Buoyant Discharge Research Articles

Responses of Coastal Upwelling to Tidally Induced Bottom Friction Dynamics and Plume-Modulated Geostrophy: A Process-Oriented Modeling Study

Abstract We used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the alongshore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.

Simulations of the Concentration Fields of Rosette-Type Multiport Buoyant Discharges Using Combined CFD and Multigene Genetic Programming Techniques

Rosette-type diffusers are becoming popular nowadays for discharging wastewater effluents. Effluents are known as buoyant jets if they have a lower density than the receiving water, and they are often used for municipal and desalination purposes. These buoyant effluents discharged from rosette-type diffusers are known as rosette-type multiport buoyant discharges. Investigating the mixing properties of these effluents is important for environmental impact assessment and optimal design of the diffusers. Due to the complex mixing and interacting processes, most of the traditional simple methods for studying free single jets become invalid for rosette-type multiport buoyant discharges. Three-dimensional computational fluid dynamics (3D CFD) techniques can satisfactorily model the concentration fields of rosette-type multiport buoyant discharges, but these techniques are typically computationally expensive. In this study, a new technique of simulating rosette-type multiport buoyant discharges using combined 3D CFD and multigene genetic programming (MGGP) techniques is developed. Modeling the concentration fields of rosette-type multiport buoyant discharges using the proposed approach has rarely been reported previously. A validated numerical model is used to carry out extensive simulations, and the generated dataset is used to train and test MGGP-based models. The study demonstrates that the proposed method can provide reasonable predictions and can significantly improve the prediction efficiency.

Subglacial Discharge and Its Down‐Fjord Transformation in West Greenland Fjords With an Ice Mélange

AbstractBuoyant freshwater released at depth from under Greenland's marine‐terminating glaciers gives rise to vigorous buoyant discharge plumes adjacent to the termini. The water mass found down fjord formed by mixing of buoyant subglacial freshwater and ambient fjord water and subsequent modification by glacial ice melt in the ice mélange is referred to as subglacial water. It substantially affects both the physical and chemical properties of the fjords' marine environment. Despite the importance of this freshwater source, many uncertainties remain regarding its transformation and detection. Here we present observations close to a marine‐terminating glacier in a fjord with substantial ice mélange and follow the down‐fjord changes of the subglacial discharge plume. Heat brought to the surface by entrainment of warm ambient fjord water into the rising plume causes intense melting of the ice mélange close to the plume pool. This results in an increase of glacial ice melt fraction to total glacial meltwater from 1–2% in the plume pool to ~18% eleven kilometers down‐fjord, with the largest increase being observed within the first few kilometers. Down‐fjord of the ice mélange two temperature minima bound the layer containing subglacial water. The upper bound is linked to the adjacent ice mélange and down‐fjord runoff sources, whereas the lower bound is linked to the stratification of the ambient water. We show that similar bounds can be observed in other marine‐terminating glacier fjords along West Greenland that contain an ice mélange, suggesting that similar processes work in other fjords.

Three-dimensional flow structure and mixing of the side thermal buoyant jet discharge in cross-flow

Numerical investigation of a side thermal buoyant discharge in the cross-flow is presented based on unsteady Reynolds-averaged Navier–Stokes equations closed with the realizable k- $$\upvarepsilon $$ turbulence model. Emphasis is placed on the detailed three-dimensional flow evolution and scalar mixing in an incompressible turbulent environment. The present study covers the cases with different jets to cross-flow velocity ratios (R) and initial temperatures. Moreover, various flow characteristics, including vortical structures, jet trajectories, jet streamlines, and intrinsic instabilities, are examined. Mixing ability is quantified by the decay rate of scalar temperatures and velocity magnitude, the probability density function, the spatial mixing deficiencies (SMDs), power spectral density analysis, and temporal mixing deficiencies (TMDs). Comparing the simulation results with the experimental data of Abdelwahed (Surface jets and surface plumes in cross-flows. Ph.D. thesis, Department of Civil Engineering and Applied Mechanics, McGill University, Montreal, Quebec, 1981), it shows good agreement in terms of the temperature half-width and half-thickness profiles, recirculation zone size, and jet trajectory. The numerical results of three-dimensional structures indicate a shear-layer, horseshoe, and surface roller vortices near the side-channel exit and secondary flows after the recirculation zone at the free surface of the main channel. The instantaneous temperature contours exhibit a vortex shedding phenomenon and gaps between the cross-flow and discharge jet at the shear layer. The maximum velocity magnitude location approaches the outer wall toward the main-channel downstream by increasing R. It is found that as the densimetric Froude number ( $$\hbox {Fr}_{{0}})$$ and R increase, the temperature dilution (S) generally decreases. The statistical analysis of TMD and SMD indicates a direct relationship between the mixing efficiency and buoyancy flux ( $${F}_{{0}})$$ .

