Affiliations: Department of Theoretical and Applied Mechanics,
University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA. E-mail:
[email protected] | Department of Theoretical and Applied Mechanics,
University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA
Abstract: High-resolution particle-image velocimetry (PIV) measurements are
made in the streamwise – wall-normal plane of turbulent channel flow at Re_{τ}=566,
1184 and 1759, facilitating documentation of the population trends and
core diameters of small-scale spanwise vortices. Swirling strength, an
unambiguous vortex-identification criterion and hence a local marker of
rotation, is used to extract small-scale spanwise vortex cores from the
instantaneous velocity fields. Once the small-scale vortices are properly
extracted from the PIV realizations, their characteristics are studied in
detail. The present results indicate that the very-near-wall region (y <
0.1 h) is densely populated by spanwise vortices with clockwise (negative)
rotation. This behavior supports the notion that hairpin-like vortices are
generated very close to the wall and grow into the outer layer as they advect
downstream. In contrast, counterclockwise (positive) spanwise vortices are
scarce in the very-near-wall region, but their presence steadily increases
within the logarithmic layer presumably due to a localized generation
mechanism. The average core diameter of negative spanwise vortices is found to
be larger than the average diameter of positive vortices, with few positive
vortices having core diameters exceeding 80 y_*.
