Le but de cette étude expérimentale est de caractériser la dynamique de particules solides, à flottabilité nulle, incluse dans un écoulement turbulent isotrope en décroissance libre. Les particules utilisées sont de forme sphérique et ont un diamètre de 4 à 5 fois plus grand que l'échelle spatial de Kolmogorov de l'écoulement. De part leur taille, les particules ont également un nombre de Stokes proche de l'unité. On s'attend alors à ce que ces particules aient une dynamique différente de celle du fluide environnant. Dans cette étude, ont se propose de quantifier les différences de vitesses entre les deux phases à l'aide d'une technique de vélocimétrie simultanée / In this experimental study, the focus is made on the characterization of the dynamics of solid neutrally buoyant particles embedded in a freely decaying, nearly isotropic turbulence, with a weak mean flow. The particles are spherical with diameters several times larger than the Kolmogorov scale. The study of this flow configuration is still challenging both theoretically and numerically. Due to large particle sizes, the local flow around particles can not be considered as uniform and due to fluid-particle density ratio of around unity, the history and Basset forces cannot be neglected in comparison with the viscous drag force. Particle equation of motion is then fully non-linear, in contrast to the equation for heavy particles with diameters smaller then the Kolmogorov scale, for which only the Stokes drag is considered. In several experimental and numerical studies, the effect of particle size on velocity and acceleration statistics has been investigated (Homann and Bec 2010 ; Qureshi et al. 2008 ; Ouellette et al. 2008 ; Xu and Bodenschatz 2008). In the case of isotropic turbulence, Homann and Bec (2010) show that while the PDF of the particle velocity normalized by the square root of its variance does not vary with particle size, the variance itself is size dependent. A scaling relation for particle velocity variance has been proposed by using the Faxen correction (Gatignol 1983) which takes into account the non uniformity of the fluid flow at the scale of the particle. The aim of our research is to further study the dependence of particle dynamics on particle size. To that purpose, a turbulence generator has been set-up and the resulting turbulence is characterized. Then the flow was seeded with millimeter sized, neutrally-buoyant particles and the velocity of the two phases have been measured simultaneously. Simultaneous measurements of particle and surrounding fluid velocities show that although the global velocity statistics of the two phases have comparable values, the particles may have different local velocity from the velocity of the neighboring fluid

No description available.

