The main task of the ventilation system in a building is to maintain the air quality and (together with the heating or cooling system) the thermal climate at an acceptable level within the building. This means that a sufficient amount of ventilation air at the appropriate temperature and quality must be supplied to satisfy thermal comfort and air quality demands and that this air is distributed to the parts of the building where people reside. Air movements caused by buoyancy forces can determine the distribution of ventilation air within buildings. The purpose of this thesis is to advance the state of knowledge of buoyancydriven air movements within buildings and to determine their importance both for ventilation air distribution and the maintenance of thermal comfort and air quality in buildings. The work is focused on studying thermally-driven air movements through large openings, both horizontal and vertical (i.e. doorways). The properties of a special type of thermally-driven currents, so called gravity currents, have also been explored. Large vertical openings like doorways are important for air exchange between rooms within a building. Air movements through doorways separating rooms with different air temperatures are often bidirectional and the buoyancy-driven flow rates are often greater than those caused by the mechanical ventilation system alone. Bidirectional flows through doorways can effectively spread contaminants, for example, from a kitchen or a hospital rooms, yet the results of this study indicate that the conversion of a thermally-driven bidirectional flow to a unidirectional flow via an increase of the mechanically forced flow rate requires forced flows that are more than three times greater than the thermally-driven flows. Experiments conducted in this project indicate that the resistance to buoyancy-driven flows in horizontal openings is significantly greater than that in vertical openings. Model tests have shown, however, that this problem may be mitigated if a simple model of a staircase located in the centre of the room (being ventilated) is linked to the horizontal ventilation opening. Gravity currents in rooms occur in connection with so called displacement ventilation as cool gravity currents propagate along the floor that are driven by the density difference of the ventilation air and the ambient, warmer air within the room. As these gravity currents easily pass obstacles and to a certain extent are self-controlling, they can effectively distribute the cool air within rooms in a building. Likewise, warm gravity currents occur when warmer air introduced in a room rises and spreads along the ceiling plane. One application where warm gravity currents may be used to advantage is when converting buildings from electric heating to district hot water heating thus, avoiding the introduction of an expensive hydronic heating system. This report includes a full-scale laboratory study of the basic properties of thermally-driven warm air gravity currents in a residential building and examines the possibilities of using the resulting air movements for the distribution of ventilation air as well as heat. Results from laboratory tests show that this conversion method may prove effective if certain conditions on the layout of the building are fulfilled. / QC 20100705

The sinking of dense shelf waters down the continental slope (or “cascading”) contributes to oceanic water mass formation and carbon cycling. Cascading is therefore of significant importance for the global overturning circulation and thus climate. The occurrence of cascades is highly intermittent in space and time and observations of the process itself (rather than its outcomes) are scarce. Global climate models do not typically resolve cascading owing to numerical challenges concerning turbulence, mixing and faithful representation of bottom boundary layer dynamics. This work was motivated by the need to improve the representation of cascading in numerical ocean circulation models. Typical 3-D hydrostatic ocean circulation models are employed in a series of numerical experiments to investigate the process of dense water cascading in both idealised and realistic model setups. Cascading on steep bottom topography is modelled using POLCOMS, a 3-D ocean circulation model using a terrain-following s-coordinate system. The model setup is based on a laboratory experiment of a continuous dense water flow from a central source on a conical slope in a rotating tank. The descent of the dense flow as characterised by the length of the plume as a function of time is studied for a range of parameters, such as density difference, speed of rotation, flow rate and (in the model) diffusivity and viscosity. Very good agreement between the model and the laboratory results is shown in dimensional and non-dimensional variables. It is confirmed that a hydrostatic model is capable of reproducing the essential physics of cascading on a very steep slope if the model correctly resolves velocity veering in the bottom boundary layer. Experiments changing the height of the bottom Ekman layer (by changing viscosity) and modifying the plume from a 2-layer system to a stratified regime (by enhancing diapycnal diffusion) confirm previous theories, demonstrate their limitations and offer new insights into the dynamics of cascading outside of the controlled laboratory conditions. In further numerical experiments, the idealised geometry of the conical slope is retained but up-scaled to oceanic dimensions. The NEMO-SHELF model is used to study the fate of a dense water plume of similar properties to the overflow of brine-enriched shelf waters from the Storfjorden in Svalbard. The overflow plume, resulting from sea ice formation in the Storfjorden polynya, cascades into the ambient stratification resembling the predominant water masses of Fram Strait. At intermediate depths between 200-500m the plume encounters a layer of warm, saline AtlanticWater. In some years the plume ‘pierces’ the Atlantic Layer and