Current climate models often predict fractional cloud cover on the basis of a diagnostic probability density function (PDF) describing the subgrid-scale variability of the total water specific humidity, qt, favouring schemes with limited complexity. Standard shapes are uniform or triangular PDFs the width of which is assumed to scale with the gridbox mean qt or the grid-box mean saturation specific humidity, qs. In this study, the qt variability is analysed from large-eddy simulations for two stratocumulus, two shallow cumulus, and one deep convective cases. We find that in most cases, triangles are a better approximation to the simulated PDFs than uniform distributions. In two of the 24 slices examined, the actual distributions were so strongly skewed that the simple symmetric shapes could not capture the PDF at all. The distribution width for either shape scales acceptably well with both the mean value of qt and qs, the former being a slightly better choice. The qt variance is underestimated by the fitted PDFs, but overestimated by the existing parameterisations. While the cloud fraction is in general relatively well diagnosed from fitted or parameterised uniform or triangular PDFs, it fails to capture cases with small partial cloudiness, and in 10 – 30% of the cases misdiagnoses clouds in clear skies or vice-versa. The results suggest choosing a parameterisation with a triangular shape, where the distribution width would scale with the grid-box mean qt using a scaling factor of 0.076. This, however, is subject to the caveat that the reference simulations examined here were partly for rather small domains and driven by idealised boundary conditions.

<p dir="ltr">Grid convergence and scaling analyses have not been done rigorously for practical large-eddy simulations (LES). The challenge arises from the fact that there are two grid-related length scales: grid size and LES filter width. It causes the numerical and model errors in LES to be inherently coupled, making the convergence of either error difficult to analyze. This study works to overcome the challenge by developing scaling laws that can be used to guide the convergence analysis of errors in LES. Three different convergence cases are considered, and their respective scaling laws are developed by varying the ratio between grid size and filter width. A pool fire is adopted as a test case for the convergence analysis of LES. Qualitative and quantitative assessments of the LES results are made first to ensure reliable numerical solutions. In the subsequent scaling analysis, it is found that the results are consistent with their respective scaling laws. The results provide strong support to the developed scaling laws. The work is significant as it proposes a rigorous way to guide convergence analysis of LES errors. In a world where LES already has a wide range of applicability and is still becoming more prominent, it is imperative to have a thorough understanding of how it works including its convergence and scaling laws with respect to the change of grid size and filter width.</p>

More and larger wind farms are planned offshore. As the most suitable build sites are limited wind farms will be constructed near to each other in so called wind farm clusters. Behind the wind turbines in these farms there is a disrupted flow of air called a wake that is characterized by reduced wind speed and increased turbulence. These individual turbine wakes combine to form a farm wake that can travel a long distance. In wind farm clusters farm to farm interaction will occur, i.e. the long distance wake from one wind farm will impact the wind conditions for other farms in the surrounding area. The thesis contains numerical studies of these long distance wakes. In this study Large Eddy Simulations (LES) using an Actuator Disc method (ACD) are used. A prescribed boundary layer is used where the wind shear is introduced using body forces. The turbulence, based on the Mann model, is introduced as fluctuating body forces upstream of the farm. A neutral atmosphere is assumed. The applied method has earlier been used for studies of wake effects inside farms but not for the longer distances needed for farm to farm interaction. Numerical studies are performed to get better knowledge about the use of this model for long distance wakes. The first study compares the simulation results with measurements behind an existing farm. Three parameter studies are thereafter setup to analyze how to best use the model. The first parameter study examines numerical and physical parameters in the model. The second one looks at the extension of the domain and turbulence as well as the characteristics of the flow far downstream. The third one gathers information on the downstream development of turbulence with different combinations of wind shear and turbulence level. The impact of placing the turbines at different distances from the turbulence plane is also studied. Finally a second study of an existing wind farm is performed and compared with a mesoscale model. The model is shown to be relevant also for studies of long distance wakes. Combining LES with a mesoscale model can be of interest.

