Cette thèse porte sur l'agrégation de la convection dans le modèle de circulation générale LMDZ5A à l'équilibre radiatif-convectif (RCE). L'instabilité du RCE mise en évidence nous permet d'étudier les mécanismes d'initiation de l'agrégation et leur dépendance à la température de surface océanique (SST). A basse SST, l'agrégation résulte d'un couplage entre la circulation grande-échelle et les effets radiatifs des nuages bas. A haute SST, elle provient d'un couplage entre la circulation de grande-échelle et les flux turbulents à la surface. Le couplage de l'atmosphère avec une couche de mélange océanique rend l'initiation de l'agrégation moins dépendante de la SST et des mécanismes d'initiation, à l'exception des effets radiatifs des nuages hauts. L'impact de l'agrégation sur la sensibilité climatique et la température de surface est aussi analysé. En favorisant la formation de zones ciel clair sèches, l'agrégation refroidit fortement le système climatique. Toutefois, cet effet est limité par l'effet des changements de gradients de SST et de fraction de nuages bas qui tendent au contraire à faire augmenter la sensibilité climatique. Aux plus courtes échelles temporelles, en revanche, le couplage entre océan et agrégation de la convection est à l'origine d'une boucle de rétroaction stabilisatrice qui contrôle l'agrégation et renverse complètement son effet. Ainsi, l'effet de l'agrégation sur la sensibilité climatique est assez faible par rapport à ce que laissent penser les simulations où le couplage océan-atmosphère est absent. Ces résultats montrent l'importance de considérer le couplage océan-atmosphère dans l'étude du rôle de l'agrégation dans le climat. / This thesis focuses on the study of convective aggregation in LMDZ5A general circulation model, used in Radiative-Convective Equilibrium (RCE) configuration. The instability of the RCE allows us to look at the mechanisms controlling the initiation of convective aggregation and its dependence on sea surface temperatures (SST). At low SSTs, a coupling between the large-scale circulation and the radiative effects of low clouds is needed to trigger self-aggregation. At high SSTs, the coupling between the large-scale circulation and the surface fluxes controls this initiation. When the atmosphere is coupled to a slab ocean mixed layer, SST gradients facilitate the initiation of convective aggregation. Except for the high-cloud radiative effects, triggering mechanisms are less crucial. Convection also becomes less dependent on the SST.The impact of convective aggregation on the climate sensitivity and surface temperature is also analyzed. Convective aggregation is found to increase the area of dry clear-sky zones. Thus, it tends to cool the system very efficiently. However, the negative feedback associated with an increase in aggregation is generally balanced by offsetting changes in SST gradients and low clouds that tend to increase the climate sensitivity. In contrast, at shorter timescales, the coupling between ocean and convective aggregation also controls the strength of convective aggregation and overturn its effect. Thus the impact of convective aggregation may not be as strong as what can be inferred from experiments with uniform SSTs.These results emphasize the importance of considering ocean-atmosphere coupling when studying the role of aggregation in climate.

