Beim kontinuierlichen Stranggießen von Stahl werden elektromagnetische Felder zur Strömungsbeeinflussung eingesetzt. In dieser Arbeit wird die Wirkung eines statischen Magnetfeldes auf die Kokillenströmung in einem Modellexperiment untersucht. Das statische Magnetfeld strukturiert die Strömung um, kann lokal die Strömungsgeschwindigkeiten erhöhen und verändert die Ausbildung und Anzahl der für Brammenkokillen typischen großskaligen Wirbel. Es zeigt sich weiterhin, dass die elektrische Leitfähigkeit der Kokillenwände einen entscheidenden Einfluss auf die Wirkung einer elektromagnetischen Bremse hat. Unter isolierenden Wänden werden räumliche Oszillationen des Flüssigmetallstrahles initiiert und es bildet sich zwischen den beiden Kokillenhälften eine asymmetrische Strömung aus. Leitfähige Wände verhindern die Oszillationen und die Kokillenströmung ist symmetrisch. Eine eindeutige bremsende Wirkung der elektromagnetischen Bremse auf den Durchfluss konnte jedoch in beiden Fällen nicht festgestellt werden.

This thesis describes experiments that were carried out using focused wave groups in the UK Coastal Research Facility (UKCRF). Considerable effort was put into calibrating the UKCRF to determine the relationship between the input signals sent to the paddles and the waves generated in the facility. Focused wave groups of various sizes and phases, based on NewWave theory were generated, and measurements were made of the resulting surface elevation data, water particle kinematics, wave runup and overtopping volumes. NewWave theory models the profile of extreme waves in a Gaussian (random) sea. The thesis describes the first time this model has been applied in the context of coastal wave transformation. A method for the separation of the underlying harmonic structure of a focused wave group is described and results presented. This technique has been used in relatively deep water but is shown to work successfully in the coastal zone until wave overturning. A method has been devised to provide a theoretical Stokes-like expansion of the free and bound waves to model the surface elevation and water particle kinematics of the focused wave groups. Satisfactory agreement is achieved between the theoretical predictions of UKCRF measurements. Suggestions are made for an improved model. The underlying harmonic structure of the focused wave groups is presented as stacked time histories that give insight into the wave transformation process from deep to shallow water. Particular attention is paid to the low frequency wave generated as the wave group interacts with the beach. This is compared to the low frequency wave that is generated by a solitary wave in the UKCRF. Runup and overtopping measurements are in reasonable agreement with predictions based on certain empirical formulae, but not others. These comparisons are useful in identifying those formulae able to predict runup and overtopping of extreme waves in the coastal zone.

