This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations. / QC 20100820

This thesis deals with the near-field of confluent jets, which can be of interest in many engineering applications such as design of a ventilation supply device. The physical effect of interaction between multiple closely spaced jets is studied using experimental and numerical methods. The primary aim of this study is to explore a better understanding of flow and turbulence behavior of multiple interacting jets. The main goal is to gain an insight into the confluence of jets occurring in the near-field of multiple interacting jets. The array of multiple interacting jets is studied when they are placed on a flat and a curved surface. To obtain the boundary conditions at the nozzle exits of the confluent jets on a curved surface, the results of numerical prediction of a cylindrical air supply device using two turbulence models (realizable 𝑘 − 𝜖 and Reynolds stress model) are validated with hot-wire anemometry (HWA) near different nozzles discharge in the array. A single round jet is then studied to find the appropriate turbulence models for the prediction of the three-dimensional flow field and to gain an understanding of the effect of the boundary conditions predicted at the nozzle inlet. In comparison with HWA measurements, the turbulence models with low Reynolds correction (𝑘 − 𝜖 and shear stress transport [SST] 𝑘 − 𝜔) give reasonable flow predictions for the single round jet with the prescribed inlet boundary conditions, while the transition models (𝑘 − 𝑘l − 𝜔𝜔 and transition SST 𝑘 − 𝜔) are unable to predict the flow in the turbulent region. The results of numerical prediction (low Reynolds SST 𝑘 − 𝜔 model) using the prescribed inlet boundary conditions agree well with the HWA measurement in the nearfield of confluent jets on a curved surface, except in the merging region. Instantaneous velocity measurements are performed by laser Doppler anemometry (LDA) and particle image velocimetry (PIV) in two different configurations, a single row of parallel coplanar jets and an inline array of jets on a flat surface. The results of LDA and PIV are compared, which exhibit good agreement except near the nozzle exits. The streamwise velocity profile of the jets in the initial region shows a saddle back shape with attenuated turbulence in the core region and two off-centered narrow peaks. When confluent jets issue from an array of closely spaced nozzles, they may converge, merge, and combine after a certain distance downstream of the nozzle edge. The deflection plays a salient role for the multiple interacting jets (except in the single row configuration), where all the jets are converged towards the center of the array. The jet position, such as central, side and corner jets, significantly influences the development features of the jets, such as velocity decay and lateral displacement. The flow field of confluent jets exhibits asymmetrical distributions of Reynolds stresses around the axis of the jets and highly anisotropic turbulence. The velocity decays slower in the combined regio  of confluent jets than a single jet. Using the response surface methodology, the correlations between characteristic points (merging and combined points) and the statistically significant terms of the three design factors (inlet velocity, spacing between the nozzles and diameter of the nozzles) are determined for the single row of coplanar parallel jets. The computational parametric study of the single row configuration shows that spacing has the greatest impact on the near-field characteristics.

A direct method is developed for solving the 2-D planar/axisymmetric Euler equations. The Euler equations are discretized using a finite-volume method with upwind flux splitting schemes, and the resulting nonlinear system of equations are solved using Newton&amp / #8217 / s Method. Both analytical and numerical methods are used for Jacobian calculations. Numerical method has the advantage of keeping the Jacobian consistent with the numerical flux vector without extremely complex or impractical analytical differentiations. However, numerical method may have accuracy problem and may need longer execution time. In order to improve the accuracy of numerical method detailed error analyses were performed. It was demonstrated that the finite-difference perturbation magnitude and computer precision are the most important parameters that affect the accuracy of numerical Jacobians. A relation was developed for optimum perturbation magnitude that can minimize the error in numerical Jacobians. Results show that very accurate numerical Jacobians can be calculated with optimum perturbation magnitude. The effects of the accuracy of numerical Jacobians on the convergence of flow solver are also investigated. In order to reduce the execution time for numerical Jacobian evaluation, flux vectors with perturbed flow variables are calculated for only related cells. A sparse matrix solver based on LU factorization is used for the solution, and to improve the Jacobian matrix solution some strategies are considered. Effects of different flux splitting methods, higher-order discretizations and several parameters on the performance of the solver are analyzed.

