AbstractDropshafts are commonly used as flood discharge structures in urban drainage systems and hydraulic structures. Plunging and vortex flows are the two main flow regimes occurring in the dropshaft with a tangential intake. However, the effects of the intake contraction on the flow regime transition have not been studied. This study aims to present the flow characteristics in the transition process between the plunging and vortex flows in dropshafts, through a series of three-dimensional computational fluid dynamic simulations. With the decrease in the intake contraction ratio, the ratio of rotation and falling velocities along the annular dropshaft wall increases gradually, and the vortex flow can remain a long distance in the vertical dropshaft. The flow regime transition from the plunging flow to the vortex flow is analyzed quantitatively based on two specific velocity ratios of rotation and falling velocities. In addition, for large flow discharges, the effects of intake contraction on the flow rotation performance are more obvious than that for small flow discharges. These findings provide reference for the hydraulic design in dropshaft structures.

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