Exact coherent states with hairpin-like vortex structure in channel flow

Hairpin vortices are widely studied as an important structural aspect of wall turbulence. The present work describes, for the first time, nonlinear travelling wave solutions to the Navier-Stokes equations in the channel flow geometry - exact coherent states (ECS) - that display hairpin-like vortex structure. This solution family comes into existence at a saddle-node bifurcation at Reynolds number Re = 666. At the bifurcation, the solution has a highly symmetric quasi-streamwise vortex structure similar to that reported for previously studied ECS. With increasing distance from the bifurcation, however, both the upper and lower branch solutions develop a vortical structure characteristic of hairpins: a spanwise-oriented 'head' near the channel centreplane where the mean shear vanishes connected to counter-rotating quasi-streamwise 'legs' that extend toward the channel wall. At Re = 1800, the upper branch solution has mean and Reynolds shear-stress profiles that closely resemble those of turbulent mean profiles in the same domain.