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Article

Aspect ratio effects in turbulent duct flows studied through direct numerical simulation

Authors: Vinuesa, R., Noorani , A. N., Lozano-Durán, A., El Khoury, G.E.K., Schlatter, P., Fischer, P F, Nagib, H.M.
Document Type: Article
Pubstate: Published
Journal: J. of Turbulence
Volume: 15   677–706
Year: 2014

Abstract

Three-dimensional e ffects in turbulent duct flows, i.e., side-wall boundary layers and secondary motions, are studied by means of direct numerical simulation (DNS). The spectral element code Nek5000 is used to compute turbulent duct flows with aspect ratios 1 to 7 (at Reb,c = 2800, Ret,c = 180) and aspect ratio 1 (at Reb,c = 5600, Ret,c = 330), in streamwise-periodic boxes of length 25h. The total number of grid points ranges from 28 to 145 million, and the pressure gradient is adjusted iteratively in order to keep the same bulk Reynolds number in the centerplane with changing aspect ratio. Turbulence is initiated via a trip forcing active during the initial stages of the simulation, and the statistical convergence of the data is discussed both in terms of transient approach and averaging period. Spanwise variations in wall shear, mean-flow pro files and turbulence statistics are analyzed as a function of aspect ratio, and also compared with the spanwise-periodic channel (as idealization of an in finite aspect ratio duct). The computations show good agreement with experimental measurements carried out in parallel at IIT (Chicago), and highlight the relevance of side-wall boundary layers and secondary vortices in the physics of the duct flow. The rich array of secondary vortices extending throughout the upper and lower walls of the duct, and their dependence on Reynolds number and aspect ratio, had not been reported in the literature before.