Marine Wastewater Discharges from Multiport Diffusers. IV: Stratified Flowing Water

Experiments on the near-field mixing of wastewater discharged from multiport diffusers into stratified flowing waters are reported for conditions typical of actual ocean sewage outfalls. Dilutions were measured by a newly developed three-dimensional laser-induced fluorescence system combined with refractive-index matching and by a microconductivity probe. The plume dynamics are complex. Depending on port spacing, plumes discharged from the upstream diffuser side may merge first with themselves, and then with the plumes discharged from the downstream side. Or the plumes that are horizontally opposed may first merge, followed by lateral merging. In all cases, however, the wastefield eventually becomes laterally homogeneous. The results are analyzed in terms of line or point-source parameters and it is found that they can be predicted by assuming the discharge to be a line plume when s/lb<1.0 and as point plumes for s/lb ≥ 6.0. Semiempirical equations to predict the near-field dilution, near-field length, and plume rise height are presented.     

Marine Wastewater Discharges from Multiport Diffusers. I: Unstratified Stationary Water

Laboratory experiments on the near-field mixing of buoyant plumes discharged from multiport diffusers into unstratified stationary water are reported. Dilution was measured by a newly developed three-dimensional laser-induced fluorescence system and a microconductivity probe. Significant additional mixing (and dilution) occurs beyond the point where the plume impacts the water surface. This mixing ceases when the turbulence generated by the plumes collapses in the surface spreading layer. The port spacing, s, was varied through a range encompassing line to point source conditions. In all cases, the concentration distribution in the surface layer eventually becomes laterally uniform. Measurements of the near-field dilution, length, and layer thickness, and semiempirical equations to predict them are presented. The discharge behaves as a line plume when s/H ? 0.3, and as a point plume when s/H ≥ 1.0. The additional near-field mixing for a point plume is much greater than for a line plume. Basing diffuser design on near-field dilution rather than impact-point dilution allows the use of far fewer ports, or risers, with considerable potential cost savings, particularly for tunneled outfalls.     

Marine Wastewater Discharges from Multiport Diffusers. III: Stratified Stationary Water

Experiments on the near-field mixing of wastewater discharged from multiport diffusers into a stationary density-stratified environment are reported for conditions typical of ocean sewage outfalls. Dilutions were measured with a microconductivity probe in which the test tank was stratified with a nonconducting solution; geometrical characteristics such as rise height and layer thickness were measured by planar laser-induced fluorescence in which the test tank was stratified with refractive-index matched fluids. The port spacing was varied over a wide range encompassing rapidly merging to nonmerging plumes. The end of the near-field occurs at a distance of order one plume rise height from the diffuser. At this point the turbulence and mixing induced by the discharge is effectively damped by the ambient stratification, and little near-field mixing occurs beyond it. The results were analyzed in terms of line and point-source parameters to deduce the ranges of port spacings for which line and point plume solutions apply. Equations to predict the near-field characteristics are presented.     

Experiments on Marine Wastewater Diffusers with Multiport Rosettes

The dispersion capabilities of two types of marine wastewater diffusers, one with risers containing eight radial ports (rosette) and the other with the ports uniformly distributed, were directly compared in laboratory experiments for three scenarios: stationary and flowing unstratified, and flowing stratified. Tracer concentration fields were measured with a three-dimensional laser-induced fluorescence system. The dispersion capabilities, particularly dilution, of the two diffuser types were similar. The primary differences occurred with no current where the rosette diffuser plumes were bent inwards, causing a dynamic interaction. This resulted in a small reduction in near field dilution, an increase in the spreading layer thickness, and a decrease of the near field length. These differences were virtually eliminated by a flowing current when the eight individual plumes from a rosette first merged with themselves and then with those from neighboring risers to become laterally quite uniform at the end of the near field. The major near field characteristics were primarily determined by the buoyancy flux per unit length. Temporal concentration fluctuations were high close to the diffuser and then decreased and leveled off near the end of the near field where the discharge-induced turbulence collapses. The fluctuations do not go to zero, however, owing to remnants of previous fluctuations.     

