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

Abstract

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 openchannel flow) and how this depends on 1) roughness virtual origin /h? (scaled by the channel height h), 2) roughness size k/h?, 3) roughness shape? and 4) Reynolds number Re τ ? 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 160 to 106. To study (3) and ( 4) the step-down case at Re τ 395 was compared with a backward-facing step case at Re τ 527, and DNS of square rib rough-to-smooth case at Re τ 1160 (Ismail et al., J. Fluid Mech., vol. 843, 2018, pp. 419-449). Results showed that /h affects the departure from equilibrium by a large extent, while k/h, roughness shape and Re τ 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 /h, i.e. higher /h leads to higher amplification.

Experimental study of a turbulent boundary layer with a rough-to-smooth change in surface conditions at high Reynolds numbers

“…In fact, the power-law exponent itself is a very sensitive quantity to assess. The streamwise extent where the fit is performed, the noise and scatter in the data points, and the step height effect at small fetches (see Rouhi et al 2019b, for example) can all affect the resulting power-law exponent. As shown by the green symbols in figure 12(a), the δ i versusx trend of Antonia & Luxton (1972) is very similar to that in the present study.…”

“…As shown in the inset of figure 1, immediately downstream of the rough-to-smooth change, a region exists where we would expect a recirculation region to form, as a result of the step in the surface elevation between the rough and smooth walls which produces vortex shedding from the roughness crests. Similar to backward facing steps (Kostas, Soria & Chong 2002;Barri et al 2010;Wu, Ren & Tang 2013;Rouhi, Chung & Hutchins 2019b), this region (which we refer to as the 'roughness trailing wake') persists over a streamwise fetch that scales on the roughness height, before the intensity attenuates and falls below the local turbulence intensity of the existing turbulent boundary layer.…”

Modeling of high-Re, incompressible, non-equilibrium, rough-wall boundary layers for naval applications under NATO-AVT349

“…The model can reproduce the depressed log-layer offset characteristic of this non-equilibrium flow that is not unlike that seen in some adverse pressure gradient or decelerated flows. The active terms in the mean-momentum balance are not the mean pressure gradient nor the mean advection but the balance between the viscous and altered Reynolds shear stress [32]. Next to streamwise changes in roughness, predicting the effect of spanwise changes in roughness is also associated with uncertainties.…”