Given the interest on underwater axisymmetric cylindrical bodies for the development of high-speed underwater weapons, characterization of the boundary layer flow-induced noise received by a Sound NAvigation and Ranging (SONAR) is very important to improve sonar detection ranges. The debate on generating mechanisms of the flow induced noise received at the stagnation point is still on as there is no experimental evidence conclusively suggesting whether it is a near-field or far-field phenomenon, thereby introducing an element of uncertainty in the prediction models. Further, the models developed thus far were based on low Reynolds numbers involving flows in water tunnels and buoyant vehicles. Therefore, the main focus of the thesis is to measure the flow induced noise using a sonar fitted at the most forward stagnation point of an underwater axisymmetric body as realistically as possible and predict the same theoretically for identifying a suitable flow noise model for future use by designers. In order to meet the stated goal, two exclusive experiments were conducted at sea using an underwater autonomous high-speed axisymmetric vehicle fitted with a planar hydrophone array (8X8) in its nose cone which measured the flow noise signature. Two different sets of existing models are used in characterizing the flow noise received by the array, while the first set comprises of models developed based on the Turbulent Boundary Layer induced noise and other is based on the transition zone radiated noise model. Through this study, it was found that the transition zone radiated noise model is in close agreement with the measured data.

In this work we study the magnetic field modeling of realistic non-axisymmetric plasma equilibrium configurations and the heat flux patterns on the plasma facing components of tokamak divertor discharges. We start by establishing the relation between generic magnetic configurations and Hamiltonian dynamical systems. We apply the concept of magnetic helicity, used to establish topological bounds for the magnetic field lines in ideal plasmas, and to understand the self-consistency of reconnected magnetic surfaces in non-axisymmetric configurations. After this theoretical discussion, we present some results on magnetohydrodynamic equilibrium and the use of analytical solutions to the Grad-Shafranov equation for describing real tokamak discharges based on the experimental diagnostics and realistic boundary conditions. We also compare the equilibrium reconstruction of a DIII-D discharge obtained with a numerical reconstruction routine, developed as part of this research, and the EFIT code used by several tokamak laboratories around the world. The magnetic topology and plasma profiles obtained with our method are in considerable agreement with the numerical reconstruction performed with the other code. Then, we introduce a simplified description of the generic non-axisymmetric magnetic field created by known sources and implement it numerically for describing the magnetic field due to external coils in tokamak devices. After that, we use this routines to develop a numerical procedure to adjust a suitable set of non-linear parameters of internal filamentary currents, which are intended to model the plasma response based on the magnetic field measurements outside the plasma. Finally, these methods are used to model the magnetic field created by a slowly rotating plasma instability in a real DIII-D discharge. The plasma response modeling is based on the magnetic probe measurements and allow us to calculate the magnetic field in arbitrary locations near the plasma edge. Using this information we determine the non-axisymmetric plasma edge through the magnetic invariant manifolds routine developed during this work. The intersection of the calculated invariant manifold with the tokamak chamber agrees considerably well with the heat flux measurements for the same discharge at the divertor plates, indicating the development of a rotating manifold due to the internal asymmetric plasma currents, giving quantitative support to our simplified description of the magnetic field and the plasma edge definition through the invariant manifolds. / Neste trabalho estuda-se a modelagem do campo magnético em configurações realistas de plasmas em equilíbrio não-axissimétrico e o fluxo de calor nos componentes em contato com o plasma em descargas de tokamaks com desviadores poloidais. Começa-se estabelecendo a relação entre configurações magnéticas arbitrárias e sistemas dinâmicos Hamiltonianos. Então aplicamos o conceito de helicidade magnética, que é usado para estabelecer limitações topológicas sobre as linhas de campo magnético em plasmas ideais, assim como para compreender a auto-consistência das superfícies magnéticas reconectadas em configurações não-axissimétricas. Após esta discussão teórica, apresentam-se alguns resultados sobre o equilíbrio magnetohidrodinâmico e o uso de soluções analíticas à equação de Grad-Shafranov para descrever descargas reais em tokamaks, com base em diagnósticos experimentais e condições de contorno realistas. Também realiza-se uma comparação entre a reconstrução do equilíbrio de uma descarga do DIII-D, obtida mediante uma rotina numérica desenvolvida para esta pesquisa, com a obtida mediante o código EFIT, usado amplamente em diversos tokamaks. Após isso, apresenta-se uma descrição simplificada do campo magnético não-axissimétrico, criado por fontes determinadas, e a sua implementação para descrever o campo magnético devido às correntes externas em tokamaks. Então, usam-se estas rotinas para desenvolver um procedimento numérico que ajusta um conjunto adequado de parâmetros não-lineares de correntes filamentares internas, com as quais pretende-se modelar a resposta do plasma com base nas medidas de campo magnético fora do plasma. Finalmente, estes métodos são utilizados para modelar o campo magnético criado por uma instabilidade com rotação lenta numa descarga do DIII-D. Com base nas medidas das sondas magnéticas é possível modelar os campos criados em regiões arbitrárias próximas da borda do plasma. Usando esta informação é possível determinar a borda não-axissimétrica do plasma mediante as invariantes magnéticas calculadas com a utilização de uma rotina desenvolvida durante este trabalho. A intersecção da superfície invariante com a câmara do tokamak coincide satisfatoriamente com as medidas de fluxo de calor nas placas do divertor para a mesma descarga, indicando o desenvolvimento de uma variedade giratória criada pelas correntes de plasma não-axissimétricas, e sustentando quantitativamente a nossa descrição simplificada do campo magnético, assim como a definição da borda do plasma mediante as invariantes magnéticas.

