Three-Dimensional Modeling of Negatively Buoyant Flow in Diverging Channels

Diverging channels, also known as diffusers, represent common natural and industrial outlets to lakes, reservoirs, and rivers. If the outflow in a diffuser has a larger density than the ambient water, the inflow may plunge and form a density underflow. In this paper, a three-dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom. Of particular interest is the formation of separated flows such as wall-jet and free-jet flows. Various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric Froude numbers are considered. The model successfully simulates the formation of attached flow, wall jets, and free jets in a negatively buoyant environment. The onset, evolution, and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail. Computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel.     

Physical Model Study of an Alternating Diffuser for Thermal Discharge

Physical model tests were conducted of the mixing of heated water from a proposed thermal diffuser. Dilutions were measured and flows imaged by a three-dimensional laser-induced fluorescence system that provides vastly more data than conventional thermocouple techniques. The flows were quite three-dimensional. For zero current speed, the effluent mixed over the water depth, but only in a limited region, and a two-layer stratified flow developed toward the edges of the effluent field and farther downstream. For zero and slow currents, the lowest surface dilution occurred where the jet centerline intersected the water surface and could be reasonably predicted by simple free-jet calculations. The prediction of an entrainment model, UM3, was close to the observed results because the flow is three-dimensional and the entraining water can be supplied from the ends. CORMIX considerably underestimated the observed dilution because it uses a two-dimensional analysis and neglects the effect of source momentum flux. Two-dimensional analyses and sectional physical models of diffusers with finite lengths should be used with caution until their limitations are better known.     

Computational Fluid Dynamics Simulation of Supersonic Oxygen Jet Behavior at Steelmaking Temperature

Supersonic oxygen jets are used in steelmaking and other different metal refining processes, and therefore, the behavior of supersonic jets inside a high temperature field is important for understanding these processes. In this study, a computational fluid dynamics (CFD) model was developed to investigate the effect of a high ambient temperature field on supersonic oxygen jet behavior. The results were compared with available experimental data by Sumi et al. and with a jet model proposed by Ito and Muchi. At high ambient temperatures, the density of the ambient fluid is low. Therefore, the mass addition to the jet from the surrounding medium is low, which reduces the growth rate of the turbulent mixing region. As a result, the velocity decreases more slowly, and the potential core length of the jet increases at high ambient temperatures. But CFD simulation of the supersonic jet using the k−ε turbulence model, including compressibility terms, was found to underpredict the potential flow core length at higher ambient temperatures. A modified k-ε turbulence model is presented that modifies the turbulent viscosity in order to reduce the growth rate of turbulent mixing at high ambient temperatures. The results obtained by using the modified turbulence model were found to be in good agreement with the experimental data. The CFD simulation showed that the potential flow core length at steelmaking temperatures (1800 K) is 2.5 times as long as that at room temperature. The simulation results then were used to investigate the effect of ambient temperature on the droplet generation rate using a dimensionless blowing number.     

Circulation in Stratified Lakes due to Flood-Induced Turbidity Currents

The river inflow in a natural lake with important suspended sediment load during floods, can impact water quality by mobilizing dissolved matters like phosphorous from deep to surface waters. Generally due to thermal stratification in prealpine lakes, the water column is stable. It does not mix vertically unless acted on by outside forces, for example, currents or winds. Since Lake Lugano has a strong thermal stratification, river inflow exhibits different modes of density currents, from surface flows and thermocline intrusion to bottom currents. Turbidity currents are the direct cause of the downward water flow, and at the same time at the origin of upward directed flow. In this study, the impact of river born turbidity currents in Lake Lugano under varying ambient conditions was investigated using field measurements at the inflow river and inside the lake, together with a full three-dimensional numerical model of the entire lake. The paper characterizes the induced circulation of the turbidity plume and gives some indications on the relevance of turbidity currents on the lake.     

Three-Dimensional CFD Simulation Coupled with Thermal Contraction in Direct-Chill Casting of A390 Aluminum Alloy Hollow Billet

Metallurgical and Materials Transactions B - A three-dimensional CFD model coupled with melt flow, heat transfer, and thermal contraction was developed to simulate the direct-chill (DC) casting...     

