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Entrapped air pockets reduce flood conveying capacity of a surcharged drainage tunnel, and could be released explosively as geysers from vertical dropshafts. Nevertheless, the impact of dropshaft inflow on air pocket expulsion has hitherto not been fully understood. In this work, experiments was performed on a simplified drainage system consisting of a dropshaft with inflow. An air pocket was introduced into the tunnel; its dynamics in the dropshaft was measured with a high speed camera and pressure transducers. A heuristic analysis on the kinematics of the air pocket showed that when the dropshaft inflow is greater than a critical flow dependent on the dropshaft diameter, the increased compression leads to stronger air pocket expulsion, as spewing and/or ejection of air-water mixture. Observed occurrence of the phenomena generally agrees with the prediction. © 2022 American Society of Civil Engineers.

Chlorine is commonly used in disinfection processes in wastewater treatment plants. The chlorine solution is typically dosed by turbulent mixing through multiport jet diffusers. The chlorine reacts with the inorganic and organic nitrogenous compounds at a fast rate (in the order of 1 s or less), and a significant portion of the dosed chlorine can be rapidly consumed by the reactions instead of destroying the pathogens. Despite extensive previous research, the chlorine consumption in the complex sewage flow remains an elusive problem. Field-scale experiments have been carried out to study the chlorine consumption in the turbulent mixing of dense chlorine jets with the primary treated sewage effluent of the Stonecutter's Island Sewage Treatment Works (serving a population of 5.7 million). Based on jet theory and the field tests, it is proposed to discharge the chlorine jets at a location of highest velocity to minimize the contact time of the sewage with high concentration chlorine. Full-scale in-plant experiments demonstrated that significant savings in chlorine can be achieved with optimal jet diffuser design and placement; at nominal applied chlorine dosages of 12 and 20 mg/L in winter and summer, respectively, the chlorine demand can be reduced by up to 30% compared with the conventional dosing design. The relatively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmful impact to the environment, and improves plant operation reliability. The chlorine demands from daily operations are well-correlated with the results of the field-scale models. Predictions of the chlorine demand as a function of applied dosage and temperature were obtained from the collective field data.

Beach water quality forecast models can be useful tools for public health protection. Despite two decades of research, there has been hitherto no comparative field validation of statistical vs mechanistic model forecasts against operational data for marine beaches. This paper reports a novel performance assessment of a three-dimensional (3D) deterministic hydrodynamic model versus a multiple linear regression (MLR) over a wide range of hydro-meteorological and pollution source conditions. The two models are used concurrently to provide continuous daily water quality forecasts for eight marine bathing beaches directly impacted by a major sewage outfall discharging 2.5 million m3/d of chemically enhanced primary treatment effluent. The predicted Escherichia coli concentrations on the beaches located in complex flow environments are studied over a period of significant changes in treatment levels and sewage flows (2011–2018). The performance of both models is found to be superior to the current advisory based purely on past observations. While the 3D model is process-based and the MLR model is data-driven, both models have comparable performance with about 70–80% accuracy. The model forecasts of E. coli concentrations are significantly correlated with field data. In general, MLR models have slightly higher overall accuracy, while the 3D model provides better prediction for the observed exceedances of water quality standard (model sensitivity). The 3D model is however indispensable in addressing issues of emergency response, setting of effluent standards and disinfection dosage optimisation. The two models serve useful complementary roles in a real time beach water quality forecast system for smart environmental management. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.

Beach water quality forecast models can be useful tools for public health protection. Despite two decades of research, there has been hitherto no comparative field validation of statistical vs mechanistic model forecasts against operational data for marine beaches. This paper reports a novel performance assessment of a three-dimensional (3D) deterministic hydrodynamic model versus a multiple linear regression (MLR) over a wide range of hydro-meteorological and pollution source conditions. The two models are used concurrently to provide continuous daily water quality forecasts for eight marine bathing beaches directly impacted by a major sewage outfall discharging 2.5 million m3/d of chemically enhanced primary treatment effluent. The predicted Escherichia coli concentrations on the beaches located in complex flow environments are studied over a period of significant changes in treatment levels and sewage flows (2011–2018). The performance of both models is found to be superior to the current advisory based purely on past observations. While the 3D model is process-based and the MLR model is data-driven, both models have comparable performance with about 70–80% accuracy. The model forecasts of E. coli concentrations are significantly correlated with field data. In general, MLR models have slightly higher overall accuracy, while the 3D model provides better prediction for the observed exceedances of water quality standard (model sensitivity). The 3D model is however indispensable in addressing issues of emergency response, setting of effluent standards and disinfection dosage optimisation. The two models serve useful complementary roles in a real time beach water quality forecast system for smart environmental management. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.