Depuis la célèbre classification d’Edwin Hubble dans les années 30, il est coutume de définir unegalaxie comme appartenant soit au groupe des galaxies dites de type tardif (late-type galaxiesabr´eg´e LTGs) soit à celui des galaxies dites de type précoce (early-type galaxies ou ETGs). Lafamille des LTGs est principalement composée de galaxies spirales (S) quand la famille des ETGsregroupe les galaxies lenticulaires (S0) et elliptiques (E). L’étude morphologique de ces galaxies arévélé qu’environ 60% des LTGs et 45% des S0 présentent une barre. Par ailleurs, il a été montréque dans l’Univers local les galaxies pouvaient être séparées en deux grands ensembles : le nuagebleu composé majoritairement de LTGs et la séquence rouge peuplée principalement par les ETGs.Plusieurs mécanismes sont à l’origine de cette distribution et l’évolution séculaire en est évidemmentun point majeur. Un nombre important de recherches ont montré l’importance des barres sur ladynamique et l’évolution d’une galaxie. Le but de ma th`ese est d’´etudier `a quel point la formationd’une barre et l’évolution qui s’ensuit influe sur l’évolution des ETGs. Pour ce faire j’ai réalisédes simulations à N-corps de galaxies barrées (et non barr´ees) qui m’a permis d´étudier les pointssuivants.Je me suis tout d’abord penché sur l’impact de la présence d’une barre dans une galaxie sur unemodélisation de cette dernière par un modèle supposant une ditribution de masse axisymmétrique.Ce genre de modélisation permettant de déterminer le rapport masse/luminosité M/L et donc lamasse d’une galaxie observée mais ´egalement son inclinaison et son anisotropie, il est importantd’estimer l’impact de la présence d’une barre sur ces paramètres. J’ai donc montré qu’en fonctionde l’inclinaison de la galaxie et de la position de la barre par rapport à l’observateur, le rapportM/L était très souvent surestimé avec des erreurs allant jusqu’`a 25%. La taille et la force de labarre sont également apparus comme des facteurs importants mais une étude plus approfondies’imposerait afin de quantifier ce résultat.D’autre part, je me suis intéressé à l’impact d’une barre sur la distribution de masse et de métauxdans une galaxie lenticulaire. J’ai tout d’abord confirmé que la présence d’une barre, de partson influence sur la dynamique d’un système, applatissait les gradients de métallicité. De plusj’ai montré que le degrés d’aplatissement ainsi que la position des zones affectées peuvent êtredirectement mis en relation avec la structure de la barre et notament avec la localisation desrésonances dynamiques. Néanmoins l’influence purement dynamique d’une barre n’explique pasà elle seule les gradients d’âges et de m´etallicité observés. L’étude de l’influence d’un potentielgravitationnel barré sur la dynamique du gaz et donc sur la formation stellaire est donc égalementà prendre un compte. Cela fait l’objet des dernières simulations produites qui permettront de mieuxcomprendre l’influence global d’une barre sur l’évolution séculaire des galaxies de type précoce. / Since the 30’s and Edwin Hubble’s famous classification, galaxies are usually separated in twogroups : the late-type galaxies (LTGs) and the early-type galaxies (ETGs). The LTGs family ismainly made of spiral galaxies (S) while the ETGs family is composed of elliptical (E) and lenticular(S0) galaxies. A morphological study of all these galaxies revealed that around 60% of LTGs and45% of S0 present a bar. It has also been shown that, in the local Universe, galaxies fall into twobig groups : the blue cloud mostly populated by LTGs and the red sequence mainly made of ETGs.Several mechanisms are responsible for this distribution and the secular evolution is obviously animportant one to examine, sepcially in the context of bars, as an important number of studiesshowed the importance of bars in the dynamics and evolution of a galaxy.The goal of my thesis is to study the importance of the formation and ensued bar-drivenevolution influence on ETGs evolution. In that context, I have performed N-body simulations ofbarred (and unbarred) galaxies in order to investigate the following issues.First of all, I focused on the influence of a bar in a galaxy when modelling it with a dynamicalmodel assuming an axisymmetric mass distribution. As these kinds of models allow to determine themass-to-light ratio M/L, thus the dynamical mass of an observed galaxy, but also its inclinationand its anisotropy, it is important to evalute the consequence of the presence of a bar on theseparameters. I have shown that, depending on the galaxy inclination and the bar position angle,M/L is most of the time biased and overestimated, and this can be up to 25%. The size andstrength of the bar also seem to be important factors but a deeper study has to be done to quantifythis preliminary result.In a second step, I have studied the role of bars on the mass and metallicity redistributionsin a lenticular galaxy. I confirmed that the presence of a bar, due to its influence on its hostsystem dynamics, flattens pre-existing metallicity gradients. Moreover, I showed that the degree offlattening and the position of affected regions are directly correlated with the bar structure and thelocation of the dynamical resonances. Nonetheless, this dynamical effect cannot explain the varietyof observed ages and metallicity gradients. The consequences of a barred gravitational potentialon the gas dynamics and the stellar formation should therefore be investigated. This is the topicof the last set of numerical simulations produced which will allow to better understand the globalinfluence a bar has on the secular evolution of ETGs.