La régulation des polluants a mené à la création de nouveaux systèmes de combustion. Le carburant étant stocké sous forme liquide, sa transformation jusqu’à sa combustion est complexe. La capacité de la Simulation aux grandes échelles à simuler des écoulements turbulents réactifs a été montrée sur des cas académiques comme sur des configurations industrielles, tout en prenant en compte les phénomènes multiphysiques intervenant dans ces configurations, mais les études sur la structure de flamme diphasique sont encore trop peu nombreuses. La présence de deux solveurs pour la simulation d’une phase liquide étant disponible dans le code AVBP, leur utilisation permet une comparaison et une compréhension des phénomènes en jeu combinant dispersion, évaporation, et combustion. La première partie de l’étude relate la validation du modèle d’injection FIM-UR. Ce modèle est capable de reconstruire les profils de vitesses et de granulométrie à l’injecteur sans avoir à simuler les phénomènes d’atomisation primaire et secondaire. Une validation en régime turbulent avait déjà été réalisée, et on propose ici de valider le modèle dans un cas laminaire. Des comparaisons entre simulations monodisperses et polydisperse et des expériences sont effectuées. La simulation monodisperse Lagrangienne donne une bonne structure globale mais la simulation polydisperse Lagrangienne permet de retrouver le comportement au centre du cône avec la présence des petites gouttes et à la périphérie du cône par la présence des grosses gouttes. De plus, des améliorations sont apportées au modèle pour le formalisme Eulérien et montrent de bons résultats. La partie suivante s’intéresse à caractériser un spray polydisperse par une distribution monodisperse. En effet, au cas où une approche polydisperse n’est pas possible, le choix du diamètre moyen à prendre pour une simulation monodisperse est délicat. On propose donc d’analyser le comportement d’un spray polydisperse en le comparant à ceux de sprays monodisperses. Deux configurations académiques sont choisies : des cas de Turbulence Homogène Isotrope chargée en particules pour étudier la dynamique, et des calculs d’évaporation 0D. Trois paramètres sont étudiés pour la dynamique : la concentration préférentielle (ou ségrégation), la traînée moyenne et la traînée réduite moyenne. Cette dernière et la ségrégation de la distribution polydisperse semblent affectées par les tailles de goutte les plus faibles, et la concentration préférentielle apparait alors comme la moyenne des ségrégations des classes qui la composent pondérées par l’inverse du nombre de Stokes associé à chacune de ces classes. La traînée moyenne de la simulation polydisperse possède un comportement proche des diamètres moyens D10 et D20. Ces analyses nous poussent donc à choisir le D10 pour caractériser la dynamique d’un spray polydisperse. Les calculs d’évaporation 0D ne permettent pas dans un premier temps de caractériser efficacement la masse évaporée d’un spray polydisperse par celle d’un spray monodisperse équivalent, mais la définition de nouveaux diamètres issus de la littérature des lits fluidisés comme le D50% le permet, ce qui le place autour du D32. On propose donc de caractériser l’évaporation d’un spray polydisperse par ce diamètre. Enfin, la dernière partie étudie la structure de flamme diphasique dans la chambre MERCATO, à l’aide du formalisme Lagrangien, monodisperse et polydisperse, mais aussi en utilisant le formalisme Eulérien. La validation du modèle FIM-UR du premier chapitre et ses améliorations sont utilisées pour représenter les conditions d’injection liquide. En plus d’un calcul polydisperse, deux simulations monodisperses Lagrangiennes sont réalisées en prenant les diamètres moyens D10 et D32, suite à la partie précédente. Des comparaisons qualitatives et des validations sont réalisées, en comparant des profils de vitesses gazeuses axiale et fluctuante et vitesse axiale