CFD-based prediction of vortex-induced turbulent flow in urban stormwater manholes: Insights into gravity and pressure flow dynamics

Inaccurate estimation of energy loss in manhole structures during precipitation events can weaken the operational performance of urban stormwater drainage systems, affecting the precision of flood risk assessment. The turbulent behavior, energy transfer patterns and their effects on comprehensive drainage performance of manholes under various flow conditions throughout rainfall duration remain unclear. This study utilized Computational Fluid Dynamics (CFD) simulation methods to investigate the mechanisms by which different flow states (gravity flow and pressure flow) and pipeline bending angles (90°, 105°, 120°, 135°, 150° and 180°) influence the turbulent dynamics and energy transfer patterns of stormwater manholes. The predicted results indicate that reducing the bending angle significantly determined the vortex-induced turbulent motion within the manhole interior and downstream pipeline, increasing velocity gradient differences, streamline curvature and vortex motion intensity. This enhanced the turbulent dissipation rate within the computational domain, particularly near the inverted channel of the manhole and the entrance of the downstream pipeline, resulting in higher head loss. Compared to gravity flow, the presence of an additional head under pressure flow conditions could improve the uniformity and stability of internal flow within the manhole, decreasing the corresponding head loss coefficient. Analysis of temporal characteristics reveals distinct energy transfer processes for gravity- and pressure-driven flow as the duration of rainfall increases. These findings provide insights into predicting abnormal vortex-induced turbulent flow in manholes, optimizing urban stormwater drainage network designs, and improving system performance assessment and flood risk evaluation accuracy.