CFD Modeling of Effluent Discharges: A Review of Past Numerical Studies

Effluent discharge mixing and dispersion have been studied for many decades. Studies began with experimental investigations of geometrical and concentration characteristics of the jets in the near-field zone. More robust experiments were performed using Laser-Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) systems starting in the 20th century, which led to more accurate measurement and analysis of jet behavior. The advancement of computing systems over the past two decades has led to the development of various numerical methods, which have been implemented in Computational Fluid Dynamics (CFD) codes to predict fluid motion and characteristics. Numerical modeling of mixing and dispersion is increasingly preferred over laboratory experiments of effluent discharges in both academia and industry. More computational resources and efficient numerical schemes have helped increase the popularity of using CFD models in jet and plume modeling. Numerous models have been developed over time, each with different capabilities to facilitate the investigation of all aspects of effluent discharges. Among these, Reynolds-averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) are at present the most popular CFD models employing effluent discharge modeling. This paper reviews state-of-the-art numerical modeling studies for different types and configurations of discharges, including positively and negatively buoyant discharges, which have mostly been completed over the past two decades. The numerical results of these studies are summarized and critically discussed in this review. Various aspects related to the impact of turbulence models, such as k-ε and Launder-Reece-Rodi (LRR) models, are reviewed herein. RANS and LES models are reviewed, and implications for the simulation of jet and plume mixing are discussed to develop a reference for future researchers performing numerical investigations on jet mixing and dispersion.

Dynamic coupling of a near with a far field model for outfall discharges

The dynamic/online coupling of the Distributed Entrainment Sink Approach with the model CORMIX for the near field and Delft3D-FLOW for the far field of outfall discharges is analysed in terms of hydrodynamic results. Six test cases have been performed, considering a straight channel with a steady state flow with a submerged positively buoyant discharge, varying the discharge flow and the ambient velocity applying offline and dynamic coupling. Three criteria were evaluated, defining the necessity of dynamic coupling approaches: a near field stability criterion, the distributed momentum criterion, and the ambient Froude number. Results showed that different coupling methods had led to distinct results for all tests, especially in the near field region. The three criteria demonstrated that tests with large flow rates or into almost quiescent waters led to hydrodynamic changes beyond the near field and thus should use the dynamic coupling.

Prediction of side thermal buoyant discharge in the cross flow using multi-objective evolutionary polynomial regression (EPR-MOGA)

Abstract The capability to predict the distribution of pollutants in water bodies is one of the most important issues in the design of jet outfalls. Three-dimensional computational fluid dynamics (CFD) model and multi-objective evolutionary polynomial regression (EPR-MOGA) are used and compared in modeling the temperature field in the side thermal buoyant discharge in the cross flow. The input variables used for training the EPR-MOGA models are spatial coordinates (x, y, z), jet to cross flow velocity ratio (R), depth of the channel (d), and the temperature excess (T0). A previous experimental study is used to verify and compare the performance of the EPR-MOGA and CFD models. The results show that the EPR-MOGA model predicts the thermal cross section of the flow and the spread of pollutants at the surface with a better accuracy than the CFD model. However, the CFD method performs significantly better than EPR-MOGA in predicting temperature profiles. The uncertainty analysis indicated that the EPR-MOGA model had lower mean prediction error and smaller uncertainty band than the CFD model. The relationships achieved by the EPR-MOGA model are very useful to predict temperature profiles, temperature half-thickness, and temperature spread on surface in practice.

Does reduced sediment load contribute to increased outbreaks of harmful algal blooms off the Changjiang Estuary?