Ce travail s’intéresse à l’analyse expérimentale du développement de la zone de mélange turbulente (ZMT) produite par une instabilité de Richtmyer-Meshkov (IRM). Les expériences sont réalisées au sein d’un tube à chocs vertical, et l’analyse s’appuie sur des mesures simultanées mettant en œuvre des techniques expérimentales de type capteurs de pression pariétaux, visualisations strioscopiques résolues en temps et mesures de vitesse par Vélocimétrie Laser Doppler (LDV). Une caractérisation de l’installation expérimentale est tout d’abord effectuée en situation homogène (air pur, sans mélange), afin de déterminer la qualité de l’écoulement de base et connaître le niveau de turbulence de fond du tube à chocs. Les configurations de mélange, principalement entre de l’air et de l’hexafluorure de soufre (SF6), sont ensuite abordées. On s’intéresse dans un premier temps aux caractéristiques globales de la zone de mélange : en particulier à l’évolution de son épaisseur et à son taux de croissance. Plusieurs configurations de mélange sont étudiées en faisant varier différents paramètres expérimentaux tels que la hauteur de la veine d’essais du tube à chocs, la forme de la perturbation initiale de l’interface entre les deux gaz et le nombre d’Atwood, dans le but de déterminer leur influence sur le développement de la ZMT. On montre ainsi une sensibilité du taux de croissance post-rechoc à plusieurs de ces paramètres. Des comparaisons avec des simulations numériques réalisées par nos partenaires du Commissariat à l’Énergie Atomique (CEA) montrent des tendances similaires entre expériences et simulations sur ce point. L’étude est ensuite complétée par une caractérisation plus locale de la ZMT, en mesurant les niveaux de turbulence en différents points de la veine d’essais à l’aide de la LDV. Après avoir quantifié les contraintes de convergence statistique imposées par l’expérience pour ce type de mesures, on donne une estimation des intensités turbulentes produites par l’écoulement de mélange à différents stades de son développement. / This experimental study sheds some light on the development of the turbulent mixing zone (TMZ) arising from a Richtmyer-Meshkov instability (RMI). The experiments are conducted in a vertical shock tube, and the analysis relies on simultaneous measurements involving pressuretransducers, time-resolved Schlieren visualizations and Laser Doppler Velocimetry (LDV). In a first step, a thorough characterization of the experimental apparatus is conducted in order to qualify the basic flow configuration corresponding to homogeneous situations (pure air withoutmixing), and to evaluate the « background » turbulence level of the shock tube. Mixing configurations (mainly between air and sulfur hexafluoride, SF6) are then investigated. We first focus on a global description of the mixing zone such as the time evolution of its thickness and the corresponding growth rate. We consider several mixing configurations, varying the length of the test section, the shape of the initial interface between the two gases and the Atwood number. A clear influence of some of these parameters is shown on the the post-reshock increasing rate of the mixing zone, in good accordance with numerical results obtained from the Commissariat à l’Energie Atomique (CEA, french atomic energy commission). A more local description of the flow is then obtained in a second step by measuring the turbulence levels at different locations inside the test section thanks to the LDV technique. After quantifying the issues linked to the statistical convergence of the turbulent quantities in such specific configurations, we provide an estimation of the turbulent intensities produced by the mixing at various stages of its development.

To improve optical measurements, which are degraded by optical distortions, wavefront correction systems can be used. Generally, these systems evaluate a guide star in transmission. The guide star emits wellknown wavefronts, which sample the distortion by propagating through it. The system is able to directly measure the distortion and correct it. There are setups, where it is not possible to generate a guide star behind the distortion. Here, we consider a liquid jet with a radially open surface. A Mach–Zehnder interferometer is presented where both beams are stabilized through a fluctuating liquid jet surface with the Fresnel guide star (FGS) technique. The wavefront correction system estimates the beam path behind the surface by evaluating the incident beam angle and reflected beam angle of the Fresnel reflex with an observer to control the incident angle for the desired beam path. With this approach, only one optical access through the phase boundary is needed for the measurement, which can be traversed over a range of 250 μm with a significantly increased rate of valid signals. The experiment demonstrates the potential of the FGS technique for measurements through fluctuating phase boundaries, such as film flows or jets.

Beim kontinuierlichen Stranggießen von Stahl werden elektromagnetische Felder zur Strömungsbeeinflussung eingesetzt. In dieser Arbeit wird die Wirkung eines statischen Magnetfeldes auf die Kokillenströmung in einem Modellexperiment untersucht. Das statische Magnetfeld strukturiert die Strömung um, kann lokal die Strömungsgeschwindigkeiten erhöhen und verändert die Ausbildung und Anzahl der für Brammenkokillen typischen großskaligen Wirbel. Es zeigt sich weiterhin, dass die elektrische Leitfähigkeit der Kokillenwände einen entscheidenden Einfluss auf die Wirkung einer elektromagnetischen Bremse hat. Unter isolierenden Wänden werden räumliche Oszillationen des Flüssigmetallstrahles initiiert und es bildet sich zwischen den beiden Kokillenhälften eine asymmetrische Strömung aus. Leitfähige Wände verhindern die Oszillationen und die Kokillenströmung ist symmetrisch. Eine eindeutige bremsende Wirkung der elektromagnetischen Bremse auf den Durchfluss konnte jedoch in beiden Fällen nicht festgestellt werden.

