Numerical study on the effect of tangential intake design and inflow discharge on vertical dropshaft assessment using pressure and velocity distributions

Abstract

Tangential intake structures with vertical dropshafts are commonly employed in stormwater and urban drainage systems. Vortex and plunging flows are the two typical flow patterns observed in dropshafts, with the former being the desired due to its favourable drainage performance and enhanced safety. The transition between these flow patterns is primarily influenced by the structural design and rate of flow discharge. To investigate the interception performance and the transition criteria between vortex and plunging flows, a series of numerical simulations were conducted. The effects of the approach channel slope, contraction ratio, and flow discharge were analyzed by comparing the evolution of the water surface, pressure and velocity distributions, and swirling distance across various scenarios. With a higher flow rate of 20 L/s, obvious water congestion was observed in dropshafts with contraction ratios below 0.4 and horizontal approach channels. However, the inflow channel was not congested in cases with sloping approach channels, indicating higher drainage efficiency of intake structures with slopping channels. Two hydraulic factors, namely, the minimum to maximum wall pressure ratios (Pmin/Pmax) and the minimum Vr/Vz ratios (Vr/Vz)min, were employed to differentiate between vortex flow and nonrotational flow. These two factors consistently led to the same conclusions under both subcritical and supercritical inflow conditions, indicating their validity and their potential application in the design and optimization of intake structures. The criteria of Pmin/Pmax > 0.2 and (Vr/Vz)min > 0.2 can be used as criteria for the formation of stable vortex flow.

Keywords

• Intake structures
• vertical dropshaft
• flow transition
• pressure
• distribution
• velocity ratio