Turbulence dispersion is a phenomenon of practical importance in many multiphase flow systems. It has a strong effect on the distribution of the dispersed phase. Physically, this phenomenon is a result of interactions between individual particles of the dispersed phase and the continuous phase turbulence eddies. In a Lagrangian simulation, a particle-eddy interaction sub-model can be introduced and the effect of turbulence dispersion is automatically accounted for during particle tracking. Nevertheless, tracking of particleturbulence interaction is extremely expensive for the small time steps required. For this reason, the Lagrangian method is restricted to small-scale dilute flow problems. In contrast, the Eulerian approach based on the continuum modeling of the dispersed phase is more efficient for densely laden flows. In the Eulerian frame, the effect of turbulence dispersion appears as a turbulent diffusion term in the scalar transport equations and the so-called turbulent dispersion force in the momentum equations. The former vanishes if the Favre (mass-weighted) averaged velocity is adopted for the transport equation system. The latter is actually the total account of the turbulence effect on the interfacial forces. In many cases, only the fluctuating effect of the drag force is important. Therefore, many models available in the literature only consider the drag contribution. A new, more general derivation of the FAD (Favre Averaged Drag) model in the multi-fluid modeling framework is presented and validated in this report.

Cette étude sur la stabilité d’une bulle unique de cavitation acoustique s’inscrit dans le cadre d’un projet ANR démarré en septembre 2009 (SONONUCLICE ANR-09-BLAN-0040-02). Elle se situe dans la continuité des travaux sur l’optimisation du procédé de lyophilisation de produits pharmaceutiques menés par l’équipe « Transferts couplés de matière et de chaleur » du laboratoire LAGEP (ESCPE/UCB, Lyon), équipe porteuse du projet, et des travaux sur la cristallisation assistée par ultrasons du laboratoire RAPSODEE. L’application des ultrasons de puissance dans un liquide produit des milliards de bulles. Ce phénomène est appelé cavitation acoustique. Les bulles formées ne font pas toutes la même taille, leurs oscillations ne sont pas en phase, et leur densité dans le fluide est très inhomogène : ce phénomène très complexe implique donc de nombreuses variables difficiles à isoler. Même si le phénomène est chaotique, la cavitation permet d’observer des effets macroscopiques notables sur la nucléation et la croissance des cristaux de glace dans une solution sous-refroidie. Ces effets sont d’une importance capitale pour des applications de congélation ou de lyophilisation. Bien que les effets des ultrasons présentent des intérêts certains sur la cristallisation, leur origine reste mal connue. L’observation directe des milliards de bulles ne fournit aucune piste sur les mécanismes microscopiques mis en jeu. Afin d’isoler l’acteur essentiel de ces effets, l’étude menée vise à isoler une bulle de cavitation acoustique. Pour cela, une cellule de lévitation carrée en verre a été conçue. Le verre a été retenu comme matériau pour sa rigidité et sa transparence. Dans cette cellule, une onde de pression acoustique est imposée par un piézoélectrique collé à la base de la cellule. Il a été possible de reconstruire la dynamique de la bulle. Les étapes d’expansion, d’implosion et de rebonds sont clairement visibles. En vue de l’étude de la cristallisation, un principe de détection des cristaux a été spécifiquement élaboré. Il repose sur le suivi de la modification de la périodicité de la bulle (mesurée par un microphone) provoquée par l’apparition d’un corps étranger à son voisinage. Une méthode utilisant la corrélation de signaux acoustiques du microphone filtré à la fréquence d’excitation du PZT et les harmoniques du signal du microphone directe a été développée. Elle permet de connaître le régime d’oscillation de la bulle et de détecter toutes les modifications de sa dynamique. Des expériences de perturbation de la bulle ont été menées à l’aide d’une micro fibre de 7 μm. Le principe de détection est alors mis en oeuvre pour déclencher l’enregistrement d’images par une caméra rapide lors des derniers instants d’existence de la bulle. Cette méthode devrait permettre de détecter l’apparition des premiers cristaux au voisinage de la bulle. Autour de la cellule de lévitation, différents systèmes ont été développés. Un système de dégazage et de remplissage de la cellule de cavitation ont permis de travailler avec de l’eau ayant des teneurs en gaz dissous de l’ordre de 20 % de la saturation. Un système d’éclairage avec une LED de puissance et un jeu de lentilles optiques a été conçu pour visualiser correctement la bulle. / This study of the stability of an acoustic cavitation bubble is part of an ANR project started in September 2009 (SONONUCLICE ANR-09-BLAN-0040-02). It takes place in the continuity of the works on the optimization process of lyophilisation of pharmaceutical products conducted by the “Transferts couplés de matière et de chaleur” team of LAGEP (ESCPE/UCB, Lyon) laboratory, which is the project’s team leader, and the studies of ultrasound-assisted crystallization in the RAPSODEE Centre. The application of power ultrasound into liquids produces thousands of bubbles. This phenomenon is called acoustic cavitation. The bubbles formed don’t have the same size, their oscillations are not in phase, and their spatial density in the fluid is not homogeneous: this phenomenon is very complex and involves multiple variables very difficult to isolate. Even if this phenomenon is chaotic, it allows to observe macroscopic effects on the nucleation and crystal growth of ice in undercooled solutions. These effects have a capital importance for industrial applications such as freezing and lyophilisation (also called freeze drying). Although ultrasound has a noticeable influence on crystallization, the origin of these effects remains unclear. The multi-bubble approach doesn’t give any hint on the microscopic mechanisms involved. In order to isolate the main actor of these effects, this study aims at isolating a single cavitation bubble. To do that, a cubic levitation cell made of optical glass was build. In this cell, an acoustic pressure is applied by a piezoelectric glued to the bottom’s external face of the cell. With this cell is possible to rebuild all the oscillations states of the bubble, and in combination with our optical system we can see the bubble’s dynamics and its stages like: expansion, collapse and rebounds. For the crystallization part of this study, a crystal’s detection system was developed. It is based on the variations of the bubble’s periodicity (measured by a microphone pill) introduced by the sudden appearance of a foreign body in its vicinity. This method requires the correlation of the signals from a filtered microphone and the harmonics signals from a microphone, in order to known the oscillation state of the bubble and detect variations on the bubble’s dynamics. Experiments of bubble perturbations by a thin wire were made. The detection system was used to trigger the image recording of a fast camera, in order to capture the final moments of the bubble. This method should be allowing the early detection of new crystals in the proximity of the bubble. Around the levitation cell, various systems have been developed. A degassing and filling system for the cavitation cell allow us to work with degased water around the 20 % of its saturated concentration of air. An illumination system based in a power LED and a set of optical lenses was used to view the bubble correctly.

