Mass transfer of solutes in turbulent wall bounded flows reacting with the conduit surface

This thesis focuses on the decay of chlorine in pipes of drinking water distribution networks due to wall and bulk demand. Accurate prediction of chlorine decay is important, as both chlorine concentrations which are too low and too high pose serious health risks, the former due to pathogen formation and the latter due to the formation of disinfection by-products. Water quality models used for the prediction of chlorine decay make use of parameterisations for the wall demand in the form of Sherwood number Sh correlations, which couple the wall mass flux to a Reynolds number Re, Schmidt number Sc and wall roughness. These correlations are subject to significant uncertainty, particularly for turbulent flows. A combined analytical and numerical approach is taken to study in detail the interaction between flow, turbulence and mass transport, with the aim of improving the understanding and accuracy of wall demand parameterisations for chlorine. Simulations of the chlorine decay in an axisymmetric pipe with hydraulically smooth walls were performed for Re = 104 to 106 and Sc = 1000 using Reynolds averaged conservation equations. These values are typical for chlorine transport in distribution networks. The simulations confirmed that the assumptions made in water quality models for chlorine wall demand are valid. Asymptotic solutions for high Sc solutes were developed which are applicable both to linear and nonlinear wall reactions. Results showed that the Sh correlation is independent of the reaction type. For rough walls, the two main wall demand parameterisations are mutually inconsistent: one is valid for low and the other for high wall demand coefficients only. Numerical simulation of flow and high Sc mass transport over a dtype rough surface at Re = 2.5×105 showed that the inconsistency between the two parameterisations was caused by the geometry. For low wall demand coefficients, the existence of roughness elements causes higher wall demand than for a smooth wall. However, at high wall demand coefficients the maximum wall demand achievable in the cavities was much smaller than for the crests. Hence, the effective surface area and therefore the wall demand became lower than for a smooth wall. A parameterisation was developed which reproduced the solute mass decay over the entire range of wall demand coefficients. Most of the solutions and parameterisations developed in this thesis are on the same level of description as water quality models. The findings of this thesis can be used as supportive evidence for the validity of assumptions made for water quality models, and to inform how processes should be modelled when these assumptions are violated

FSM: FBS Set Management, An energy efficient multi-drone 3D trajectory approach in cellular networks

Nowadays, Unmanned Aerial Vehicles (UAVs) have been significantly improved, and one of their most important applications is to provide temporary coverage for cellular users. Terrestrial Base Station cannot service all users due to disasters or events such as ground BS breakdowns, bad weather conditions, natural disasters, transmission errors, etc. The UAV can be sent to the target location and establishes the necessary communication links without requiring any predetermined infrastructure and covers that area. Finding the optimal location and the appropriate number (DBS) of drone-BS in this area is a challenge. In this paper, the optimal location and optimal number of DBSs are distributed in the current state of the users and the subsequent user states determined by the prediction. Finally, the DBS transition is optimized from the current state to the predicted future locations. The simulation results show that the proposed method can provide acceptable coverage on the network

Coupled heat-mass transport modelling of radionuclide migration from a nuclear waste disposal borehole

Disposal of radioactive waste originating from reprocessing of spent research reactor fuel typically includes stainless steel canisters with waste immobilised in a glass matrix. In a deep borehole disposal concept, waste packages could be stacked in a disposal zone at a depth of one to potentially several kilometres. This waste will generate heat for several hundreds of years. The influence of combining a natural geothermal gradient with heat from decaying nuclear waste on radionuclide transport from deep disposal boreholes is studied by implementing a coupled heat-solute mass transport modelling framework, subjected to depth-dependent temperature, pressure, and viscosity profiles. Several scenarios of waste-driven heat loads were investigated to test to what degree, if any, the additional heat affects radionuclide migration by generating convection-driven transport. Results show that the heat output and the calculated radioactivity at a hypothetical near-surface observation point are directly correlated; however, the overall impact of convection-driven transport is small due to the short duration (a few hundred years) of the heat load. Results further showed that the calculated radiation dose at the observation point was very sensitive to the magnitude of the effective diffusion parameter of the host rock. Coupled heat-solute mass transport models are necessary tools to identify influential processes regarding deep borehole disposal of heat-generating long-lived radioactive waste

Comparison and verification of turbulence Reynolds-averaged Navier-Stokes closures to model spatially varied flows

The robustness and accuracy of Reynolds-averaged Navier–Stokes (RANS) models was investigated for complex turbulent flow in an open channel receiving lateral inflow, also known as spatially varied flow with increasing discharge (SVF). The three RANS turbulence models tested include realizable k–ε, shear stress transport k–ω and Reynolds stress model based on their prominence to model jets in crossflows. Results were compared to experimental laser Doppler velocimetry measurements from a previous study. RANS results in the uniform flow region and farther from the jet centreline were more accurate than within the lateral inflow region. On the leeward side of the jet, RANS models failed to capture the downward velocity vectors resulting in major deviations in vertical velocity. Among RANS models minor variations were noted at impingement and near the water surface. Regardless of inadequately predicting complex characteristics of SVF, RANS models matched experimental water surface profiles and proved more superior to the theoretical approach currently used for design purposes

Large eddy simulation of turbidity currents in a narrow channel with different obstacle configurations

