Large-eddy simulation of turbulent flow and structures in a square duct roughened with perpendicular and V-shaped ribs

In this paper, highly disturbed turbulent flows confined within a square duct with uniformly spaced V-shaped ribs mounted on one wall are investigated using large-eddy simulation (LES). Two V-shaped ( 6 0 ° and 4 5 ° ) rib cases are studied in comparison with the perpendicular ( 9 0 ° ) rib case. The LES results are validated against a set of water-channel measurement data newly acquired by the authors. The effects of different shaped ribs on turbulent flow are systematically studied in terms of the mean velocity field, turbulence statistics, and coherent structures. The results show that strong secondary flows in the pattern of a pair of large symmetrical streamwise-elongated vortices exist in all three rib cases. The impacts of rib geometry on turbulent coherent structures are investigated using vortex identifiers, temporal autocorrections, spatial two-point autocorrelations, and velocity spectra.