225 research outputs found
Contact Tank Design Impact on Process Performance
No full textIn this study three-dimensional numerical models were refined to predict reactive processes in disinfection contact tanks (CTs). The methodology departs from the traditional performance assessment of contact tanks via hydraulic efficiency indicators, as it simulates directly transport and decay of the disinfectant, inactivation of pathogens and accumulation of by-products. The method is applied to study the effects of inlet and compartment design on contact tank performance, with special emphasis on turbulent mixing and minimisation of internal recirculation and short-circuiting. In contrast to the conventional approach of maximising the length-to-width ratio, the proposed design changes are aimed at addressing and mitigating adverse hydrodynamic structures, which have historically
led to poor hydraulic efficiency in many existing contact tanks. The results suggest that water treatment facilities can benefit from in-depth analyses of the flow and kinetic processes through computational fluid dynamics, resulting in up to 38% more efficient pathogen inactivation and 14% less disinfection by-product formation
Developing a multi-scale parallelised coupled system for wave-current interactions at regional scales
No full textAt coastal areas, the interplay between waves and currents is crucial. This interaction impacts many phenomena and applications, highlighting the necessity for accuracy and speed
in the numerical representation of Wave-Current Interactions (WCI). These applications
encompass a wide spectrum, including coastal morphology, sediment transport, offshore
structure scouring, pollutant mixing, infrastructure design, marine energy projects, and
storm surges. The complexity in representing WCI stems from incorporating multi-scale
processes with diverse temporal and spatial scales. For example, wind wave periods range
from seconds to hours, while the wavelengths span from centimetres to kilometres. In
contrast, tides showcase much larger scales with periods in the order of hours and wave-lengths in the order of thousands of kilometres. Practically, reconciling all these processes
and scales within a single model is improbable, leading to the need for coupled systems
to address this challenge.
This study presents the development of a Python-interfaced multi-scale parallelised coupled modelling system for WCI. It is formed by coupling the spectral wave model Simulat ing WAves Nearshore (SWAN) with the 2-D shallow-water equation hydrodynamics model
Thetis. The coupling is facilitated by the Basic Model Interface (BMI), a lightweight
generic coupling interface. The impact of waves on current is introduced via the radiation
stress formulation, accompanied by the integration of wave-roller effects. Two coupling
options are offered: online and offline. The online choice supports both one-way and
two-way coupling, while the offline alternative is focused on one-way coupling.
Considering that only few existing WCI models report on validation in controlled environments, a suite of benchmarking scenarios is established consisting of analytical and
experimental scenarios in quasi 1-D and 2-D configurations. In these cases, sensitivity
analyses are performed spanning various parameters in both models. The results underscore the importance of customising each coupled configuration when WCI are prominent,
rather than solely relying on recommended or “default” values. Calibrated results align
well with the data and often showcase the same level of accuracy as other 3-D WCI. This
efficiency means less computational cost, as the developed model converges faster and
requires less CPU time compared to alternative options.
A month-long numerical representation of the field configuration located in Duck, North
Carolina, investigates the coupled system’s performance under moderate wind conditions.
This scenario serves to assess the influence of various coupling approaches on its predictions. Since this area is primarily influenced by waves and features low current speeds, the
coupling modes have minor impact on wave predictions. However, with coupling modes
transitioning from no to two-way coupling, the hydrodynamics predictions exhibit substantial improvement in regions where WCI are evident. The improved accuracy does not
encompass areas characterised by rip currents or other processes that require a vertical
discretisation for their hydrodynamics. Discrepancies between online and offline one-way
coupling configurations are evident, with the most pronounced differences observed in the
SWAN-to-Thetis coupling. They can be attributed to different interpolation methodologies.