Sensitivity Analysis and Comparative Performance of Outfalls with Single Buoyant Plumes

This paper presents a new approach to analyze the performance of outfalls with single buoyant plumes in flowing ambient sea water, taking into account all three dilution mechanisms: the initial, the dispersion, and the effective dilution due to the decay of nonconservative substances. Simultaneous consideration of all dilution mechanisms, study of their functional relationships, sensitivity analysis of outfall behavior, and graphical presentation of the results, allows recognition of patterns of practical importance that remain otherwise obscure. The generated graphs afford an overview of the relative performance of outfalls with a single port and perpendicular line diffusers over a broad range of operating conditions, and portray the sensitivity of the single-port outfall behavior. The results show that, contrary to common belief, outfalls with a single port outperform those with perpendicular line diffusers over a broad range of operating conditions. They also show that higher rather than lower current speed is the critical factor in the design of single-port outfalls. These findings affect the construction cost of outfall systems and bear a significant impact on the sizing of new outfalls as well as on the performance evaluation of existing ones.     

Mixing in Stratified Jets

Experiments were performed to measure the mixing characteristics of turbulent momentum jets discharged horizontally into a linearly-stratified, stationary environment. These flows can occur when sewage is discharged into water bodies such as lakes. The centerline dilution was found to follow the results for an unstratified jet up to the point where the jet begins to collapse under the influence of the stratification. The distance at which this occurs is slightly longer than that reported previously from visual observations. The dilution then continues to increase with distance, but more slowly. The results are interpreted in terms of stratified turbulence collapse, and a model is proposed for the initial and final collapse of the turbulence in the jet. Implications for mixing zone models are discussed.     

Buoyant Surface Discharges into Water Bodies. I: Flow Classification and Prediction Methodology

Buoyant surface discharges into ambient water bodies can exhibit multiple complex flow processes, which cover the spatial range from the near field with initial jet mixing to the far field with passive ambient diffusion. Multiple flow phenomena can occur, such as buoyant collapse motions, bottom attachment, deflection by the ambient current, and dynamic shoreline interaction, in the near field, and lateral and/or upstream spreading motions and turbulent diffusion processes, in the far field. Efficient and reliable predictive techniques covering the whole range of these processes are needed for the design and prediction of wastewater effluents that are subject to water quality regulations that can apply in either near and/or far field. A new comprehensive classification framework distinguishes among ten hydrodynamically distinct flow classes within four major flow categories: free jets, shoreline-attached jets, wall jets, and upstream intruding plumes. A prediction methodology for these discharges has been presented that covers the entire spatial domain from the near to the far field. It is based on the linkage of separate predictive modules in form of the expert system CORMIX3. These hydrodynamic modules are implemented by specific flow protocols and transition criteria determine their spatial extent. The methodology, corroborated by numerous detailed laboratory and some field data sources, constitutes a simple and efficient, yet accurate and robust, tool with few data requirements for surface discharge design and mixing analysis.     

Field Observations of Ipanema Beach Outfall

Field observations of the Ipanema Beach, Rio de Janeiro, Brazil, ocean sewage outfall were obtained by adding dye tracers to the effluent and simultaneously measuring oceanographic conditions. Four experiments were performed, two during unstratified conditions, and two during stratified conditions. When stratified, the plume was trapped below the thermocline with low dilution, around 35 to 1, when unstratified, the plume surfaced and the dilution increased to more than 100 to 1. The results were compared with predictions of some commonly used near field plume mathematical models: UM3, RSB, and CORMIX. With suitable assumptions, all the models reasonably predict near field dilution. RSB predicts near field results directly; for UM3 and CORMIX, it was assumed that the end of the near field occurs when the plume reaches its terminal rise height or impacts the free surface. Different assumptions about the shape of the density profiles in CORMIX resulted in widely differing predictions of dilution. While the gross properties of the plume can be reasonably predicted by plume models, there remain many aspects which cannot be, particularly the patchy nature of the wastefield that has been observed here and in other field tests.     