Tsunami are a fascinating but potentially devastating natural phenomena that have occurred regularly throughout history along New Zealand's shorelines, and around the world. With increasing population and the construction of infrastructure in coastal zones, the effect of these large waves has become a major concern. Many natural phenomena are capable of creating tsunami. Of particular concern is the underwater landslide-induced tsunami, due to the potentially short warning before waves reach the shore. The aims of this research are to generate a quality benchmark dataset suitable for comprehensive comparisons with numerical model results and to increase our understanding of the physical processes involved in tsunami generation. The two-dimensional experimental configuration is based on a benchmark configuration described in the scientific literature, consisting of a semi-elliptical prism sliding down a submerged 15° slope. A unique feature of these experiments is the method developed to measure water surface variation continuously in both space and time. Water levels are obtained using an optical technique based on laser induced fluorescence, which is shown to be comparable in accuracy and resolution to traditional electrical point wave gauges. In the experiments, the landslide density and initial submergence are varied and detailed measurements of wave heights, lengths, propagation speeds, and shore run-up are made. Particle tracking velocimetry is used to record the landslide kinematics and sub-surface water velocities. Particular attention is paid to maintaining a high level of test repeatability throughout the experimental process. The experimental results show that a region of high pressure ahead of the landslide forces up the water over the front half of the landslide to form the leading wave crest, which propagates ahead of the landslide. The accelerating fluid above, and the turbulent wake behind, the moving landslide create a region of low pressure, which draws down the water surface above the rear half of the landslide to form the leading trough. Differences in the phase and group velocities of the components in the wave packet cause waves to be continually generated on the trailing end of the wave train. The downstream position that these waves form continually moves downstream with time and the wave packet is found to be highly dispersive. The interaction of the landslide pressure field with the free surface wave pressure field is important, as the location of the low pressure around the landslide relative to the wave field acts to reinforce or suppress the waves above. This has a substantial effect on the increase or decrease in wave potential energy. When the low pressure acts to draw down a wave trough, the wave potential energy increases. When the low pressure is below a wave crest, it acts to suppress the crest amplitude, leading to an overall decrease in wave potential energy. Measurements of the efficiency of energy transfer from the landslide to the wave field show that the ratio of maximum wave potential energy to maximum landslide kinetic energy is between 0.028 and 0.138, and tends to increase for shallower initial landslide submergences and heavier specific gravities. The ratio of maximum wave potential energy to maximum landslide potential energy ranges between 0.011 and 0.059 and tends to be greater for shallower initial submergences. For two experimental configurations the ratio of maximum wave potential energy to maximum fluid kinetic energy is estimated to be 0.435 and 0.588. The wave trough initially generated above the rear end of the landslide propagates in both onshore and offshore directions. The onshore-propagating trough causes a large initial draw-down at the shore. The magnitude of the maximum draw-down is related to the maximum amplitude of the offshore-propagating first wave trough. A wave crest generated by the landslide as it decelerates at the bottom of the slope causes the maximum wave run-up observed at the shore. A semi-analytical model, based on inviscid and irrotational theory, is used to investigate the wave generation process of a moving submerged object in a constant depth channel. The simplified geometry allows a variety of phenomena, observed during the experimental tests, to be investigated further in a more controlled setting. The variations in the growth, magnitude, and decay of energy as a function of time is due the interaction of the pressure distribution surrounding the moving slider with the wave field, in particular, the leading crest and trough. The largest energy transfer between slider kinetic energy and wave potential energy occurs when there is prolonged interaction between the slider's low pressure region and the leading wave trough. The generation of onshore propagating waves by a decelerating landslide is confirmed, and the magnitude of the maximum wave run-up is found to be dependent on the magnitude of the slider deceleration. The model also shows that slides with Froude number close to unity convert substantial amounts of energy into offshore propagating waves. The onshore propagating wave potential energy is not as sensitive to Froude number. A further result from the model simulations is that the specific shape of the slider has only a minor influence on the wave response, provided the slider's length and area are known. A boundary element model, based on inviscid and irrotational theory, is used to simulate the laboratory experiments. Model predictions of the wave field are generally accurate, particularly the magnitude and range of wave amplitudes within the wave packet, the arrival time of the wave group, the amplitude of the run-up and run-down at the shore, the time the maximum run-down occurs, and the form and magnitude of the wave potential energy time history. The ratios of maximum wave potential energy to maximum slider kinetic energy are predicted to within ± 29%. The model predictions of the crest arrival times are within 3.6% of the measured times. The inability of the inviscid and irrotational model to simulate the flow separation and wake motions lead to a 45% under prediction of the maximum fluid kinetic energy. Both the semi-analytical and BEM models highlight the need for the correct specification of initial slider accelerations in numerical simulations in order to accurately predict the wave energy.

The thesis describes state of the art volumetric measurement techniques and applies a 3D measurement technique, 3D Scanning Particle Tracking Velocimetry, to the transitional backward facing step flow. The measurement technique allows the spatial and temporal analysis of coherent structures apparent at the backward facing step. The thesis focusses on the extraction and interaction of coherent flow structures like shear layers or vortical structures.

