Measurements of the velocity and turbulence structure of a rotor tip vortex

The turbulent how structure of a tip vortex generated by a hovering rotor was investigated with the use of three-dimensional laser Doppler velocimetry. The velocity field was measured at five wake ages, with the three components of turbulence being measured at the three earliest ages. The tangential (swirl) velocity component inside the tip vortex was found to have a self-similar behavior for all wake ages. The viscous core size was estimated to be approximately 2.5% of the blade chord just after formation, growing with the square root of vortex age to about 13% of blade chord at one rotor revolution. At early wake ages, radially symmetric peaks in the turbulence intensities were measured inside the vortex core, whereas only a single peak appeared in the turbulence distribution at later ages. The radial component of turbulence was found to be the most dominant, with the axial component being the smallest. Additional turbulence quantities, such as Reynolds' stress, were also measured. Finally, the evolutionary features of the turbulent kinetic energy production are discussed.