Fragmentation of rivers due to the rapid expansion of hydropower plants is one of the major factors responsible for the decline in the Atlantic salmon (Salmo salar L.) in the Baltic Sea. Current methods to aid downstream migrating salmon smolt still struggle with effectively stopping the smolt from going through the hydropower turbines, by guiding them towards the available salmon ladders. A promising guiding method is using a barrier of bubbles which may serve as a non-intrusive alternative to conventional guiding structures. This study evaluated the risk of artificial selection caused using bubble barriers and compared the results with an earlier study.  The three hypotheses tested were: i) the salmon display a scototaxis behaviour related to their boldness, ii) temperature, and size of arena affect the scototaxis of the salmon, and iii) the salmons reaction to a bubble barrier is correlated to their boldness. The hypotheses were tested in a controlled laboratory setting by conducting three scototaxis tests and an experiment with several bubble barriers, where the reaction of salmon to bubbles could be assessed. There was significant positive correlation (r = 0,62 & p = 0,012) between the boldness of an individual and the time it took to swim through the bubbles. Even though a significant difference in scototaxis as a result of lowered temperature were discovered, the difference was too small to have a noticeable effect on the overall results. A bubble barrier may therefore put artificial selection pressure on the salmon population.

Bostadsmarknaden har varit väldigt uppmärksammad de senaste åren på grund av de ökade priserna och lägre bolåneräntor som blivit resultatet av en sänkt reporänta. I detta arbete har skuldsättningen av olika hushållstyper exemplifierats och simulerats utifrån att marknadsvariabler förändras såsom marknadsvärden på bostäder och ränteläget. Utöver detta har skuldsanering satts i relation till en ny typ av skuldlättnad som skulle kunna vara ett alternativ till skuldsanering men också samtidigt förhindra reaförluster och ostabila hushåll under verkningar av exempelvis en ökad räntebörda som hushåll inte klarar av. Denna skuldlättnad är en typ av skuldsanering som bankerna blir obligerade att genomföra då de varit för optimistiska i sin utlåning. Detta skulle göra att bostaden i sig blir mer intressant för bankerna i stället för att endast fokusera på hushållets betalningsförmåga. På detta sätt har bankerna ytterligare ett incitament till att vara försiktiga i sin utlåning, samtidigt som i de fall då denna skuldnedskrivning skulle aktualiseras skulle även hushållen kunna bo kvar i sina bostäder och inte behöva sälja till ett pris möjligen lägre än vad som finns kvar på bolånet. Detta leder till en mer stabil marknad i det fall att priserna skulle sjunka och räntan öka, då inga större realisationsförluster skulle behöva äga rum utan i stället räddas hushållens förmåga att kunna betala för sin bostad. / The housing market has been much discussed the last few years as a consequence of the dramatically increasing prices and the lower interest rates on housing mortgages, which in turn was the consequence of the lowered repo rate. In this thesis the debt constellation of different household types have been exemplified and simulated from market variables such as market values and interest rate levels. Beyond this, debt restructuring has been modified into another type of debt relief that could be an alternative to debt restructuring but also being able to prevent capital losses and instability on the housing market as a consequence of too high interest rates which may force households to sell their house under mortgage levels. This type of debt relief could obligate the banks to relieve the households of the excess debt that they cannot meet. By doing this the banks will have a stronger incentive of making a proper valuation of the residence instead of only focusing on the solvency of the household. This will have the effect that once the household can’t pay for their mortgage they will get the chance to be relieved of excess debt so that they still can live in their residence and not having to sell the property at a low market price so that mortgage still remains after the sell, as this could result in more instability on the housing market. The losses for the banks on debt relief are calculated in these cases to be significantly smaller than the potential capital losses on properties. As a result, fluctuations on the market could be contained to a certain degree by obligating banks to make debt relieves for the households that otherwise would have had to sell, and thus, offering more stability on an instable market.

