Negatively buoyant jets are turbulent flows that are frequently employed by the desalination industry to disperse reject brines into oceanic environments. Although such brines are characterised by elevated concentrations of the same elemental components as the discharge environment contains, there is significant potential for marine ecosystem damage if this waste is not diluted properly. Numerous workers have analysed the dilution and spatial characteristics of negatively buoyant jets, but published data demonstrates notable inconsistencies. An important reason for these discrepancies is the variety of bottom-boundary conditions employed. This complicates comparison with predictions by integral models typically employed for discharge design, as these generally have not been developed with consideration to boundary interaction. In the present study, negatively buoyant jet experimental data is collected where bottom boundary distances are sufficiently large to avoid boundary influence at the point where the discharge returns to its source height (the return point). Near-field centreline dilution data is measured under still ambient conditions, for the source inclinations of 15–75°. Considerable attention is paid to experimental data quality, and all relevant issues are mitigated where possible. In order to ensure the boundary has no influence, source heights in this study range between 2.33 d F0 and 8.07 d F0. A variety of time-averaged and temporal statistics are calculated, and these statistics are compared with published experimental data and predictions by integral models. Normalised trajectory and dilution data from the source through to the return point collapses well at each inclination. The attention to signal quality and the self-consistency of derived experimental results in this study suggest a high level of accuracy, and large distances to the bottom boundary ensure that results are not confused by boundary interaction. Data for dilution rate at the return point supports the use of higher source inclinations (60° and 75°) to maximise dilution capability. A new ‘forced jet’ model is developed that incorporates the concept of a reducing buoyancy flux as the flow rises to maximum height. While this model is not applicable above source inclinations of 60°, predictions at other inclinations are reasonable. Dilution predictions are notably improved when compared to those from existing integral models. Finally, CFD simulations of negatively buoyant jets are conducted using the k-ε turbulence model. Despite the sophistication of this model, the quality of spatial and dilution bulk flow predictions at the centreline maximum height are no better than those obtained from the forced jet model or analytical solutions of Kikkert et al. (2007).

In urban areas, discharging wastewater into rivers is a common way to dispose of contaminants, and it is usually the most economical. Accurate information about how effluents are distributed in the receiving water body is desirable when designing industrial plants. Flow structures will be influenced by an effluent’s dilution processes during the mixing. Meanwhile, the cross-stream motions resulting from the streamline curvature can redistribute both the velocity and the shear stress, which favors the mixing behavior compared to a straight channel. However, the interactions between jet mixing behavior and the bend flow requires further investigation. In the present study, jets with different densities were discharged horizontally into a laboratory flume with a 135-degree open channel bend, and both the main and secondary flow behaviors in the bend were observed after the introduction of effluents. The acquired three- dimensional velocity data were used to validate numerical models of the effluent-bend flow. Numerical turbulence models such as the standard k-ε eddy viscosity model, non-linear k-ε model (Shih quadratic k-ε), and the k-ω SST (shear stress transport) model were employed to evaluate their accuracy. OpenFOAM was selected in the analysis for proposing better numerical models since it gives high-quality results to individualized complex fluid flows, and as an open source CFD software it can be beneficial to further develop and maintain. The first part of this study presents the implementation of the physical modelling of the proposed problems. Detailed descriptions of the experimental process were elaborated. Specifically, the three velocity components at four cross-sectional planes in the bend section were measured with and without saltwater jets by using the stereo Particle Image Velocimetry (PIV) ii technique in the laboratory flume. The experimental results show that the more pronounced effects with the jets were found at the beginning and exit of the bend. Although the jets had little effect on the maximum streamwise velocity, it was found that the occurrence of the negatively buoyant jets would affect the patterns and properties of the secondary flow in the bend. The second part of this study investigated the mechanisms underlying the two cells system, particularly when interacting with a discharged effluent jet. Detailed experimental data were used in interpreting the large center-region cell as well as small structures in the 135-degree open channel bend. A term-by-term analysis of the downstream vorticity equation was executed to investigate the various mechanisms underlying these cross-stream flow motions considering the influence of the negatively buoyant jets. The results indicated the generation and the dissipation of the streamwise vorticity with the effective terms of the vorticity equation. The third part of this study evaluated the performance of three different turbulence models with the experimental measurements. It can be concluded that fully 3D numerical models are capable of simulating the primary flow pattern in a strongly curved channel with the presence of a negatively buoyant jet. The comparison also shows that, although the outer bank cell was not predicted, the k-omega SST model can satisfactorily predict some of the smaller flow features in bend flow, such as the inner bank circulation cell and the overall form of the vorticity distributions. The results enable more reliable predictions for the characteristics and development of jets in a bend.