Scroll vortex intakes are commonly used in water supply, drainage and sewerage systems. Water flows into the intake via an eccentric approach channel, forming a swirling vortex flow with a stable air core along the dropshaft. Previous design theories have largely been based on unjustified assumptions on the velocity and pressure distributions of the vortex flow at the intake bellmouth. To clarify these assumptions, a validated three-dimensional computational fluid dynamics model has been developed. Model predictions compare well with independent measurements of flow profile, velocity and air core size. While the vortex flow in the scroll chamber can be described by a free vortex with constant circulation equal to that at the inlet, the tangential velocity and the pressure distribution at the bellmouth outlet have to be approximated by a forced vortex. A revised theory is proposed, providing significant improvement to the prediction of air core sizes and head–discharge relationship. © 2022 International Association for Hydro-Environment Engineering and Research.

In subtropical coastal waters, the explosive growth of phytoplankton under favorable conditions can lead to water discolouration and massive fish kills. Manual field sampling and laboratory analysis of chlorophyll-a concentration (Chl-a) as an indicator to algal biomass, is resources intensive and time consuming, delaying responses to disastrous harmful algal blooms. Cloudy weather often precludes the use of satellite images for water quality and algal bloom monitoring. This study aims at developing an estimator algorithm for quantitative mapping of surface Chl-a for coastal waters, based on surface reflectance measurement from an Unmanned Aerial Vehicle (UAV) with a five-band multispectral camera. The surface reflectance is obtained from calibrated multispectral images which are radiometric-corrected against incoming solar radiation. It is found that Chl-a has an inverse correlation with the Normalized Green-Red Difference Index (NGRDI). A regression estimator model for Chl-a from NGRDI is developed, showing excellent performance for fish farms in coastal waters with different characteristics. The technology is demonstrated for mapping the spatial and temporal variation of Chl-a during an algal bloom, offering a useful complement to traditional field monitoring for fisheries management and emergency response.

Tangential vortex intakes are compact hydraulic structures commonly used in water supply, drainage and sewerage systems to convey water from high to low elevations efficiently. For certain intake design, due to the complex three-dimensional (3D) flow transition, hydraulic jump and shock waves may form. This paper presents an experimental and 3D computational fluid dynamics (CFD) modeling of the flow in a tangential vortex intake with a steep-slope (sloping angle = 45 degrees) tapering section. Swirling velocity field was measured using laser Doppler anemometry (LDA) for discharges with typical flow features. CFD predictions were most encouraging in the good agreement with measured head-discharge relationship, air core size and velocity. It was found that the flow regimes are determined by the control at different sections under different discharges, forming a complex flow transition with an inclined hydraulic jump at the tapering section. While the swirling flow in the dropshaft is highly asymmetrical, the local tangential velocity is similar to that of a stable vortex intake with Rankine vortex behaviour. Flow energy dissipation is caused by the hydraulic jump at the tapering section and the friction loss at the dropshaft. The present study offers comprehensive insights to the design of tangential vortex intake structures.

In drainage and sewerage systems, vertical dropshafts are commonly used to convey water flow from high to low elevations. When the underground tunnel is surcharged, the lower part of the shaft is submerged. The impact of falling water on the free surface in the dropshaft entrains a vast amount of air, leading to the transport or entrapment of air in the tunnel, causing undesirable effects such as the reduction in flow conveyance capacity or air blowback. In this study, a comprehensive series of experiments was performed to study the influence of flow variables on the flow regimes in a submerged dropshaft connected to a horizontal tunnel. The air-water flow in the dropshaft was recorded using high-speed imaging, with air concentration and high-frequency pressure measurements. The experiments showed that the flow in the dropshaft-tunnel system can be classified into four regimes: (I) bubbly column flow, (II) rising slugs, (III) instability and surging, and (IV) net air transport in the tunnel, based on the dropshaft Froude number and the ratio of dropshaft diameter to tunnel diameter. © 2021 American Society of Civil Engineers.