Assessment of a River Reach for Environmental Fluid Dynamics Studies

Turbulence is the fundamental mechanism governing energy transfer in river flows that was conventionally examined in laboratory flumes. Recently, a trend has been observed for constructing larger scale and outdoor facilities that tend to avoid the problems of upscaling of experimental results. This paper presents the results of an experimental study performed on a river reach used as an environmental field laboratory. The study is focused on the understanding of the spatial arrangement of the flow structure and its dependency on the temporal variability of the flow. Detailed measurements were taken using acoustic Doppler velocimeters and their analysis was completed applying the theory of open-channel flows. The obtained results reveal that the flow structure on the river reach resembles characteristics of a typical three-dimensional open-channel flow. Away from the riverbanks, the flow behaves as a quasi-two-dimensional fully developed turbulent open-channel flow thus providing conditions favorable for field experimental studies of shallow mixing layers and flows over patches of submerged aquatic plants. An interesting observation in the seasonal dynamics of turbulent shear stress patterns was that the height of the roughness layer was reduced in the central part of the flow, though the overall roughness coefficient was increased. At the same time, the structure of the secondary flow near the banks was also substantially altered as the secondary circulations observed at low water levels were replaced by flow separation and internal boundary layers at medium water levels.     

Near-Field Mixing Downstream of a Multiport Diffuser in a Shallow River

Near-field mixing downstream of a multiport diffuser in a wide shallow river was studied with a field dye test. Dye concentrations at different depths and lateral locations were measured. The near-field mixing was analyzed in four zones: the free jet zone, the jet surface-impingement zone, the merging zone, and the vertical mixing zone. Analytical models were proposed to derive the three-dimensional concentration field after the jets impinged the water surface. After the impingement, the dye concentration can be predicted well by treating the multiple jets as a simple mathematical summation of individual jets. The vertical mixing zone was dominated by the riverbed friction-induced turbulence, with little effect from the effluent momentum and buoyancy. The results of the field data were also used to validate the applicability of some existing models for multiport diffusers.     

Graphical Sizing and Analysis of Ocean Outfalls with Buoyant Plumes

A graphical model is developed that offers a convenient solution to wastewater disposal problems through submarine outfalls. The model is intended to perform preliminary analysis under worst-case dilution conditions for outfalls with line diffusers, currents from any direction, stratified or unstratified ambient, and submerged or surfacing plumes, and it is also valid for outfalls with single horizontal or vertical ports. The model is thus applicable for most outfall configurations and ambient conditions of practical interest. The model simultaneously considers three dilution mechanisms: initial dilution, dilution through dispersion, and effective dilution due to decay of nonconservative substances. This enables analysis of functional dependencies and grouping of model variables in a way that permits a generalized graphical solution. It also allows one to study the sensitivity of the overall dilution to current speeds and compare the relative performance of perpendicular and parallel line diffusers. The results showed that, contrary to what is widely believed, in most situations of practical interest, high rather than low current speeds are critical in outfall design and parallel rather than perpendicular line diffusers often deliver the best overall performance.     

Dynamic Model for Subcritical Combining Flows in Channel Junctions

A one-dimensional theoretical model for subcritical flows in combining open channel junctions is developed. Typical examples of these junctions are encountered in urban water treatment plants, irrigation and drainage canals, and natural river systems. The model is based on applying the momentum principle in the streamwise direction to two control volumes in the junction together with overall mass conservation. Given the inflow discharges and the downstream depth, the proposed model solves for each of the upstream depths. The interfacial shear force between the two control volumes, the boundary friction force, and the separation zone shear force downstream of the lateral channel entrance are included. Predictions based on the proposed approach are shown to compare favorably with existing experimental data, previous theories, and conventional junction modeling approaches. The main advantages of the proposed model are that the proposed model does not assume equal upstream depths and that the dynamic treatment of the junction flow is consistent with that of the channel reaches in a network model.     

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.     

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.     

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.     

Heat transfer of calcium cored wires and CFD simulation on flow and mixing efficiency in the argon-stirred ladle

A three-dimensional CFD simulation model of 160-tonne LF ladle based on gas–liquid multiphase flows and k-ε mixture turbulence model was developed on FLUNET platform considering the agitation effect of argon blowing on the mixing performance of gaseous calcium in the liquid steel. Taking the volume fraction of gaseous calcium mixed in the molten steel as the quantified indicator of mixing efficiency, the flow field in the bottom argon-stirred ladle and the influence of cored-wire feeding position, feeding depth, feeding velocity, argon flow on the mixing efficiency were investigated. By utilising the element birth and death technique on MSC.MARC platform, the transient heat transfer between the calcium cored wire and the molten steel was simulated, and the effect of the feeding speed and the wire structure on the evaporation depth of the calcium was presented.     

Nonlinear Mixing Length Model for Prediction of Secondary Currents in Uniform Channel Flows

A nonlinear turbulence model for numerical solution of uniform channel flow is presented. Turbulent stresses are evaluated from a nonlinear mixing length model that relates turbulent stresses to quadratic products of the mean rate of strain and the mean vorticity. The definition of the mixing length, based on a three-dimensional integral measure of boundary proximity, eliminates the need for solution of additional transport equations for the turbulence quantities. Experimental data from the literature for closed and open-channel flows are utilized to validate the model. The model produced the secondary flow vortices successfully. Velocity field and wall shear stresses affected by secondary flow vortices are accurately computed. Bulging of velocity contour lines toward the corners and dipping phenomena of maximum velocity are successfully simulated.     