Ultimately, the WCI system is applied in a regional configuration within the Orkney
archipelagos, UK. Specifically, the model simulates the waters of Westray Firth, a region
known for its energetic tidal conditions, to assess its capacity for effectively depicting
WCI phenomena in regional scales. Our predictions correlate well with the observations,
accurately mirroring the sinusoidal pattern of the measured wave parameters, usually
attributed to tidal effects. Furthermore, our model showcases similar precision to a 3-D
WCI coupled system implemented in the same region at lower computational cost.
The coupled system developed during this thesis presents an efficient tool for incorporating WCI phenomena across various scales, exhibiting performance comparable to its 3-D
counterparts. Its efficiency is highlighted by: (a) minimising computational resource usage, as evidenced by a 38% reduction in the number of cores employed during the Westray
Firth application; (b) reducing elapsed real times; and (c) accelerating convergence, such
as achieving convergence 1.4 to 18 times faster in benchmarking scenarios. It provides
a crucial foundation for researchers and stakeholders that seek to adopt a precise and
efficient solution, independent of the 3-D nature of WCI. This unlocks new opportunities for its versatile employment in a range of applications spanning from initial research
and decision-making stages to optimisation studies and to the development of forecasting
systems
Instantaneous transport of a passive scalar in a turbulent separated flow
No full textThe results of large-eddy simulations of flow and transient solute transport over a backward facing step and through a 180° bend are presented. The simulations are validated successfully in terms of hydrodynamics and tracer transport with experimental velocity data and measured residence time distribution curves confirming the accuracy of the method. The hydrodynamics are characterised by flow separation and subsequent recirculation in vertical and horizontal directions and the solute dispersion process is a direct response to the significant unsteadiness and turbulence in the flow. The turbulence in the system is analysed and quantified in terms of power density spectra and covariance of velocity fluctuations. The injection of an instantaneous passive tracer and its dispersion through the system is simulated. Large-eddy simulations enable the resolution of the instantaneous flow field and it is demonstrated that the instabilities of intermittent large-scale structures play a distinguished role in the solute transport. The advection and diffusion of the scalar is governed by the severe unsteadiness of the flow and this is visualised and quantified. The analysis of the scalar mass transport budget quantifies the mechanisms controlling the turbulent mixing and reveals that the mass flux is dominated by advection
Calibration and validation of a shared space model: case study
No full textShared space is an innovative streetscape design that seeks minimum separation between vehicle traffic and pedestrians. Urban design is moving toward space sharing as a means of increasing the community texture of street surroundings. Its unique features aim to balance priorities and allow cars and pedestrians to coexist harmoniously without the need to dictate behavior. There is, however, a need for a simulation tool to model future shared space schemes and to help judge whether they might represent suitable alternatives to traditional street layouts. This paper builds on the authors’ previously published work in which a shared space microscopic mixed traffic model based on the social force model (SFM) was presented, calibrated, and evaluated with data from the shared space link typology of New Road in Brighton, United Kingdom. Here, the goal is to explore the transferability of the authors’ model to a similar shared space typology and investigate the effect of flow and ratio of traffic modes. Data recorded from the shared space scheme of Exhibition Road, London, were collected and analyzed. The flow and speed of cars and segregation between pedestrians and cars are greater on Exhibition Road than on New Road. The rule-based SFM for shared space modeling is calibrated and validated with the real data. On the basis of the results, it can be concluded that shared space schemes are context dependent and that factors such as the infrastructural design of the environment and the flow and speed of pedestrians and vehicles affect the willingness to share space
A multi-objective GA-based optimisation for holistic Manufacturing, transportation and Assembly of precast construction
No full textResource scheduling of construction proposals allows project managers to assess resource requirements, provide costs and analyse potential delays. The Manufacturing, transportation and Assembly (MtA) sectors of precast construction projects are strongly linked, but considered separately during the scheduling phase. However, it is important to evaluate the cost and time impacts of consequential decisions from manufacturing up to assembly. In this paper, a multi-objective Genetic Algorithm-based (GA-based) searching technique is proposed to solve unified MtA resource scheduling problems (which are equivalent to extended Flexible Job Shop Scheduling Problems). To the best of the authors' knowledge, this is the first time that a GA-based optimisation approach is applied to a holistic MtA problem with the aim of minimising time and cost while maximising safety. The model is evaluated and compared to other exact and non-exact models using instances from the literature and scenarios inspired from real precast constructions
On the values for the turbulent Schmidt number in environmental flows
No full textComputational Fluid Dynamics (CFD) has consolidated as a tool to provide understanding and quantitative information regarding many complex environmental flows. The accuracy and reliability of CFD modelling results oftentimes come under scrutiny because of issues in the implementation of and input data for those simulations. Regarding the input data, if an approach based on the Reynolds-Averaged Navier-Stokes (RANS) equations is applied, the turbulent scalar fluxes are generally estimated by assuming the standard gradient diffusion hypothesis (SGDH), which requires the definition of the turbulent Schmidt number, Sct (the ratio of momentum diffusivity to mass diffusivity in the turbulent flow). However, no universally-accepted values of this parameter have been established or, more importantly, methodologies for its computation have been provided.