<p>    </p> <p>Cerebral aneurysms are presented in 3-5% of the population and account for approximately 10% of all strokes. The clinical decision on treating unruptured aneurysms should not be taken lightly because a majority of the asymptomatic cerebral aneurysm will not rupture, while both endovascular and microsurgical treatments carry the risk of morbidity and mortality. Thus, there is a need for objective risk assessment to reliably predict the high-risk aneurysms to intervene. Recent studies have found that the blood flow hemodynamic metrics such as pressure and wall shear stress (WSS) are related to the growth and rupture of the aneurysms. 4D flow magnetic resonance imaging (MRI) measures time-resolved three-dimensional velocity fields in the aneurysms <em>in vivo</em>, allowing for the evaluation of hemodynamic parameters. This work presents the developments of flow-physics constrained data enhancement and augmentation methods for 4D flow MRI to assist the risk stratification of cerebral aneurysms. First, a phase unwrapping and denoising method is introduced to enhance the dynamic range and accuracy of 4D flow MRI velocity measurement by incorporating the divergence-free constraint of incompressible flow. Moreover, methods are developed to improve the estimation of hemodynamic parameters from 4D flow data including pressure and WSS. The pressure reconstruction method is also applied to the flow data acquired using particle imaging velocimetry (PIV) and particle tracking velocimetry (PTV) and shows superior performance as compared to the existing methods by solving the pressure Poisson equation. We also proposed a framework to estimate the uncertainty of the PIV/PTV based pressure estimation by propagating the velocity uncertainty. In addition, a multi-modality approach is introduced to enhances the resolution and accuracy of 4D flow data with sparse representation, which improves the reliability of the hemodynamic evaluation. Finally, we present a method to measure the left ventricular flow propagation velocity from cardiac imaging to help in assessing the diastolic function. </p>

Seit mehr als einem Jahrhundert ist ein wachsendes wissenschaftliches Interesse an Tropfen und den Vorgängen bei deren Aufprall auf die verschiedensten Substrate zu verzeichnen, wohl auch durch die Fotografien von Worthington (1908) ausgelöst. Inzwischen wurden viele Erkenntnisse durch große Fortschritte bei der experimentellen Untersuchung (z.B. mittels Hochgeschwindigkeitsaufnahmen) und durch theoretische und computergestützte Untersuchung (z.B. durch skalenfreie und numerische Modellierung) gewonnen. Trotzdem bleibt durch die Vielfältigkeit und Komplexität der Phänomene während des Tropfenaufpralls sowie wegen der ständig erweiterten Anwendungsbereiche dieses Forschungsgebiet hochaktuell. Insbesondere sehr kleine und gleichzeitig sehr schnelle Tropfen (Tropfendurchmesser 10µm bis 100µm, Tropfengeschwindigkeit 10m/s bis 100m/s) kommen in vielen modernen Anwendungen vor (z.B. Verbrennungsmotoren, Tintenstrahldrucker, Reinigung von Oberflächen). In diesem wichtigen, aber für Untersuchungen schwer zugänglichen Parameterbereich gibt es immer noch offene Fragen. Die vorliegende Arbeit beschäftigt sich daher mit diesen schnellen Mikrotropfen in Bezug auf ihre Herstellung und den Aufprallvorgang auf ein festes, trockenes oder benetztes Substrat. Zunächst wird eine Methode zur Erzeugung eines Hochgeschwindigkeitssprays realisiert, welche auf dem durch Ultraschall gesteuerten Plateau-Rayleigh-Zerfall eines Flüssigkeitsstrahls beruht. Sie ermöglicht es, sowohl Tropfengröße als auch –geschwindigkeit präzise und mit hoher Reproduzierbarkeit über den gesamten oben angegebenen Parameterbereich einzustellen. Durch gezielte Manipulation eines Einzeltropfens durch elektrische Felder wird anschließend der Tropfenaufprall auf Substrate unterschiedlicher Benetzbarkeit mit sehr hoher zeitlicher Auflösung (ca. 100 Mio. Bilder pro Sekunde) bei gleichzeitig hoher räumlicher Auflösung (< 1µm) untersucht. Es zeigt sich, dass bekannte Modelle für langsamere und größere Tropfen im Millimeterbereich auch für schnelle Mikrotropfen Gültigkeit behalten. Somit ist bei gleichen dimensionslosen Kennzahlen (z.B. Reynolds-Zahl, Weber-Zahl, Ohnesorge-Zahl) eine skalenfreie Beschreibung des Tropfenaufpralls möglich. Schließlich wird die Methode zur Tropfenerzeugung auf einen für Anwendungen in der Reinigung relevanten Fall übertragen. Hierbei geht es um den Tropfenaufprall auf ein von einem Flüssigkeitsfilm überströmten Substrat. Es werden die während des Auftreffvorgangs auftretenden Geschwindigkeiten in der sich bildenden radialen Strömung in Abhängigkeit von verschiedenen Prozessparametern bestimmt. Aus den Ergebnissen lassen sich Aussagen über die zu erwartende Reinigungswirkung durch derartige Tropfen und den Einfluss der Prozessparameter treffen.