sinks into the deep Fram Strait while in other years it remains ‘arrested’ at Atlantic Layer depths. It has been unclear what parameters control whether the plume pierces the Atlantic Layer or not. In a series of experiments we vary the salinity ‘S’ and the flow rate ‘Q’ of the simulated Storfjorden overflow to investigate both strong and weak cascading conditions. Results show that the cascading regime (piercing, arrested or ‘shaving’ - an intermediate case) can be predicted from the initial values of S and Q. In those model experiments where the initial density of the overflow water is considerably greater than of the deepest ambient water mass we find that a cascade with high initial S does not necessarily reach the bottom if Q is low. Conversely, cascades with an initial density just slightly higher than the deepest ambient layer may flow to the bottom if the flow rate Q is high. A functional relationship between S/Q and the final depth level of plume waters is explained by the flux of potential energy (arising from the introduction of dense water at shallow depth) which, in our idealised setting, represents the only energy source for downslope descent and mixing. Lastly, the influence of tides on the propagation of a dense water plume is investigated using a regional NEMO-SHELF model with realistic bathymetry, atmospheric forcing, open boundary conditions and tides. The model has 3 km horizontal resolution and 50 vertical levels in the sh-coordinate system which is specially designed to resolve bottom boundary layer processes. Tidal effects are isolated by comparing results from model runs with and without tides. A hotspot of tidally-induced horizontal diffusion leading to the lateral dispersion of the plume is identified at the southernmost headland of Spitsbergen which is in close proximity to the plume path. As a result the lighter fractions in the diluted upper layer of the plume are drawn into the shallow coastal current that carries Storfjorden water onto the Western Svalbard Shelf, while the dense bottom layer continues to sink down the slope. This bifurcation of the plume into a diluted shelf branch and a dense downslope branch is enhanced by tidally-induced shear dispersion at the headland. Tidal effects at the headland are shown to cause a net reduction in the downslope flux of Storfjorden water into deep Fram Strait. This finding contrasts previous results from observations of a dense plume on a different shelf without abrupt topography. The dispersive mechanism which is induced by the tides is identified as a mechanism by which tides may cause a relative reduction in downslope transport, thus adding to existing understanding of tidal effects on dense water overflows.

Since 1996, super-critical CO$_2$ has been injected at a rate of $\sim$0.85~Mt~yr$^{-1}$ into a pristine, saline aquifer at the Sleipner carbon capture and storage project. A suite of time-lapse, three-dimensional seismic reflection surveys have been acquired over the injection site. This suite includes a pre-injection survey acquired in 1994 and seven post-injection surveys acquired between 1999 and 2010. Nine consistently bright reflections within the reservoir, mapped on all post-injection surveys, are interpreted to be thin layers of CO$_2$ trapped beneath mudstone horizons. The areal extents of these CO$_2$ layers are observed to either increase or remain constant with time. However, volume flux of CO$_2$ into these layers has proven difficult to measure accurately. In addition, the complex planform of the shallowest layer, Layer 9, has proven challenging to explain using reservoir simulations. In this dissertation, the spatial distribution of CO$_2$ in Layer~9 is measured in three dimensions using a combination of seismic reflection amplitudes and changes in two-way travel time between time-lapse seismic reflection surveys. The CO$_2$ volume in this layer is shown to be growing at an increasing rate through time. To investigate CO$_2$ flow within Layer~9, a numerical gravity current model that accounts for topographic gradients is developed. This vertically-integrated model is computationally efficient, allowing it to be inverted to find reservoir properties that minimise differences between measured and modelled CO$_2$ distributions. The best-fitting reservoir permeability agrees with measured values from nearby wells. Rapid northward migration of CO$_2$ in Layer~9 is explained by a high permeability channel, inferred from spectral decomposition of the seismic reflection surveys. This numerical model is found to be capable of forecasting CO$_2$ flow by comparing models calibrated on early seismic reflection surveys to observed CO$_2$ distributions from later surveys. Numerical and analytical models are then used to assess the effect of the proximity of an impermeable base on the flow of a buoyant fluid, motivated by the variable thickness of the uppermost reservoir. Spatial gradients in the confinement of the reservoir are found to direct the flow of CO$_2$ when the current is of comparable thickness to the reservoir. Finally, CO$_2$ volume in the second shallowest layer, Layer~8, is measured using structural analysis and numerical modelling. CO$_2$ in Layer~8 is estimated to have reached the spill point of its structural trap by 2010. CO$_2$ flux into the upper two layers is now $\sim$40\% of total CO$_2$ flux injected at the base of the reservoir, and is increasing with time. This estimate is supported by observations of decreasing areal growth rate of the lower layers. The uppermost layers are therefore expected to contribute significantly to the total reservoir storage capacity in the future. CO$_2$ flow within Layer~9 beyond 2010 is forecast to be predominantly directed towards a topographic dome located $\sim$3~km north of the injection point. This dissertation shows that advances in determining the spatial distribution and flow of CO$_2$ in the sub-surface can be made by a combination of careful seismic interpretation and numerical flow modelling.