Inclined negatively buoyant jets are commonly used to dispose brine effluent produced by desalination plants. Desalination and associated research has expanded in recent years due to the continued depletion and degradation of natural potable water sources. Desalination plants are the preferred option for meeting water demand deficits in many countries around the world. Inclined negatively buoyant jets are produced when the brine is discharged at an upward inclined angle via an offshore pipeline and diffuser system. Previous experimental studies have focused on the rapid mixing and dilution achieved by these discharges, as well as geometric parameters. Dilution measurements between these experimental studies vary significantly, which is possibly due to variations in the location of a lower boundary on observed flow behaviour. In the present study, velocity field information is experimentally measured for inclined negatively buoyant jets and compared to integral model predictions. Experiments are conducted with and without a lower boundary influencing observed flow behaviour, thus allowing the effects of a lower boundary to be determined. The particle tracking velocimetry experimental technique is employed to measure near field velocities of these discharges. Firstly, discharges with source angles between 15\degree and 75\degree are investigated without boundary influence in stationary ambient conditions. The source was a minimum of 655 mm above the bottom of the experimental tank to ensure there was no lower boundary influence on observed behaviour. Time-averaged and fluctuating data are extracted along the trajectory of discharges. All non-dimensionalised geometric and centreline velocity parameters are found to collapse. Empirical coefficients are compared to previous experimental studies and integral model predictions. A new detrainment model is developed to predict the behaviour of inclined negatively buoyant jets without boundary influence. The model further develops recent attempts to allow for buoyancy flux reduction along the flow path. The reduction in buoyancy flux is dependent on the local parameters of the flow and simulates experimentally observed detrainment. Dilution, geometric, and velocity predictions are found to be improved over previous models when compared to experimental data. Finally, a raised platform was placed inside the experimental tank to determine the influence of a lower boundary on inclined negatively buoyant jets. Source angles of 30\degree, 45\degree, and 60\degree are investigated at three different non-dimensional source heights. The lower boundary is horizontal and ambient conditions are again stationary. Discharges impinge the lower boundary before forming a radially spreading layer along the boundary. Geometric and velocity data are compared to the first set of experiments in this study to determine the influence of the lower boundary on observed flow behaviour. Empirical coefficients at maximum height are similar with and without the influence of the boundary, whereas coefficients are substantially influenced at the return point when the boundary is present.

Dense wastewater discharges into marine environments can severely impact water bodies. This study addresses the disposal of hypersaline brines from desalination plants through multiport diffusers into seas and oceans. Accurate prediction of the mixing of discharges with the receiving water bodies is crucial for the optimal design of outfall systems. Designers can enhance mixing and increase dilution by modifying outfall properties. However, the interaction of discharges from multiport diffusers poses a significant challenge, impairing the mixing process. The main aim of this study is to improve multiport diffuser designs by limiting the negative effects of jet interaction on mixing. This research applies a three-dimensional numerical model, the Launder, Reece, and Rodi (LRR) turbulence model, to evaluate the predictive capabilities of the Reynolds Stress Models (RSM) for multiple dense jets and to explore the mixing characteristics and merging process of multiple jets. To validate the model, its predictions are compared with available experimental data. The LRR model showed good agreement with the experimental measurements, and the model outperformed the standard and re-normalization group (RNG) 𝑘−𝜀 turbulence models, making it a promising tool for studying the mixing behavior of multiport diffusers. This study proposes multiport diffusers with non-uniform port orientation as a means for mitigating the negative effect of jet mering on the mixing process and increasing dilution. Using the validated numerical model and the laser-induced fluorescence (LIF) technique, the effect of non-uniform port orientation on the mixing process is explored. The numerical results indicated that the orientation of adjacent jets significantly affected the behavior of individual jets. An individual jet exhibited a longer trajectory and higher dilution when its neighboring jets were disposed of with a different angle, compared to that of uniform discharges. Laboratory experiments on uniform and non-uniform diffusers, with varying port angles in the range of highest reported dilution rates for single discharges (40o-70o), are reported, and the major flow properties and merging processes are compared. Investigations revealed that non-uniform diffusers achieved overall higher mean dilutions due to different mixing behavior in the interaction zones. Non-uniform port orientation provided more space between the jets to expand before interacting with their neighbors, resulting in higher dilutions. This study challenges the application of formulae obtained from single discharge experiments for multiport diffuser designs and emphasizes the importance of considering source characteristics specific to multiport diffusers, such as angle difference, for efficient desalination outfall. The new data and analysis provided in this study can benefit the design of desalination discharge systems with considerable potential cost savings, especially for tunneled outfalls, due to shorter diffusers with non-uniform port orientations and environmental risk reductions.