liquide issus de l’expérience. / Regulations on pollutants have led to the creation of new combustion systems. Giving that fuel is stored in a liquid form, its evolution until combustion is complex. The ability of Large Eddy Simulation has been demonstrated on academic cases, as well as on industrial configurations, by taking into account the multi-physics phenomena, but there is a lack of studies about two-phase flow flame structures. Two solvers for the simulation of two-phase flows are available in the AVBP code, hence both simulations are performed to compare and increase understanding of the phenomena involved such as dispersion, evaporation and combustion. The first part of the study focuses on the validation of the FIM-UR injection model. This model is able to build velocity and droplet profiles at the injector, without simulating primary and secondary break up. A validation in a turbulent case has already been done, and this study validates the model in a laminar case. Comparisons between monodisperse and polydisperse simulations, and experiments are performed. The monodisperse Lagrangian simulation shows good results but the polydisperse simulation is able to represent profiles in the center of the cone by small droplets and at the peripheral part of the cone, by big ones. Moreover, improvements in the Eulerian model exhibit good results. The next section tries to evaluate the impact of polydispersion. Indeed, when a polydisperse approach is not available, choosing the mean diameter can be tricky. A comparison between the behavior of polydisperse spray and monodisperse sprays ones is realised. Two academic cases are studied: Homogeneous Isotropic Turbulence with particles to analyze the dynamics, and 0D evaporation cases. For the dynamics, preferential concentration, mean drag and reduced mean drag are studied. The latter and preferential concentration are affected by small droplets, and the preferential concentration of a polydisperse spray is equivalent to the average of preferential concentration of classes, extracted from the polydisperse distribution, weighted by the inverse of the Stokes number of each class. The mean drag behaves like the D10 and D20 mean drags. This analysis allows us to choose the D10 to characterize a polydisperse distribution for the dynamics. Zero-D evaporation simulations cannot characterize the polydisperse spray evaporated mass by the evaporated mass of monodisperses sprays. New definitions of diameters from fluidized bed literature enable the use of D50%, which is close to D32. We propose to use this diameter to characterize the evaporation of a polydisperse spray. Finally, the last section studies the structure of two-phase flames in the MERCATO bench, using the Lagrangian formalism, monodisperse and polydisperse but also using the Eulerian formalism. The validation of FIM-UR model and improvements from the first section are used to represent liquid injection conditions. A polydisperse simulation is realized and two monodisperse simulations are computed using mean diameters D10 and D32, thanks to the previous section. Qualitative comparisons and validations are realized, comparing gaseous velocity profiles and liquid velocity profiles. Good agreements are found and the mean diameter D32 seems to be close to the polydisperse spray. A comparison between mean flames is done with an Abel transform of the flame from the experiments. The flame has an "M shape", anchored by small recirculation zones out of the swirler, and by a point at the tip of the central recirculation zone. Then, the impact of droplet distributions is analyzed. Even if few bigger droplets from the polydisperse distribution are convected in the hot gases due to bigger particular time and evaporation time, two-phase flow flame structures are equivalent. Different combustion regimes appeared with premixed flames and pockets of fuel burning in the hot gases.

