Study of the Super-Late Stage Temporal Transition in a Plane Channel Flow

Abstract

In this work a temporal laminar-turbulent K-type transition at a subcritical Reynolds number (Re = 5000) is simulated employing Direct Numerical Simulation (DNS). The configuration is a temporal developing channel flow driven by a constant flow rate. The study focuses in the description of flow features before and after the time that corresponds to the peak in the evolution of the friction coefficient during the transition. Specifically, the evolution of the friction coefficient is described employing the FIK identity, vortex structures, and the turbulent kinetic energy budget. Results show that during the peak zone the intensity of the Reynolds shear stress term of the FIK identity is greater than that computed in the fully developed turbulent regime. Also, a peak is found in the temporal evolution of the magnitude of the turbulent dissipation in this zone. This agrees with the larger population of vortices found in this time span with respect to that computed in the fully turbulent regime. Additionally, we show that the presence of coherent hairpin vortices upstream the peak zone (i.e. before the peak) play a key role in the peak of the friction coefficient because they generate streamwise vortices (small scales). Downstream of the peak zone (i.e. after the peak), these vortices lose their coherence and the generation of small scales decays to the level found in the fully developed turbulent regime.