This thesis presents an experimental study which investigates the behaviour of gas bubbles rising in a liquid metal and the related bubble-driven flow under the influence of external DC magnetic fields. The experimental configuration considered here concerns a cylindrical container filled with the eutectic alloy GaInSn. Argon gas bubbles are injected through a single orifice located at the container bottom in the centre of the circular cross-section. A homogeneous magnetic field was generated by a Helmholtz configuration of a pair of water-cooled copper coils. The magnetic field has been imposed either in vertical direction parallel to the main bubble motion or in horizontal direction, respectively. A vertical magnetic field stabilizes and damps the liquid metal flow effectively. The temporal variations of the fluid velocity with time become smaller with increasing magnetic induction. The velocity magnitudes are decreased, and the velocity distributions along the magnetic field lines are smoothed. The flow field keeps the axisymmetric distribution. A horizontal magnetic field destabilizes and enhances the flow within a range of moderate Hartmann numbers (100 < Ha < 400). The flow becomes non-axisymmetric due to the non-isotropic influence of the magnetic field. In the meridional plane parallel to the field lines, the flow changes its direction from a downward to an upward motion. Enhanced downward flows were observed in the meridional plane perpendicular to the field lines. The liquid velocity in both planes shows strong, periodic oscillations. The fluid motion is dominated by large-scale structures elongated along the magnetic field lines over the entire chord lengths of the circular cross-section.

<p>Raman spectroscopy is a molecular spectroscopy technique that uses monochromatic light to provide a fingerprint to identify structural components and chemical composition. Depending on the changes in the unit-cell parameters and volume under the application of stress and temperature, the Raman spectrum undergoes changes in the wavenumber of Raman-active modes that allow identification of sample characteristics. Due to the various advantage of mechanical Raman spectroscopy (MRS), the use of this technique in the characterization and modeling of chemical changes under stress and temperature have gained popularity. </p> <p> Quantitative information regarding the local behavior of interfaces in an inhomogeneous material during shock loading is limited due to challenges associated with time and spatial resolution. Recently, we have extended the use of MRS to high-strain rate experiments to capture the local thermomechanical response of mock energetic material and obtain material properties during shock wave propagation. This was achieved by developing a novel method for <i>in‑situ</i> measurement of the thermo‑mechanical response from mock energetic materials in a time‑resolved manner with 5 ns resolution providing an estimation on local pressure, temperature, strain rate, and local shock viscosity. The results show the solid to liquid phase transition of sucrose under shock compression. The viscous behavior of the binder was also characterized through measurement of shock viscosity at strain rates higher than 10⁶/s using microsphere impact experiments.</p> <p> This technique was further extended to perform Raman spectral imaging over a microscale domain of the sample with a nano-second resolution. This was achieved by developing a laser-array Raman spectral imaging technique where simultaneous deconvolution of Raman spectra over the sample domain was achieved and Raman spectral image was reconstructed on post-processing. We developed a Raman spectral imaging system using a laser array and analysis was performed over the interface of sucrose crystals bonded using an epoxy binder. This study provides the Raman spectra over the microstructure domain which enabled the detection of localized melting under shock compression. The distribution of shock pressure and temperature over the microstructure was obtained using mechanical Raman analysis. The study shows the effects of an actual interface on the propagation of shock waves where a higher dissipation of shock energy was observed compared to an ideal interface. This increase in shock dissipation is accompanied by a decrease in both the maximum temperature, as well as the maximum pressure within the microstructure during shock wave propagation.</p>

