Nature-based solutions (NBS) are increasingly popular infrastructure protection options, particularly in coastal engineering. These systems have shown the ability to provide similar coastal protection services to traditional hard schemes while providing other ecological and economic benefits, and a capacity to adapt to changing contexts. One prominent example of coastal NBS are saltmarshes: fields of flexible or semi-flexible vegetation, which have been found to significantly reduce damage to local communities under daily and storm conditions. Scientific study of these complex, multi-faceted structures is growing in volume, but there remain many knowledge gaps in the field. Numerical modelling is a powerful tool for investigating both large- and small-scale behaviours of saltmarshes. Numerical models provide a controlled, repeatable, and easily variable method for testing how a marsh impacts local hydrodynamic climates and how incident flow or wave conditions affect the behaviour of their constitutive vegetation. Small-scale plant behaviour is the focus of this thesis. Literature on the subject has been chiefly limited to greatly simplified vegetation modelling, reducing plants' behaviour to that of straightforward rigid cylinders. While this can be effective, it requires significant calibration to measured data and may not provide an accurate picture of the intricate flow dynamics surrounding an individual plant, let alone a full marsh system. Recently, numerical models capable of modelling flexible structures have been developed and used by researchers. However, studies applying these tools have focussed on replicating the more significant hydrodynamic effects of marshes, such as mixing or wave attenuation. By doing so, the calibration requirements of the rigid-type models remain, and the way the plants themselves are modelled loses physical meaning beyond their hydrodynamic impacts. The work presented in this thesis aims to expand on current flexible plant modelling research by evaluating a new numerical modelling tool in the open-source software REEF3D for replicating in situ saltmarsh plant behaviour in terms of drag force and motion response to hydrodynamic forcing. Three experimental programs were designed and conducted in order to thoroughly evaluate both aspects of the model. The first, based on a flume study performed by Paul et al. (2016), tested the drag force response to regular wave action. The second, based on the work of Tschisgale & Fröhlich (2020), further investigated the drag force response using closed- and open-channel flow, as well as solitary waves. The third, based on a flume study performed with live vegetation by Markov et al. (2023), evaluated the accuracy of the motion response to irregular waves. Consistent through all three programs was an overestimation of the examined behaviour and, in the third case, persistent model breakdown. These results demonstrate that, as tested, the evaluated tool is unsuitable for this purpose. It is suggested that this is due to the foundational assumptions of the model, namely that the material of the flexible structure is of a linearly viscoelastic type, whereas a nonlinear elastic material would be more appropriate for this application. These results highlight the difficulty of numerically modelling these systems and the need for further research developing and applying practical modelling tools for marshes.

<p>One of the most fundamental human needs is fresh air. It has been estimated that people spend comparatively much time in indoor premises. That creates an elevated need for high-quality ventilation systems in buildings. The ventilation airflow rate is recognised as the main parameter for measuring the indoor air quality. It has been shown that the ventilation airflow rates have effects on respiratory diseases, on “sick building syndrome” symptoms, on productivity and perceived air quality. Ventilation is necessary to remove indoor-generated pollutants by diluting these to an acceptable level. The choice of ventilation airflow rate is often based on norms or standards in which the airflow rate is determined based on epidemiological research and field or laboratory measurements. However, the determination of ventilation flow rate is far more complex. Indoor air quality in the occupied zone can be dependent of many factors such as outdoor air quality, airflow rate, indoor generation of pollutants, moisture content, thermal environment and how the air is supplied into the human occupied zone. One needs to acknowledge the importance of air distribution which clearly affects the comfort of occupants. To design a ventilation system which considers all aspects of room ventilation can only be achieved by computer modelling. The objective of this thesis is to investigate air diffusion, indoor air quality and comfort issues by CFD (computational fluid dynamics) modelling. The crucial part of the CFD modelling is to adopt BCs (boundary conditions) for a successful and accurate modelling procedure. Assessing the CFD simulations by validated BCs enabled constructing the ventilation system virtually and various system layouts were tested to meet given design criteria. In parallel, full-scale measurements were conducted to validate the diffuser models and the implemented simplified particle-settling model. Both the simulations and the measurements reveal the full complexity of air diffusion coupled with solid contaminants. The air supply method is an important factor for distribution of heat, air velocity and solid contaminants. The influence of air supply diffuser location, contaminant source location and air supply method was tested both numerically and by measurements to investigate the influence of different parameters on the efficiency of room ventilation. As example of this, the well-known displacement ventilation is not fully able to evacuate large 10 μm airborne particles from a room. Ventilation should control the conditions in the human breathing zone and therefore the ventilation efficiency is an important parameter. A properly designed ventilation system could use less fresh air to maintain an acceptable level of contaminant concentration in the human breathing zone. That is why complete mixing of air is not recommended as the ventilation efficiency is low and the necessary airflow rate is relatively high compared to other ventilation strategies. Especially buoyancy-driven airflows from heat sources are an important part of ventilation and should not be hampered by supply airflow from the diffusers. All the results revealed that CFD presently is the only reliable method for optimising a ventilation system considering the air diffusion and contaminant level in all locations of any kind of room. The last part of the thesis addresses the possibility to integrate the CFD modelling into a building design process where architectural space geometry, thermal simulations and diffuser BCs could be embedded into a normal building design project.</p>

