EVIDENCE FOR HAIRPIN PACKET STRUCTURE IN DNS CHANNEL FLOW

Experimental studies of boundary layers and channel flows with 2-D PIV measurements have revealed that wall turbulence is thickly populated with hairpin vortices that mostly appear in groups as "packets". A recent numerical simulation study of the evolution of a single hairpin-like vortex has disclosed the mechanism for the generation of near-wall hairpin vortex packets. However, there are very few observations of hairpins in existing DNS studies, and none have revealed packets. The present study demonstrates, for the first time, the existence of hairpin vortex packets in DNS of turbulent flow. Vortex packets are found to appear frequently in DNS of fully developed turbulent channel flows at moderate Reynolds number, Re_τ = 300. A vortex detection method based on the imaginary part of the eigenvalue of the velocity gradient tensor has helped to identify the vortices in velocity fields. The vortex packet structure found in present study is consistent with and substantiates the observations and the results from previous 2-D PIV measurements. This evidence substantiates 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 DNS velocity field and vortices also shows the close association of hairpin packets with long low-momentum streaks