The mesoscale organization and structure of convective clouds is thought to be rooted in the thermodynamic properties of the atmosphere and in the turbulent to mesoscale dynamics of the flow. Such structure may contribute to the transition between shallow and deep convection. The thermodynamic state of the boundary layer is forced by the amount of surface fluxes from below. Conversely, landscape patterns and land-cover heterogeneity may equally give rise to focused regions for deep convection triggering, in particular when patch sizes exceed 10 km. Since the convective boundary layer has a mediating function between the surface and deep storm clouds, the connection between surface and upper atmosphere is not straightforward. It is generally believed to involve local erosion of the capping inversion layer, the build-up of a moist energy supply, gradual humidification of the lower-free troposphere that reduces dry air entrainment into burgeoning deeper clouds, and thermal mesoscale circulations that can generate moisture convergence and locally forced ascent. To what extent microscale realistic surface heterogeneity and an interactive surface response matter to shallow and deep convection and its organization remains an open question. In this dissertation, we describe the coupling of a physiology-based vegetation model (HYBRID) and of a sea surface flux algorithm (COARE) to the cloud-resolving Active Tracer High-resolution Atmospheric Model (ATHAM). We investigate the full diurnal cycle of convection based on the example of the Hector storm over Tiwi Islands, notably the well-characterized event on 30th November 2005. The model performs well in terms of timing and cloud dynamics in comparison to a range of available observations. Also, ATHAM-HYBRID seems to do well in terms of flux partitioning. Whilst awaiting more thorough flux validation, we remain confident that the interactive surface response of both HYBRID and COARE is suited for the purpose of simulating convective-scale processes. We find the storm system evolution in 3D simulations to be robust with respect to differences in surface configuration and initialization. Within our 3D sensitivity runs, we could not identify a strong dependence on either realistic surface heterogeneity in the island landscape or on the interactive surface response. We conclude that in our case study at least, atmospheric (turbulent) dynamics likely dominate over surface heterogeneity effects, provided that the bulk magnitude of the surface energy fluxes, and their partitioning into sensible and latent heat (Bowen ratio), remain unaltered. This is consistent with 2D sensitivity studies, where we find model grid-spacing and momentum diffusion, governing the dynamics, to have an important influence on the overall evolution of deep convection. Fine grid-spacing is necessary, as the median width of updraught cores mostly does not exceed 1000 m. We associate this influence with the dry air entrainment rate in the wake of rising parcels, and with how resolution and diffusion act on coherent structures in the flow. In 2D sensitivity studies with differences in realistic heterogeneities of surface properties, we find little evidence for a clear deterministic influence of these properties on the transition between shallow and deep convection, in spite of largely different storm evolutions across the various runs. In these runs, we tentatively ascribe triggering to stochastic features in the flow, without discarding the relevance of convergence lines produced by mesoscale density currents, such as the sea breeze and cold pool storm outflows.

Convective heat transfer to CO2 flowing upward in electrically heated vertical tubes at supercritical pressures was studied for wall heat fluxes q within ranges that included values corresponding to the onset of heat transfer deterioration (HTD). The inlet pressure was P = 8.35 MPa, the mass flux was in the range 200 kg/m2s ≤ G ≤ 1500 kg/m2s, and the inlet temperature was in the range 0 ◦C ≤ Tin ≤ 35 ◦C. Wall temperature measurements were collected in three tubular test sections, having inner diameters of D = 4.6, 8, and 22 mm. The abilities of three different HTD identification methods to separate the entire data set into deteriorated and normal heat transfer modes were tested. Two types of buoyancy parameters were tested as HTD detection methods, and correction factors for changes in mass flux were devised. The minimum heat flux at HTD onset was found to follow a power law of mass flux with the same exponent for all three sections and the same proportionality coefficient for the two smaller sections but a smaller one for the larger test section. For heat flux values that were larger than this minimum, HTD was found to occur only within a limited range of Tin, whose width increased with increasing heat flux. The heat transfer coefficient for normal heat transfer was expressed as an exponential function of the diameter.

Le test et la calibration des MEMS sont des enjeux complexes à cause de leur nature multi-domaines. Ils nécessitent l'application de stimuli physiques, en utilisant des équipements de test coûteux, afin de tester et de calibrer leurs spécifications. L'objectif de cette thèse est de développer des méthodes alternatives et purement électriques pour tester et calibrer un accéléromètre MEMS convectif. Premièrement, un modèle comportemental du capteur est développé et validé en se basant sur des simulations FEM. Il inclut l'influence de tous les paramètres géométriques sur la sensibilité du capteur. Deuxièmement, le modèle est utilisé pour simuler des fautes dans le but d'identifier la corrélation qui peut exister entre la sensibilité du capteur à l'accélération et certains paramètres électriques. Troisièmement, cette corrélation est exploitée pour développer des méthodes de test et de calibration alternatives où la sensibilité est estimée en effectuant uniquement des mesures électriques et sans appliquer de stimuli physiques (accélérations). L'efficacité de ces méthodes est ainsi démontrée. Finalement, deux architectures permettant l'auto-test et l'auto-calibration sur puce sont proposées. / MEMS test and calibration are challenging issues due to the multi-domain nature of MEMS devices. They therefore require the application of physical stimuli, using expensive test equipments, to test and to calibrate their specifications. The main objective of this thesis is to develop alternative electrical-only test and calibration procedures for MEMS convective accelerometers.First, a behavioral model that includes the influence of sensor geometrical parameters on sensitivity is developed and validated with respect to FEM simulations. Second, the model is used to perform fault simulations and to identify correlation that may exist between device sensitivity to acceleration and some electrical parameters. Third, this correlation is exploited to develop alternative test and calibration methods where the sensitivity is estimated using only electrical measurements and without applying any physical stimulus (acceleration). The efficiency of these methods is demonstrated. Finally, two architectures that allow on-chip test and calibration are proposed.