The phenomenon of spray in crossflow is of relevance in gas turbine combustor development. The current work focuses on spray in crossflow rather than liquid jet in crossflow from the standpoint of enhancing fuel dispersion and mixing. Specifically, the first part of the work involves study of spray structure, droplet sizing, and velocimetry for sprays of water and ethanol in a crossflow under ambient conditions. Laser-based diagnostic techniques such as Particle/Droplet Image Analysis (PDIA) and Particle Tracking Velocimetry (PTV) are utilized. Using spray structure images, trajectory equations are derived by multi-variable regression. It is found that the spray trajectory depends only on the two-phase momentum ratio and is independent of other flow parameters. A generalized correlation for the spray trajectory is proposed incorporating the liquid surface tension, which is found to be effective for our data, with water and ethanol, as well as data on Jet-A from the literature for a wide variety of operating conditions. An interesting phenomenon of spatial bifurcation of the spray is observed at low Gas-to-Liquid ratios (GLRs). The reason for this phenomenon is attributed to the co-existence of large and highly deformed ligaments along with much smaller droplets at low GLR conditions. The smaller droplets lose their vertical momentum rapidly leading to lower penetration, whereas the larger ligaments/droplets penetrate much more due to their larger momentum leading to a spatial separation of the two streams. The second part of the study focuses on evaporating sprays in preheated crossflow. Experiments are conducted using ethanol, decane, Jet-A1 fuel, and a two-component surrogate for Jet-A1 fuel. The crossflow air is heated up to 418 K and the effect of evaporation is studied on spray trajectory and droplet sizes. Measured droplet sizes and velocities at two successive locations are used to estimate droplet evaporation lifetimes. Evaporation constant for the d2 law derived from the droplet lifetimes represents the first-ever data for the above-mentioned liquids under forced convective conditions. This data can be used to validate multi-component droplet evaporation models. The last part of the study focuses on Large Eddy Simulations (LES) of the spray in crossflow. The near-nozzle spray structure is investigated experimentally to obtain droplet size and velocity distributions that are used as inputs to the computational model. For the spray in crossflow under ambient conditions, trajectory and droplet sizes at different locations are compared with experimental results. While the predicted trajectory is found to be in good agreement with data, the predicted droplet sizes are larger than the measured values. This is attributed to the implicit assumption in the secondary breakup model that the droplets are spherical, whereas the experimental data in the near-nozzle region clearly shows presence of mostly ligaments and non-spherical droplets, especially for the low GLR cases. A modified breakup model is found to lead to improved agreement in droplet sizes between predictions and measurements. Overall, the experiments and computations have provided significant insight into spray in crossflow phenomenon, and have yielded useful results in terms of validated spray trajectory correlations, droplet evaporation lifetimes under forced convective conditions, and a methodology for simulation of airblast sprays.

Evaluating aerodynamic noise from aircraft engines is a design stage process, so that it conform to regulations at airports. Aerodynamic noise is also a principal source of structural vibration and internal noise in short/vertical take off and landing and rocket launches. Acoustic loads may be critical for the proper functioning of electronic and mechanical components. It is imperative to have tools with capability to predict noise generation from turbulent flows. Understanding the mechanism of noise generation is essential in identifying methods for noise reduction. Lighthill (1952) and Lighthill (1954) provided the first explanation for the mechanism of aerodynamic noise generation and a procedure to estimate the radiated sound field. Many such procedures, known as acoustic analogies are used for estimating the radiated sound field in terms of the turbulent fluid flow properties. In these methods, the governing equations of the fluid flow are rearranged into two parts, the acoustic sources and the propagation terms. The noise source terms and propagation terms are different in different approaches. A good description of the turbulent flow field and the noise sources is required to understand the mechanism of noise generation. Computational aeroacoustics (CAA) tools are used to calculate the radiated far field noise. The inputs to the CAA tools are results from CFD simulations which provide details of the turbulent flow field and noise sources. Reynolds-Averaged Navier Stokes (RANS) solutions can be used as inputs to CAA tools which require only time-averaged mean quantities. The output of such tools will also be mean quantities. While complete unsteady turbulent flow details can be obtained from Direct Numerical Simulation (DNS), the computation is limited to low or moderate Reynolds number flows. Large eddy simulations (LES) provide accurate description for the dynamics of a range of large scales. Most of the kinetic energy in a turbulent flow is accounted by the large-scale structures. It is also the large-scale structures which accounts for the maximum contribution towards the radiated sound