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Substructures in a turbulent spot

Published online by Cambridge University Press:  21 April 2006

R. Sankaran ,
M. Sokolov  and
R. A. Antonia
R. Sankaran
Affiliation:
Department of Mechanical Engineering, University of Newcastle, New South Wales, 2308, Australia
M. Sokolov
Affiliation:
Department of Fluid Mechanics & Heat Transfer, Tel Aviv University, Tel Aviv, 69978, Israel
R. A. Antonia
Affiliation:
Department of Mechanical Engineering, University of Newcastle, New South Wales, 2308, Australia
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Abstract

Substructures within a turbulent spot which develops in a slightly heated laminar boundary layer have been identified using arrays of cold wires aligned in either a streamwise direction or in a direction normal to the wall. At any given streamwise distance from the spot origin, histograms of the number of detected substructures exhibit a peak, defining the most probable spot or the spot with the most likely number of substructures. The number of substructures in the most probable spot increases with streamwise distance but all substructures are convected at approximately the same velocity for any given distance from the wall. This velocity is approximately equal to that of the leading edge of the spot and increases slightly with distance from the wall. The increase in the number of substructures accounts for the streamwise growth of the spot. A simple relation is derived for determining the number of substructures at a particular streamwise station and a geometrical construction is proposed for identifying the origin of a new substructure. There is sufficient evidence for suggesting that the new substructures are formed near the trailing edge of the spot. The convection velocity, inclination and lengthscales of the substructures compare favourably with the corresponding characteristics of hairpin vortices.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 197 , December 1988 , pp. 389 - 414
Copyright
© 1988 Cambridge University Press

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