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Flow Visualizations and Extended Thrust Time Histories of Rotor Vortex Wakes in Descent

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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

Affiliations: University of California Berkeley, Berkeley, CA 94720-1740

Publication date: 2005-07-01

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  • The Journal of the American Helicopter Society is the world's only scientific journal dedicated to vertical flight technology. It is a peer-reviewed technical journal published quarterly by AHS International and presents innovative papers covering the state-of-the-art in all disciplines of rotorcraft design, research and development. (Please note that AHS members receive significant discounts on articles and subscriptions.)

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