Impact of Rib Spacing on Secondary Flows and Near-Wall Turbulence in Channel Flow: Insights from Large Eddy Simulation (LES)

This study employs Large Eddy Simulation (LES) to investigate secondary flows and near-wall turbulence, brought about by the surface-mounted longitudinal triangular ribs in a channel flow at a Reynolds number of 220, based on the channel height H and friction velocity. The rib aspect ratio has been taken as 2, with rib height as 0.1H. Two different rib spacings, such as 0.3H (low rib spacing) and 0.6H (high rib spacing) have been considered. The spanwise domain size has been taken as 3.6H, including 12 ribs for 0.3H spacing and 6 ribs for 0.6H spacing. Results show two counter-rotating vortices between adjacent ribs in both cases. For the low rib spacing case, secondary velocity impact is concentrated near the wall. For the high rib spacing case, it extends towards the top wall. The roughness function is relatively high for the case of low rib spacing. Wall shear stress is relatively high at the mid-plane of the ribs as compared to the spacing between the ribs. Near the wall and below the rib, for the case of low rib spacing, relatively lower production and dissipation of turbulent kinetic energy and normal Reynolds stresses are observed. The converse is observed above the rib height. Normal Reynolds stress exhibits strong anisotropic behaviour near the wall for both the cases, overlapping above 0.4H. For the low rib spacing case, two production peaks are observed, while for the case of high rib spacing only one peak is identified. The ratio of production to dissipation is found close to unity in the log-law region for both the cases.