Harmful algal blooms (HABs) have been increasingly frequent in coastal waters around the world over the last several decades. Accelerated coastal eutrophication, resulting from the increased anthropogenic loadings of nutrients, is commonly assumed to be the primary cause of this increase. However, although important, accelerated coastal eutrophication may not be the only explanation for the increasing blooms or toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than nutrients, such as sediment load, may significantly affect biological activities and HAB incidence in estuarine and coastal waters. A case study off the Changjiang (Yangtze River) Estuary indicated that with the increasing riverine loadings of nutrients, the sediment load from the Changjiang River has been reduced by 70% over the past four decades. A comparison of long-term data revealed that the phytoplankton biomass maximum has expanded to a region of much lower salinity due to the drastic reduction in riverine sediment load and the subsequent improvement in light penetration in the Changjiang River plume. Furthermore, there was an apparent mirror-image relationship between the sediment load from the Changjiang River and the HAB incidence off the Changjiang Estuary over the past four decades, and the number of HAB incidents was significantly negatively correlated with the sediment load. Therefore, it is argued that the drastic decline in sediment load from the Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to the increased frequency of HABs in the buoyant discharge plumes.

Warm discharges in cold fresh water: 2. Numerical simulation of laminar line plumes

The behaviour of a discharge of warm water upwards into a homogeneous body of cold fresh water was investigated by means of a numerical model. The discharge has a parabolic velocity profile, with Reynolds number $$Re=50$$ , Prandtl number $$Pr=7$$ and Froude number varied over the range $$0.2 \le {\rm Fr} \le 2.5$$ . Water density is taken to be a quadratic function of temperature, so that an initially positively buoyant discharge will experience buoyancy reversal as it mixes with an ambient below the temperature of maximum density. The resulting plume has some similarities to a fountain resulting from injection of negatively buoyant fluid upward into a less dense ambient. The plume is initially symmetric, but then its head detaches as it approaches its maximum height. The detached head is denser than the fluid in the plume below it, and the interaction between the sinking head and the rising plume causes a sideways deflection; as this cycle is repeated, the plume displays side-to-side flapping motion and vertical bobbing. As Froude number is increased (i.e. buoyancy reduced) the growth of the plume becomes slower, but the plume eventually reaches a greater height. We obtain empirical power-law scalings for maximum height and time taken to reach that height as functions of Froude number; these scalings are simlar to those for fountains with a linear dependence of density on temperature in the very weak regime.

Dynamics of the Block Island Sound Estuarine Plume

AbstractBuoyant discharge of freshwater from Long Island Sound (LIS) forms a seasonal buoyant plume outside Block Island Sound (BIS) between the coast of Long Island and the denser shelf waters. The plume’s seasonal variability and its response to tides, winds, and surface heating are investigated through a series of process-oriented experiments using the Regional Ocean Modeling System (ROMS). Results show the importance of river discharge, wind directions, and surface heating in the seasonal variation of the BIS buoyant plume. In winter and spring, the plume is intermediate with a large surface offshore extension detached from the bottom. From winter to spring, the river discharge increases; meanwhile, upwelling-favorable winds keep dominating. They compete with the increase of surface heating and generate a broader buoyant plume in spring than in winter. In summer, the plume is bottom advected with most of its width in contact with the bottom and is featured with the steepest isopycnals and narrowest plume, which is driven by a combination of strong insolation, weak buoyant discharge from LIS, and feeble winds. In fall, although the river discharge is comparable to that in winter, the upwelling-favorable wind is relatively weaker, corresponding to a narrower intermediate plume.

Modelling the Zambezi River plume

A model for the Zambezi River plume, the largest on the Indian Ocean coast of Africa, is presented and the results of experiments with different discharges and wind forcings are analysed. Although the river plays an important role in the southern African economy through power generation on large dams, artisanal fisheries, and frequent flooding events that impact greatly on local populations, the plume has not been well studied. Observations during the period 2004–2007, when the winds were mainly easterly or south-easterly, indicated that the plume waters can extend both downstream (equatorwards) and upstream (polewards) of the Zambezi Delta with a recirculating bulge near the river mouth. The model is constructed using the Regional Ocean Modeling System (ROMS), with a 40-km long, 3-km wide river discharging into a rectangular coastal ocean with a linearly sloping bottom. When the model is forced only by a constant river discharge of 1 000 m3 s−1 (typical of observed discharge amounts in summer), the Kelvin and Froude numbers for the resulting plume imply a ‘large-scale’ buoyant discharge with a coastal current that is close to being in geostrophic balance with the across-shore pressure gradient and a recirculating ageostrophic bulge near the mouth. The distributions of the bulge and plume waters are found to be relatively insensitive to the discharge amount. Under constant wind forcing, the plume distribution changes dramatically. Northerly and easterly winds produce the largest changes with the latter able to deflect the plume up to 180° due to Ekman drift. When sea breeze-like winds are imposed, accumulation of water in the bulge occurs with substantial spreading upstream. Stronger sea breezes lead to less downstream spreading of the plume than gentle winds. When the winds are mainly across-shore, Ekman drift dominates, but the dynamics become almost geostrophic when the winds are roughly aligned to the coast. These experiments suggest that the Zambezi River plume is sensitive to the winds on diurnal to synoptic time-scales.