As society becomes more and more sedentary new solutions must be researched. One such possible solution is exergames.They are games that require physical movement to be played.By playing exergames a player can become less sedentary and get a bit of physical exercise. This thesis will examine the viability of a bubble shooter game as an exergame in order to potentially find another kind. This is to find if a bubble shooter is a game type that fits well with being made into an exergame. To do this a bubble shooter was developed and turned into an exergame. Then a small user test wascarried out in order to gather data on the viability of the game. These results showed that a bubble shooter works well as an exergame, and it has the potential to work really well.

Svenska bostadsmarknaden har de senaste åren varit ett hett diskussionsämne i media. Kanske beror det på att bostadspriserna har stigit i en snabbare takt de senaste 15 åren jämfört med de senaste 60 åren, vilket har skapat diskussioner kring en eventuell prisbubbla som skulle orsaka stora problem för många. När nobelpristagaren i ekonomi 2013 Robert Shiller samtidigt som det blev officiellt att han blivit tilldelad priset, gick ut med att han tror att risken för en bubbla är stor på Svenska bostadsmarknaden, var ingenting som fick debatten att upphöra. Efter uttalandet har bostadspriserna gått upp med ännu raskare takt än vad de gjort tidigare och gör frågan än mer aktuell än tidigare. Syftet med denna rapport är att undersöka vilka faktorer som är tillämpliga vid en prisbubblas bildande samt undersöka huruvida risken är stor idag för att en sådan ska inträffa med fokus på vad som vad som skedde under 90-talet när en prisbubbla senast uppstod på den Svenska bostadsmarknaden. Har vi idag en stor risk för en bostadsbubbla i Sverige? Samt vilka faktorer tyder på att risken är stor respektive vilka tyder på att risken är mindre? Det är några av frågorna som ska bli besvarade i uppsatsen För att kunna undersöka huruvida bubbla föreligger har personer med lång erfarenhet inom byggbranschen intervjuats som också har en relevant befattning inom respektive institution eller företag. Resultatet visar att många faktorer pekar på att det inte finns någon bubbla i Sverige, samtidigt som det fortfarande inte går att förkasta att risken finns, då vissa faktorer pekar på just det. Det som visar sig mest troligt är dock en mer jämn marknad där priserna varken går upp eller ner. / Swedish housing market in recent years has been a hot topic in the media. Perhaps it is because housing prices have risen at a faster pace over the past 15 years compared to the past 60 years, creating discussions about a possible price bubble that could cause major problems for many. When the Nobel Prize in economics in 2013, Robert Shiller, while it became official that he has been awarded the prize, went out with that he believes that the risk of a bubble is big at the Swedish housing market, this did not put an end of the debate. After the statement, housing prices have gone up even more rapidly than what it has previously and makes the issue even more relevant than before. The purpose of this report is to examine the factors that are relevant at a housing bubble formation and examine whether the risk is big today to such a thing to occur, with a focus on what happened in the 90s when a price bubble later arose on the Swedish housing market. Do we have a high risk of a housing bubble today on Swedish housing market? What factors suggest that the risk is big and which suggest that the risk is less? These are some question that will be answered during the essay In order to investigate whether a bubble exists or not, people with long experience in the construction industry were interviewed who also has a relevant position within the respective institution or company. The result shows that many factors indicate that there is no bubble on the Swedish housing market, while it still does not reject that possibility exists, certain factors indicate just that. It turns out that most likely, however, a more stable market where prices either go up or down.