Super Duplex Stainless Steels have superior corrosion resistance properties and strength compared to conventional steels. However, these properties are influenced by the different phases that precipitate during the heat treatment process. The conventional way of studying the time and temperature effects on the properties and micro-structure of SDSS is to prepare many samples at different temperatures and holding times. The welding research group at Production Technology Center, Trollhättan, Sweden, has recently developed a unique heat treatment method to produce a wide range of temperature by using a stationary TIG arc heat source. It results in a graded micro-structure in a single sample at a specific time period. The accuracy of the results ob-ained from this process is highly related to the accuracy of the temperature field model next to weld pool. In this work, a model was developed by using OpenFOAM CDF code, to predict the temperature field of the super duplex stainless steel samples that have been subjected to this novel TIG arc heat treatment process. The developed model was able to capture the trend in the overall temperature field in the heat affected zone. However, there was some mismatch between the modelled and experimental temperature profiles in certain locations in the heat affected zone. Further improvements have to be done to the developed model in order to take the phase transformation effect into account. A preliminary investigation has been carried out on how to implement this in the current model and reported in the thesis.

No description available.

One of the most fundamental human needs is fresh air. It has been estimated that people spend comparatively much time in indoor premises. That creates an elevated need for high-quality ventilation systems in buildings. The ventilation airflow rate is recognised as the main parameter for measuring the indoor air quality. It has been shown that the ventilation airflow rates have effects on respiratory diseases, on “sick building syndrome” symptoms, on productivity and perceived air quality. Ventilation is necessary to remove indoor-generated pollutants by diluting these to an acceptable level. The choice of ventilation airflow rate is often based on norms or standards in which the airflow rate is determined based on epidemiological research and field or laboratory measurements. However, the determination of ventilation flow rate is far more complex. Indoor air quality in the occupied zone can be dependent of many factors such as outdoor air quality, airflow rate, indoor generation of pollutants, moisture content, thermal environment and how the air is supplied into the human occupied zone. One needs to acknowledge the importance of air distribution which clearly affects the comfort of occupants. To design a ventilation system which considers all aspects of room ventilation can only be achieved by computer modelling. The objective of this thesis is to investigate air diffusion, indoor air quality and comfort issues by CFD (computational fluid dynamics) modelling. The crucial part of the CFD modelling is to adopt BCs (boundary conditions) for a successful and accurate modelling procedure. Assessing the CFD simulations by validated BCs enabled constructing the ventilation system virtually and various system layouts were tested to meet given design criteria. In parallel, full-scale measurements were conducted to validate the diffuser models and the implemented simplified particle-settling model. Both the simulations and the measurements reveal the full complexity of air diffusion coupled with solid contaminants. The air supply method is an important factor for distribution of heat, air velocity and solid contaminants. The influence of air supply diffuser location, contaminant source location and air supply method was tested both numerically and by measurements to investigate the influence of different parameters on the efficiency of room ventilation. As example of this, the well-known displacement ventilation is not fully able to evacuate large 10 μm airborne particles from a room. Ventilation should control the conditions in the human breathing zone and therefore the ventilation efficiency is an important parameter. A properly designed ventilation system could use less fresh air to maintain an acceptable level of contaminant concentration in the human breathing zone. That is why complete mixing of air is not recommended as the ventilation efficiency is low and the necessary airflow rate is relatively high compared to other ventilation strategies. Especially buoyancy-driven airflows from heat sources are an important part of ventilation and should not be hampered by supply airflow from the diffusers. All the results revealed that CFD presently is the only reliable method for optimising a ventilation system considering the air diffusion and contaminant level in all locations of any kind of room. The last part of the thesis addresses the possibility to integrate the CFD modelling into a building design process where architectural space geometry, thermal simulations and diffuser BCs could be embedded into a normal building design project. / QC 20101007