Fluid flows occurring in moving and/or deforming environments are influenced by the transient nature of their containment. In Computational Fluid Dynamics (CFD), simulating such flow fields requires effort to maintain the geometric integrity of the transient flow domain. Convective fluxes in such domains are evaluated with respect to the motion of the boundaries of the control volume. These simulations demand conservation of space in addition to the conservation of mass, momentum and energy as the solution continues in time. The Space Conservation Law in its continuous form can be inferred by using the rules of fundamental calculus. However, implementing it in a discrete form poses substantial challenges. During mesh motion, the surfaces enclosing the control volumes sweep through three-dimensional space. As per the Space Conservation Law, the change in the control volume has to match the sum of the swept volumes of all its faces exactly. The Space Conservation Law must be satisfied accurately and consistently in order to avoid the occurrence of non-physical masses and to prevent the violation of the continuity equation. In this work we have attempted to address the consistency issues surrounding the implementation of the Space Conservation Law in OpenFOAM. The existing method for calculation of swept volumes falls short in terms of consistency. Moreover, its capabilities are limited when it comes to complex three-dimensional mesh motions. The existing method of calculation treats swept volumes as net fluxes emanating from cell faces. We have implemented an alternate algorithm in which the swept volumes are treated as intermittent virtual cells whose volumes can be calculated in a unique and consistent manner. We will conclude by validating our approach for mesh motions of varying degrees of complexity.

Convective snow bands forming over the Baltic Sea can result in heavy precipitation along the Swedish east coast. Forecasting these events well is of great importance to prevent road traffic injuries, increased pressure at hospitals and cancelled bus traffic. This thesis project aims to evaluate the performance of the high-resolution non-hydrostatic convection permitting model HIRLAM ALADIN Reasearch on Mesoscale Operational NWP In Euromed (HARMONIE) in simulating convective snow bands. Its horizontal respresentation of precipitation rates, area, placement and timing was examined in a case study of two events. The case of 2007 during the 12-14th of November, formed over the Gulf of Finland and resulted in Nynäshamn receiving ≥ 55 mm precipitation during the 37 hours long event. Areas south of this precipitation maxima also recieved heavy precipitation. The second case, 21-23rd of March 2008, formed over the Gulf of Bothnia during north-easterly  winds and resulted in a total precipitation of 10 mm at Gävle. The precipitation maxima was observed offshore, north of Gävle, with ≥ 16 mm precipiation in 34 hours. Convective snow bands were also observed over Vänern and Vättern in both cases.  HARMONIE simulated convective snow bands well in terms of intensity, timing, placement and area, in both cases, compared to datasets from radar, radar with merged gauge data and separate observational data from weather stations. Areas south of the local maxima at Nynäshamn in the case of 2007 were however overestimated by a total of 10-15 mm, most likely due to a simulated shift in wind direction during some of the most intense hours. In the case of 2008 the model also captured weak convective snow bands at Vänern and Vättern accurately in terms of precipitation area, timing and accumulated precipitation. Further case studies of snow bands are however necessary to obtain a more comprehensive view of the performance of HARMONIE.