Les courants de gravité, écoulements issus de la présence d’un contraste de densité dans un fluide ou de la présence de fluides de densités différentes, sont rencontrés dans de nombreuses situations naturelles ou industrielles. Quelques exemples de courants de gravité sont les avalanches, les marées noires et les courants de turbidité. Certains courants de gravité peuvent représenter un danger pour l’homme ou l’environnement, il est donc nécessaire de comprendre et de prédire leur dynamique. Cette thèse a pour objectif d’étudier l’évolution de courants de gravité de masse fixée, et notamment l’influence d’une forme initiale non-axisymétrique sur la dynamique, effet jusque-là peu abordé dans la littérature. Pour cela, une large gamme de paramètres est couverte, incluant le rapport de masse volumique entre le fluide ambiant et le fluide dans le courant, le rapport de forme initiale, la forme de la section horizontale de la colonne de fluide (circulaire, rectangulaire ou en forme de croix), le nombre de Reynolds (couvrant jusqu’à 4 ordres de grandeur) et la nature du fluide lourd (salin ou chargé en particules). Deux campagnes d’expériences ont été menées et complétées par des simulations numériques hautement résolues. Le résultat majeur est que la propagation du courant et le dépôt de particules (lorsque particules il y a) sont fortement influencés par la forme initiale de la colonne de fluide. Dans le cas de la colonne initialement rectangulaire le courant se propage plus vite et dépose plus de particules dans la direction initialement de plus courte dimension. Ce comportement non-axisymétrique est observé dans une large gamme des paramètres étudiés ici. Pourtant les modèles analytiques existants et notamment le modèle dit de boîte (box model) qui prédit avec succès le comportement des courants de gravité/turbidité dans les cas plan et axisymétrique ne sont pas capables de reproduire ce phénomène. C’est pourquoi une extension du box model a été développée ici, et est en mesure de décrire la dynamique de courants de gravité de masse fixée dont la forme initiale est arbitraire. Le cas plus général d'un courant de gravité évoluant sur un plan incliné a été abordé et une dynamique intéressante a été observée. / Gravity currents are buoyancy driven flows that appear in a variety of situations in nature as well as industrial applications. Typical examples include avalanches, oil spills, and turbidity currents. Most naturally occurring gravity currents are catastrophic in nature, and therefore there is a need to understand how these currents advance, the speeds they can attain, and the range they might cover. This dissertation will focus on the short and long term evolution of gravity currents initiated from a finite release. In particular, we will focus attention to hitherto unaddressed effect of the initial shape on the dynamics of gravity currents. A range of parameters is considered, which include the density ratio between the current and the ambient (heavy, light, and Boussinesq currents), the initial height aspect ratio (height/radius), different initial cross-sectional geometries (circular, rectangular, plus-shaped), a wide range of Reynolds numbers covering 4 orders of magnitude, as well as conservative scalar and non-conservative (particle-driven) currents. A large number of experiments have been conducted with the abovementioned parameters, some of these experiments were complemented with highly-resolved direct numerical simulations. The major outcome is that the shape of the spreading current, the speed of propagation, and the final deposition profile (for particle-driven currents) are significantly influenced by the initial geometry, displaying substantial azimuthal variation. Especially for the rectangular cases, the current propagates farther and deposits more particles along the initial minor axis of the rectangular cross section. This behavior pertaining to non-axisymmetric release is robust, in the sense that it is observed for the aforementioned range of parameters, but nonetheless cannot be predicted by current theoretical models such as the box model, which has been proven to work in the context of planar and axisymmetric releases. To that end, we put forth a simple analytical model (an extension to the classical box model), well suited for accurately capturing the evolution of finite volume gravity current releases with arbitrary initial shapes. We further investigate the dynamics of a gravity current resulting from a finite volume release on a sloping boundary where we observe some surprising features.

