Numerical Investigation of Geometrical Effects on the Flow Hydrodynamics in Tangential Vortex Drop Shaft

Vortex drops are compact hydraulic structures used in surface water and sewer collection systems to convey runoff from higher to lower elevations by creating a rotational flow inside vertical shafts. These structures are composed of three main parts: the intake, drop shaft, and dissipation chamber. Tangential intake is a steep tapering channel that generally has a junction with a rectangular approach channel with the horizontal bottom at the beginning and a narrow slot at the connection with the drop shaft. Many factors need to consider in the design of vortex drop shafts with proper hydraulic performance. The review of previous studies and the guideline designs for this structure indicates that most design relations were obtained either based on simplified assumptions or by conducting limited tests on laboratory scale models, which can cause desirable operation in practice. These conditions have forced engineers to set up laboratory models or numerical simulations of the initial design to evaluate the proper performance of the structure in big projects. With this introduction, one of the problems in the surface water collection network of Tehran is conveying high volumes of runoff from the surface of streets at the highway intersections to a lower level in the underground tunnels or pipes. Therefore the authorities pay more attention to necessary considerations in the design and use of these types of structures for the safe transfer of runoff downstream. In this paper, using numerical modeling, the hydrodynamics of flow in a real vortex drop shaft with tangential intake has been studied. In the design stage, a vortex drop structure in Tehran's urban drainage has been selected and evaluated by the Flow-3D numerical model. Based on the latest available design methods, Several tangential intakes with different geometry were assessed separately. Finally, the performance of the final drop shaft was simulated and analyzed using the numerical model. The final design simulation results showed that the flow in the tangential intake would enter the vertical shaft without forming a hydraulic jump. The flow in the vertical shaft is spirally attached to the wall with a central air core. A key design parameter is the ratio of the air core area to the drop shaft cross-sectional area that was greater than 0.49. The efficiency of energy losses at the tangential intake is about (9-15%), in the vertical shaft is about (23-40%), and in the energy dissipating chamber is (70-71%) depending on the flow rate. The energy loss efficiency in the whole structure was about (80-84%). The depth size needed to create a water cushion in the energy dissipation chamber was considered for three depths of 0.4D, 0.5D, and 0.6D. After numerical modeling, the appropriate depth for the water cushion was determined to be 0.6D. The results of the simulations indicated that the use of existing design methods only sometimes leads to optimal hydraulic performance in the structure. Therefore, reviewing the existing design methods, simulating the flow in the designed drop shaft, or setting up a laboratory model before finalizing the design is necessary.