Experimental Comparison of Dividing Open-Channel with Duct Flow in T-Junction

To aid numerical modeling of T-junction flows, this paper presents experimental differences in velocity and water-surface elevation between open-channel free surface and duct symmetry plane. The data were taken in an open-channel flow with a width-to-depth ratio of 1:2, and were compared to published data for a duct flow with a width-to-depth ratio of 1:4. It was found that at a discharge ratio of 0.81 between the branch and upstream main channel, there is up to 0.36Ux/Uo difference in the near-surface velocity (the symmetry plane velocity in duct flow) at certain corresponding (same normalized x, y, and z) locations. The differences in velocity are attributed to secondary currents in open-channel flow that are not present in the duct flow. It is also found that as the flow turns to enter the branch, centrifugal forces caused up to 2.1% superelevation of the water surface. The observations suggest that caution is needed when assuming that flow in an open-channel cross section is equivalent to that in one-half of a closed conduit.