Dredge spoil is commonly disposed in estuarine and coastal waters through submerged pipelines from a barge in the form of concentrated mixture of sediment and water. The discharge resembles a downward dense turbulent plume under the negative buoyancy of the sediment particles. Sediment discharge can increase the turbidity and suspended solid level of coastal water, causing damage the marine ecosystem. It is important to understand the mixing of a sediment plume with the ambient water in order to properly assess the environmental impact of disposal operation. This chapter presents a computational fluid dynamics (CFD) model of a sediment plume in a non-stratified stagnant ambient using the two-phase Eulerian approach. The axisymmetric two-phase continuity and momentum equations are solved with the drag force term accounting for the interaction between phases. The standard k-ε model is used for turbulence closure for sediment-water mixture. The radial turbulent dispersion of particles is modeled by a drift velocity term related to the concentration gradient of the particle phase. The model prediction is validated against experiments of a companion work and independent experimental data, with a wide range of particle sizes (68–1500 μm) and plume sediment volume fraction (maximum of 60%). The model predicted cross-sectional distribution of sediment concentration, plume fluid velocity and the slip velocity of particle to fluid can be well described by Gaussian profiles. The reduction of plume spreading rate with increasing particle size and settling velocity is also well predicted by the model. The CFD model results shed light on the development of a simple integral model for predicting the mixing of sediment plumes.

Lead (Pb) is a toxic metal that can be present in drinking water; exposure to lead (WHO standard C <10 μg/l) can result in learning deficits in children and increased blood pressure in adults. In 2015 a public outcry was caused by high excess lead concentrations found in the drinking water of public rental housing estates in Hong Kong. Similar incidents have been reported elsewhere. Drinking water supply systems in densely populated high-rise buildings consist of complex labyrinths of copper piping fitted with solder joints and brass fixtures that contain lead. The prediction of lead concentration at the consumer tap is most challenging: the lead release mechanism due to galvanic corrosion and the interaction with the diffusion and transport in the pipe flow is largely unknown; the accurate measurement of the low concentrations of concern is also demanding and costly. This paper presents a comprehensive experimental and theoretical investigation of lead contamination and transport in high rise buildings. Systematic tests on full-scale water supply chains dismantled from public rental housing estate buildings are performed. The main lead sources are determined; leaching experiments are performed on the key components to study the Pb leaching rate (Rt) over time. A 3D diffusion model is developed to model the leaching process assuming a constant Pb concentration next to the pipe wall (E0), and the subsequent tap water concentration under flowing condition for a real-life water supply chain is successfully predicted by a numerical solution of the contaminant transport for the first time.

Chlorine is extensively used in disinfection processes in water and wastewater treatment. In the chemically-enhanced primary treatment (CEPT) plant of the Hong Kong Harbour Area Treatment Scheme, high concentration (10 percent) chlorine solution is dosed into the treated sewage flow (2 × 106 m3/d) by jet mixing. Due to the fast reaction of chlorine with organic and inorganic nitrogen compounds, field observations have demonstrated significant loss of chlorine within a short distance or travel time from the dosing point (in the order of 0.1 m or 1 second or less). It is essential to understand the mixing and reaction of chlorine jet with ammonia nitrogen for disinfection dosage optimization. In this paper, an integral reacting chlorine jet model for predicting the changes in free chlorine, chloro-amine and ammonia nitrogen is developed for the first time. The model is validated against measurements in a bench-scale “toy” model experiment of chlorine jet discharging in coflowing ammonia nitrogen solution and CEPT effluent. The results suggest that in ammonia solution, the dosed chlorine predominantly reacts with ammonia to form combined chlorine with negligible chlorine demand. In CEPT effluent, the chlorine demand is mainly due to the preferential oxidation of organic debris by free chlorine. Combined chlorine is then formed due to the reaction of remaining free chlorine with ammonia nitrogen under jet mixing.

Scroll vortex intakes are vortex drop structures commonly used in water supply, drainage and sewerage systems, characterized by a vortex chamber with its wall curling inwards to the dropshaft and a horizontal bottom. The stormwater flows into the intake via an eccentrical approach channel, which imparts vortex motion to the flow, forming a swirling vortex flow with a stable air core through the center of dropshaft. Over past decades, much effort has been devoted to investigating the scroll intake vortex flow, yet the understanding and predictions of the vortex flow is still far from complete due to a lack of detailed investigation on its velocity field and air core characteristics. In this work, a three-dimensional (3D) computational fluid dynamics (CFD) model using the Volume-of-Fluid (VOF) method is used for investigating the complex flow of a scroll vortex intake. The CFD model predictions are validated with detailed flow profile, flow velocity and air core measurements on a physical hydraulic model. It is found that the vortex flow in the scroll chamber resembles a free vortex and the circulation is approximately equal to that at the inlet to the chamber, with a thin bottom boundary layer. For the vortex flow at the bellmouth outlet, the tangential velocity distribution satisfies a Rankine vortex. Furthermore, the vortex flow at the bellmouth outlet possesses a circulation constant which is smaller than that in the chamber.