No description available.

The thesis presents a systematic experimental and numerical study on the interactions among porous fence, airflow, and windblown snowdrifts, a knowledge that will contribute to optimize the performance of porous wind shielding system in Cold Regions. A comprehensive review of the concepts, theories, techniques, and key findings associated with the research work has been undertaken. The key technical parameters influencing fence performance have been systematically studied by means of wind tunnel experimental investigations and Computational Fluid Dynamics (CFD) simulations. The study has found that porosity is the most influential structural parameter affecting the performance of porous fences in many aspects. Fence height stands a significant positive position in terms of its performance. It was found that fence performance is not sensitive to the changes of approaching atmospheric airflow velocity. Nevertheless, a bottom gap can improve snow fence trap efficiency. All of those findings agree with most of the findings of other researchers, which affirms that the research methodology adopted in this research is sound. Physical experimental work was performed to assess the reliability and credibility of the numerical models. Those models have been intentionally simplified, which made them easier to construct and quicker to obtain numerical solutions at a lower computational cost. Furthermore, the numerical models demonstrate the level of competence acquired through this research that is implemented in the optimisation of fence design. Special attention has been paid to the issues where elaborate research work has not been systematically reached in the open literature, this includes areas such as the effects of arrangement of porous holes, fence surface shear, and directions of wind load with respect to the fence, etc. Correlation between the reattachment length, the shelter distance, and the creation and distribution of fence surface shear is reported, to the author's knowledge, for the first time in the open literature. General guidelines for the design of shelters based on porous fences have been established through this study. For example, the desirable size of hole range should be identified beforehand, and porous holes with sharp angular corners should usually be avoided in the fence design. It is recommended to place the fence within an angle of 30° to the wind load, where the effective shelter distance can be estimated in a linearized equation, and the normal drag coefficient can be described as a function of cos2θ. Optimal design of the arrangement of porous holes will maximize the fence performance, especially when the close fence environment is of concern. Although the definition of fence effective zone is still vague in the research field, the key factors influencing the fence effective zone have been investigated by evaluating the reduction of wind velocity leeward of the fence in this thesis. It is found that the fence effective zone is not sensitive to the change of approaching airflow velocity, and that increasing fence height will increase the physical size of the fence effective zone, but not in a proportional manner. It is also concluded that fence effective zone will be significantly reduced when the non-normal wind load is inclined at an angle greater than 30° to the fence. The effective zone increases effectively when the fence porosity is optimal. In contrast to the majority of published research work, the transient snow transport model presented in this work considers the snow transport rate as a whole without distinguishing the rate in saltation and suspension layer. The numerical study indicated that the position of the snow crest is mainly determined by the fence height, while porosity and bottom gap mainly affect the downwind deposition length. The optimal porosity for snow fences is in the range of 0.4 to 0.5, which is greater than the one for wind fences, which lies in the range from 0.25 to 0.35. Two snow crests have been observed leeward the fence at the onset of snow deposition, when the fence was placed without a bottom gap to the snow ground. This finding has not been encountered in any of the reported research work. Wind tunnel simulations of snowdrift around the fences have marginally under-predicted the sizes of snow deposition. The numerical predictions were quantitatively and qualitatively in good agreement with the field observations. This incompetence of wind tunnel experiments on porous fences implies that numerical modelling can play a more important role in snow fence research.

