An experimental study was performed on a three‐bladed rotor model in two water tanks. Blade pitch, rotational velocity, descent angle, and descent speed were varied to simulate a wide range of rotorcraft operating states, focusing on cases where the rotor was operating in vortex
ring state. Flow visualization used flow markers injected tangentially from the blade tips to mark vortex cores, showing the development of shortwave and longwave instabilities on the helical vortices in the wake. Strain gages recorded transient loads, allowing a correlation between rotor
thrust and wake development. Chord Reynolds numbers were of order 105. The data indicate that as the instabilities develop, adjacent vortices merge, forming thick vortex rings during descent. Periodic shedding of these rings from the wake associated with
vortex ring state was observed, resulting in peak‐to‐peak thrust fluctuations of up to 95% of the mean that occured at regular intervals of 20–50 rotor revolutions, depending on flow parameters.
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Document Type: Research Article
University of California Berkeley, Berkeley, CA 94720-1740
Publication date: 2005-07-01
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