Improved Discharge Configurations for Brine Effluents from Desalination Plants

Sea water desalination plants discharge a concentrated brine effluent into coastal waters. Modern, large capacity plants require submerged discharges, in the form of a negatively buoyant jet, that ensure a high dilution in order to minimize harmful impacts on the marine environment. Existing design practice favors a steep discharge angle of 60° above horizontal, a practice based on limited and outdated laboratory data for dilutions at the level of maximum rise. Examination of more recent laboratory data and the parametric application of a jet integral model suggest that flatter discharge angles of about 30–45° above horizontal may have considerable design advantages. These relate to better dilution levels at the impingement location, especially if bottom slope and port height are taken into account, there is better offshore transport of the mixed effluent during weak ambient current conditions, and there is the ability to locate in more shallow water near shore.     

Modeling Mamala Bay Outfall Plumes.?I: Near Field

The near-field behavior of the Sand Island, Hawaii, ocean outfall plume was modeled. The model, a modified version of the Environmental Protection Agency Roberts-Snyder-Baumgartner model, used as input data simultaneous measurements through the water column of currents obtained from Acoustic Doppler Current Profilers and density profiles obtained from thermistor strings. More than 20,000 simulations were run for a modeling period of almost one year, and frequency distributions of plume characteristics were obtained. The currents and density stratification change widely and rapidly, resulting in extreme variability in plume behavior. Rise height was predicted to vary from deeply submerged to surfacing, and near-field dilution was predicted to vary from around 100 to several thousands within a few hours. The length of the near field, or hydrodynamic mixing zone, also varies considerably, so that a fixed regulatory mixing zone may sometimes encompass all of the near field and some of the far field and sometimes only part of the near field. The combination of oceanographic data with suitable mathematical models represents a significant improvement in our ability to predict the statistical variability of ocean outfall plume behavior.     

Mixing of a Rosette Jet Group in a Crossflow

Partially treated wastewater is often discharged into coastal waters through an outfall diffuser fitted with clustered ports on risers. On each riser the effluent is discharged through two to eight ports arranged circumferentially, in the form of a rosette-shaped buoyant jet group. The near field mixing of such a jet group in a tidal flow is determined by the merging and interaction of coflowing, oblique-flowing, cross-flowing, and counterflowing jets. Despite numerous studies, a general predictive method for such complex jet groups has not been reported; ocean outfall design is often based on comprehensive physical model experiments. The mixing of merging nonbuoyant and buoyant jets issuing from a rosette outfall riser into an ambient current is studied experimentally by using the laser-induced fluorescence technique. Detailed cross-sectional measurements of the scalar concentration field downstream of the bent-over jets are made. The trajectories of multiple and individual jets discharging at various angles are measured. For typical outfall designs, the dynamic interaction of adjacent jets is found to be negligible. The average dilution of the jet group can be predicted by accounting for jet merging and plume overlapping. Theoretical predictions using the Lagrangian VISJET model are in excellent agreement with the experimental data and also results of previous studies in a stratified crossflow. The model correctly predicts the changes in near field dilution as a function of the number of nozzles on a riser, or the number of risers on a diffuser, and helps to resolve observed anomalies in previous studies.     

Performance of Tee Diffusers in Shallow Water with Crossflow

The dilution and plume trajectory of the tee diffuser has been investigated via the collection of experimental data for a wide range of ambient current conditions. A new dilution equation in which the stagnation effect between ambient current and diffuser discharge is assumed to be a function of the ratio of the ambient momentum to the discharge momentum, mr, is proposed modifying the conventional theory of Adams that significantly underpredicts mixing for large mr. A simple equation for the plume trajectory including the dependency of the momentum ratio is also derived by dimensional analysis. Experimental results on the near field dilution show that when mr < 1 the dilution decreases with mr, whereas when mr > 1 it increases with increasing mr, and approaches the stagnant water dilution for very large values of mr. The equation is applied to aid the preliminary design of a diffuser discharging heated water from a power station in Korea.     

