Mechanisms for generating coherent packets of hairpin vortices in channel flow

The evolution of a single hairpin vortex-like structure in the mean turbule nt field of a low-Reynolds-number channel flow is studied by direct numeric al simulation. The structure of the initial three-dimensional vortex is ext racted from the two-point spatial correlation of the velocity field by line ar stochastic estimation given a second-quadrant ejection event vector. Ini tial vortices having vorticity that is weak relative to the mean vorticity evolve gradually into omega-shaped vortices that persist for long times and decay slowly. As reported in Zhou, Adrian & Balachandar (1996), initial vo rtices that exceed a threshold strength relative to the mean flow generate new hairpin vortices upstream of the primary vortex. The detailed mechanism s for this upstream process are determined, and they are generally similar to the mechanisms proposed by Smith et al. (1991), with some notable differ ences in the details. It has also been found that new hairpins generate dow nstream of the primary hairpin, thereby forming, together with the upstream hairpins, a coherent packet of hairpins that propagate coherently. This is consistent with the experimental observations of Meinhart & Adrian (1995). The possibility of autogeneration above a critical threshold implies that hairpin vortices in fully turbulent fields may occur singly, but they more often occur in packets. The hairpins also generate quasi-streamwise vortice s to the side of the primary hairpin legs. This mechanism bears many simila rities to the mechanisms found by Brooke & Hanratty (1993) and Bernard, Tho mas & Handler (1993). It provides a means by which new quasi-streamwise vor tices, and, subsequently, new hairpin vortices can populate the near-wall l ayer.