In a historical view, house prices have developed at the same rate as inflation. In the end of the 20th century the prices on the housing market started to differ from the inflation rate, and today the deviation is strong and the real price increase has been very strong. The financial crisis in 2008, which was a result of a subprime mortgage crisis on the American real estate market, has made several agents on the housing market in Sweden question today’s prices. Robert Schiller, an American economist who predicted the mortgage crisis in 2008, claimed during the Nobel Prize ceremony 2013, that Sweden shows signs of a financial bubble – "I think that people here in Sweden have an illusion that increasing prices is a lasting trend, but that is more suggestive of a bubble". Sweden has been in a recession for the last years, which has led to low interest rates and beneficial conditions on the real estate market. Despite the high unemployment and the recession, prices on the housing market in Stockholm County have continued to increase. This bachelor thesis aims to analyze the housing market in Stockholm County and investigate if there is a price bubble or not, or if the high prices can be explained by fundamental factors. The Thesis suggests that there is no price bubble on the housing market in Stockholm County. The increasing prices are based on fundamental factors, such as low interest rate, low supply on the housing market in combination with an increasing demand. Therefore it is unlikely that there is no existing pricing bubble on the housing market in Stockholm County.

Recent technological developments, mostly in the fields of concentrated solar power and microelectronics, have driven heat transfer requirements higher than current heat exchangers are capable of producing. Processing power is increasing, while processor size simultaneously decreases and the heat flux requirements of concentrating solar power plants are being driven up by the high temperatures that produce the best thermal efficiency. Heat transfer in microchannels, specifically when utilising flow boiling, has been shown to produce significantly higher heat fluxes than their macro-scale counterparts and could have a large impact on many industrial fields. This high heat transfer characteristic is caused by a number of factors, including the large difference between the sensible and latent heat of the working fluid and the evaporation of a thin liquid film that forms between the microchannel walls and the vapour bubbles. These phenomena occur at incredibly small scales. Flow visualisations, temperature and pressure measurements are therefore difficult to obtain. Many experiments that cover a wide range of microchannel sizes, shapes and orientations, and utilise different working fluids and heat fluxes have been reported. However, the correlations between confined boiling, heat flux and pressure drop have mostly been produced for macro-scale flow. Many different criteria have been developed to distinguish the macro scale from the micro scale, but the general consensus is that macro-scale heat transfer correlations do not perform well when used in the micro scale. Heat transfer correlations are typically created by performing physical experiments over a wide range of parameters and then quantifying the effect that varying these parameters has on the performance of the system. The small scale and high complexity of microchannel-based heat exchangers make visualising the flow within them difficult and inaccurate because both the working fluid and the microchannel walls distort light. The use of numerical modelling via computational fluid dynamics software allows phenomena that occur within the channel to be simulated, which provides valuable insight into how rapid bubble growth affects the surrounding fluid, which can lead to the design of better heat exchangers. This study focused on numerically modelling the growth of a single bubble during the flow boiling of FC-72 in a microchannel with a hydraulic diameter of 0.9 mm and an aspect ratio of 10. The numerical domain was limited to a 10 mm section of the microchannel where bubble nucleation and detachment were observed in an experimental study on a similar microchannel setup. The high cost of 3D simulations was offset by an interface-tracking mesh refinement method, which refined cells not only at the interface, but also a set distance on either side of the interface. To focus on the effects of gravity, a simplified approach is used, which isolates certain phenomena. Density gradients, material roughness and multiple bubble interaction are ignored so that the effects of buoyancy and bubble detachment can be analysed. Simulations are first performed in a 2D section through the centre of the microchannel, and then in the full 3D domain. In both the 3D numerical and experimental cases (Meyer et al., 2020), the bottom heated case had the lowest maximum temperature and the highest heat transfer characteristics, which were influenced by the detachment of the bubble from the heated surface. This observation indicates that the gravitational orientation of the channel can have a significant effect on the heat transfer characteristics of microchannel-based heat exchangers, and that more investigation is required to characterise the extent of this effect. / Dissertation (MEng)--University of Pretoria, 2019. / Mechanical and Aeronautical Engineering / MEng / Unrestricted