Pollutant Transport and Mixing Zone Simulation of Sediment Density Currents

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in “flow lane” or “open water” disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.     

Distributed Entrainment Sink Approach for Modeling Mixing and Transport in the Intermediate Field

In densely populated coastal cities in Asia, wastewater outfalls are often located not far from sensitive areas such as beaches or shellfisheries. The impact and risk assessment of effluent discharges poses particular technical challenges, as pollutant concentration needs to be accurately predicted both in the near field and intermediate field. The active mixing close to the discharge can be modeled by proven plume models, while the fate and transport far beyond the mixing zone can be well-predicted by three-dimensional (3D) circulation models based on the hydrostatic pressure approximation. These models are usually applied separately with essentially one-way coupling; the action of the plume mixing on the external flow is neglected. Important phenomena such as surface buoyant spread or source-induced changes in ambient stratification cannot be satisfactorily addressed by such an approach. A Distributed Entrainment Sink Approach is proposed to model effluent mixing and transport in the intermediate field by dynamic coupling of a 3D far field shallow water circulation model with a Lagrangian near-field plume model. The action of the plume on the surrounding flow is modeled by a distribution of sinks along the plume trajectory and an equivalent diluted source flow at the predicted terminal height of rise. In this way, a two-way dynamic link can be established at grid cell level between the near and far-field models. The method is demonstrated for a number of complex flows including the interaction of a confined rising plume with ambient stratification, and the mixing of a line plume in cross flow. Numerical predictions are in excellent agreement with basic laboratory data. The general method can be readily incorporated in existing circulation models to yield accurate predictions of mixing and transport in the intermediate/far field.     

Enhanced Surface Cooling as an Alternative for Thermal Discharges

Excess heat is an unavoidable by-product of electricity generation from fossil and nuclear fuels. In most cases, excess heat is transferred to a cooling water stream and discharged to a local receiving water body, or processed through on-site cooling towers. In many cases existing discharges are potentially responsible for significant ecological impacts, and regulatory authorities are mandating the construction of cooling towers, often at significant expense. Most existing cooling water discharges are designed to reduce excess temperatures through rapid dilution. Enhanced surface cooling is an alternative approach which involves the development of a thin surface plume, while limiting mixing of the discharge with ambient waters. This process encourages rapid transfer of heat to the atmosphere while limiting impacts to sensitive benthic environments and most of the volume of the receiving water body. This discharge approach may be particularly effective for receiving water bodies which have limited natural flushing, such as enclosed bays, estuaries, reservoirs and some river environments. A preliminary case study of a thermal discharge into Mt. Hope Bay (Massachusetts/Rhode Island) is discussed.     

Prediction of Near Field Plume Characteristics Using Far Field Circulation Model

3-D numerical models are being used more commonly to predict changes in coastal water quality associated with point discharges such as sewage outfalls. Because these “far field” models use grid sizes which are orders of magnitude larger than the scale of near field entrainment processes, it is of interest to compare their predictive capability with that of initial mixing models and to identify ways in which the two model types can be coupled. Comparisons between the 3-D circulation model ECOMsi and the Environmental Protection Agency's mixing model RSB suggest that the former does a reasonable job predicting plume trap height and volumetric dilution but often overpredicts plume width. Results are sensitive to source representation and parameterization of horizontal and vertical diffusion. The success results from the fact that initial dilution is governed in part by gravitational exchange flow (a large-scale phenomenon that can be resolved in a far field model) in addition to plume entrainment (which is clearly subgrid scale), as well as the self-regulating relationship between plume trap height and initial dilution. Overprediction of plume width is attributed to numerical diffusive effects. Several procedures for improving predictions by coupling near and far field models are explored, ranging from the use of the near field model to dynamically adjust far field mixing parameters, so that the far field model simulates the correct trap height, to simply using the near field model to assign source location and dimension for the far field.     

