Wind Tunnel Measurements of Full-Scale UH-60A Rotor Tip Vortices

A full-scale UH-60A rotor was tested in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel in May 2010. During the test, flow field velocities were measured using stereo Particle Image Velocimetry (PIV) in a stationary cross-flow plane located at approximately 90 deg rotor azimuth. At each test condition, blade position relative to the measurement plane was varied. The region of interest was approximately 3.5-ft high by 14-ft wide and covered the outer half of the blade radius. Vortices from at least 3 blades, the shear layer, and vortices generated by the ends of the trim tab were captured in the flow field. Two data processing techniques were applied to significantly improve spatial resolution and accuracy. The first technique applied a Gaussian mask to the image interrogation window; the second technique extracted vortex properties from the measured velocity fields using a planar least-squares fit to an assumed vortex model. Mean and standard deviations of vortex center locations, core radius, and circulation were extracted from the velocity fields. The comprehensive rotorcraft analysis CAMRAD II was used to predict vortex trajectory and strength. For the conditions presented, the predicted trajectory of the vortices in the PIV measurement plane descended more steeply than the measured trajectory. Also, the circulation contained within the measured tip vortex was significantly lower than predicted because the predictions did not account for a separate vortex that formed inboard of the blade tip. A method for estimating measurement uncertainty using a particle image combined with a synthetic flow field was demonstrated.