Characterization of unsteady multiphase flows in stormwater management applications
Type of DegreePhD Dissertation
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Intense rain events create many issues in urban areas, including flooding, overflows in stormwater systems, sediment pollution, etc. In many circumstances, flows are unsteady and characterized by multiphase flow conditions, which may lead to many difficulties when trying to understand flow characteristics and solve eventual operational problems. To understand the flow characteristics under such conditions, computational fluid dynamics (CFD) simulations, as well as field-scale experiments, were performed in this research. Uncontrolled air pockets released from water-filled shafts can lead to geysering in stormwater systems. Such occurrences are deleterious from a public health and environmental standpoint and can cause property and structural damage. Causes, frequency, magnitude, and location of geysering events remain poorly understood, and pose practical difficulties for the design of dropshafts geometries that are less likely to experience such events. This work presents numerical investigations on air-related geysers that aims to gain insights on the mechanisms of entrapped air pocket releases and water displacement in vertical shafts. A CFD model was calibrated with experimental data and then was subsequently used in a larger geometry that allowed for the evaluation of air pocket release kinematics for a wider range of conditions. Among various findings, it was shown that water displacement was linked to entrapped air pocket volumes, initial water level in shafts and shaft diameter. In worst case conditions the displacement of free surface reached over 300% of the initial water level in the shaft. A retrofit strategy for vertical shaft geometries is proposed and evaluated with a CFD model for geyser mitigation. Manhole cover displacement is also a hazardous operational issue that may occur in stormwater systems undergoing rapid filling during intense rain events. In various instances, the water free surface within vertical shafts can change rapidly. It is possible that the air located in the headspace of manholes will pressurize when the manhole is insufficiently ventilated. Air pressurization or direct water impact can lead to the displacement of manhole covers, with obvious impacts to the safety of pedestrians and vehicular traffic. Yet, investigations on this topic have been limited. This work presents a study on the conditions for manhole cover displacement in shafts undergoing inertial oscillations or experiencing sudden release of entrapped air pockets. Numerical results showed that pressures associated with the sudden release of air pockets have much greater potential to cause manhole cover displacements when compared to air pressurization created by inertial oscillations within shafts. It is expected that results from this work could help mitigate such hazardous conditions. Another application regarding unsteady multiphase flows is on flows in sediment basins. Sediment basins provide quiescent conditions that enable settling of fine particles present in runoff, mitigating environmental impacts created by excessive sediment discharges. The design of sediment basins is mostly based on empirically-based recommendations. Yet, details of the flows and settling conditions during filling and dewatering processes in sediment basins are not fully understood. The present work collected experimental data of the turbidity distribution and variation, and performed a particle size characterization in a large-scale test sediment basin to evaluate the performance of various basin treatment configurations. The result indicated that the Lamella High Rate Settler treatment with the longitudinal flow direction configuration presented the highest relative decrease in turbidity values. It also presented a more pronounced drop in turbidity between the basin inlet and outlet regions over a sequence of runs where sediments were not cleaned in the basin. The research results indicate that the small-scale high-rate settler (SSHRS) combined with the use of flocculant (PAM) can significantly reduce the turbidity of water discharged through skimmer. A series of CFD simulations were performed to assess the flow conditions for different basin length-width aspect ratios, as well as the effect of porous baffles on sediment basin flows. One outcome of this work was a model that can be used to evaluate flow conditions in sediment basins with porous baffles during filling stages. In addition to the settling process in sediment basins, the settled particles are susceptible to undesired resuspension if, during dewatering of the basin, new flows are admitted and create an increase in velocities, shear forces, and turbulence near the basin bottom. A numerical study was performed to evaluate the benefits of confinement cells as a lining strategy for the bottom of sediment basins. CFD modeling was used to determine which geometries of confinement cells and flow conditions were more likely to succeed in decreasing turbulence and shear forces within confinement cells. Through a comparison between experimental turbidity results and CFD modeling results, it was found a significant decrease in effluent turbidity was linked to the flow patterns in which two circulation zones appeared within the cells. In summary, the present research mainly explores the use of CFD, as well as field experiments, to describe flows in the application of stormwater management. The major contributions include: 1) to understand the flow characteristics of air-water interactions in stormwater systems undergoing rapid filling and provide guidelines for designers on solving relevant problems associated with air-water flows; 2) to test the performance of various configurations of sediment basins and confinement cell systems in reducing water turbidity and to develop CFD models that aid in the description of flow conditions.