exhaust flow, wind gusts, blockage of the fuselage, tail rotor, etc. Furthermore, neither approach enables a variation of the Reynolds number independently of the rotational speed and the rotor dimensions. Therefore, we chose DLR’s High Pressure Wind Tunnel Göttingen (HDG) for measurements with a varying Reynolds number. The current work discusses the experimental methodology and sample results from a pretest.