Limitations of Depth-Averaged Modeling for Shallow Wakes

Large-scale horizontal vortical structures are generic features of shallow flows which are often modeled using the two-dimensional (2D) depth-averaged equations. Such modeling is investigated for the well-defined case of shallow wakes of a conical island of small side slope for which a three-dimensional (3D) boundary-layer (3DBL) model has previously been validated through comparison with experiment. The 3DBL model used a 3D, two-mixing-length, eddy-viscosity turbulence model with a vertical mixing length of classical Prandtl form and a horizontal mixing length some multiple of this. A multiple of six gave good predictions. This mixing length approach is reduced to depth-averaged form, giving a horizontal mixing length of about half the water depth. The shallow wakes may be vortex shedding or steady/stable and are conventionally defined by a stability parameter. The critical value above which a stable wake is formed is considerably overestimated by the depth-averaged model (for a range of mixing lengths) and the length of stable wake bubble is considerably underestimated. It seems likely that this is because the amplification of friction coefficient due to horizontal strain rates is not represented. However, when vortex shedding is prominent the 2D and 3DBL wake structures are quite similar. These results show, for example, the limitations of depth-averaged models for the prediction of solute dispersion.     

Numerical modelling studies of flow and mixing phenomena in gas stirred steel ladles

Abstract

A three-dimensional computational fluid dynamics (CFD) model was developed to simulate the fluid flow and mixing phenomena in a gas stirred ladle. Particular attention was paid to incorporate the effect of slippage between rising gas bubbles and surrounding fluid in the numerical model, to capture the relevant flow physics in a more effective manner. Various parametric studies were undertaken to examine the effects of gas flow rate, bottom nozzle configurations and tracer addition locations on mixing time. It was observed that the arrangement of bottom nozzles has a great effect on the mixing behaviour in a gas stirred ladle, with off centric gas injection producing shorter mixing time. Mixing time was found to be sensitive to the tracer addition position, particularly for the axisymmetric bottom gas injection system. The predicted results were compared with reported experimental observations and a very good agreement was observed in this regard, thereby establishing the authenticity of the proposed formulation.     

Computational Fluid Dynamics Modeling of Supersonic Coherent Jets for Electric Arc Furnace Steelmaking Process

Supersonic coherent gas jets are now used widely in electric arc furnace steelmaking and many other industrial applications to increase the gas–liquid mixing, reaction rates, and energy efficiency of the process. However, there has been limited research on the basic physics of supersonic coherent jets. In the present study, computational fluid dynamics (CFD) simulation of the supersonic jet with and without a shrouding flame at room ambient temperature was carried out and validated against experimental data. The numerical results show that the potential core length of the supersonic oxygen and nitrogen jet with shrouding flame is more than four times and three times longer, respectively, than that without flame shrouding, which is in good agreement with the experimental data. The spreading rate of the supersonic jet decreased dramatically with the use of the shrouding flame compared with a conventional supersonic jet. The present CFD model was used to investigate the characteristics of the supersonic coherent oxygen jet at steelmaking conditions of around 1700 K (1427 °C). The potential core length of the supersonic coherent oxygen jet at steelmaking conditions was 1.4 times longer than that at room ambient temperature.     

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.     

Validation of a Three-Dimensional Computational Fluid Dynamics Model of a Contact Tank

A three-dimensional (3D) computational fluid dynamics (CFD) model of a contact tank is presented in this paper. The model results are compared against 3D velocities and flow through curve (FTC) data, representing a tracer concentration profile, from a 1:8 scale physical model. The objective is to demonstrate that CFD models can simulate both the FTC and the 3D velocity field quite well. Simultaneous validation of velocities and FTC is important in ascertaining the predictive capabilities of CFD models, as physical model studies indicate that different baffle arrangements can lead to similar FTCs. Therefore, a good prediction of only FTC, as presented in previous 3D CFD model studies, does not necessarily imply a correct simulation of the flow field.     

Exchange Processes between a River and Its Groyne Fields: Model Experiments

The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that is formed at the river-groyne-field interface. The large eddies formed in the mixing layer are the major cause of this exchange. Second, under certain conditions, even larger eddies are shed from the upstream groyne tip. Distortions in the flow field caused by such intermittent structures cause a much larger exchange than that by the mixing layer alone. The occurrence of large shed eddies depends on the presence of a sufficiently large, stationary, secondary gyre located at the upstream corner of the groyne field. The overall exchange of matter could be characterized as a first-order process, in accordance with the dead-zone-theory. The corresponding exchange coefficients agreed reasonably well with the results of earlier experiments and the effective coefficients as found in experiments in real river flows.