CFD modeling and analysis of the behavior of 30° and 45° inclined dense jets – new numerical insights

A three-dimensional numerical model of inclined turbulent jets with negatively buoyant discharge into stationary ambient water is presented in this paper to study certain jet parameters with turbulence schemes that have not been employed before in this context such as standard Boussinesq gradient diffusion hypothesis and general gradient diffusion hypothesis to account for the buoyancy-induced turbulence generation. Two jet discharge angles have been chosen for this study: 30° and 45° with the horizontal. These two angles are chosen in this study due to lower terminal rise heights for 30° and 45°, a fact which is critically important for discharges of effluent into shallow waters compared to higher angles than these values. The spatio-temporal jet evolutions for these cases have been modeled using OpenFOAM open-source CFD code, which is based on Finite-Volume Method. Results presented in this paper deal with the geometrical and flow properties of the inclined dense jets. The densimetric Froude number of the effluent at the nozzle ranges between 10 and 34. Two Reynolds-Averaged Navier–Stokes turbulence models are applied to evaluate the accuracy of the numerical predictions: the realizable k–ε (a two-equation model) and the Launder Reece Rodi (a Reynolds Stress Model – RSM).

Modified reduced buoyancy flux model for desalination discharges

Large-scale desalination facilities are increasingly employed to supplement potable water supplies for many cities, where the demand for water is having a negative impact on the sustainability of natural water resources. A primary environmental concern with the establishment of these large-scale facilities is the effective disposal of the hyper-saline effluent brine, so that harmful effects on the marine environment are minimised. In countries with effective effluent discharge regulations, the negatively buoyant brine is typically released through submerged diffusers, where the ports are inclined towards the ocean surface to aid dispersion processes. Predictive models provide an inexpensive method of considering different sources and ambient design parameters. It has been demonstrated that predictive models developed primarily for positively buoyant discharges significantly underestimate dilution measured by physical studies, when applied to negatively buoyant desalination discharges. It has been shown recently that predicted dilution and geometric parameters can be improved if a reduction in the buoyancy flux of the main flow is incorporated into these models. Here, the reduced buoyancy flux (RBF) approach is modified through the use of a physically based buoyancy loss mechanism. Improvements in the predictive capabilities of the new model are demonstrated through comparisons with predictions from existing models and an extensive range of results from physical studies for geometric, dilution, and velocity parameters.

Large-eddy simulation of horizontally discharging sediment-laden jets

Horizontally discharging jets laden with sediment particles are investigated numerically. The computation of the fluid phase is conducted by Large Eddy Simulation (LES) while Lagrangian particle tracking is used to calculate the motion of sediment particles. Both momentum and buoyant discharge cases of sediment-laden jets are simulated. The computational results reveal that the advecting large eddies are responsible for the formation and the transport of high-concentration particle patches in the jet flows. For a horizontally discharging buoyant jet, it is observed that there exists a significant correlation of particle abundance and high turbulence intensity in the lower outer layer of the jet flow when its trajectory is deflected by buoyancy. The investigation of the instantaneous vorticity fields shows that the particle transport in horizontal sediment jets are large associated with the large eddies in the flow.

Characteristics of the plume formed by the buoyant discharges from the river

Density currents formed by buoyancy discharges from rivers are numerically studied using non-dimensional two layer model including Coriolis acceleration, bottom stress, interfacial friction. Some typical numbers such as Froude number, densimetric Froude number and Kelvin number are obtained and some characteristic scales are defined as a result of non-dimensionalization of the governing equations. Besides the Coriolis effect, the configurations of bottom topography, bottom friction coefficient and interfacial friction are found to significantly affect the propagation of the warm water plume. Frontal position can fastly propagate in the case of large density difference between the two layers and small interfacial friction. Left side boundary current is easily formed under the small interfacial friction. With large Kelvin number, both right and left side boundary currents are formed. Wave-like disturbances and eddies are easily formed under the high Froude number.

