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Effect of surface morphological changes on flow over a sphere

Citation

Norman, Adam Keith (2010) Effect of surface morphological changes on flow over a sphere. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:05212010-124044645

Abstract

An experimental investigation was undertaken to examine the effect of a morphing surface on the flow over a sphere in the Reynolds number range of 5x10^4 to 5x10^5. Here, a morphing surface is defined as a continuous surface that undergoes small amplitude changes in order to excite flow instabilities, rather than utilizing large mechanical changes to the overall shape as with traditional aerodynamic control surfaces. The sphere was chosen as an ideal geometry for testing morphing surfaces, because of the well-known sensitivity of the flow to small asymmetries on the surface. In this study, an approximation of a morphing surface was made by dynamically moving a small isolated roughness element along the sphere, thus producing small amplitude time-dependent changes to the surface shape. An experimental apparatus was designed that produced the actuation with an internal motor, which moved the roughness element via magnetic interaction. A three-component piezoelectric force sensor placed inside the sphere allowed for accurate, instantaneous measurements of the global effect of the actuator on the flow. It was found that the moving roughness could produce an instantaneous lateral force as large as the drag. Simultaneous force and particle image velocimetry measurements in the subcritical regime were used to show that there is a relatively long timescale associated with the instability growth, entrainment of fluid, and local change of the position of separation. This allowed the roughness to trip an extended region of the flow at once. It is shown that the three-dimensionality of the disturbance leads to the production of two helical counter-rotating vortices in the wake. In addition, it is demonstrated that a mean side force can be obtained by oscillating the roughness element about a point, producing a lateral force an order of magnitude larger than the force caused by a stationary roughness element. Finally, the results from the dynamic roughness were used to help interpret the underlying physical mechanisms that govern the forcing on a smooth sphere.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:sphere; separated flow; vortex; morphing surface; roughness; unsteady; actuation
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Awards:The Donald Coles Prize in Aeronautics, 2010
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • McKeon, Beverley J.
Thesis Committee:
  • McKeon, Beverley J. (chair)
  • Hussain, Fazle
  • Leonard, Anthony
  • Pullin, Dale Ian
Defense Date:16 April 2010
Author Email:norman (AT) caltech.edu
Funders:
Funding AgencyGrant Number
National Science Foundation0747672
Record Number:CaltechTHESIS:05212010-124044645
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:05212010-124044645
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:5821
Collection:CaltechTHESIS
Deposited By: Adam Norman
Deposited On:25 May 2010 16:14
Last Modified:26 Dec 2012 03:25

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