Roughness and Reynolds Number Effects on the Flow Past a Rough-to-Smooth Step Change
AuthorRouhi, A; Chung, D; Hutchins, N
EditorOrlu, R; Talamelli, A; Peinke, J; Oberlack, M
Source TitleSpringer Proceedings in Physics
PublisherSpringer International Publishing
Document TypeConference Paper
CitationsRouhi, A., Chung, D. & Hutchins, N. (2019). Roughness and Reynolds Number Effects on the Flow Past a Rough-to-Smooth Step Change. Orlu, R (Ed.) Talamelli, A (Ed.) Peinke, J (Ed.) Oberlack, M (Ed.) iTi 2018: Progress in Turbulence VIII, 226, pp.81-86. Springer International Publishing. https://doi.org/10.1007/978-3-030-22196-6_13.
Access StatusOpen Access
ARC Grant codeARC/DP160102279
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 ϵ/hϵ/h? (scaled by the channel height h), (2) roughness size k / h?, (3) roughness shape? and (4) Reynolds number ReτReτ? 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≃ks+≃ 160 to k+s≃106ks+≃106. To study (3) and (4) the step-down case at Reτ≃395Reτ≃395 was compared with a backward-facing step case at Reτ≃527Reτ≃527, and DNS of square rib rough-to-smooth case at Reτ≃1160Reτ≃1160 (Ismail et al., J. Fluid Mech., vol. 843, 2018, pp. 419–449). Results showed that ϵ/hϵ/h affects the departure from equilibrium by a large extent, while k / h, roughness shape and ReτReτ 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 ϵ/hϵ/h, i.e. higher ϵ/hϵ/h leads to higher amplification.
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