© 2020, The Author(s). Turbidity currents are frequently observed in natural and man-made environments, with the potential of adversely impacting the performance and functionality of hydraulic structures through sedimentation and reduction in storage capacity and an increased erosion. Construction of obstacles upstream of hydraulic structures is a common method of tackling adverse effects of turbidity currents. This paper numerically investigates the impacts of obstacle’s height and geometrical shape on the settling of sediments and hydrodynamics of turbidity currents in a narrow channel. A robust numerical model based on LES method was developed and successfully validated against physical modelling measurements. This study modelled the effects of discretization of particles size distribution on sediment deposition and propagation in the channel. Two obstacles geometry including rectangle and triangle were studied with varying heights of 0.06, 0.10 and 0.15 m. The results show that increasing the obstacle height will reduce the magnitude of dense current velocity and sediment transport in narrow channels. It was also observed that the rectangular obstacles have more pronounced effects on obstructing the flow of turbidity current, leading to an increase in the sediment deposition and mitigating the impacts of turbidity currents

Large eddy simulation of turbidity currents in a narrow channel with different obstacle configurations

Turbidity currents are frequently observed in natural and man-made environments, with the potential of adversely impacting the performance and functionality of hydraulic structures through sedimentation and reduction in storage capacity and an increased erosion. Construction of obstacles upstream of hydraulic structures is a common method of tackling adverse effects of turbidity currents. This paper numerically investigates the impacts of obstacle’s height and geometrical shape on the settling of sediments and hydrodynamics of turbidity currents in a narrow channel. A robust numerical model based on LES method was developed and successfully validated against physical modelling measurements. This study modelled the effects of discretization of particles size distribution on sediment deposition and propagation in the channel. Two obstacles geometry including rectangle and triangle were studied with varying heights of 0.06, 0.10 and 0.15 m. The results show that increasing the obstacle height will reduce the magnitude of dense current velocity and sediment transport in narrow channels. It was also observed that the rectangular obstacles have more pronounced effects on obstructing the flow of turbidity current, leading to an increase in the sediment deposition and mitigating the impacts of turbidity currents

New models for energy beam machining enable accurate generation of freeforms

We demonstrate that, despite differences in their nature, many energy beam controlled-depth machining processes (e.g. waterjet, pulsed laser, focused ion beam) can be modelled using the same mathematical framework – a partial differential evolution equation that requires only simple calibrations to capture the physics of each process. The inverse problem can be solved efficiently through numerical solution of the adjoint problem, and leads to beam paths that generate prescribed three-dimensional features with minimal error. The viability of this modelling approach has been demonstrated by generating accurate freeform surfaces using three processes that operate at very different length scales and with different physical principles for material removal: waterjet, pulsed laser and focused ion beam machining. Our approach can be used to accurately machine materials that are hard to process by other means for scalable applications in a wide variety of industries

Towards a digital twin for characterising natural source zone depletion: A feasibility study based on the Bemidji site

Natural source zone depletion (NSZD) of light non-aqueous phase liquids (LNAPLs) may be a valid long-term management option at petroleum impacted sites. However, its future long-term reliability needs to be established. NSZD includes partitioning, biotic and abiotic degradation of LNAPL components plus multiphase fluid dynamics in the subsurface. Over time, LNAPL components are depleted and those partitioning to various phases change, as do those available for biodegradation. To accommodate these processes and predict trends and NSZD over decades to centuries, for the first time, we incorporated a multi-phase multi-component multi-microbe non-isothermal approach to representatively simulate NSZD at field scale. To validate the approach we successfully mimic data from the LNAPL release at the Bemidji site. We simulate the entire depth of saturated and unsaturated zones over the 27 years of post-release measurements. The study progresses the idea of creating a generic digital twin of NSZD processes and future trends. Outcomes show the feasibility and affordability of such detailed computational approaches to improve decision-making for site management and restoration strategies. The study provided a basis to progress a computational digital twin for complex subsurface systems

Towards characterizing LNAPL remediation endpoints

Remediating sites contaminated with light non-aqueous phase liquids (LNAPLs) is a demanding and often prolonged task. It is vital to determine when it is appropriate to cease engineered remedial efforts based on the long-term effectiveness of remediation technology options. For the first time, the long term effectiveness of a range of LNAPL remediation approaches including skimming and vacuum-enhanced skimming each with and without water table drawdown was simulated through a multi-phase and multi-component approach. LNAPL components of gasoline were simulated to show how component changes affect the LNAPL\u27s multi-phase behaviour and to inform the risk profile of the LNAPL. The four remediation approaches along with five types of soils, two states of the LNAPL specific mass and finite and infinite LNAPL plumes resulted in 80 simulation scenarios. Effective conservative mass removal endpoints for all the simulations were determined. As a key driver of risk, the persistence and mass removal of benzene was investigated across the scenarios. The time to effectively achieve a technology endpoint varied from 2 to 6 years. The recovered LNAPL in the liquid phase varied from 5% to 53% of the initial mass. The recovered LNAPL mass as extracted vapour was also quantified. Additional mass loss through induced biodegradation was not determined. Across numerous field conditions and release incidents, graphical outcomes provide conservative (i.e. more prolonged or greater mass recovery potential) LNAPL remediation endpoints for use in discussing the halting or continuance of engineered remedial efforts