Artificial intelligence techniques have been widely used for prediction in various areas of sciences and engineering. In the thesis, applications of AI techniques are studied to predict the dilution of industrial outfall discharges. The discharge of industrial effluents from the outfall systems is broadly divided into two categories on the basis of density. The effluent with density higher than the water receiving will sink and called as negatively buoyant jet. The effluent with density lower than the receiving water will rise and called as positively buoyant jet. The effluent discharge in the water body creates major environmental threats. In this work, negatively buoyant jet is considered. For the study, ANFIS model is taken into consideration and incorporated with algorithms such as GA, PSO and FFA to determine the suitable model for the discharge prediction. The training and test dataset for the ANFIS-type models are obtained by simulating the jet using the realizable k-ε turbulence model over a wide range of Froude numbers i.e. from 5 to 60 and discharge angles from 20 to 72.5 degrees employing OpenFOAM platform. Froude number and angles are taken as input parameters for the ANFIS-type models. The output parameters were peak salinity (Sm), return salinity (Sr), return point in x direction (xr) and peak salinity coordinates in x and y directions (xm and ym). Multivariate regression analysis has also been done to verify the linearity of the data using the same input and output parameters. To evaluate the performance of ANFIS, ANFIS-GA, ANFIS-PSO, ANFIS-FFA and multivariate regression model, some statistical parameters such as coefficient of determination (R2), root mean squared error (RMSE), mean absolute error (MAE) and average absolute deviation in percentage are determined. It has been observed that ANFIS-PSO is better in predicting the discharge characteristics.

Discharge of brine wastewater produced from industrial plants into adjacent coastal water bodies is considered as a preferable and common method currently used in many offshore industrial plants. Therefore, it is important to carefully study the behavior of jets and their environmental impacts on water bodies close to the discharge points, especially when the density is different between the jets and the receiving water. The main goal of this study is to improve the understanding of the mixing behaviour of jet trajectories for positively (offset) and negatively (inclined) buoyant jets when density is considered a significant factor, and also to examine the accuracy of some RANS turbulence models and one type of artificial neural network in predicting jet trajectory behaviours. In the first part of this study, experiments using a PIV system for offset buoyant jets were conducted in order to study the effect of the density differences (due to salinity [nonthermal] or temperature [thermal]) between the discharge and the receiving water body on the jet behavior, and the results showed that the nonthermal jets behaved differently as compared to the thermal jets, even though the densimetric Froude numbers (Frd) and density differences (∆ρ) were similar. In addition, a Reynolds-averaged Navier-Stokes (RANS) numerical model was performed using open-source CFD code (OpenFOAM) with a developed solver (modified form of the pisoFoam solver). The realizable k-ε model showed the best prediction among the models. Secondly, an extensive experimental study of an inclined dense jet for two angles (15°and 52°) was conducted to study the effect of these angles on the jets’ geometrical characteristics in the presence of a wide range of densimetric Froude numbers as well as with different discharge densities. More experimental data were obtained for these angles to be added to the previous data for the purpose of calibrating, validating, and comparing the various numerical models for future studies. The results of these experiments are used to evaluate the performance of a type of artificial neural network method called the group method of data handling (GMDH), and the GMDH results are then compared with existing analytical solutions in order to prove the accuracy of the GMDH method in simulating mixing behaviors in water bodies. Thirdly, a comprehensive study on predicting the geometrical characteristics of inclined negatively-buoyant jests using GMDH approach was conducted. The superiority of this model was demonstrated statistically by comparing to several previous analytical models. The results obtained from this study confirm that the GMDH model was highly accurate and was the best among others for predicting the geometrical characteristics of inclined negatively-buoyant jests.