A Novel Dual Inlet Tangential Vortex Drop Shaft: An Analytical and Numerical Study

Abstract

Urban drainage systems are undergoing substantial upgrades to enhance resilience to increased urbanisation, population growth and more intense rainfall events. Typically, in these hydraulic structures, the challenge of dropping water flows over significant elevation differences is overcome using vortex drop shaft structures. Many options are available for transferring flows from a single upstream channel to a lower level using independent vortex drop shafts. However, there are many cases where a manhole is to convey discharge from multiple flow inlets. In this work, a novel dual inlet tangential vortex drop shaft was developed during a full-scale commercial project proposal. The paper presents analytical and multiphase numerical modelling studies of various geometries of the structure. Analytical methods were used to derive conditions to maximize the free drainage discharge of inlets and were tested using a three-dimensional multiphase numerical model. The performance of the structure was evaluated for two drop tube diameters with scenarios including (1) equal flows in symmetric inlets, (2) unequal flows in asymmetric inlets and (3) independent inlet flows. The results demonstrate that the analytical model overestimates the free-drainage discharge. Nonetheless, interaction between flows and opposing inlets in the design did not appear to pose any significant stability or performance issues. Moreover, when tested at extreme flow rates up to flooding conditions, the numerical results demonstrate that the vortex air core remains intact. To conclude, a cost-benefit analysis of this type of configuration is presented which provides an economic argument for consideration of the dual inlet drop shaft in future schemes.