Scalar and momentum transfer in a low-Reynolds-number channel flow after a rough-to-smooth step change
Phys. Rev. Fluids 11, 084611 – Published 31 August, 2026
DOI: https://doi.org/10.1103/39bs-fhf8
Abstract
Relaxation of the turbulent momentum and passive scalar fields generated by a specific staggered roughness distribution in the entrance region of an initially laminar channel flow is studied through direct numerical simulations. The computational domain is necessarily large, and a new and fast algorithm is introduced to efficiently resolve the flow characteristics. Staggered roughness elements of height 0.27 times the half width of the channel recover the entry zone either entirely or partly, prior to a smooth channel labeled as the control channel (CC) wherein both the turbulent flow and scalar fields decay, because the Karman number is significantly low and it is close to the subcritical limit. Despite the low Reynolds numbers, the roughness elements are able to trigger the bypass transition mechanism leading to massive clusters of coherent quasistreamwise vortices entering into the smooth channel. The characteristics of the turbulence decay at the CC centerline collapse reasonably well with those of a homogeneous isotropic turbulence. The inner layer responds relatively rapidly to the rough-smooth step change in the control channel while the outer layer recovery is gradual. The decays of the fluctuating streamwise velocity and of the passive scalar intensities are particularly slow. Despite the fact that the turbulent activity decreases and that the flow approaches the relaminarization, the Nusselt number averaged over large streamwise distances in the smooth channel remains close to an equivalent fully developed turbulent smooth-channel flow.
