Affiliations: Laboratory for Turbulence and Complex Flow, Department
of Theoretical and Applied Mechanics, University of Illinois at
Urbana-Champaign, Urbana, Illinois 61801, USA.
Abstract: A number of experimental studies have inferred the existence of
packets of inclined, hairpin-like vortices in wall turbulence on the basis of
observations made in two-dimensional x-y planes using visualization and
particle image velocimetry (PIV). However, there are very few observations of
hairpins in existing three-dimensional studies made using direct numerical
simulation (DNS), and no such study claims to have revealed packets. We
demonstrate, for the first time, the existence of hairpin vortex packets in DNS
of turbulent flow. The vortex packet structure found in the present study at
low Reynolds number, Re_ = 300, is consistent with and substantiates the
observations and the results from two-dimensional PIV measurements at higher
Reynolds numbers in channel, pipe and boundary layer flows. Thus, the evidence
supports the view that vortex packets are a universal feature of wall
turbulence, independent of effects due to boundary layer trips or critical
conditions in the aforementioned numerical studies. Visualization of the DNS
velocity field and vortices also shows the close association of hairpin packets
with long low-momentum streaks and the regions of high Reynolds shear
stress.