Flow Measurement of Unsteady Flow Confluences Using Particle Image Velocimetry Technique

Unsteady flow at confluences is not uncommon in nature, for reasons such as sudden flooding or dam operation in the upstream tributary of a river confluence. Previous studies have mainly focused on the flow dynamics, sediment transport and bed morphology of steady flow confluences. Knowledge regarding unsteady flow confluences is sparse. Herein, a 60-degree confluence flume was fabricated having a flow circulation system, which can produce cosine-curve variable discharge inflows in the main channel, automatically controlled by a computer. Two sets of tests were investigated, i.e. the flow structure of an unsteady flow confluence and the steady flow confluence, using the particle image velocimetry (PIV) technique. Velocity fields were measured at four water depths (z = 4, 8, 12, and 16 cm measured from the bottom), the average depth of flow being 20 cm. The data from the PIV enabled us to analyze the instantaneous and the time-averaged flow structure, e.g., flow velocity, kinetic energy, and vorticity at an unsteady confluence and contrast them with the steady flow confluence results. It was observed that, even though the flow in the tributary was steady, the water level in it followed a cosine-curve variation due to the blocking effect of the varying discharge in the main stream. The water-level variation in the tributary is almost out of phase with that of the main stream. The kinetic energy of the unsteady flow confluence was observed to be greater than that of the steady case, notwithstanding identical average discharges in the two. The variation of vorticity over time indicated the evolution of the vortex within the shear layer and the separation zone. The shear layer, demarcated by the vorticity field, sways as the discharge in the main stream changes with time, and the extent of the sway is approximately equal to 12.5% of the channel width.