Yes / A mass transfer model was developed for CO2 capture from a binary gas mixture of N2/CO2 in hollow fibre membrane contactors under laminar flow conditions. The axial and radial diffusions through membrane and convection in tube and shell sides with chemical reaction were investigated. COMSOL software was used to numerically solve a system of non-linear equations with boundary conditions by use of the finite element method. Three different amine solutions of monoethanolamine (MEA), diethanolamine (DEA) and n-methyldiethanolamine (MDEA) were chosen as absorbent in lumen to consider the mass transfer rate of CO2 and compare their removal efficiency. The modelling results were compared with experimental data available in the literature and a good agreement was observed. The CFD results revealed that MEA had the best performance for CO2 removal as compared to DEA and MDEA under various operating conditions due to the different CO2 loading factor of absorbents. Furthermore, efficiency of CO2 removal was highly dependent on the absorbent concentration and its flow rate, increasing of the gas flow rate caused a reduction in gas residence time in the shell and consequently declined CO2 mass transfer. The modelling results showed the influence of the absorbent concentration on the CO2 mass transfer has improved due to availability of absorbent reactants at the gas-liquid interface.

Thesis (MScEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The use of computational fluid dynamics (CFD) as a design tool is becoming increasingly popular in the water resources field. This thesis aims to extend the knowledge of CFD and determine the usefulness of current CFD programs as a modelling tool. This thesis also seeks to determine the accuracy of CFD modelling when compared to physical modelling, the more established form of model testing. It is important that research is conducted on the validation of CFD because with an increase in computer power, processing speed and continual development in the programs used to generate the models, CFD could become an essential tool for the hydraulic engineer. A current key difficulty faced by CFD programs is the mapping of the free surface level of a body of fluid in a two-phase (water and air) flow condition. This is further complicated by the existence of three-dimensional flow over a labyrinth weir and a fluctuating nappe, which at times requires a free surface level to be mapped both above and below the nappe. This thesis begins by detailing the design methods and actual design of a typical labyrinth weir. It then describes the construction of a 1:20 scale physical model, testing procedures, goals, and the results of the physical model tests. Following the physical model study, the thesis discusses the development of a three-dimensional CFD model, designed in a way that matched the physical model. Simulation results obtained from the CFD model are then compared to those from the physical model study and the accuracy and suitability of CFD modelling as a design tool are evaluated. This evaluation considers the surcharge upstream of the weir and transient pressures on the weir. The thesis concludes with recommendations for further research in this field. The results achieved show that the CFD model was able to accurately map the movement of particles within the domain, to fully develop a flow profile, and to accurately predict the water surface level. The pressure readings obtained during CFD modelling were in the same order as those obtained during physical modelling. However, the CFD modelling pressure readings did not often accurately correspond with the physical modelling data, with the average error being 92%. These results indicate that there is still further development required in CFD before it can be relied upon as a design tool independent of other experimental methods. The difficulty and the length of time taken to generate the results also indicate that, at this stage and in this particular scenario, the engineer would be better served through the use of a physical model. / AFRIKAANSE OPSOMMING: Die gebruik van gerekenariseerde vloeidinamika (CFD) as ’n ontwerpinstrument het toenemend gewild begin raak op die gebied van waterhulpbronne. Die doel van hierdie verslag is om kennis van CFD uit te brei en die nut van huidige CFD-programme as ’n modelleringsinstrument te bepaal. Daar word voorts ook gepoog om die akkuraatheid van CFD-modellering te bepaal in vergelyking met fisiese modellering – die meer gevestigde vorm van modeltoetsing. Dit is noodsaaklik dat navorsing gedoen word oor die bekragtiging van CFD, want met ’n toename in rekenaarkrag, verwerkingsnelheid en deurlopende ontwikkeling in die programme wat gebruik word om die modelle te genereer, sal CFD ’n noodsaaklike instrument vir die hidroulika-ingenieur word. ’n Belangrike probleem wat CFD-programme tans inhou, is die kartering van die vry oppervlak van ’n liggaam vloeistof in ’n tweefasse vloeitoestand (water en lug). Dit word verder bemoeilik deur die bestaan van driedimensionele vloei oor ’n labirint-stuwal en ’n skommelende “nappe”, wat by tye vereis dat ’n vry oppervlak sowel bo as onder die “nappe” gekarteer met word. Die verslag begin met ’n uiteensetting van die ontwerpmetodes en fisiese ontwerp van ’n tipiese labirintstuwal. Die bou van ’n 1:20-skaal- fisiese model, toetsprosedures, doelwitte en die resultate van die toetse op die fisiese model word dan beskryf. Ná die studie van die fisiese model, word die ontwikkeling van ’n driedimensionele CFD-model bespreek, wat ontwerp is om by die fisiese model te pas. Die simulasie-resultate van die CFD-model word dan vergelyk met dié van die studie van die fisiese model en die akkuraatheid en geskiktheid van CFD-modellering as ’n ontwerpinstrument word geëvalueer. In hierdie evaluering word die opdamming stroomop van die stuwal en druk op die stuwal ondersoek. Die verslag word afgesluit met aanbevelings vir verdere navorsing op hierdie gebied. Die resultate toon dat die CFD-model die beweging van partikels in die domein akkuraat kon karteer ten einde ’n volledige vloeiprofiel te ontwikkel en die watervlak akkuraat te voorspel. Die drukke wat tydens CFD-modellering verkry is, stem egter nie ooreen met die lesings wat tydens fisiese modellering verkry is nie. Die gemiddelde fout is 92%. Hierdie resultate toon dat verdere ontwikkeling in CFD nodig is voordat daarop staat gemaak kan word as ’n ontwerpinstrument wat onafhanklik van ander eksperimentele metodes gebruik kan word. Die moeilikheidsgraad en die lang tydsduur betrokke by die generering van resultate is ook ’n aanduiding dat die gebruik van ’n fisiese model die ingenieur op hierdie stadium en in hierdie spesifieke scenario beter tot diens sal wees.