Behavior of Nonbuoyant Jets in a Wave Environment

This paper presents experimental results of a turbulent neutrally buoyant jet vertically discharged in a stagnant ambient and of the same jet discharged in a flow field of regular waves, in the intermediate range between deep and shallow water. A number of such studies have been performed on this configuration in the past, but mostly limited to concentration (dilution) measurements. Thus one could argue the need for a detailed analysis of this problem with modern instrumentation. This paper will address the deficiency and will present the results of a new experimental study into wave-induced mixing. Velocity jet components were measured with a backscatter, two-component four-beam laser Doppler anemometer system. For the cases of jets with waves, ensemble-averaged velocities were obtained by phase averaging the measured signals separated by the wave period over about 500 waves. The main results of the present study indicate the following: (1) the entrained flow is higher in the case of jet with waves; (2) oscillating velocity components cannot be described by classic wave motion theories; (3) comparison of the root-mean square of turbulent velocity components indicates the effect of wave presence; (4) in the case of jet-wave interaction, the reduction of the jet momentum shown in literature can be explained by the presence of the turbulent Reynolds normal stresses (which are not as small as in the case of stagnant ambient and, consequently, cannot be ignored) and the wave Reynolds normal stresses; and (5) the analysis of the wave Reynolds shear stresses shows that it is not always right to apply a wave motion theory to obtain the statistical contribution of the wave field; a conclusion particularly interesting in order to enable the improvement of mathematical models present in literature.     

Double-Plume Integral Models for Near-Field Mixing in Multiphase Plumes

We present a generalized integral model for multiphase plumes in stratified ambient conditions based on the double-plume approach, where the plume is composed of a rising, multiphase core plume surrounded by a counterflowing outer ring plume of dense fluid. The generalized model captures as limiting cases the current approaches in the literature, including two-fluid and mixed-fluid equations, continuous and discrete detrainment, dispersed-phase mass transfer, and two models for entrainment in the counterflow region. These modeling approaches are compared and validated against both laboratory and field-scale data. In unstratified conditions, all model formulations perform equally well. In stratification, entrainment in the counterflow region is best represented by correlation to the inner plume velocity instead of the difference between the inner and outer plume velocities. The vertical distribution of the exchange between the inner and outer plumes in the models differs from that measured in the prototype due to enhanced entrainment at the detrainment zone and forced entrainment from the collapsing intrusion layer. Nonetheless, the models predict well the length scales and volume fluxes at the detrainment zone and intrusion layer. Applications are demonstrated for reservoir air bubble plumes. The mass transfer and near-field mixing in the double-plume integral model prove sufficiently accurate to predict the depth of maximum plume rise (both the locations of total dissolution of the bubbles and the maximum height of the decelerating plume) and the volume flux, dissolved constituent mass flux, and trap height of the intrusion.     

Physical Simulation of Nozzle Electromagnetic Brake in Twin‐Roll Strip Casting

This paper presents the Nozzle Electromagnetic Brake (N‐EMBR) technology for twin‐roll strip continuous casting. N‐EMBR consists of imposing a stationary magnetic field coupled with direct current inside the nozzle to control the flow and suppress the free surface fluctuation. A low melting point metal model was set up to examine the magnetic field and additional current effect on the velocity near the meniscus and free surface fluctuation. The experimental results showed that the velocity near the meniscus, the amplitude and main frequency of the free surface fluctuation were all decreased with the N‐EMBR technology. It was found that the N‐EMBR technique can be applied successfully in twin‐roll strip casting to suppress the flow near the meniscus and free fluctuation.     

Surface Wave Forces Acting on Submerged Logs

Drag forces exerted on cylindrical simulated logs by flowing water are measured in a laboratory flume. Drag coefficient values calculated from these data are presented for a range of log submergence and slenderness values. Drag coefficients for large submergence values are consistent with those previously published. However, at submergence values less than eight element diameters, the observed drag coefficients are consistently higher than those previously published. This discrepancy is due to the additional drag created by stationary surface waves which causes the drag coefficient to increase considerably when a log is positioned near the free surface. This phenomenon, which has not been accounted for in previous studies on large woody debris, is shown to depend on log slenderness, submergence, and Froude number.