Dans cette étude, nous proposons une étude théorique et numérique de l'influence des vibrations de hautes fréquences et de faibles amplitudes sur la séparation thermo-gravitationnelle des constituants d'un mélange binaire saturant un milieu poreux. La cellule considérée est horizontale, de grand rapport d'aspect et placée dans le champ de pesanteur. La formulation mathématique est obtenue en utilisant le formalisme des équations moyennées. Les conditions aux limites imposées au niveau des parois horizontales diffèrent de celles du problème de Rayleigh Bénard. On considère respectivement le cas où un flux constant est imposé sur ces parois puis le cas où un flux thermique constant est imposé sur la paroi inférieure alors qu'une température constante est imposée sur la paroi supérieure. Dans les deux configurations, onmet en évidence de solutions stables monocellulairesconduisant à la séparation des constituants du mélange et ce pour une large gamme des paramètres adimensionnels régissant le problème. Les résultats analytiques et de simulations numériques directes sont en très bon accord. Dans les deux cas, une étude de stabilité linéaire de la solution d'équilibre et de la solution monocellulaireest réalisée par méthode spectrale. / In this study, the influence of vertical vibrations on species thermo-gravitational separation of a binary fluid, saturating a porous medium, is presented. The cell is horizontal of large aspect ratio and situated in the gravity field. A formulation using time average equations is used. Two configurations have been considered and compared. In the first one, a constant heat flux is imposed on the horizontal walls and in the second case, a constant heat flux is imposed on the bottom wall while a constant temperature is imposed on the top wall. For each configuration, stable unicellular solutions leading to species separation are obtained, depending on the dimensionless parameters of the problem. The analytical results are in good agreement with those obtained by direct numerical simulations. In both cases, a linear stability analysis of the equilibrium solution and the unicellular one is presented using a spectral method.

Natural turbulent convection studies encompass a wide range of flows occurring in nature, for example, atmospheric and oceanic flows, con-vection in the Earth’s mantle, convection in the stars and also in many engineering applications. Rayleigh-Benard convection (RBC), i.e. con-vection in a horizontal fluid layer confined between two plates with a temperature differential maintained across them, has been a proto-type problem in the studies of turbulent natural convection. Many small scale and global features of the flow in the turbulent regime of RBC are known, yet the flow dynamics is not fully understood, es-pecially at high Rayleigh numbers (Ra). Present work comprises of experimental investigations of a different type of flow, high Rayleigh number turbulent convection in a long vertical tube (abbreviated as tube convection or TC). The tube of aspect ratio (length to diameter) of about 10, open at both the ends interconnects two large tanks. The flow driven by an unstable density difference created between the two tanks, has some unique features, different from RBC. The net flow at any tube cross-section is zero and the time averages of the velocities, the Reynolds shear stress and the mean shear are also zero. Turbu-lent energy production is therefore solely due to buoyancy. The flow is axially homogeneous and axisymmetric. In the homogeneous region, the mean density gradient is linear. Rayleigh number in TC is conve-niently defined based on the mean (linear) density gradient (denoted by Rag). Two sets of experiments are carried out. In one set of experiments, the density difference is created using brine and fresh water and in another set, it is created using heat. The ranges of Rag achieved are 3 × 108 < Rag < 8 × 109 in the experiments using salt (Schmidt number, Sc ≈ 600) and 5 × 104 < Rag < 5 × 106 in the experiments using heat (Prandtl number, P r ≈ 6). From the measured salt and heat fluxes in both the sets of experiments, the non dimensional flux 1 1 scaling above a certain value of Rag is obtained as N ug ∼ Rag2 P r 2 and from the velocity measurements in the experiments using salt, the 1 Reynolds number scaling is obtained as Re ∼ Rag2 P r− 12 . Both these are as per the predicted scalings by the mixing length model proposed by Arakeri et al. (2000) for high Rag convection in the vertical tube. The flux scaling N u ∼ (RaP r)2 , also known as the ‘ultimate regime’ of convection, expected at very high Ra but not yet observed in the experiments in classical RBC, is