An advanced laser-Doppler velocimeter is designed to acquire fully-resolved turbulence structural measurements in high Reynolds number two- and three-dimensional turbulent boundary layers. The new instrument combines, for the first time, new techniques allowing for the direct measurement of particle acceleration and sub-measurement-volume-scale position resolution so that second-order 3D particle trajectories may be measured at high repetitions. Using these measurements, several terms in the Reynolds stress transport equations may be directly estimated, giving new data for modeling and understanding the processes leading to the transport of turbulence in boundary layer flows. Due to the unique performance of the probe, many aspects of LDV instrumentation development were addressed. The LDV configuration was optimized for lowest uncertainties by considering the demanding applications of particle position and acceleration measurements. Low noise light detection and signal conditioning was specified for the three electronic channels. A high-throughput data acquisition system allows for exceptional burst rate acquisition. Signal detection and processing algorithms have been implemented which draw from previous techniques but also address distinctive problems with the current system. In short, the instrument was designed to advance the state-of-the-art in LDV systems. Measurements presented include turbulence dissipation rate and fluctuating velocity-pressure gradient correlations that have been measured in 2D and 3D turbulent boundary layers using the unique capabilities of the CompLDV--many of these measurements are the first of their kind ever acquired in high Reynolds number turbulent flows. The flat-plate turbulent boundary layer is studied at several momentum thickness Reynolds numbers up to 7500 to examine Reynolds numbers effects on terms such as the velocity-pressure gradient correlation and the dissipation rate in the Reynolds transport equations. Measurements are also presented in a pressure-driven three-dimensional turbulent boundary layer created upstream from a wing-body junction. The current results complement the extensive data from previous studies and provide even richer depth of knowledge on the most-completely-documented 3D boundary layer flow in existence. Further measurements include the wakes of three circular-cylinder protuberances submerged in a constant pressure turbulent boundary layer. / Ph. D.

No description available.

L’étude présentée dans cette thèse relève de la mécanique des fluides expérimentale et numérique appliquée aux écoulements pariétaux de refroidissement de chambres de combustion aéronautiques. En présence de phénomènes thermo-acoustiques, comme les instabilités de combustion, il est important d’évaluer si les capacités de l’écoulement pariétal à protéger les parois de chambre restent suffisantes. C’est ainsi que nous nous sommes intéressés aux écoulements de paroi multiperforée soumis à une excitation acoustique. Dans ce but, le banc d’essais MAVERIC a été amélioré grâce à l’installation d’un système qui permet de forcer acoustiquement l’écoulement transverse dans lequel les jets pariétaux débouchent. Nous avons pu alors mettre en évidence la forte sensibilité de ce type d’écoulements à l’excitation acoustique. Le bon accord entre les résultats expérimentaux et les simulations numériques aux grandes échelles (LES) effectuées est très encourageant dans le cas d’un forçage par onde stationnaire. Le forçage par onde progressive, étudié uniquement par simulations numériques, s’est révélé être capable de modifier significativement la topologie de l’écoulement. Enfin, à partir de l’outil numérique AVBP-AVTP qui permet le couplage de calculs fluide-solide, nous avons réalisé une étude de l’influence de la présence d’une excitation acoustique sur le comportement thermique de l’écoulement autour d’une paroi multiperforée de chambre de combustion. / This experimental and numerical study in the field of fluid mechanics deals with jets-in cross flow configurations that are relevant for the cooling of aero engine combustion chambers. Indeed, in presence of instabilities it is important to determine to which extent the film cooling is able to do its job of preserving the combustion chamber walls from the thermal load. The test facility MAVERIC has been upgraded in order to acoustically force the crossflow in which the jets are discharging. The strong sensitivity of the overall flow unsteady properties to the presence of the acoustic forcing has been clearly evidenced. The agreement between the experimental results and large-eddy simulations proved to be quite encouraging for a stationary acoustic wave whereas the case of a propagating acoustic wave investigated only numerically reveals also quite a significant change of the flow topology. In this context, the effect of the acoustic forcing on the wall thermal behavior has been analyzed thanks to the use of the fluid-solid coupled AVBP-AVTP solver.