Carbon dioxide (CO2) is a natural, low cost refrigerant with good thermo-physical properties. CO2 is a good alternative for replacing HFC refrigerants that possess high global warming potential and reducing the direct impacts of refrigeration systems on the environment. However, CO2 refrigeration systems operate at relatively high condenser/gas cooler pressures and this imposes special design and control considerations. The gas cooler is a very important part of the system and can have significant influence on its performance. In sub-critical operation, good gas cooler/condenser design can reduce the condenser pressure and delay switching to supercritical operation which increases system efficiency. In supercritical operation optimum design and control can enable the system to operate at pressures that maximise system efficiency. In air cooled systems, gas coolers/condensers are of the finned-tube type. This type of heat exchanger is well established in the HVAC and refrigeration industries. The large changes in the CO2 properties in the gas cooler, however, during supercritical operation impose special design and manufacturing considerations. This research project considered the influence of the unique heat transfer characteristics of CO2 on the design and performance of finned tube air cooled condensers/gas coolers for CO2 refrigeration applications. A combined experimental and modelling approach using Computational Fluid Dynamics (CFD) was employed. A CO2 condenser/gas cooler test facility was developed for the experimental investigations. The facility employs a ‘booster’ hot gas bypass CO2 refrigeration system, with associated condenser/gas cooler test rig and evaporator load simulation facility. A series of experimental tests were carried out with two gas coolers which incorporated horizontal and horizontal-vertical slit fins and was obtained adequate experimental data concerning gas cooler performance. CFD modelling was used to study the performance of the gas coolers. The model was validated against test results and was shown to predict the air outlet temperature and heat rejection of the gas cooler with an accuracy of within ±5%. The model was subsequently used to evaluate the effect of a fin slit between the 1st and 2nd row of tubes of the gas cooler as well as a vertical slit on the 1st row before the last tube of the section. The results showed a 6%-8% increase in the heat rejection rate of the gas cooler compared to the performance without the horizontal slit. The vertical slit in the fin of the last tube has resulted in an additional increase in heat rejection over and above that for the horizontal slit of 1%-2%. CFD modelling was also used to investigate the variation of the refrigerant side, air side and overall heat transfer coefficient along the heat exchanger. The results showed that the refrigerant heat transfer coefficient increases with the decreasing of bulk refrigerant temperature and reaches its maximum when the specific heat of the refrigerant is highest. Furthermore, increasing the refrigerant mass flux, increases the refrigerant side heat transfer coefficient and heat rejection. This can reduce the size of the gas cooler for a given capacity at the expense of higher pressure drop and compressor power consumption. Air side and overall heat transfer coefficient correlations were developed for the specific gas cooler designs which were investigated and showed the heat transfer coefficients increase with increasing Reynolds Number.