Flow dynamics near 60° divergent ribs detached from a wall in a square duct

The turbulent wake behind detached 60° divergent ribs mounted on one wall of a square duct was investigated with regard to turbulent motion, coherent structures, vortex dynamics and temporal organization, as well as the spatial and temporal characteristics of large-scale turbulent structures. Time-resolved particle image velocimetry (TR-PIV), hot-wire anemometry (HWA), and laser Doppler anemometry (LDA) were employed for varying clearance-to-rib-height ratios in the range of 0.1 to 1.0 at a Reynolds number (based on the rib height and mean bulk velocity) of 5000. The turbulent flow field in the wake exhibits an anisotropic turbulent state. Vortical structures originate predominantly from the upper and lower separated shear layers. In the duct symmetry plane, the secondary-flow-induced downwash restricts their vertical growth, producing a regular chain of compact vortices that are largely confined below the rib crest. Power spectral density (PSD) analysis revealed regular vortex motion in the wake for c / k r ≥ 0.3 , with the Strouhal number increasing from 0.17 to 0.22 as clearance decreased, indicating stronger spatial confinement of the vortices. Although PSD amplitude peaks weaken for the largest clearance ratio, bimodal distributions of the vertical velocity confirm persistent organized shedding. Away from the symmetry plane, weaker downwash and lateral transport disrupt this organization, resulting in smaller, denser, and more irregular vortical structures. However, turbulent motion relevant for fluid transport between the core and the wake remains unaffected by the loss of vortex regularity.