This paper firstly presents a review of previous studies about Sct in environmental flows, involving both water and air systems. Secondly, three case studies are presented where the key role of a correct parameterization of the turbulent Schmidt number is pointed out. These include: (1) transverse mixing in a shallow water flow; (2) tracer transport in a contact tank; and (3) sediment transport in suspension. An overall picture on the use of the Schmidt number in CFD emerges from the paper
Long-range collision avoidance for shared space simulation based on social forces
No full textShared space is an innovative approach to improve environments where both pedestrians and vehicles are present, with integrated layouts to balance priority. The Social Force Model (SFM) was used to visualise pedestrian and car trajectories so that peaks of density and pressure at critical locations are avoided. This paper extends the SFM to consider a long-range collision detection and collision resolution strategy. The determination of potential conflicts is enhanced using principle component analysis for a set of agent's prior speeds and directions. This long-range collision avoidance strategy results in more realistic SFM-based trajectories for pedestrians and cars in shared spaces
Effect of three-dimensional mixing conditions on water treatment reaction process
No full textThe performance of water disinfection facilities traditionally relies on Hydraulic Efficiency Indicators (HEIs), extracted from experimentally derived Residence Time Distribution (RTD) curves. This approach has often been undertaken numerically through computational fluid dynamics (CFD) models, which can be calibrated to predict accurately RTDs, enabling the assessment of disinfection facilities prior to the construction of disinfection tanks. However, a significant drawback of the conventional efficiency methodology prescribed for disinfection tanks is associated with the respective indicators, as they are predominantly linked to the internal flow characteristics developed in the reactor, rather than the disinfection chemistry which should be optimized. In this study three-dimensional numerical models were refined to simulate the processes of chlorine decay, pathogen inactivation and the by-product formation in disinfection contact tanks (CTs). The main objective of this study was to examine the effect of three-dimensional mixing on the reaction processes which were modelled through finite-rate kinetic models. Comparisons have been made between pathogen inactivation and disinfection by-product accumulation results produced by a RANS approach against the findings of a Segregated Flow Analysis (SFA) of conservative tracer transport. CFD Results confirm that three-dimensional mixing does have an effect on the reaction processes, which, however, is not apparent through the SFA approach
Freshwater variability in the Arctic Eurasian Shelf seas: satellite sea surface salinity and inter-annual variability to predict Arctic system change
No full textEurasian Rivers provide a quarter of total fresh water to the Arctic, maintaining a persistent fresh layer that covers the surface Arctic Ocean. This freshwater export controls Arctic Ocean stratification, circulation, and basin-wide sea ice concentration. The Russian Arctic receives around 2/3 of the river runoff to the Arctic, primarily from the Ob, Yenisei and Lena Rivers which outflow into the Kara and Laptev Sea as a particularly shallow plume.
