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Journal of Flow Visualization and Image Processing
LARGE-SCALE STRUCTURE IDENTIFICATION OF A TURBULENT SPOT BY TAKING BLASIUS VELOCITY PROFILE AS REFERENCE
School of Mechanical Engineering, University of Technology, Sydney, P. O. Box 123, Broadway, NSW 2007, Australia
This paper addresses the large-scale structure identification of an artificially generated turbulent spot that simulates the spontaneous turbulent intermittency in a Blasius boundary layer. The Blasius velocity profile of the laminar boundary layer was taken as a reference to observe the turbulent spot. This differs from the conventional method that takes the wall as a reference. The structure of the spot was identified with the profile of the velocity perturbation that is the deviation of the ensemble-averaged velocity of the interior spot from the velocity of the unperturbed laminar flow surrounding the spot. The velocity perturbations near the central streamline on a y-t plane are taken as a sample to describe the identification method. The identified large-scale structure of the spot is a spanwise vortex moving downstream (forward) and rolling backward in the laminar boundary layer. The streamwise velocity perturbation of the turbulent spot from the experiments is compared with the horizontal velocity component of a vortex numerically generated by a CFD code in order to verify the number of vortices contained in a turbulent spot. Significance of the large-scale structure of turbulent spots is analyzed. The role of turbulent spots under zero and adverse pressure gradients is discussed briefly.
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