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Identification of thermally-tagged coherent structures in the zero pressure gradient turbulent boundary layer

Citation

Rought, Rebecca Lynn (2013) Identification of thermally-tagged coherent structures in the zero pressure gradient turbulent boundary layer. Engineer's thesis, California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:06172013-144232973

Abstract

A zero pressure gradient boundary layer over a flat plate is subjected to step changes in thermal condition at the wall, causing the formation of internal, heated layers. The resulting temperature fluctuations and their corresponding density variations are associated with turbulent coherent structures. Aero-optical distortion occurs when light passes through the boundary layer, encountering the changing index of refraction resulting from the density variations. Instantaneous measurements of streamwise velocity, temperature and the optical deflection angle experienced by a laser traversing the boundary layer are made using hot and cold wires and a Malley probe, respectively. Correlations of the deflection angle with the temperature and velocity records suggest that the dominant contribution to the deflection angle comes from thermally-tagged structures in the outer boundary layer with a convective velocity of approximately 0.8U∞. An examination of instantaneous temperature and velocity and their temporal gradients conditionally averaged around significant optical deflections shows behavior consistent with the passage of a heated vortex. Strong deflections are associated with strong negative temperature gradients, and strong positive velocity gradients where the sign of the streamwise velocity fluctuation changes. The power density spectrum of the optical deflections reveals associated structure size to be on the order of the boundary layer thickness. A comparison to the temperature and velocity spectra suggests that the responsible structures are smaller vortices in the outer boundary layer as opposed to larger scale motions. Notable differences between the power density spectra of the optical deflections and the temperature remain unresolved due to the low frequency response of the cold wire.

Item Type:Thesis (Engineer's thesis)
Subject Keywords:Boundary Layer, Turbulence
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • McKeon, Beverley J.
Thesis Committee:
  • McKeon, Beverley J. (chair)
  • Colonius, Timothy E.
  • Ravichandran, Guruswami
Defense Date:21 May 2013
Funders:
Funding AgencyGrant Number
United States Air Force Office of Scientific ResearchFA9550-09-1-0701
Record Number:CaltechTHESIS:06172013-144232973
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:06172013-144232973
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7900
Collection:CaltechTHESIS
Deposited By: Rebecca Rought
Deposited On:18 Jun 2013 21:13
Last Modified:15 Jul 2013 18:20

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