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DNS OF A TRANSITIONALLY ROUGH CHANNEL FLOW WITH A 3-D ROUGHNESS

Mehdi Seddighi
School of Engineering, University of Aberdeen, Aberdeen AB24 3UE; Department of Mechanical Engineering, University of Sheffield, Mappin Street, Sheffield,S1 3JD, UK; School of Engineering, Technology and Maritime Operations, Liverpool John Moores University, Liverpool, L3 3AF, United Kingdom

S. He
School of Engineering and Physical Sciences, University of Aberdeen, Aberdeen AB24 3UE; Department of Mechanical Engineering, University of Sheffield, Mappin Street, Sheffield,S1 3JD, UK

Alan E. Vardy
Division of Civil Engineering University of Dundee Dundee, UK, PH14 9SS

Tom O'Donoghue
School of Engineering University of Aberdeen Aberdeen, UK, AB24 3UE

Dubravka Pokrajac
School of Engineering University of Aberdeen Aberdeen, UK, AB24 3UE

Resumo

A DNS study is performed to investigate turbulence in a steady-state channel flow at transitionally rough region (ks+ = 8 − 41) with a smooth top surface and a roughened bottom surface made of close-packed pyramids at several Reynolds numbers. Simulations in a channel with all smooth walls are also conducted to facilitate direct comparison. The skin friction factor results show an inflection point at ks+ ~ 15. Reynolds stress and normal vorticity show a similarity in the logarithmic region. In the outer layer, however, it appears that small- and large-scales structures exhibit different sensitivities to the existence of roughness. While the dissipation and vorticity, which are representing the small-scale structures, collapse to the corresponding smooth-wall values, Reynolds stress quantities containing great contributions of the large-scale structures do not exhibit this behaviour for δ/kt = 20 cases. The rough-case with δ/kt = 40, however, represents greater tendency to the similarity. Flow field patterns in the roughness sublayer demonstrate a significant dependence on Reynolds number. Also, at y/k = 0.4 above the crest, the flow structure of the case Reτ = 670, ks+ = 41 is very similar to that of Reτ = 3520, δ/kt = 54.4, ks+ = 95 in the experimental fully-rough results Hong et al. (2011).