Experimental analysis of turbulent flow structure in a fully developed rib-roughened rectangular channel with PIV

An experimental study was carried out to investigate the turbulent water-flow structure over one-side micro-repeated ribs in a narrow two-dimensional rectangular channel by particle image velocimetry (PIV). Two rib pitch-to-height ratios p/k of 10 and 20 were investigated while the rib height was held constant at 4 mm. The rib height-to-channel equivalent diameter ratio k/D-e was 0.1. The streamwise mean velocity and turbulent kinetic energy fields in the fully developed flow region of the channel were calculated at three different positions of x=0, 3.8, and -6.2 mm, which corresponded to center, downstream, and upstream of the rib, respectively, and for two Reynolds numbers Re of 7,000 and 20,000. A large-scale turbulent eddy was generated by the rib promoter and then propagated into the mainstream flow, which led to the deformation of the velocity profile. Downstream of the rib, rotating and counter-rotating eddies were also generated by the rib promoter. The enhancement of the turbulent kinetic energy was not changed when the Reynolds number increased from 7,000 to 20,000 between p/k=20 and 10. The reattachment length L-R was measured from velocity vector fields in the developing, fully developed, and exit regions of the flow over the range Re=1,400-50,000. The results showed that the ratio p/k and the Reynolds number had no significant effect on the reattachment length beyond a critical value of Re=15,000, where LR was found to be approximately 4 times the rib height.