field. The results from LES can be used as an input to a suitable CAA tool to calculate the sound field. Numerical prediction of turbulent flow field, the acoustic sources and the radiated sound field is at the focus of this study. LES based on explicit filtering method is used for the simulations. The method uses a low-pass compact filter to account for the sub-grid scale effects. A one-parameter fourth-order compact filter scheme from Lele (1992) is used for this purpose. LES has been carried out for four different flow situations: (i) round jet (ii) plane jet (iii) impinging round jet and (iv) impinging plane jet. LES has been used to calculate the unsteady flow evolution of these cases and the Lighthill’s acoustic sources. A compact difference scheme proposed by Hixon & Turkel (1998) which involves only bi-diagonal matrices are used for evaluating spatial derivatives. The scheme provides similar spectral resolution as standard tridiagonal compact schemes for the first spatial derivatives. The scheme is computationally less intensive as it involves only bi-diagonal matrices. Also, the scheme employs only a two-point stencil. To calculate the radiated sound field, the Helmholtz equation is solved using the Green’s function approach, in the form of the Kirchhoff-Helmholtz integral. The integral is performed over a surface which is present entirely in the linear region and covers the volume where acoustic sources are present. The time series data of pressure and the normal component of the pressure gradient on the surface are obtained from the CFD results. The Fourier transforms of the time series of pressure and pressure gradient are then calculated and are used as input for the Kirchhoff-Helmholtz integral. The flow evolution for free jets is characterised by the growth of the instability waves in the shear layer which then rolls up into large vortices. These large vortical structures then break down into smaller ones in a cascade which are convected downstream with the flow. The rms values of the Lighthill’s acoustic sources showed that the sources are located mainly at regions immediately downstream of jet break down. This corresponds to the large scale structures at break down. The radiated sound field from free jets contains two components of noise from the large scales and from the small scales. The large structures are the dominant source for the radiated sound field. The contribution from the large structures is directional, mainly at small angles to the downstream direction. To account for the difference in jet core length, the far field SPL are calculated at points suitably shifted based on the jet core length. The peak value for the radiated sound field occurs between 30°and 35°as reported in literature. Convection of acoustic sources causes the radiated sound field to be altered due to Doppler effect. Lighthills sources along the shear layer were examined in the form of (x, t) plots and phase velocity pattern in (ω, k) plots to analyse for their convective speeds. These revealed that there is no unique convective speeds for the acoustic sources. The median convective velocity Uc of the acoustic sources in the shear layer is proportional to the jet velocity Uj at the center of the nozzle as Uc ≈ 0.6Uj. Simulations of the round jet at Mach number 0.9 were used for validating the LES approach. Five different cases of the round jet were used to understand the effect of Reynolds number and inflow perturbation on the flow, acoustic sources and the radiated sound field. Simulations were carried out for an Euler and LES at Reynolds number 3600 and 88000 at two different inflow perturbations. The LES results for the mean flow field, turbulence profiles and SPL directivity were compared with DNS of Freund (2001) and experimental data available in literature. The LES results showed that an increase in inflow forcing and higher Reynolds number caused the jet core length to reduce. The turbulent energy spectra showed that the energy content in smaller scale is higher for higher Reynolds number. LES of plane jets were carried out for two different cases, one with a co-flow and one without co-flow. LES of plane jets were carried out to understand the effect of co-flow on the sound field. The plane jets were of Mach number 0.5 and Reynolds number of 3000 based on center-line velocity excess at the nozzle. This is similar to the DNS by Stanley et al. (2002). It was identified that the co-flow leads to a reduction in turbulence levels. This was also corroborated by the turbulent energy spectrum plots. The far field radiation for the case without co-flow is higher over all angles. The contribution from the low frequencies is directional, mainly towards the downstream direction. The range of dominant convective velocities of the acoustic sources were different along shear layers and center-line. The plane jet results were also used to bring out a qualitative comparison of flow and the radiation characteristics with round jets. For the round jet, the center-line velocity decays linearly with the stream-wise distance. In the plane jet case, it is the square of the center-line velocity excess which decays linearly with the stream-wise distance. The turbulence levels at any section scales with the center-line stream-wise velocity. The decay of turbulence level is slower for the plane jet and hence the acoustic sources are present for longer distance along the downstream direction. Subsonic impinging jets are composed of four regions, the jet core, the fully developed jet, the impingement zone and the wall