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Stereoscopic particle image velocimtery (PIV) was used to characterize a hovering rotor wake at four collective pitch settings in the world's largest wind tunnel test section. The PIV measurements are a subset of a comprehensive dataset acquired during the hover test of the HVAB rotor. Substantial effort was made to cross-validate PIV results with other test measurements and fluid mechanic theories to ensure accuracy in the reported HVAB dataset. Blade coning and flap bending were validated against early tip vortex locations. Tip vortex trajectory was compared against shadowgraphy results and free-jet boundaries. Tip vortex circulation was evaluated using a line-integral approach and least-squares curve-fit to a vortex model. Downwash velocity was compared against momentum theory values. Best practices were followed to correct inherent tip vortex aperiodicity. PIV-specific challenges were exacerbated by testing in a large facility, such as identifying and removing noisy vector fields caused by inadequate seeding. Towards this end, two new filtering methodologies were developed: (1) Modal Outlier Method (MOM), and (2) Projection on Phaseaverage (POP). Significant reduction in standard deviation was observed when outlier vector fields were removed. Lastly, inverse Betz theory was applied on PIV flow fields to relate trailed wake circulation and the sectional bound circulation. The resulting PIV-based loading distribution was used with lifting-line calculations and Helios simulations to analyze wake evolution. Blade-vortex interaction played a significant role in the airloads distribution that, in turn, affected the strength and evolution of the tip vortices themselves. The similarities and differences found among the PIV-, Helios-, and surface pressure sensor-based airloads were analyzed in detail to help plan future experiments.
Ramasamy et al. (Tue,) studied this question.