Water Tunnel Flow Diagnostics of Wake Structures Downstream of a Model Helicopter Rotor Hub

A scaled model of a notional rotor hub design was tested at one-third- and two-thirds scale Reynolds number with reference to a large helicopter at an advance ratio of 0.2 in the 48” Garfield Thomas Water Tunnel (GTWT) at the Applied Research Laboratory (ARL) at Penn State University. The main objectives of the experiment were to understand the spatial- and temporal content of the unsteady wake downstream of a rotor hub up to a distance corresponding to the empennage of a large helicopter. Measurements included total hub drag and flow diagnostics involving PIV, SPIV and LDV at several locations downstream of the model rotor hub. Computations of the rotor hub flow were also performed by Sikorsky Aircraft and compared with the experimental results. Various flow structures were identified and linked to geometric features of the hub model. The most prominent structures were two-per-revolution (scissors) and four-per-revolution (main hub arms) vortical structures shed by the hub. Both the two-per-revolution and four-per-revolution structures persisted far downstream of the hub, but the rate of dissipation was greater for the four-per-rev structure. This work provides an extensive dataset for enhanced understanding of the fundamental physics underlying rotor hub flows and serves as validation data for future CFD analyses in the rotorcraft community.

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