jet. The presence of the second region (fully developed free jet) depends on the distance of the wall from the nozzle and the length of the jet core. In impinging jets, reflection from the wall and the wall jet are additional sources of noise compared to the free jets. The results are analysed for the contribution of the different regions of the flow towards the radiated sound field. LES simulations of impinging round jets and impinging plane jet were carried out for this purpose. In addition, the results have been compared with equivalent free jets. The directivity plots showed that the SPL levels are significantly higher for the impinging jets at all angles. For free jets, a typical time scale for the acoustic sources is the ratio of the nozzle size to the jet velocity. This is ro/Uj for round jets and h/Uj for plane jets. For impinging jets, the non-dimensionlised rms of Lighthill’s source indicates that the time scale for acoustic sources is the ratio of the height of the nozzle from the wall to the jet velocity be L/Uj. LES of impinging round jets was carried out for two cases with different inflow perturbations. The jets were at Reynolds number of 88000 and Mach number of 0.9, same as the free jet cases. The impingement wall was at a distance L = 24ro from the nozzle exit. For impinging round jets, the SPL levels are found to be higher than the equivalent free jets. From the SPL levels and radiated noise spectra it was shown that the contribution from the large scale structures and its reflection from the wall is directional and at small angles to the wall normal. The difference in the range of angles where the radiation from the large scale structures were observed shows the significance of refraction of sound waves inside the flow. The rms values of the Lighthill’s sources indicate two dominant regions for the sources, just downstream of jet breakdown and in the impingement zone. The LES of impinging plane jet was done for a jet of Mach number 0.5 and Reynolds number of 6000. The impingement wall was at a distance L = 10h from the nozzle exit. The radiated sound field appears to emanate from this impingement zone. The directivity and the spectrum plots of the far field SPL indicate that there is no preferred direction of radiation from the impingement zone. The Lighthill’s sources are concentrated mainly in the impingement zone. The rms values of the sources indicate that the peak values occur in the impingement zone. The results from the different flow situations demonstrates the capability of LES with explicit filtering method in predicting the turbulent flow and radiated noise field. The method is robust and has been successfully used for moderate Reynolds number and an Euler simulation. An important feature is that LES can be used to identify acoustic sources and its convective speeds. It has been shown that the Lighthill source calculations, the calculated sound field and the observed radiation patterns agree well. An explanation for these based on the different turbulent flow structures has also been provided.

Atmospheric boundary layer flows are characterized by the coexistence of a broad range of scales. These scales cover from synoptic- (100-5000 km) and meso-scales (1-100 km) up to three-dimensional micro-scale turbulence (less than a few kilometers). This multiscale nature inherent to atmospheric flows clearly determines the behaviour of the atmospheric boundary layer, whose structure and evolution are of major importance for the wind energy community. This PhD thesis is focused on the development of a numerical methodology that allows to include contribution from all the above mentioned scales, with the purpose of improving the representation of boundary layer processes. The multiscale numerical methodology is developed based on a numerical weather prediction (NWP) model, the Weather Research and Forecasting (WRF) model.<p><p>Prior to the development of the multiscale numerical methodology, one-year of sonic anemometer and wind LiDAR measurements from the FINO1 offshore platform are analyzed. A comprehensive database of offshore measurements in the lowest 250 m of the boundary layer is developed after quality data check and correction for flow distortion effects by the measurement mast, allowing the characterization of the offshore conditions at FINO1. Spectral analysis of high frequency sonic anemometer measurements is used to estimate a robust averaing time for the turbulent fluxes that minimizes non-universal contributions from mesoscale structures but captures the contribution from boundary layer turbulence, employing the Ogive function concept. A stability classification of the measurements is carried out based on the Obukhov length. Results compare well to other surface layer observational studies while vertical wind speed profiles exhibit the expected stability-dependency.<p><p>Although NWP models have been extensively used for weather forecasting purposes, a comprehensive analysis of its suitability to meet the wind energy requirements needs to be carried out. The applicability of the WRF mesoscale model to reproduce offshore boundary layer characteristics is evaluated and validated against field measurements from FINO1. The ability of six planetary boundary layer (PBL) parameterizations to account for stability effects is analyzed. Overall, PBL parameterizations are rather accurate in reproducing the vertical structure of the boundary layer for convective and neutral stabilities. However, difficulties are found under stable stratifications, due to the general tendency of PBL formulations to be overdiffusive and therefore, not capable to develope the strong vertical gradients found in the observations. A low-level jet and a very shallow boundary layer cases are simulated to provide further insights into the limits of the parameterizations.