The explosive growth of phytoplankton under favorable conditions in subtropical coastal waters can lead to water discolouration and massive fish kills. Traditional water quality monitoring relies on manual field sampling and laboratory analysis of chlorophyll-a (Chl-a) concentration, which is resources intensive and time consuming. The cloudy weather of Hong Kong also precludes using satellite images for algal blooms monitoring. This study for the first time demonstrates the use of an Unmanned Aerial Vehicle (UAVs) to quantitatively map surface water Chl-a distribution in coastal waters from a low altitude. An estimation model for Chl-a concentration from visible images taken by a digital camera on a UAV has been developed and validated against one-year field data. The cost-effective and robust technology is able to map the spatial and temporal variations of Chl-a concentration during an algal bloom. The proposed method offers a useful complement to traditional field monitoring for fisheries management. © 2020 Elsevier Ltd

Tangential vortex intakes are compact hydraulic structures commonly used in water supply, drainage and sewerage systems to convey water from high to low elevations efficiently. For certain design of tangential vortex intakes, flow instability can occur in the approach channel and the vortex dropshaft, resulting in undesirable hydraulic jump and shock waves. Due to the complex three-dimensional (3D) flow in the tangential vortex intake, current theoretical models are not sufficiently complete to interpret the flow process reliably. This paper presents an experimental and 3D Computational Fluid Dynamics (CFD) modeling study of an unstable tangential vortex intake flow. The CFD predictions are in good agreement with detailed point velocity and air core size measurements. Despite of the hydraulic jump at the tapering channel, the swirling flow at the vortex drop shaft is similar to that of a stable vortex intake with Rankine vortex behaviour.

Compact intake structures are used for diverting turbulent supercritical flow on steep catchments in the hinterland of the urban area of Hong Kong to an underground flood diversion system through a number of vortex dropshafts. Bottom racks are placed at the entrance of the intake to exclude debris from entering the system. The rack bars interact with the supercritical inflow and creates a highly turbulent air-water mixture. This paper presents a three-dimensional (3D) computational fluid dynamics (CFD) modeling study to predict the complex flow details and air concentration of the bottom rack intake structure, using the volume-of-fluid (VOF) technique. Numerical simulations are conducted with different inflow rates and bottom rack bar shapes. The water depth, velocity and air concentration agree well with experimental measurement. Model results show that the rack interception induces an energy loss and increase the flow depth above the rack. The rack interception also gives rise to a sheet jet beneath the rack and results in air entrainment. In the rack chamber, the flow consists of a wall jet that impinges on a spiral circulation of aerated flow, inducing significant turbulence and air entrainment. The average air concentration in the rack ranges from 20% - 50% and decreases with increasing discharge. The air concentration in the chamber appears to be little affected by the presence of bottom rack or the shape of rack.

The Hong Kong West Drainage Tunnel (HKWDT) system consists of 34 storm water intake structures designed to intercept storm water runoffs in upland catchments. During storm water events, each of these intake structures intercepts and diverts the upstream supercritical flow into a bottom rack chamber connected to a supercritical vortex drop. To satisfy the minimum environmental flow requirement in dry weather, small surface runoffs are intercepted by an inclined barrier across the main channel, and conveyed to the downstream drainage system through a 300mm wide low flow channel along one side of the intake. When the intakes are in operation during rainstorms, observations suggest that the storm water runoffs being conveyed via the low flow channel might be more than originally designed. The objective of the present study is to assess the hydraulic performance of a typical intake structure in HKWDT via three-dimensional computational fluid dynamic (CFD) modelling. Numerical simulations are carried out using the volume-of-fluid (VOF) method in FLUENT software for a range of storm inflow condition. The un-intercepted flow rate are determined from CFD results and compared with experimental results of a 1:12 undistorted model in laboratory. The predicted water level and un-intercepted flow agree well with measurement. It is found that the ratio of un-intercepted flow to the total flow decreases with increasing inflow. The model results show that in typical rainstorm event the supercritical inflow impinges on the barrier and is deflected upwards. Part of the flow spills to low flow channel, resulted in undesirable higher flow to the downstream. In large flow rate scenarios, the effect of the barrier is less significant due to the increase in flow depth.