easily achieved in TC at relatively lower values of Ra. The fluxes and Reynolds numbers in TC are orders of magnitude higher as compared to those obtained in RBC for similar values of Ra and P r. In the lower range of Rag values for P r ≈ 6, a transition to a new flux scaling, N u ∼ (RaP r)0.29 is found. Similar transitions are also found to be present in the results of Tovar (2002) for Sc ≈ 600 and in the DNS results of Schmidt et al. (2012) for P r = 1, at different values of Rag. Collecting all these data, it is shown that the transition occurs at a fixed Grashof number of 1.6 × 105, independent of P r. Velocity measurements are carried out using particle image velocime-try (PIV) in the salt experiments. Kinetic energy spectra computed from the velocity fields are presented for the locations from the tube axis to the wall, for the lowest and the highest values of Rag achieved in the experiments. The spatial energy spectrum of lateral velocity at the tube axis follows Kolmogorov-Obukhov (KO) scaling (−5/3 scaling exponent) while the spatial spectrum of longitudinal velocity shows a scaling slightly higher than −5/3 but lower than −11/5 (the Bolgiano-Obukhov (BO) scaling). The scalar spectra is computed from the concentration fields obtained from planar laser induced fluorescence (PLIF) in the experiments using salt, and also from the temperature measurements from the experiments using heat. Both the concentra-tion and temperature fluctuations spectra show some evidence of dual scaling - BO scaling (−7/5 scaling exponent) in the inertial subrange followed by Obukhov-Corrsin (OC) scaling (−5/3 scaling exponent) over a narrow range of scales. Light propagation through the buoyancy driven turbulent flow in TC has also been experimentally investigated. Light propagation through convective turbulence is encountered in many situations. In some cases e.g. in observational astronomy it is undesirable, while in some other cases it is useful, e.g. in remote sensing of meteorological parameters. In the present study, light intensity and angle of arrival fluctuations in a parallel beam of light are measured. Laser shadowgraphy is used in the intensity measurements while the angle of arrival is obtained by measuring deflections of narrow laser beams, created by passing collimated laser light through a mask having equispaced grid of holes. Background oriented schlieren (BOS) measurements have also been carried out to obtain the displacements, which are proportional to the angle of arrivals. The equations for frequency spectrum of intensity and angle of arrival from the literature, developed for isotropic, ho-mogeneous turbulent media, are modified for the flow in the present case and the asymptotic scalings for high and low frequency ranges are obtained. The scalings in the frequency spectra computed from the measurements of intensity and angle of arrival fluctuations are com-pared with the obtained asymptotic scalings. The results from the present work are also compared with results from studies in the atmo-sphere and lab experiments.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-03-02T23:01:37Z No. of bitstreams: 1 RafaelaCarreiroDias_DISSERT.pdf: 7493649 bytes, checksum: 18d66dcedc96371bd5f40830e9aec1ba (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-03-04T21:54:12Z (GMT) No. of bitstreams: 1 RafaelaCarreiroDias_DISSERT.pdf: 7493649 bytes, checksum: 18d66dcedc96371bd5f40830e9aec1ba (MD5) / Made available in DSpace on 2016-03-04T21:54:12Z (GMT). No. of bitstreams: 1 RafaelaCarreiroDias_DISSERT.pdf: 7493649 bytes, checksum: 18d66dcedc96371bd5f40830e9aec1ba (MD5) Previous issue date: 2014-03-06 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico - CNPq / O ru?do s?smico tem sido tradicionalmente considerado como uma perturba??o n?o desejada do ambiente que ?contamina? a aquisi??o de dados de terremotos. Mas ao longo da ?ltima d?cada tem sido mostrado que informa??es coerentes sobre a estrutura do subsolo podem ser extra?das a partir de correla??es cruzadas do ru?do s?smico de ambiente. Neste contexto, as regras s?o reversas, sendo os terremotos o que necessitamos excluir dos dados. Os principais causadores do ru?do s?smico de ambiente s?o os microssismos oce?nicos e perturba??es atmosf?ricas. A per?odos menores que 30 s, o espectro do ru?do s?smico de ambiente ? dominado por energia micross?smica. O microssismo ? o sinal s?smico mais cont?nuo da Terra e pode ser classificado como prim?rio (observado na faixa 10-20 s) e secund?rio (observado na faixa 5-10 s). A fun??o de Green do meio de propaga??o entre dois receptores pode ser reconstru?da atrav?s da correla??o cruzada do ru?do s?smico de ambiente registrado