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Role of the tip vortex in the force generation of low-aspect-ratio normal flat plates

Published online by Cambridge University Press:  22 May 2007

MATTHEW J. RINGUETTE ,
MICHELE MILANO  and
MORTEZA GHARIB
MATTHEW J. RINGUETTE
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
MICHELE MILANO
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
MORTEZA GHARIB
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
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Abstract

We investigate experimentally the force generated by the unsteady vortex formation of low-aspect-ratio normal flat plates with one end free. The objective of this study is to determine the role of the free end, or tip, vortex. Understanding this simple case provides insight into flapping-wing propulsion, which involves the unsteady motion of low-aspect-ratio appendages. As a simple model of a propulsive half-stroke, we consider a rectangular normal flat plate undergoing a translating start-up motion in a towing tank. Digital particle image velocimetry is used to measure multiple perpendicular sections of the flow velocity and vorticity, in order to correlate vortex circulation with the measured plate force. The three-dimensional wake structure is captured using flow visualization. We show that the tip vortex produces a significant maximum in the plate force. Suppressing its formation results in a force minimum. Comparing plates of aspect ratio six and two, the flow is similar in terms of absolute distance from the tip, but evolves faster for aspect ratio two. The plate drag coefficient increases with decreasing aspect ratio.

Type
Papers
Information
Journal of Fluid Mechanics , Volume 581 , 25 June 2007 , pp. 453 - 468
Copyright
Copyright © Cambridge University Press 2007

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