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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN Print: 1-56700-135-1

CHARACTERISTICS OF VORTEX PACKETS IN WALL TURBULENCE

Abstract

Large-scale motions are studied in a Reθ ~ 7700 turbulent boundary layer using 2-D PIV measurements in the streamwise-spanwise plane. It is known that inclined low momentum regions grow from the wall at this Reynolds number, extending over 2 δ in x and out to 0.8 δ in y, and evidence suggests these structures are induced by packets of hairpin-like vortices aligned in the streamwise direction. The current experiment is designed to investigate the characteristics of these structures. It uses a horizontal plane at y/δ = 0.2 to intersect mature motions and a wide field of view, permitting observation of several large-scale motions on a single realization while adequately resolving the individual structures associated with these motions. Data reveals large-scale low momentum regions elongated in the streamwise direction, bordered by rotating velocity vector patterns and vorticity, suggesting the presence of hairpin or cane-like vortices. Characteristics of these large-scale motions are investigated, including streamwise and spanwise size, shape, and spanwise spacing. Characteristics of the associated hairpin vortex packets investigated include number of vortices, streamwise alignment, and the nature, size, and symmetry of individual member vortices.
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