[pt] Correntes de turbidez ocorrem tanto na natureza como em situações criadas pelo homem. Segundo alguns pesquisadores, grande parte das reservas de petróleo conhecidas no mundo estão armazenadas nos reservatórios de hidrocarbonetos formados a partir dos sistemas turbidíticos. Tendo em vista a importância dessas correntes, este trabalho pretende propor um modelo numérico consistente e com baixo custo computacional, capaz de auxiliar na previsão de sedimentação de bacias nos processos de modelagem estratigráfica no contexto do simulador numérico de sedimentação tridimensional com ênfase nos processos deposicionais em ambientes de talude plataforma e bacia, chamado Steno, e validar o modelo numérico proposto através de simulações físicas conduzidas no Instituto de Pesquisas Hidráulicas da Universidade Federal do Rio Grande do Sul. O modelo numérico proposto é baseado nas equações de Navier-Stokes, que são resolvidas tirando-se a média na profundidade do perfil de velocidades característico. Além disso, uma aplicação gráfica tridimensional para simulação numérica de correntes de turbidez, chamada Turb3D, foi desenvolvida com base nas equações propostas. O programa possui uma interface gráfica amigável para o usuário no que diz respeito à entrada de dados, solução e visualização dos resultados. Os resultados obtidos mostraram que apesar do modelo numérico possuir um bom desempenho computacional, se comparado com os complexos modelos numéricos existentes, não foi possível reproduzir com precisão a evolução das correntes de turbidez e seus depósitos. Contudo, acredita-se que a abordagem dada ao problema de simulação numérica de correntes de turbidez apresentada neste trabalho pode fornecer bons resultados. Entretanto, é fundamental que seja dada continuidade a pesquisa, de modo que ajustes e melhorias ao modelo proposto possam ser realizadas. / [en] Turbidity currents occur in both natural and man-made situations. In agreement with some researchers, most of the world’s oil reserves are stored in hydrocarbon reservoir built by turbidity systems. Because of the importance of these currents, this work proposes a consistent and efficient numerical method for simulations of turbidity currents for basin sedimentations predictions in the stratigraphic modelling process, which will be incorporated to the numerical simulator of sedimentation 3D called Steno. In order to validate the proposed numerical model experiments were conducted at the Hydraulics Research Institute of Federal University of Rio Grande do Sul. The proposed numerical model is based on Navier-Stokes equations that are solved in the depth-average. Moreover, a three-dimensional graphic application for numerical simulations of turbidity currents called Turb3D was developed. The application user interface provides a common, user-friendly, graphical environment for pre-processing, solution and post-processing. Despite the good computational performance achieved by using this approach, the method presented did not reproduce accurately the evolution of turbidity current and their deposits. However, it is believed that the approach to the numeric simulation of the turbidity current problem given in this work can provide better results, although this research should continue and improvements should be made.

Submitted by PPG Engenharia e Tecnologia de Materiais (engenharia.pg.materiais@pucrs.br) on 2018-06-05T13:28:29Z No. of bitstreams: 1 frantz2018simulacoes.pdf: 23131075 bytes, checksum: e748910d1820968a07c86be9461b7489 (MD5) / Approved for entry into archive by Sheila Dias (sheila.dias@pucrs.br) on 2018-06-12T12:40:17Z (GMT) No. of bitstreams: 1 frantz2018simulacoes.pdf: 23131075 bytes, checksum: e748910d1820968a07c86be9461b7489 (MD5) / Made available in DSpace on 2018-06-12T12:49:08Z (GMT). No. of bitstreams: 1 frantz2018simulacoes.pdf: 23131075 bytes, checksum: e748910d1820968a07c86be9461b7489 (MD5) Previous issue date: 2018-03-09 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / This work investigates the method of large-eddy simulation (LES) in the context of gravity currents, which is found necessary since it allows a substantial increase in the order of magnitude of the characteristic Reynolds number used in numerical simulations, approaching them with natural scales, in addition to significantly reducing the computational cost. The implicit large eddy simulation (ILES) methodology, based on the spectral vanishing viscosity model, is unprecedentedly employed in the context of gravity currents, is compared against with explicit methods such as the static and dynamic Smagorisnky. The evaluation of the models is performed based on statistics from a direct numerical simulation (DNS). Results demonstrate that the first model based purely on numerical dissipation, introduced by means of the second order derivative, generates better correlations with the direct simulation. Finally, experimental cases of the literature, in different flow configurations, are reproduced numerically showing good agreement in terms of the front position evolution. / Este trabalho investiga o m?todo de simula??o de grandes escalas (LES) no contexto de correntes gravitacionais. O mesmo se faz necess?rio, visto que possibilita um aumento substancial da ordem de grandeza do n?mero de Reynolds caracter?stico utilizado em simula??es num?ricas, aproximando os mesmos de escalas naturais, al?m de reduzir significativamente o custo computacional dos c?lculos. A avalia??o dos modelos ? realizada utilizando uma base de dados de simula??o num?rica direta (DNS). A metodologia de simula??o de grandes escalas impl?cita (ILES), baseada no modelo de viscosidade turbulenta espectral, ? colocado a prova de maneira in?dita no contexto de correntes de gravidade com m?todos expl?citos dispon?veis na literatura. Resultados demonstram que o mesmo, baseado puramente em dissipa??o num?rica introduzida por meio do comportamento dos esquemas de derivada de segunda ordem, gera melhores correla??es com as estat?sticas baseadas em campos m?dios da simula??o direta. Por fim, casos experimentais da literatura, em diferentes configura??es de escoamento, s?o reproduzidos numericamente.