simultaneamente nesses dois receptores. A reconstru??o da fun??o de Green ? geralmente proporcional ? por??o de ondas de superf?cie do campo de onda s?smico, j? que a energia micross?smica viaja principalmente como ondas de superf?cie. Neste trabalho, s?o apresentadas 194 fun??es de Green obtidas a partir de correla??es cruzadas de 1 m?s de registro da componente vertical do ru?do s?smico de ambiente para diferentes pares de esta??es s?smicas do Nordeste do Brasil. As correla??es cruzadas di?rias foram empilhadas utilizando a t?cnica n?o linear tf-PWS que real?a sinais coerentes fracos atrav?s da redu??o de ru?do incoerente. As correla??es cruzadas mostram que o sinal emergido ? dominado por ondas Rayleigh nas componentes verticais e que as velocidades de dispers?o podem ser medidas confiavelmente para uma faixa de per?odos entre 5 e 20 s. O estudo inclui tanto esta??es permanentes para monitoramento s?smico, quanto esta??es tempor?rias de experimentos passivos na regi?o, formando uma rede combinada de 33 esta??es separadas por dist?ncias entre 60 e 1311 km, aproximadamente. Estas medidas de velocidades de dispers?o de ondas Rayleigh em seguida s?o usadas na elabora??o de imagens tomogr?ficas da Prov?ncia Borborema do Nordeste do Brasil. As tomografias de ru?do s?smico obtidas aqui permitem mapear satisfatoriamente fei??es estruturais existentes na regi?o. As imagens tomogr?ficas de per?odos mais curtos (~5 s) mostram a estrutura crustal rasa e claramente definem as bacias sedimentares marginais e intracontinentais, bem como as partes de zonas de cisalhamento importantes que atravessam a Prov?ncia Borborema. As imagens tomogr?ficas de per?odos mais longos (10 - 20 s) atingem profundidades da crosta superior e a maior parte das anomalias desaparece. Algumas delas localizada no interior da Prov?ncia Borborema, no entanto, persistem. A evolu??o Cenoz?ica da Prov?ncia Borborema foi marcada por epis?dios de vulcanismo Cenoz?ico e eleva??o, mas nenhuma correla??o ? observada com estas caracter?sticas Cenoz?icas e as anomalias profundas. As anomalias n?o se correlacionam com mapas dispon?veis de fluxo de calor superficial na Prov?nica Borborema, e a origem das anomalias profundas permanece enigm?tica. / Ambient seismic noise has traditionally been considered as an unwanted perturbation in seismic data acquisition that "contaminates" the clean recording of earthquakes. Over the last decade, however, it has been demonstrated that consistent information about the subsurface structure can be extracted from cross-correlation of ambient seismic noise. In this context, the rules are reversed: the ambient seismic noise becomes the desired seismic signal, while earthquakes become the unwanted perturbation that needs to be removed. At periods lower than 30 s, the spectrum of ambient seismic noise is dominated by microseism, which originates from distant atmospheric perturbations over the oceans. The microsseism is the most continuous seismic signal and can be classified as primary ? when observed in the range 10-20 s ? and secondary ? when observed in the range 5-10 s. The Green?s function of the propagating medium between two receivers (seismic stations) can be reconstructed by cross-correlating seismic noise simultaneously recorded at the receivers. The reconstruction of the Green?s function is generally proportional to the surface-wave portion of the seismic wavefield, as microsseismic energy travels mostly as surface-waves. In this work, 194 Green?s functions obtained from stacking of one month of daily cross-correlations of ambient seismic noise recorded in the vertical component of several pairs of broadband seismic stations in Northeast Brazil are presented. The daily cross-correlations were stacked using a timefrequency, phase-weighted scheme that enhances weak coherent signals by reducing incoherent noise. The cross-correlations show that, as expected, the emerged signal is dominated by Rayleigh waves, with dispersion velocities being reliably measured for periods ranging between 5 and 20 s. Both permanent stations from a monitoring seismic network and temporary stations from past passive experiments in the region are considered, resulting in a combined network of 33 stations separated by distances between 60 and 1311 km, approximately. The Rayleigh-wave, dispersion velocity measurements are then used to develop tomographic images of group velocity variation for the Borborema Province of Northeast Brazil. The tomographic maps allow to satisfactorily map buried structural features in the region. At short periods (~5 s) the images reflect shallow crustal structure, clearly delineating intra-continental and marginal sedimentary basins, as well as portions of important shear zones traversing the Borborema Province. At longer periods (10 ? 20 s) the images are sensitive to deeper structure in the upper crust, and most of the shallower anomalies fade away. Interestingly, some of them do persist. The deep anomalies do not correlate with either the location of Cenozoic volcanism and uplift - which marked the evolution of the Borborema Province in the Cenozoic - or available maps of surface heat-flow, and the origin of the deep anomalies remains enigmatic.