Geysers are explosive eruptions of air-water mixture from manholes in drainage systems. When the design capacity of urban storm water drainage systems is exceeded during extreme rainfall, rapid inflows into the drainage network can lead to air-water interactions that give rise to geysers—causing damage to the water infrastructure and threatening human lives. Although extensive research has revealed the role of entrapped air in causing large pressure transients in drainage tunnels, the mechanism of geyser formation remains elusive. In this study, an unsteady three-dimensional (3D) computational fluid dynamics (CFD) model is developed to simulate the pressure transients and air-water interactions during geyser events using the volume-of-fluid (VOF) technique. Extensive numerical simulations are conducted to study the air-pocket dynamics caused by release of trapped air from a horizontal tunnel into a vertical riser. Model predictions of the air-water interface in the vertical shaft are in good agreement with laboratory measurements by a high-speed camera; the mechanism for the formation of geysers is elucidated. It is found that during a geyser event, compression of the air pocket in the riser can lead to rapid acceleration of the overlying water column and its expulsion from the riser; the air-pocket pressure is significantly higher than the hydrostatic pressure of the overlying water column, and the velocity is greater than that of a Taylor bubble. Comprehensive numerical modeling has been conducted to study the effect of the vertical shaft diameter, the upstream pressure head and the air-pocket volume on the air-pocket dynamics; the results show that geyser formation is primarily controlled by the riser to tunnel diameter ratio, Dr/DDr/D. 3D CFD simulations have also been carried out for an idealized prototype drainage system; it is shown that the geyser behavior can be characterized by extremely large vertical air velocities that result in dispersed air

A compact bottom rack structure is used for diverting storm water flow on steep catchments in the hinterland of a densely built city into a drainage tunnel through a vortex dropshaft. This study investigates the complex three-dimensional, turbulent and aerated flow of the bottom rack intake structure by comprehensive experiments and three-dimensional Computational Fluid Dynamics (CFD) modeling. Extensive physical model tests were conducted on a 1:9.5 Froude scale model over a wide range of discharges and different rack bar shapes. The water depth, velocity and air concentration were measured. As the rack interception induces an energy loss, the depth of the supercritical flow increases as it passes across the racks. The rack interception also gives rise to a sheet jet beneath it. In the rack chamber, the flow consists of a wall jet that impinges on a spiral circulation of aerated flow, inducing significant turbulence and air entrainment. The average air concentration ranges from 20% to 50% and decreases with increasing discharge. The air concentration in the chamber appears to be little affected by the presence of the bottom racks or the cross-sectional shape of the rack bars. The complex flow features and air concentration distribution in the rack chamber are satisfactorily predicted by the 3D numerical model. © 2018 International Association for Hydro-environment Engineering and Research, Asia Pacific Division

A concentrated 10% chlorine solution (in sodium hypochlorite) with a specific gravity of 1.2 is used for the disinfection of treated primary effluent in the Stonecutters Island Sewage Treatment Works (SCISTW) in Hong Kong. The chlorine solution is injected in the form of coflowing dense jets into a sewage flow of approximately 1.4 million m3/day, with a target dosing rate of 10-20 mg/L (or an initial dilution of 5,000 to 10,000). The large sewage flow to chlorine dosing flow ratio has resulted in insufficient mixing and high chlorine demand as a result of the fast reaction of chlorine with organic and inorganic substances in the sewage. An integral jet model is developed to predict the jet mixing in the initial contact region of chlorine and sewage, accounting for chemical kinetics of the chlorine-sewage reaction. The model is calibrated using field data obtained from experiments using real sewage and chlorine solution in a field-scale physical model inside the sewage treatment works. The mathematical model is used to guide experimentation of various alternatives to reduce the chlorine demand. It is found that under the same chlorine dosage, diluting the concentrated chlorine solution by four times (i.e., containing 2.5% available chlorine) and doubling the number of injection jets can result in the reduction of chlorine demand by 10-15%. The chlorine dosage optimization was supported by extensive field measurements and three-dimensional (3D) computational fluid dynamics (CFD) simulations. © 2017 American Society of Civil Engineers.