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.

L'objectif de cette thèse est de réaliser une étude approfondie sur la vélocimétrie par suivi 3D de particules (3D Particle Tracking Velocimetry, 3D PTV) pour l'air intérieur convective thermique. Ce travail se concentre principalement sur l'étude de la littérature, l'évaluation des performances des algorithmes de mesure de la 3D PTV, et l'étude expérimentale pour l'air intérieur convective thermique en utilisant la technologie 3D PTV. Tout d'abord, la technologie 3D PTV typique et ses principales applications précédentes pour l'étude de d'air intérieur sont examinés. Ensuite, les performances de différents algorithmes de mesure de la 3D PTV sont évalués numériquement et expérimentalement. Deux sections se compose de cette partie, on est de comparer les performances de mesure de l'algorithme de PIV typique et l'algorithme de 3D PTV, une autre est de comparer les performances des sept algorithmes complets de 3D PTV. Enfin, sur la base de l'analyse présentée dans la thèse, l'étude expérimentale de l'écoulement d'air intérieur généré par la méthode de ventilation mélange est réalisée / The objective of this thesis is to conduct a comprehensive study on 3D Particle Tracking velocimetry (PTV) for thermal convective indoor airflow. This work mainly concentrates on the literature survey, the performance evaluation of 3D PTV measurement algorithms, and the experimental investigation for thermal convective indoor airflow using 3D PTV measurement technology. First, typical 3D PTV technology and its main previous applications for indoor airflow study are carefully reviewed. Then, the performances of different 3D PTV measurement algorithms are evaluated numerically and experimentally. Two sections consist of this part, one is to compare the measurement performances of typical PIV algorithm and 3D PTV algorithm, another is to compare the performances of seven complete 3D PTV algorithms. Last, based on the analysis in the thesis, the experimental investigation for indoor airflow generated by mixing ventilation method is conducted

Daniel Duda Visualization of selected flows of superfluid helium using solid hydrogen tracer particles 5 Thesis title: Visualization of selected flows of superfluid helium using solid hydrogen tracer particles Author: Bc. Daniel Duda Department: Department of Low Temperature Physics, Supervisor: prof. RNDr. Ladislav Skrbek, DrSc, Department of Low Temperature Physics, Faculty of Mathematics and Physics, Charles University in Prague. Consultant: Dr. Marco La Mantia, PhD. Abstract: Quantum turbulence generated in thermal counterflow of He II is studied experimentally by visualization. The statistical properties of the motion of micron size solid deuterium particles are studied by using the particle tracking velocimetry technique at length scales comparable to the mean distance between quantized vorti- ces. The probability density function (PDF) of the longitudinal velocity displays two peaks that correspond to two velocity fields of the two-fluid description of He II. The PDF of the transversal velocity displays a classical-like Gaussian core with non- classical power-law tails, confirming the quantum nature of turbulence in counter- flowing He II. The distribution of the particle acceleration is found to be similar in shape to the classical one, in the range of investigated parameters. The observed de-...

The thesis describes state of the art volumetric measurement techniques and applies a 3D measurement technique, 3D Scanning Particle Tracking Velocimetry, to the transitional backward facing step flow. The measurement technique allows the spatial and temporal analysis of coherent structures apparent at the backward facing step. The thesis focusses on the extraction and interaction of coherent flow structures like shear layers or vortical structures.