Previous in-situ and modelling studies suggest that local wind forcing is a driver of variability in Eurasian Arctic sea surface salinity (SSS) but there is no consensus on the roles river runoff and sea ice cover have in contributing to this variability or on the dominant driver of variability. The dominant controls on SSS variability have also been suggested to vary regionally, with suggested differences in the Kara and Laptev Sea.
Until recently, satellite SSS retrievals were insufficiently accurate for use in the Arctic. However, retreating sea ice cover and continuous progress in satellite product development have significantly improved SSS retrievals. This thesis first shows the value and potential of satellite SSS as a useful tool to strengthen our understanding of Arctic SSS dynamics. Satellite SSS is found to agree well with in situ data (r ≥ 0.81) with notably better agreement than reanalysis products and in situ data (r ≤ 0.76).
Satellite SSS is then used in combination with reanalysis and in-situ products to first compare and contrast the processes controlling the interannual variability of summer SSS, sea surface temperature (SST) and sea ice concentration (SIC) variability and their interactions in the Laptev and Kara Sea and in the Vilkitsky Strait.
In the Laptev Sea, zonal wind is the dominant driver of offshore/alongshore Lena River plume transport, with eastward wind driving alongshore transport (and westward wind driving offshore transport). This drives differences in both SSS and SST and spatial variability in SIC across the Laptev and East Siberian Sea. Conversely, Lena runoff does not appear to play a role in controlling interannual variability in SSS, SST, or SIC in the Laptev and East Siberian Sea.
In the Kara Sea, zonal (and meridional) wind and Ob and Yenisei runoff all appear to be key drivers of whether the fresh plume is transported offshore or alongshore. Whilst eastward wind forcing is the dominant driver of alongshore transport, as is true in the Laptev Sea, a high ratio of summer Yenisei runoff/ spring Ob runoff can accentuate the low SSS anomalies offshore driven by westward wind forcing. Zonal wind forcing also has an influence on SST but is not the dominant driver of variability in SST, as it is in the Laptev Sea. Therefore, SSS and SST are notably less closely coupled in the Kara Sea and the zonal wind does not drive differences in Kara SIC.
In the Vilkitsky Strait, strong eastward wind drives buoyancy driven transport of the Ob-Yenisei plume through Vilkitsky Strait and into the western Laptev Sea and can occur over one summer season. Plume transport has a consistent SST signature, suggesting co-variability between SSS and SST but is not drive a notable difference in SIC. After a summer of westward wind forcing, plume transport through the strait appears to occur over winter. However, differences in timing drive very different SSS/SST patterns and in turn stratification dynamics.
The dominant controls on Eurasian-wide interannual variability are then identified and the implications of these on sea ice persistence and Arctic-wide freshwater storage are assessed. The Arctic Oscillation Index (AOI) is found to be a dominant control on local wind forcing in all three regions, and drives a consistent pattern of freshwater transport across all Eurasian shelf seas, which appears to be accelerating in recent decades. This pattern of freshwater transport persists until at least the following year and appears to have implications on autumn and spring sea ice persistence.
Finally, the implications of these findings are inferred in the context of climate change. The dominance of zonal wind and the AOI as a key driver of SSS (and SST) interannual variability suggests that understanding variability in wind stress and its changes is key to predicting future freshwater transport from the Eurasian shelf seas and its impacts on Arctic circulation
Strategic maritime container transport design in oligopolistic markets
No full textAbstractThis paper considers the maritime container assignment problem in a market setting with two competing firms. Given a series of known, exogenous demands for service between pairs of ports, each company is free to design a liner service network serving a subset of the ports and demand, subject to the size of their fleets and the potential for profit. The model is designed as a three-stage complete information game: in the first stage, the firms simultaneously invest in their fleet; in the second stage, they individually design their networks and solve the route assignment problem with respect to the transport demand they expect to serve, given the fleet determined in the first stage; in the final stage, the firms compete in terms of freight rates on each origin-destination movement. The game is solved by backward induction. Numerical solutions are provided to characterize the equilibria of the game
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