Effect of injection on bed shear stress and turbulence characteristics in a closed conduit flow

Bed shear stress and turbulence quantities are important parameters to understand sediment erosion, transport, and hydraulic processes in most hydraulic studies. An experimental investigation was conducted to understand the effect of injection on the bed shear stress and turbulence characteristics of flows during low sediment transport rate in a closed conduit, which is similar in construction to the erosion function apparatus. In particular, the effect of injection on the bed shear stress, mean velocity profiles, turbulence intensities, Reynolds shear stress, and higher-order moments of the closed conduit flows in the seepage zone as well as at the upstream edge of the seepage zone was examined. The instantaneous velocities were measured using two-dimensional particle image velocimetry (PIV) to evaluate the turbulence structure in both the seepage zone and at the upstream edge of the seepage zone. The bed shear stress estimated by the Reynolds shear stress approach was found to be more appropriate than that estimated by the usual logarithmic law approach. However, a spatial fluctuation in the bed shear stress was noticed as the injection intensity was increased. Injection was found to decrease the velocity near the bed and to increase the velocity near the center of the conduit in comparison to the no-seepage condition in both zones. The injection resulted in more of a decrease in bed stability in the seepage zone in comparison to the upstream edge of the seepage zone as the injection intensities were increased. The introduction of injection increased the magnitudes of the various turbulence parameters in comparison to the no-seepage condition in the seepage zone. The effect of injection was not only visible in the near-bed region, but in both zones as the water depth (measured from top of the sediment surface) increased.