A Visual Study of Vortex Instabilities in the Wake of a Rotor in Hover

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Abstract:

A visual study of the instability characteristics of the helical vortex filaments trailing from the tips of a three-bladed lifting rotor in a water tank is presented. The rotor diameter was 25.4 cm, and its rotation rate ranged from 4 to 12 revolutions per second. Soon after their formation, the vortex filaments developed long- and short-wave instabilities. In the long-wave instability mode, two of the three vortices coming off the rotor orbited around each other and merged in about 0.4 of the theoretical orbit time of equistrength two-dimensional vortices, after which the third vortex joined the merger to form a single, apparently turbulent helical vortex filament. The wavelengths of the short-wave instabilities were about 0.4 of the wake radius, about 17 cycles over the circumference. The short waves exhibited a linear growth rate during the first half of their orbital motion and an exponential growth prior to merging. The linear growth rate was about 0.0034 D/rad. The e-folding time for the exponential growth rate was about 0.52 rad.

Document Type: Research Article

DOI: http://dx.doi.org/10.4050/JAHS.57.042005

Publication date: October 1, 2012

More about this publication?
  • 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.)

    Journal subscribers who are AHS members log in here if you are not already logged in.

    Authors can find submission guidelines and related information on the AHS website.

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