Effects of spanwise rotation on the vorticity stretching in transitional and turbulent channel flow

We investigate, by means of direct numerical simulations, the three-dimensional (3-D) dynamics of coherent vortices in a rotating channel. We focus here on the structure of the instantaneous (absolute and relative) vorticity field. Both transitional and turbulent regimes are considered. Strong rotation is shown to suppress the transition towards turbulence (leading to two-dimensional [2-D] flow). Conversely, moderate rotation yields strong longitudinal vortices on the anticyclonic side of the channel, which trigger early transition (earlier than without rotation). In that regime, the complete transition to fully developed turbulence is compared for two values of Rossby number: \Ro((i))\ = 2 and \Ro((i))\ = 6. In the early stage of the transition, perturbations are more strongly amplified at \Ro((i))\ = 2. The saturation is, however, reached earlier in that case, and a more energetic turbulent state is achieved at \Ro((i))\ = 6. In the fully developed turbulent case, nonrotating and moderately rotating channels are compared. Relaminarization occurs on the cyclonic wall, while turbulence is observed on the anticyclonic wall. The vortex topology is shown to be strongly affected by the rotation. The enhancement of the anticyclonic perturbations level is associated with hairpin vortices which are much more inclined (up to 10 degrees to the wall) than in the nonrotating case (45 degrees). These extend until the channel center and are associated with a characteristic region of zero absolute mean vorticity. Stretching mechanisms of absolute vortex lines are carefully examined.