A supersonic ejector is used to pump a secondary gas using a supersonic primary gas flow by augmentation of momentum and energy in a variable area duct. The internal compressible flow through an ejector has many complex gas dynamic features, like compressible shear layers and associated shock interactions. In many practical applications, ejectors are operated in the choked flow regimes where higher operating pressure ratios and mass flow rates are encountered. On the other hand, rather low entrainment and subsonic secondary flow dynamics (referred as the mixed regime of operation) dominate the dilution and purging applications of ejectors. The fundamental understanding of the flow dynamics associated with gaseous mixing process in the ejector especially in the mixed operational regime is still unclear. Obtaining a comprehensive understanding of the flow through a supersonic ejector in the mixed regime through experimental investigations is the prime focus of the present study. A new supersonic ejector test facility is designed, fabricated and established in the laboratory during the course of this study. The effects of using different gases in the secondary flow have been investigated. Two novel methods to improve the ejector by enhancing mixing are also implemented. Optical diagnostic tools (Time-resolved Schlieren and laser scattering) and wall static pressure measurements are used to investigate the dynamics of mixing process inside the ejector. State of the art image processing codes are developed to determine the length in the ejector for which the primary and the secondary flows are separate, referred here as the non-mixed length from the results of the flow visualization studies. Exhaustive experiments are carried out on the two dimensional rectangular supersonic ejector by varying the mass flow rates of primary and secondary flows, primary stagnation pressure, for two locations of the nozzle in the ejector. The non-mixed length determined from quantitative flow visualization tools is found to lie within 4.5 to 5.2 times the height of the duct (20 mm). The non-mixed flow length determined from flow visualization studies corroborates well with the wall static pressure measurements. A significant reduction of non-mixed length of about 46.7% is caused by shock wave-boundary layer interactions in the supersonic nozzle at over-expanded conditions. Further, the effects of differences in molecular weight and ratio of specific heats on the performance are also studied using cylindrical supersonic ejector at low entrainment ratios (0.008 to 0.06). In these studies air is used as the primary fluid while argon and helium are used in the secondary flow segment of the ejector. The results indicate that Argon has better entrainment characteristics compared to helium. Two novel supersonic nozzles (the tip rig nozzle and Elliptic Sharp Tipped Shallow lobed nozzle) are also devel- oped to enhance mixing in the ejector. About 30% enhancement of entrainment ratio is observed with the newly designed nozzle geometries. Illustrative numerical simulations are also carried out to complement the experimental studies.

No description available.

No description available.

No description available.