Highly-disturbed turbulent flow in a square channel with V-shaped ribs on one wall

• Highly-disturbed turbulent flows in a square duct with a bottom wall roughened by V-shaped ribs are studied using PIV.
• Strong secondary flows induced by the V-shaped ribs appear as a pair of large-scale streamwise counter-rotating vortices in the cross-stream directions.
• Turbulent intensity near the channel midspan is suppressed above V-shaped ribs, while strong turbulent motions are confined below the rib height.
• The main contribution to TKE, for the V-shaped rib cases, comes from the interaction between the extra mean shear rate and Reynolds normal stresses, rather than the primary shear and Reynolds shear stress as for the perpendicular rib case.
• Two-point auto-correlation coefficient shows that near wall turbulent structures are strongly affected by secondary flows for V-shaped rib cases.

We investigate highly-disturbed turbulent flows in a square channel with V-shaped ribs mounted on one wall. A planar particle image velocimetry (PIV) system is used to measure the fully developed turbulent flow at the channel midspan and two off-center planes. V-shaped rib configurations with three different inclinations (60°, 45° and 30°) are studied and compared to the perpendicular (90°) rib case. The highly-disturbed turbulent flow in the V-shaped rib cases are studied by investigating the statistics of first and second order moments, including the mean velocity, mean shear, mean vorticity and Reynolds stresses. Owing to the geometrical skewness of the V-shaped ribs, strong secondary flows are induced which appear in the pattern of a pair of streamwise counter-rotating roll cells and have a significant impact on the momentum transport process. Near the channel midspan of the V-shaped ribs, turbulent intensity is suppressed above the ribs but enhanced below the rib height, and turbulent vortical motions exhibit isotropic patterns due to the strong disturbances from the ribs. The coherent flow structures have been systematically analyzed based on studying two-point auto-correlation coefficient, quadrant decomposition of Reynolds stresses, swirling strength, and proper orthogonal decomposition of the turbulent flow.