There are in general two different types of studies concerning nanofluids. The first one concerns itself with the study of the effective thermal conductivity and the other with the study of convective heat transfer enhancement. The study on convective heat transfer enhancement generally incorporates the study on the thermal conductivity. Not many papers have been written on the convective heat transfer enhancement and even fewer concerning the study on multi-walled carbon nanotubes in the transitional flow regime. In this paper the thermal conductivity and viscosity was determined experimentally in order to study the convective heat transfer enhancement of the nanofluids. Multi-walled carbon nanotubes suspended in distilled water flowing through a straight, horizontal tube was investigated experimentally for a Reynolds number range of a 1 000 - 8 000, which included the transitional flow regime. The tube was made out of copper and has an internal diameter of 5.16 mm. Results on the thermal conductivity and viscosity indicated that they increase with nanoparticle concentration. Convective heat transfer experiments were conducted at a constant heat flux of 13 kW/m2 with 0.33%, 0.75% and 1.0% volume concentrations of multi-walled carbon nanotubes. The nanotubes had an outside diameter of 10 - 20 nm, an inside diameter of 3 - 5 nm and a length of 10 - 30 μm. Temperature and pressure drop measurements were taken from which the heat transfer coefficients and friction factors were determined as a function of Reynolds number. The thermal conductivities and viscosities of the nanofluids were also determined experimentally so that the Reynolds and Nusselt numbers could be determined accurately. It was found that heat transfer was enhanced when comparing the data on a Nusselt number as a function of Reynolds number graph but comparing the results on a heat transfer coefficient as a function of average velocity graph the opposite effect was observed. Performance evaluation of the nanofluids showed that the increase in viscosity was four times the increase in the thermal conductivity which resulted in an inefficient nanofluid. However, a study on the performance evaluation criterion showed that operating nanofluids in the transition and turbulent flow regime due to the energy budget being better than that of the distilled water. / Dissertation (MEng)--University of Pretoria, 2012. / Mechanical and Aeronautical Engineering / unrestricted

The goal of this work is to investigate the capabilities of two different commercial codes, OpenFOAM and ANSYS CFX, to predict forced convection heat transfer in low Prandtl number fluids and investigate the sensitivity of these predictions to the type of code and to several input parameters.The goal of the work is accomplished by predicting forced convection heat transfer in two different experimental setups with the codes OpenFOAM and ANSYS CFX using three different turbulence models and varying the input parameters in an extensive sensitivity analysis. The computational results are compared two the experimental data and analyzed for qualitative and quantitative parameters, such as shape of velocity and temperature profiles, thickness of the boundary layers and wall temperatures.The results show that predictions of the temperature and velocity field are generally sufficient to good, however, the sensitivity especially to the turbulent Prandtl number has to be taken into account when computing forced convection heat transfer in low Prandtl number fluids. The results also show that methods applied to OpenFOAM cannot directly be applied to ANSYS CFX. / <p>QC 20130531</p> / GENIUS