The first step on the way to understanding the complicated dynamics of spray is to study the behavior of isolated droplets. In many industrial and natural processes such as turbulent combustion, agricultural sprays, spray cooler, falling raindrops and cloud evolution the droplet is subjected to a chaotic unsteady external flow field. The interaction between the liquid and gaseous phases results in very intricate droplet dynamics like capillary instabilities, atomization, droplet collision and coalescence and vaporization, to name a few. In this dissertation, the focus is on shape oscillations, atomization and vaporization dynamics of pendant and sessile droplets. A droplet residing on a substrate which vibrates vertically at ultrasonic frequency will exhibit different modes of shape oscillation. The competition between capillary forces and inertia forces is basically responsible for these oscillations. However, when an acoustic force field is introduced asymmetrically around the droplet, we discover with the help of ultra high-speed imaging, a new droplet spreading phase. This new method of droplet manipulation could nd application in micro fluidics and lab-on-a-chip systems. By lading the droplet with nanoparticles, the spreading rate can be easily controlled. The spreading phase is followed by an atomization phase where surface ligaments grow to disintegrate into daughter droplets; the intensity of atomization is found to decrease with increase in fluid viscosity. The ability to control atomization characteristics of droplets by lading them with nanoparticles is a powerful technique that may be implemented in spray coolers and combustors to control the spray characteristics or combustion efficiency. Both the spreading and ligament dynamics have been theoretically simulated and the physics behind the observed trends is explained. The growth rate of the ligaments is found to be governed by Weber number modified to include the acoustic pressure level of the standing wave. The frequency of ligament breakup is found to decrease with fluid viscosity and this observation is adequately supported by a theory developed based on the evolution crater on the droplet surface. Turning now to the pendant droplets, by decomposing the droplet shape into Legendre modes, the shape oscillations exhibited by a droplet hanging from the junction of cross-wire placed at the center of an air jet is studied. Both high-speed imaging and hot-wire anemometry are employed. The driving force of oscillation of droplets subjected to the air jet is proved to be the inherent pressure fluctuations in the jet. The effect of surface tension, viscosity and Reynolds number on the shape oscillation level has been examined. The first experimental evidence of viscous attenuation of lower frequencies in a particular mode in glycerol/water mixture is reported. A theoretical model was developed to simulate the droplet shape oscillations induced by different ambient flow fields like pulsatile flow, vortical flow and flow with broadband energy spectrum. The time of interaction of the droplet with an eddy in the flow is found to be very crucial in determining the amplitude of oscillation of the droplet. The shorter the interaction time, the higher are the chances of the droplet oscillation being pushed into resonance. Finally, the heat transfer and droplet regression dynamics of pendant droplets in a hot air stream of various chemical compositions (like conventional fuels, alternative fuels and nanosuspensions) have been experimentally analyzed using high speed imaging. The droplet is deployed at the junction of cross-wire at the centre of a vertical air jet. A hybrid timescale has been proposed which incorporates the effects of latent heat of vaporization, saturation vapor pressure and thermal diffusivity. This timescale in essence encapsulates the different parameters that influence the droplet vaporization rate. The analysis further permitted the evaluation of the effect of various parameters such as surrounding temperature, Reynolds number, far-field vapor presence, impurity content and agglomeration dynamics (nanosuspensions) in the droplet.

