Study and Valiation of the Spatial Laminar-Turbulent Transition in Channel Flow with Uniform Roughness

Abstract

The laminar turbulent transition on a channel flow depends on many factors such as disturbances at the channel entrance, roughness and Reynolds number between others. In the present work, the effect of the roughness is studied using Direct Numerical Simulation (DNS). The numerical tool employed is the Incompact3d code, which solves the incompressible Navier-Stokes equations and allows to model roughness at the wall with the Immersed Boundary Method (IBM). First, numerical simulations are performed for Reo= 6300 in a periodic channel in order to validate the code. Results are well compared with data from the literature. Then, three simulations (two with roughness and one with-out roughness) are performed using the same Reynolds number (i.e. the same flow rate), on a channel with an inflow-outflow boundary condition in the streamwise direction to calculate the evolution of the friction coefficient from the laminar state to the fully turbulent one. Results show that right after the entrance a disturbed laminar regime is presented. The evolution of this regime to a fully turbulent state is then compared with the case without roughness to evaluate the effect of the roughness on the transition phenomenon. It was found that the roughness increases the friction coefficient by a factor of 1.8 and accelerates the transition in the streamwise direction for approximately 18.5 channel half-heights.