Weather forecasting relies on the availability of observational data as input parameters. However,such data are not readily available, because of difficulties to collect weather data due toinaccessibility to many places in the world, such as oceans or mountain regions. For this reason,satellite surveillance is a suitable tool to observe the atmosphere in regions where it is notpossible by other means. This master thesis is a study of convective cloud cluster formation over Thailand, conductedthrough satellite image analysis. Characteristics of cloud cluster formations are investigatedthrough an implementation of the Maximum Spatial Correlation Technique (MASCOTTE),described by Carvalho and Jones (2001). This method allows tracking of convective cloud systemsthrough region based analysis of satellite images. The aim of this study is to investigate whether satellite image analysis, through the implementationof the MASCOTTE methodology, can provide characteristics of convective cloud systems,in order to discern convective systems by intensity, accurately enough to be able to discernsevere thunderstorms from ordinary thunderstorms. The annual distribution of the occurrenceof life cycles detected through the analysis is studied, as well as their monthly distribution ofmean and maximum life times. Moreover, the yearly distribution of life cycle mean and minimumbrightness temperatures are analysed, as well as the number of detected split and mergeevents. This is followed by a comparison of life cycle structural properties to investigate thepossibility to use individual parameters, alone or in combination with each other, as indicatorsof the degree of convective activity within life cycles. Yearly distributions were studied in order to verify if this method could reveal seasonal variations,such as the onset period of the wet season, in terms of the occurrence of life cycles andtheir life time. The findings of this study verified that the most convectively intense life cycles exist under theinfluence of the Inter Tropical Convergence Zone (ITCZ), during the onset and beginning ofthe monsoon season. Analysis of life cycle structural properties, showed that properties likemean and minimum brightness temperature as well as fractional convective area, could be usedas indicators to discern between life cycles with different level of convective activity. However,it is concluded that studies, including ground-based remote sensing technologies such asRADAR/LIDAR, as well as data from rawinsondes, needs to be conducted in order to clarifyif it is possible to use this methodology to successfully discern severe thunderstorms fromordinary thunderstorms. / Tillgängligheten av meteorologiska mätdata är väsentlig för att kunna prognostisera väder. Idag är tillgängligheten på dessa data relativt gles, bland annat på grund av svårigheter att mäta på många platser runt om i världen, t.ex över världshaven eller vid otillgängliga bergsområden. Därför är satellitövervakning ett bra alternativ till andra typer av väderobservationer, eftersom denna teknik kan tillhandahålla mätdata över stora områden som annars inte är möljiga att samla data från. Denna magisteruppsats är en studie om egenskaper hos konvektiv molnbildning över Thailand. Studien är genomförd med hjälp av satellitbildsanalys. Egenskaper hos olika konvektiva molnceller har studerats genom att använda en metod baserad på ”the Maximum Spatial Correlation Technique” (MASCOTTE), beskriven av Carvalho and Jones (2001). Tanken bakom denna metod är att hitta och följa utvecklingen av olika konvektiva molnceller baserat på deras storlek och temperatur. Målet med studien är att undersöka hurvida denna metoden kan ge kunskap som leder till att man kan skilja på konvektiva celler, genom intensitetsskillnader, med tillräcklig noggrannhet för att kunna urskilja vanliga konvektiva celler från intensiva celler. För att få en uppfattning om förekomsten av intensiva konvektiva system, har antalet detekterade livscykler per månad studerats. För sedan att få en bild av hurvida deras livscykler skiljer sig åt över året, har även egenskaper som medellivslängd och maximal livslängd studerats. Dessutom studerades den årliga fördelningen av livscyklernas medel och minimum temperaturer, samt förekomsten av delningar och sammanslagningar av konvektiva celler. För att finna kunskap om skillnader i intensitet mellan individuella livscykler, har egenskaper som medel och minimum temperatur analyserats. Dessutom har andelen moln med extremt låg temperatur studerats i syfte att kunna använda dessa parametrar som intensitetsindikatorer vid satellitbildsanalys. Resultaten i denna studie visar att de mest intensiva konvektiva molnsystemen (kraftigaste åskvädren), förekommer under påverkan av ITCZ (Inter Tropical Convergence Zone), under antågandet och början av regnperioden. Studier av de konvektiva systemens egenskaper visade att parametrar, som andelen extremt kallt område i molnceller (fractional convective area), och livscyklernas medel och minimum temperaturer, skulle kunna användas som intensitetsindikatorer för att skilja på olika livscykler med avseende på deras styrka i intensitet. Slutsatsen av studien är att det behövs fler studier där andra typer av meteorologiska mätdata, såsom RADAR/LIDAR och sonderingsdata är involverade, för att skaffa ytterligare kunskap om hur man genom satellitbildsanalys kan urskilja kraftiga åskväder.

Stationary spiral inserts can effectively enhance heat transfer and temperature blending in the heat convection systems. In this paper, the impact of the segment length on the performance of a stationary insert is studied for flow Re numbers from ~80 to ~7900 through numerical simulation of heat transfer in streams of cold and hot gases flowing across it. The segment length to width ratio is from 1.11 to 2.33. The temperature of the studied gas is from 300 K to 1300 K. It is shown that the insert with variable segment length is more effective in temperature blending for two compressible streams compared with an insert with constant segment length, especially for low-Re-number turbulent flows.