Measurements of the aperiodic wake of a hovering rotor

An experimental investigation was made to study the aperiodic flow characteristics of the tip vortices generated by one-bladed and two-bladed hovering rotors. Measurements of the tip vortex locations and accompanying aperiodicity statistics were established as a function of vortex age. Velocity field measurements were made using three-component laser Doppler velocimetry. The average amplitude of the aperiodicity was found to be a fraction of the measured viscous core radius, this being approximately 5% of blade chord or about 50% of the core radius for wake ages of less than two rotor revolutions. The aperiodicity appeared isotropic. A numerical analysis of the aperiodicity problem based on convolution with an assumed displacement probability function showed that for this experiment the measured tangential velocities in the tip vortices were underestimated by approximately 20% and the viscous core radii were overestimated by 20%. There was no evidence that the number of blades or the blade passage adversely influenced the aperiodicity of the rotor tip vortices.