Multiport diffusers are used in many environmental applications to distribute a discharge flow uniformly through a number of jet nozzles into the same or another fluid. For example, a sewage diffuser can be designed to achieve approximately equal port discharges along the diffuser to maximize environmental benefit, i.e., approximately the same port discharge to achieve similar dilutions for all ports. When the effluent density is smaller than that of the ambient receiving water (i.e., positively buoyant), the energy head increases with elevation (in the inshore direction); as the total diffuser flow varies, the relative port discharge distribution remains similar. However, for dense (negatively buoyant) discharges, this may not be the case. This paper presents an experimental and theoretical study of the internal hydraulics of a diffuser used for dosing sodium hypochlorite solution (specific gravity 1.2) for sewage disinfection. Because of the significant density difference, the jet discharge undergoes a flow switching (reversal of relative discharge distribution) as the total discharge decreases. The predictions were validated with full scale experiments carried out with water jets into air; the design implications for plant operation are discussed. The findings are of general interest to the design of diffuser manifolds for dense effluents. (C) 2017 American Society of Civil Engineers.

Geysers are explosive eruptions of air-water mixture from manholes in drainage systems. When the design capacity of storm water drainage systems is exceeded during heavy rainfall, rapid inflows can lead to air-water interactions that give rise to geysers, causing damages and threatening human lives. Over the past two decades, the dynamics of air pockets in vertical shafts and horizontal pipes has been extensively studied. Although the role of entrapped air in causing large pressure transients is revealed, the mechanism of geyser formation remains elusive primarily because of the lack of detailed observations. A comprehensive series of experiments has been performed on a physical model of a simplified drainage system, which consists of a vertical riser and a horizontal pipe with diameter D connected to a constant head tank. The system is filled with water; an air pocket is then released into the horizontal pipe and the trajectories of the air pockets in horizontal pipe and vertical riser are measured by videos and a high speed camera. Pressures are measured using pressure transducers near the pipe end and at the bottom of the riser. Parameters considered include riser diameter (Dr), upstream head (H0), and initial air pocket volume (Vair ). Without an external pressure head, the air pocket migration in a water-filled vertical riser is found to be similar to a slug flow. The rising velocity of the air pocket relative to the free surface (vnet) is nearly constant and close to the speed of a Taylor bubble; no geyser is observed. When an external pressure head is applied to the horizontal pipe, two types of flow are observed. For large riser diameter and small air volume, the rise of the air pocket in the vertical riser resembles a Taylor bubble, with an air pressure equal to the hydrostatic pressure of the water column above. The air breaks within the riser and no geyser is observed. For small riser diameter and large air volume, the horizontal air pocket shoots past th

Sediment-laden turbulent buoyant jets are commonly found in natural and engineered environments. Examples include volcanic eruptions, deep sea hydrothermal vents, discharge of partially-treated wastewater, and dredging operations. A horizontal sediment buoyant jet is characterized by a horizontal momentum jet, rising plume, and a horizontal surface gravity current. The settling of particles from a sediment buoyant jet depends on the complex interaction of particles with turbulent fluctuations and the mean flow-in particular particle re-entrainment due to the external irrotational flow induced by the jet. A three-dimensional (3D) stochastic particle tracking model is proposed to predict the sedimentation from an arbitrarily inclined sediment-laden buoyant jet in a stagnant ambient. The simple model predicts the entire 3D jet flow field via a coupling of semianalytical models for the mean flow as well as turbulent fluctuations. The mean flow for the turbulent buoyant jet and the surface gravity current are determined using well-validated integral models, while the external jet-induced irrotational flow field is computed by a distribution of point sinks along the jet trajectory. Turbulent velocity fluctuations are modeled by a Lagrangian particle velocity autocorrelation function that mimics the trapping and loitering of sediment particles in turbulent eddies. The turbulent fluctuations are stochastically generated from a self-similar profile of the turbulent kinetic energy derived from computational fluid dynamics (CFD) solution of jets and plumes. Predictions of the particle tracking model are in excellent agreement with experimental data over the entire jet-plume regime for a wide range of particle sizes (58-621 mu m) and properties. The model provides physical insight in delineating the modes of sediment fall out. The fraction of sediment mass fall-out from the jet lower boundary is found to be a function of the ratio between the jet momentum-buoyancy length scale

Integral models of single-phase plumes are often closed using the entrainment hypothesis, which assumes the entrainment velocity is proportional to a characteristic plume velocity, but the corresponding theoretical development for modeling particle plumes has received less attention. In this paper an integral model is developed by proposing a new spreading hypothesis for particle plumes, in which the fluid phase spreading rate is taken as that of a single-phase plume, whereas the particle phase spreading rate is a function of the particle phase and fluid phase average vertical velocity. The change in momentum flux of the particle and fluid phases are calculated by considering the cross-sectional integrated buoyant and drag forces acting on the particles, and their reaction force acting on the fluid. Mixing characteristics of the particle plume can then be determined. The model was validated by laboratory particle plume experiments conducted for various particle sizes and initial plume-to-particle-settling velocity ratios, as well as experiments in the literature. Centerline particle and fluid velocities, as well as particle concentrations were generally well predicted. The inferred entrainment coefficient for a particle plume and the importance of lift force are also discussed. © 2016 American Society of Civil Engineers.