<p><p>Large-eddy simulations (LES) based on averaged conditions from a convective episode at FINO1 are conducted to understand the mechanisms of transition and equilibration that occur in turbulent one-way nested simulations. The nonlinear backscatter and anisotropy subgrid scale model with a prognostic turbulent kinetic energy equation is found to be capable of providing similar results when performing one-way nested large-eddy simulations to a reference stand-alone domain using periodic lateral boundary conditions. A good agreement is obtained in terms of velocity shear and turbulent fluxes of heat and momentum, while velocity variances are overestimated. A considerable streamwise fetch is needed following each domain transition for appropriate energy levels to be reached at high wavelengths and for the solution to reach quasi-stationary results. A pile-up of energy is observed at low wavelengths on the first nested domain, mitigated by the inclusion of a second nested domain with higher resolution that allows the development of an appropriate turbulent energy cascade.<p><p>As the final step towards developing the multiscale capabilities of WRF, the specific problem of the transition from meso- to micro-scales in atmospheric models is addressed. The challenge is to generate turbulence on inner LES domain from smooth mesoscale inflow. Several new methods are proposed to trigger the development of turbulent features. The inclusion of adequate potential temperature perturbations near the inflow boundaries of the LES domain results in a very good agreement of mean velocity profiles, variances and turbulent fluxes, as well as velocity spectra, when compared to periodic stand-alone simulations. This perturbation method allows an efficient generation of fully developed turbulence and is tested under a broad range of atmospheric stabilities: convective, neutral and stable conditions, showing successful results in all the regimes. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

This thesis makes contributions towards the development of two different small-scale sensing systems which show promise for measurements in fluid mechanics. Well-resolved turbulent Wall Shear Stress (WSS) measurements could provide a basis for realistic computational models of near-wall turbulent flow in aerodynamic design. In aerodynamics field applications, they could provide indication of flow direction and regions of separation, enabling inputs for flight control or active control of wind-turbine blades to reduce shock and fatigue loading due to separated flow regions. Traditional thermal WSS sensors consist of a single microscale hot-film, flush-mounted with the surface and maintained at constant temperature. Their potential for fast response to small fluctuations may not be realized, as heat transfer through the substrate creates heat-exchange with fluid, leading to loss of spatial and temporal resolution. The guard-heated thermal WSS sensor is a design introduced to block this loss of resolution. A numerical flow-field with a range of length and time and scales was generated to study the response of both guard-heated and conventional single-element thermal WSS sensors. A conjugate heat transfer solution including substrate heat conduction and flow convection, provides spatiotemporal data on both the actual and the “measured” WSS fluctuations calculated from the heat transfer rates experienced due to the WSS field. For a single-element sensor in air, we found that the heat transfer through the substrate was up to six times larger than direct heat transfer from the hot-film to the fluid. The resulting loss of resolution in the single-element sensor can be largely recovered by using the guard-heated design. Spectra for calculated WSS from heat transfer response show that high frequencies are considerably better resolved in guard-heated sensors than in the single element sensor. Nanoresonators are nanowires (NWs) excited into mechanical vibration at a resonance frequency, with a change in spectral width created by gas damping from the environment, or a shift in the resonance peak frequency created by added mass. They enable a wide range of applications, from sensors to study rarefied gas flow friction to the detection of early-stage cancer. The extraordinary sensitivity of nanoresonators for disease molecule detection has been demonstrated with a few NWs, but the high cost of traditional electron-beam lithography patterning, have inhibited practical applications requiring large arrays of sensors. Field-directed assembly techniques under development in our laboratory enable a large number of devices at low cost. Electro-deposition of metals in templates yields high-quality single nanowires, but undesired clumps must be removed. This calls for separation (extraction) of single nanowires. In this work, single nanowires are extracted by using the sedimentation behavior of particles. Based on numerical and experimental analyses, the optimum time and region for extracting samples with the highest fraction of single nanowires ratio was found. We show that it is possible to take samples free of large clumps of nanowires and decrease the ratio of undesired particles to single nanowires by over one order of magnitude. / Graduate

No description available.