Αντικείμενο της διατριβής είναι η προσομοίωση της ροής και της μεταφοράς θερμότητας σε ηλιακά και ενεργειακά συστήματα. Η έμφαση δόθηκε στις δεξαμενές αποθήκευσης της θερμότητας που παράγεται στα συγκεκριμένα συστήματα, με στόχο τον χαρακτηρισμό των ενεργειακών απωλειών και την βελτιστοποίηση του σχεδιασμού τους. Κύριες δραστηριότητες της διατριβής θα είναι η περαιτέρω ανάπτυξη διαθέσιμων εργαλείων προσομοίωσης ροών φυσικής και μικτής συναγωγής, με διερεύνηση των νεώτερων εξελίξεων στην μοντελοποίηση με τη μέθοδο Προσομοίωσης Μεγάλων Δινών (LES). Αρχικά γίνεται εκτεταμένη επικύρωση με πειραματικά αποτελέσματα σε απλές γεωμετρικές διατάξεις (π.χ. ορθογωνικά κανάλια ή κοιλώματα με βαθμίδα θερμοκρασίας) από την βιβλιογραφία. Στη συνέχεια η μεθοδολογία εφαρμόζεται στον υπολογισμό ροών σε πιο ρεαλιστικές γεωμετρίες, επιλεγμένες από πρακτικές εφαρμογές, όπως οι δεξαμενές αποθήκευσης νερού. Αναλύονται σε βάθος οι δυναμικές διεργασίες και τα ροϊκά φαινόμενα τόσο κατά την προσαγωγή της θερμότητας στη δεξαμενή (φόρτιση) όσο και κατά την απαγωγή της (εκφόρτιση) και η επίδραση που έχουν αυτά στην αποδοτικότητα της αποθήκευσης με βάση κατάλληλους ποσοτικούς δείκτες. Από τα αποτελέσματα αναδεικνύεται η σημασία της μοντελοποίησης σε τέτοιου είδους συστήματα ως ένα σημαντικό εργαλείο στη διερεύνηση της απόδοσης τους, του ενεργειακού χαρακτηρισμού τους και ακολούθως στην προσπάθεια επίτευξης του βέλτιστου σχεδιασμού τους. / The subject of the thesis is the Simulation of Turbulent Flow and Heat Transfer in Solar and Energy Systems. Emphasis is given in the thermal storage component of these systems, with the aim of characterizing their energy losses and improve their design. Main activities of the thesis will be the further development of available computational tools for the simulation of flows in natural and mixed convection, incorporating some of the most recent developments in modeling, particularly in the Large Eddy Simulation (LES) method. Initially, an extensive validation with experimental results in simple geometric configurations is carried out (e.g. channels or differentially heated cavities). Subsequently, the methodology is applied in the calculation of flows for more realistic geometries selected from practical applications, such as various hot water storage tanks. Analysis is conducted of the dynamic processes and relevant physical phenomena during the heat supply (charging) to and removal (discharging) from the tank and their influence on the storage effectiveness using appropriate thermodynamic indices. From the simulation results, the significance of the flow and heat transfer modeling in these systems as a practical tool for studying their performance is demonstrated, by characterizing their energy content and significantly contributing to the process of optimizing their design.