Wake structure of a helicopter rotor in forward flight

The wake of an idealized two-bladed helicopter rotor in forward flight has been experimentally investigated using hot-wire anemometry techniques. A model was designed, constructed, installed, and tested in a wind tunnel. Time-dependent measurements of three-dimensional velocity vectors were obtained for two pitch angles corresponding to a Reynolds number based on the rotor tip rotational velocity and blade chord of 8.2×104 and 6.2×104, respectively, at an advance ratio of 0.15. The results include time- and phase-averaged data of mean velocity components and turbulence intensities, as well as spectral analysis. Profiles of the streamwise component of the velocity vector have shown the existence of an Ωz component of the vorticity vector. Analysis of the vertical and the transverse component of the velocity vector also revealed the existence of an Ωx component of the vorticity vector at both edges of the wake. Large levels of turbulent kinetic energy were found almost everywhere in the wake. Turbulence intensity due to the presence of the counterclockwise rotating vortex reached the maximum values of 22.4 and 19% for pitch angle 10 and 6 deg, respectively. Spectral analysis of the velocity components revealed peaks of power spectral densities at frequencies two or four times higher than the blade passage frequency. This suggests a possible breakup of the vortex sheets roll-up or tip vortices due to large scale instabilities.