<div><br></div><div> Today, autoinjectors are developed for more viscous drug solutions, which require larger forces for actuating the syringe and impose larger stresses on the drug solution during the administration of autoinjectors. We developed experimentally validated high-fidelity simulations to investigate the liquid jet formation, liquid slosh and cavitation during the insertion process of an autoinjector. </div><div> </div><div> The jet formed due to an acceleration-deceleration motion of syringe is found to be governed by the interplay between inertial, viscous, surface tension and gravitational forces. A scaling for the jet velocity and a criterion for the jet breakup in a simplified geometry are proposed.</div><div> </div><div> When the syringe accelerates and decelerates during the insertion, liquid slosh occurs and there is a vehement motion of the air-liquid interface. Here, we quantified the area of air-liquid interface and hydrodynamic strain rate, which increase with the air gap size, syringe velocity, tilt angle and inner wall hydrophobicity, and decrease with the solution viscosity and hardly change with the liquid column height and surface tension. The strain rate is not sufficient to unfold the protein and the air-liquid interface is more likely to cause protein aggregation.</div><div> </div><div> In a spring-driven autoinjector, the plunger is actuated by the impact of a driving rod, which generates a strong pressure wave and can cause cavitation inception. The cavtiation bubbles can be impeded by the syringe walls and form a re-entrant jet shooting toward the syringe wall. During the process, the protein molecules are focused in the jet, pushed toward the syringe wall and spread across the wall, which can be the reason for the protein aggregation and adsorption on the syringe walls. The impedance effects of the wall decreases with the wall distance and increases with the maximum bubble size. The maximum bubble radius also increases with the liquid column size and nucleus size and decreases with the air gap pressure. Since inertia effects dominate in the cavitation process, the liquid viscosity and surface tension hardly changes the cavitation bubble dynamics. Small bubbles can also form in the bulk, which may generate aggregates in the bulk solution. Bubbles in the cavitation bubble cloud may coalesce with nearby bubbles and induce a higher pressure at the collapse (up to 1000 bar). This high pressure can potentially generate hydroxyl radicals that oxidize the protein molecules.</div><div> </div><div> The current study presents a detailed picture of fluid flows in autoinjectors and provide recommendations for mitigating the liquid slosh and cavitation generated in syringes. The results can be combined with future experiments to understand the implications of fluid flows on protein drugs and the performance of autoinjectors.</div>

There exist several different numerical models for predicting bubble coalescence and breakup using computational fluid dynamics (CFD). Various combinations of these models will be employed to model a bioreactor process in a stirred reactor tank. A mass transfer coefficient, Kla, has been calculated and compared to those found experimentally by Thermo-Fisher Scientific, to validate the accuracy of currently available mathematical models for population balance equations. These include various combinations of bubble breakup and coalescence models coupled with the calculation of mass transfer coefficients.