Rapid submarine melting driven by subglacial discharge, LeConte Glacier, Alaska

We show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers. This buoyant discharge draws in warm seawater, entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice. To capture the effects of subglacial discharge on submarine melting, we conducted 4 days of hydrographic transects during late summer 2012 at LeConte Glacier, Alaska. A major rainstorm allowed us to document the influence of large changes in subglacial discharge. We found strong submarine melt fluxes that increased from 9.1 ± 1.0 to 16.8 ± 1.3 m d−1 (ice face equivalent frontal ablation) as a result of the rainstorm. With projected continued global warming and increased glacial runoff, our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems. These effects must be considered when modeling glacier response to future warming and increased runoff.

Removing the boundary influence on negatively buoyant jets

A comprehensive laboratory study of negatively buoyant discharges is presented. Unlike previous studies, here the focus is on generating data sets where influences of the bottom boundary have been eliminated. There are significant discrepancies in the published dilution data for these flows and a contributing factor is the large variation in the bottom boundary condition. A Laser-induced Fluorescence system is employed to gather flow spread, peak concentration (minimum dilution) and trajectory data for a wide range of densimetric Froude numbers and initial discharge angles. Data from these experiments are compared with previously published data, along with predictions from integral models and a revised form of the previously published semi-analytical solutions. The new data sets are not distorted by mixing processes associated with the bottom boundary and therefore provide the basis for more meaningful assessments of the predictive capabilities of existing models, given that the influences of the bottom boundary on contaminant mixing are not incorporated into these models. In general the models assessed are able to predict key geometric quantities with reasonable accuracy, but their minimum dilution predictions are conservative. Importantly dilution at the return point shows a strong dependence on the initial discharge angle and this could have important implications for the design of discharge systems.

Predicting the near-field mixing of desalination discharges in a stationary environment

A modified integral model is developed to predict the near-field behaviour of negatively buoyant discharges, which are typical of those created when releasing brine from desalination plants into a marine environment via a submerged outfall system. These predictions are compared with available laboratory data and the predictions from other integral formulations, both analytical and numerical. Based on these comparisons, it is evident that the modified model is capable of predicting the effects of the additional mixing noted in previous studies, for the range of initial conditions relevant to a submerged discharge from a desalination facility. The modified model is therefore superior to existing numerical integral formulations in this respect and is reasonably consistent with the predictions from previously published analytical solutions. A critical feature of the modified integral model is the reduction in buoyancy flux of the main flow, which provides a mechanism to account for the influences of additional mixing created by buoyancy induced instabilities on the inner side of the flow.

Mapping ocean outfall plumes and their mixing using autonomous underwater vehicles

This paper reports on developing autonomous underwater vehicle (AUV) survey methods for ocean outfall discharge plumes and new insights gained on plume mixing. Unique to the study is mapping the discharge mixing using colored dissolved organic matter (CDOM) calibrated for effluent dilution. AUV mission planning methodologies for discharge plume sampling, plume characterization using onboard temperature, salinity and optical sensors, and comparison of observational data to model results are presented for the Point Loma Ocean Outfall offshore of San Diego, CA. The results are expected to be applicable to the general theme of mixing of submerged buoyant discharges. In the near‐field, the plume is found to mix to a height consistent with the predictions of buoyant jet engineering models. At the far‐field, the fine spatial scales of the plume resolved by the vehicle suggests that shear instabilities caused by internal waves can enhance plume mixing and elevate the discharge plume above the predicted equilibrium rise height. These results suggest that even under variable oceanic conditions, properly planned missions for AUVs equipped with an optical CDOM sensor in addition to traditional physical oceanographic sensors, can accurately map the mixing of ocean outfall plumes at resolutions not possible with traditional boat‐based techniques. Variations of oceanic conditions are found to influence the mixing and fate of the plume at time scales generally not considered in the design of these discharges.

Flow Classification of Negatively Buoyant Surface Discharge in an Ambient Current

Abstract Surface discharge of negatively buoyant effluents into moving waterbodies creates a range of complex flow behavior. These complexities in the hydrodynamic phenomena develop through the interplay between a discharge's initial fluxes and the motion of the ambient current, which advects the discharge fluid downstream. In the present study, experiments were conducted for a comprehensive range of initial fluxes for negatively buoyant discharges in an ocean current through rectangular and semicircular channels. From this data, three flow regimes—the free surface jet, the shore-attached surface jet, and the plunging plume—and their range of occurrences were identified. These observations provide the basis for developing a revised flow classification diagram, which differs significantly from the positively buoyant discharge scheme as expected. A major difference is in the transition from the free surface jet regime to the shore-attached surface jet, whereas the plume-like regime has the same transition c...