This thesis paper proposes a Medium Access Control (MAC) protocol for wireless networks, termed as CD-MMAC that utilizes multiple channels and incorporates opportunistic cooperative diversity dynamically to improve its performance. The IEEE 802.11b standard protocol allows the use of multiple channels available at the physical layer but its MAC protocol is designed only for a single channel. The proposed protocol utilizes multiple channels by using single interface and incorporates opportunistic cooperative diversity by using cross-layer MAC. The new protocol leverages the multi-rate capability of IEEE 802.11b and allows wireless nodes far away from destination node to transmit at a higher rate by using intermediate nodes as a relays. The protocol improves network throughput and packet delivery ratio significantly and reduces packet delay. The performance improvement is further evaluated by simulation and analysis. / sabbir@linuxmail.org

The present work aims at studying the coupling between hydrodynamic fingering instabilities and chemical reactions at the interface between two miscible solutions. Hydrodynamic deformations of interfaces between two reactive fluids as well as flows induced by chemical reactions at the front between two initially steady fluids are encountered frequently in combustion, petroleum, chemical and pharmaceutical engineering. Most of the time, concrete applications imply a very large number of variables so that an understanding of the fundamental processes of chemo-hydrodynamic coupling is out of reach. Our goal is here to analyze a much simpler model system in which only one mechanism of hydrodynamic instability is at play and for which the chemical reactions can be modeled by a one or two-variable model. <p><p>Buoyantly unstable, autocatalytic chemical fronts, are one such model system, which can be used as prototype to study the effects of the coupling between chemical reactions and hydrodynamic fingering instabilities. Fingering processes occur whenever a fluid of high mobility displaces a less mobile one in a porous medium. The initially planar interface looses then stability and a cellular fingering deformation of the interface is observed. Such an instability has been observed, for instance, in the iodate-arsenous acid and chlorite-tetrathionate reactions, autocatalytic redox reactions known to produce a change of density across a traveling front. Fingering happens there when the heavier solution lies on top of the lighter one in the gravity field. <p> <p>Our theoretical contribution to the analysis of fingering of chemical fronts focuses on different points which we detail in this thesis along the following outline. In chapter 2, we introduce fingering phenomena occurring in porous media and distinguish the situation of viscous and density fingering of pure non reactive fluids. Chapter 3 reviews the literature on coupling between fingering and chemical reactions before studying the linear stability conditions as well as nonlinear dynamics of density fingering of isothermal iodate-arsenous acid fronts. This prototype nonlinear redox reaction is the first one on which experimental results on fingering in spatially extended set-ups have been obtained. We next analyze in chapter 4 the density fingering of another front producing autocatalytic system i.e. the chlorite-tetrathionate reaction in order to address the influence of the chemical kinetics on the dynamics observed. The influence of the exothermicity of the reaction is then presented in chapter 5. Eventually, chapter 6 analyzes what happens if the kinetics is now bistable and further compares the situation of both viscous and density fingering of bistable fronts. We then conclude and present suggestions for future work in this subject at the frontier between nonlinear chemistry, hydrodynamics and engineering. <p> / Agrégation de l'enseignement supérieur, Orientation sciences / info:eu-repo/semantics/nonPublished

This work is concerned with the fluid dynamic processes and the associated loss of acoustic energy produced by circular apertures within noise absorbing perforated walls. Although applicable to a wide range of engineering applications particular emphasis in this work is placed on the use of such features within a gas turbine combustion system. The primary aim for noise absorbers in gas turbine combustion systems is the elimination of thermo-acoustic instabilities, which are characterised by rapidly rising pressure amplitudes which are potentially damaging to the combustion system components. By increasing the amount of acoustic energy being absorbed the occurrence of thermo-acoustic instabilities can be avoided. The fundamental acoustic characteristics