Roughness and Reynolds Number Effects on the Flow Past a Rough-to-Smooth Step Change

We report direct numerical simulations (DNSs) of open-channel flow with a step change from three-dimensional sinusoidal rough surface to smooth surface. We investigate the persistence of non-equilibrium behaviour beyond this step change (i.e. departures from the equilibrium smooth open-channel flow) and how this depends on (1) roughness virtual origin epsilon/h? (scaled by the channel height h), (2) roughness size k/h?, (3) roughness shape? and (4) Reynolds number Re-tau? To study (1), the roughness origin was placed aligned with, below (step-up) and above (step-down) the smooth patch. To study (2), the equivalent sand-grain roughness of the aligned case was decreased from k(s)(+) similar or equal to 160 to k(s)(+) similar or equal to 106 . To study (3) and (4) the step-down case at Re-tau similar or equal to 395 was compared with a backward-facing step case at Re-tau similar or equal to 527 , and DNS of square rib rough-to-smooth case at Re-tau similar or equal to 1160 (Ismail et al., J. Fluid Mech., vol. 843, 2018, pp. 419-449). Results showed that epsilon/h affects the departure from equilibrium by a large extent, while k/h, roughness shape and Re-tau have a marginal influence. The departure from equilibrium was found to be related to the near-wall amplification of Reynolds shear stress, which in turn depends on epsilon/h , i.e. higher epsilon/h leads to higher amplification.