Geysers are explosive eruptions of air-water mixture from manholes in drainage systems. Due to urbanization and climate change, the design capacity of storm water drainage systems in many cities is often exceeded during extreme rainfall; rapid inflows into the drainage network can lead to air-water interactions that give rise to geysers - causing damages to the water infrastructure and threatening human lives. Although extensive research has revealed the role of entrapped air in causing large pressure transients in drainage tunnels, the mechanism of geyser formation remains elusive mainly due to the lack of detailed observations. In this study, an unsteady 3D computational fluid dynamics model is developed to simulate the pressure transients and air-water interactions during geyser events using the Volume-ofFluid (VOF) technique. Model predictions of air-water interface in the vertical shaft are in good agreement with measurements by high-speed camera; the mechanism for the formation of geysers is elucidated.

Concentrated dense sodium hypochlorite (NaOCl) solution is commonly used in sewage treatment plants for disinfection of treated sewage. In view of the importance of sewage disinfection to the environmental impact, it is essential to understand the mixing and reaction of chlorine with sewage for disinfection dosage optimization. In this study, a laboratory experiment of dense chlorine jet discharging into stationary treated sewage is conducted. To ensure safe operation of the experiment, the set-up is first tested with sugar solution with density similar to that of dense chlorine solution. Then, experiments of dense NaOCl solution discharging into tap water and treated sewage are conducted. Numerical models are also developed based on the OpenFOAM computational fluid dynamics code and the Distributed Entrainment Sink Approach (DESA). The predicted ambient total residual chlorine (TRC) profiles and dilutions at different jet cross-sections agree well with experimental observations. Measurement of ambient TRC shows that the chlorine consumption is rapid within the first three minutes of NaOCl discharge. Chemical reaction slows down as TRC accumulates in the tank.

Sediment-laden turbulent buoyant jets are commonly found in natural and engineered environments. Examples include volcanic eruptions, deep sea hydrothermal vents, discharge of partially-treated wastewater, and dredging operations. A horizontal sediment buoyant jet is characterized by a horizontal momentum jet, rising plume, and a horizontal surface gravity current. The settling of particles from a sediment buoyant jet depends on the complex interaction of particles with turbulent fluctuations and the mean flow-in particular particle re-entrainment due to the external irrotational flow induced by the jet. A three-dimensional (3D) stochastic particle tracking model is proposed to predict the sedimentation from an arbitrarily inclined sediment-laden buoyant jet in a stagnant ambient. The simple model predicts the entire 3D jet flow field via a coupling of semianalytical models for the mean flow as well as turbulent fluctuations. The mean flow for the turbulent buoyant jet and the surface gravity current are determined using well-validated integral models, while the external jet-induced irrotational flow field is computed by a distribution of point sinks along the jet trajectory. Turbulent velocity fluctuations are modeled by a Lagrangian particle velocity autocorrelation function that mimics the trapping and loitering of sediment particles in turbulent eddies. The turbulent fluctuations are stochastically generated from a self-similar profile of the turbulent kinetic energy derived from computational fluid dynamics (CFD) solution of jets and plumes. Predictions of the particle tracking model are in excellent agreement with experimental data over the entire jet-plume regime for a wide range of particle sizes (58-621 mu m) and properties. The model provides physical insight in delineating the modes of sediment fall out. The fraction of sediment mass fall-out from the jet lower boundary is found to be a function of the ratio between the jet momentum-buoyancy length scale