Les interactions entre les flammes et les phénomènes thermiques sont le fil conducteur de ce travail. En effet, les flammes produisent de la chaleur, mais peuvent aussi être affectées par des transferts ou des sources de chaleur. La Simulation aux Grandes Echelles (SGE) est utilisée ici pour étudier ces interactions, en mettant l’accent sur deux sujets principaux: le transfert de chaleur aux parois et le contrôle de la combustion. Dans un premier temps, on étudie le transfert de chaleur aux parois dans un modèle de brûleur CH4/O2 de moteur-fusée. Dans un contexte deréutilisabilité et de réduction des coûts des lanceurs, qui constituent des enjeux majeurs, de nouveaux couples de propergols sont envisagés et les flux thermiques à la paroi doivent êtreprécisément prédits. Le but de ce travail est d’évaluer les besoins et les performances des SGEpour simuler ce type de configuration et de proposer une méthodologie de calcul permettant desimuler différentes configurations. Les résultats numériques sont comparés aux donnéesexpérimentales fournies par la Technische Universität München (Allemagne). Dans un deuxième temps, le contrôle de la combustion au moyen de décharges de plasma de type NRP (en anglaisNanosecond Repetitively Pulsed) est étudié. Les systèmes de turbines à gaz modernes utilisent en effet une combustion pauvre dans le but de réduire la consommation de carburant et les émissions de polluants. Les flammes pauvres sont connues pour être sujettes à des instabilités et le contrôle de la combustion peut jouer un rôle majeur dans ce domaine. Un modèle phénoménologique qui considère les décharges de plasma comme une source de chaleur est développé et appliqué à un brûleur pauvre avec prémélange CH4/Air stabilisé par un swirler. LesSGE sont réalisées afin d’évaluer les effets des décharges NRP sur la flamme. Les résultats numériques sont comparés aux observations expérimentales faites à la King Abdulla University ofScience and Technology (Arabie Saoudite) / Interactions between flames and thermal phenomena are the guiding thread of this work. Flamesproduce heat indeed, but can also be affected by it. Large Eddy Simulations (LES) are used hereto investigate these interactions, with a focus on two main topics: wall heat transfer andcombustion control. In a first part, wall heat transfer in a rocket engine sub-scale CH4/O2 burner isstudied. In the context of launchers re-usability and cost reduction, which are major challenges,new propellant combinations are considered and wall heat fluxes have to be precisely predicted.The aim of this work is to evaluate LES needs and performances to simulate this kind ofconfiguration and provide a computational methodology permitting to simulate variousconfigurations. Numerical results are compared to experimental data provided by the TechnischeUniversität München (Germany). In a second part, combustion control by means of NanosecondRepetitively Pulsed (NRP) plasma discharges is studied. Modern gas turbine systems use indeedlean combustion with the aim of reducing fuel consumption and pollutant emissions. Lean flamesare however known to be prone to instabilities and combustion control can play a major role in thisdomain. A phenomenological model which considers the plasma discharges as a heat source isdeveloped and applied to a swirl-stabilized CH4/Air premixed lean burner. LES are performed inorder to evaluate the effects of the NRP discharges on the flame. Numerical results are comparedwith experimental observations made at the King Abdulla University of Science and Technology(Saudi Arabia).