relating to linear acoustic absorption are presented. It is shown that changes in orifice geometry, in terms of gas turbine combustion system representative length-to-diameter ratios, result in changes in the measured Rayleigh Conductivity. Furthermore in the linear regime the maximum possible acoustic energy absorption for a given cooling mass flow budget of a conventional combustor wall will be identified. An investigation into current Rayleigh Conductivity and aperture impedance (1D) modelling techniques are assessed and the ranges of validity for these modelling techniques will be identified. Moreover possible improvements to the modelling techniques are discussed. Within a gas turbine system absorption can also occur in the non-linear operating regime. Hence the influence of the orifice geometry upon the optimum non-linear acoustic absorption is also investigated. Furthermore the performance of non-linear acoustic absorption modelling techniques is evaluated against the conducted measurements. As the amplitudes within the combustion system increase the acoustic absorption will transition from the linear to the non-linear regime. This is important for the design of absorbers or cooling geometries for gas turbine combustion systems as the propensity for hot gas ingestion increases. Hence the relevant parameters and phenomena are investigated during the transition process from linear to non-linear acoustic absorption. The unsteady velocity field during linear and non-linear acoustic absorption is captured using particle image velocimetry. A novel analysis technique is developed which enables the identification of the unsteady flow field associated with the acoustic absorption. In this way an investigation into the relevant mechanisms within the unsteady flow fields to describe the acoustic absorption behaviour of the investigated orifice plates is conducted. This methodology will also help in the development and optimisation of future damping systems and provide validation for more sophisticated 3D numerical modelling methods. Finally a set of design tools developed during this work will be discussed which enable a comprehensive preliminary design of non-resonant and resonant acoustic absorbers with multiple perforated liners within a gas turbine combustion system. The tool set is applied to assess the impact of the gas turbine combustion system space envelope, complex swirling flow fields and the propensity to hot gas ingestion in the preliminary design stages.

No description available.

Phase field models have played an important role in shaping our understanding of a variety of micro structural phenomena in materials. Their attractive features include (a) their ability to capture instabilities in microstructures, and (b) their ability to handle topological transitions { such as splitting or coalescence { gracefully. Therefore, we have chosen to use a phase field model in our study of instabilities in cylindrical pores in nanoporous membranes which eventually lead to their failure. Our study is motivated by recent studies on thermal stability of nanoporous membranes of alumina, titania and zirconia. The key feature in our model is its ability to incorporate surface discussion as the mechanism for mass transport. We first benchmark the model through a critical comparison of our results on early stages of surface evolution during Rayleigh instability and grain boundary grooving with those from linear theories of these phenomena. We have then used longer simulations (which go beyond early stages, and therefore, can incorporate non-lineare effects) to study instabilities in a hollow cylinder in three different systems: single crystal or amorphous solid (which fails through Rayleigh instability), a model sys-tem with parallel grain boundaries (which fails through grain boundary grooving), and a polycrystal (whose failure depends on a combination of grain growth and grooving). In all the cases, the surface energy is assumed to be isotropic, and the operative mechanism for mass transport is assumed to be surface discussion.

This diploma thesis deals with the analysis of the IEEE 802.11ah wireless communication system. For such a purpose, an appropriate simulation model in program environment MATLAB is created. The first part of thesis focuses on the IEEE 802.11 standard. Basic blocks of the transmitter and receiver are described. Attention is also devoted on the brief description of considered transmission channels. The second part contains the description of the proposed and realized simulation model in MATLAB. Individual blocks of the simulation model are described in details. Finally, the obtained simulation results are evaluated and discussed.