To protect the water quality of nearby bathing beaches, chlorination disinfection has been provided to the treated sewage in a major sewage treatment works in Hong Kong. A ten-percent sodium hypochlorite solution (specific gravity 1.2) is injected in the form of multiple dense jets (from dosing pipes at two vertical levels) into the sewage cross flow in a flow distribution chamber (FDC). Bench scale experiments show that higher chlorine concentrations will lead to significantly higher chlorine demand. Thus, the mixing and transport of chlorine in the FDC plays an important role in the disinfection efficiency and determining the effluent quality. It is essential to achieve rapid mixing of chlorine with the sewage in order to achieve maximum disinfection efficiency in the chlorine contact culverts before discharge to the receiving water via a tunnelled sea outfall. The diurnal variation in the sewage inflow (8 - 22 m(3)/s) and tide level results in variation between pressurized and free surface flow in the hydraulic system. Furthermore, to maintain a minimum water depth, an oblique weir is installed across the middle section of the chamber-leading to complex 3D flow patterns and uneven distribution of flow and chlorine concentration downstream of the FDC. Theoretical and experimental modelling of the internal hydraulics of the dosing unit revealed a curious distribution of dense jet flows. Due to the large relative density difference between chlorine solution and sewage, the relative dosing flow between the lower and upper pipes may undergo a

A vertically upward sediment-laden buoyant jet is an important flow phenomenon in geophysical science and engineering as it resembles the dynamics of volcanic eruptions, deep sea hydrothermal vents and wastewater discharge. In an upward buoyant jet, sediment particles are carried by the rising jet fluid until they reach the maximum height of rise or the water surface. Sediment fall-out occurs at the jet edge and/or from the radial spreading current and re-entrainment of sediment occurs. For the first time, a three-dimensional stochastic Lagrangian particle tracking model for predicting the deposition and dynamics of a vertical sediment-laden buoyant jet is developed. The model solves the governing equation of particle motion in a predetermined fluid velocity field superimposed with stochastic turbulent fluctuations. The three flow regimes involved in a buoyant jet - the turbulent jet flow, jet entrainment-induced external flow and surface spreading current, are modeled using validated semi-analytical methods: (1) the jet mean flow velocity is determined using a jet integral model; (2) jet-induced external irrotational flow field is computed by the point sink approach; (3) surface spreading current is predicted using an integral modd accounting for the interfacial shear. Turbulent velocity fluctuations are modelled by an autocorrelation function that mimics the trapping of sediment particles in turbulent eddies. Root-mean-square (RMS) turbulence velocity and turbulent time scale are estimated from best-fitted self-similar profiles of turbulent kinetic energy and dissipation rate derived from a computational fluid dynamics solution of vertical buoyant jets. Model prediction shows excellent agreement with previous experimental data of bottom sediment deposition. Model prediction reveals the increase in sediment concentration due to sediment re-entrainment which has important significance for the dynamics of vertical sediment jets.

We report in this paper the first study on particle plumes in a turbulent background. A jet array in which each jet was programmed to turn on and off randomly was used to generate a roughly isotropic and homogeneous turbulence. Particles of sizes range from 0.0675 mm to 0.725 mm were discharged continuously from a submerged hourglass to produce steady plume flows into the turbulent background. The changes in plume mixing characteristics (comparing to plumes in quiescent ambient) due to the effect of turbulence is reported. The breakdown of some plumes due to the turbulence were observed and a criteria for the breaking down of plume is presented.

Bathing beaches are invaluable marine resources for Hong Kong and the protection of public health is of utmost importance. The Harbour Area Treatment Scheme (HATS) is the sewage collection and treatment scheme that serves the urban areas of Hong Kong on both sides of Victoria Harbour. The sewage receives Chemically Enhanced Primary Treatment (CEPT), and the treated sewage flow (1.4×106 m 3 /d) is discharged through a 1.2km outfall in the western Victoria Harbour. Since March 2010 the additional provision of chlorination to the treated effluent has brought significant water quality improvements to the neighbouring Tsuen Wan beaches. The WATERMAN Real-Time Coastal Water Quality Management System has been applied to formulate a disinfection dosage control strategy. A comprehensive investigation on the relationship between beach water quality, bacterial standard of effluent discharge and environmental conditions has been carried out using the extensively validated WATERMAN 3D hydrodynamic model. Detailed numerical simulations for an entire average wet year demonstrate that the Water Quality Objectives (WQO, E.coli < 180 count/100mL) can be met in the coastal receiving waters for an effluent E.coli standard of 200,000 counts/100mL for HATS Stage 2A (Q = 1.8×106 m 3 /d). The setting of an appropriate discharge standard will help to optimize the chlorine dosage, reduce energy consumption and operation cost whilst preserving the protection of the water quality of beaches, seawater intakes and other sensitive receivers. The WATERMAN system has also been applied for emergency response in pollution accidents and development of operational strategies.