Experimental investigation of flow structure due to truncated prismatic rib turbulators using particle image velocimetry

An experimental investigation of detailed flow field measurement in a rectangular duct with truncated prismatic ribs mounted on the bottom wall has been carried out using 2-dimensional particle image velocimetry (2-D PIV) technique. The truncated prismatic ribs have been manufactured by tapering the square rib from both the sides up to the center to provide rib height at the ends (h) of 0-8 mm with an increment of 2 mm. The PIV measurements have been performed in vertical as well as in horizontal streamwise planes for different Reynolds numbers and a fixed rib pitch to height ratio of 12. In addition, a number of PIV measurements in vertical symmetry plane have also been performed while covering the streamwise region of 46≤x/e≤120 to observe the nature of the flow in the test section and the flow periodicity. Overall, the emphasis is to investigate the flow structures and their dynamics in the inter-rib region in terms of time-averaged streamlines, mean velocities, vorticity fields, instantaneous velocity vector and corresponding vorticity fields, and fluctuation statistics in both the measurement planes. The close-up inspection based on critical point theory shows the significant evidence of salient critical points and limit cycles such as saddle point, stable and unstable node, bifurcation line and stable and unstable focus in both just upstream and downstream rib regions. The critical flow patterns around the rib confirm three-dimensionality and unsteady nature of the flow. The instantaneous flow characteristics reveal the presence of large-scale coherent vortices in the separated shear layer region, the existence of periodical processes such as intermittent ejection of the recirculation zone into the main flow and motion of the reattachment point. Significant reduction in the size of primary recirculation bubble and thereby reduction in reattachment length is observed for truncated prismatic ribs as compared to square rib in spanwise direction.