Segui questo link per vedere altri tipi di pubblicazioni sul tema: Cavitation.
Tesi sul tema "Cavitation"
Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili
Vedi i top-50 saggi (tesi di laurea o di dottorato) per l'attività di ricerca sul tema "Cavitation".
Accanto a ogni fonte nell'elenco di riferimenti c'è un pulsante "Aggiungi alla bibliografia". Premilo e genereremo automaticamente la citazione bibliografica dell'opera scelta nello stile citazionale di cui hai bisogno: APA, MLA, Harvard, Chicago, Vancouver ecc.
Puoi anche scaricare il testo completo della pubblicazione scientifica nel formato .pdf e leggere online l'abstract (il sommario) dell'opera se è presente nei metadati.
Vedi le tesi di molte aree scientifiche e compila una bibliografia corretta.
1
Momma, Takahiro. "Cavitation loading and erosion produced by a cavitating jet". Thesis, University of Nottingham, 1991. http://eprints.nottingham.ac.uk/14102/.
Testo completoAbstract (sommario):
The aim of the project is to investigate the detail of cavitation loading and erosion process using a submerged jet cavitation technique. Large size cavitating jet apparatus in the University of Nottingham was used with an long orifice nozzle and experiments were carried out using tap water as a test liquid with upstream pressure ranging from 8D-120bar. Distribution of the mean pressure, cavity clouds and cavitation damage on a specimen have been obtained and their mutual relation was discussed. Effects of pressures and stand off distances on the characteristics of the erosion produced by the cavitating jet were studied and the results were compared with previous investigations. These include not only the weight loss but also the size of the damage and the jet length both related with the optimum stand off distance. Indentations on soft aluminium produced by the cavitating jet were investigated. Their size distributions were obtained for various pressures and stand off distances. Variations of the total number and the average size of indentations with stand off distance were also presented. The cavitation loading pulses were successfully measured by a novel piezoelectric pressure transducer using PYDF polymer and the pulse height measurement system, both of which were developed in the present project. During the process to determine the size of the sensitive area of the transducer, its effect on the pulse height was found. Then, the loading pressure was estimated from the pulse height and the indentation size distribution. The value estimated is around 2GPa and compared with results of the other investigators using similar method with different, vibratory and water tunnel, cavitation facilities. All of them show the similar magnitude. Good correlations of the indentation counting and the pulse height analysis with erosion results were obtained in terms of the intensity of cavitation loading. Simple calibration apparatus for the pressure transducer which utilises a pencil lead break.
2
Peterson, Ashley Thomas. "Cavitation prediction". Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612813.
Testo completo
3
Johansen, Kristoffer. "Stable-inertial cavitation". Thesis, University of Glasgow, 2018. http://theses.gla.ac.uk/30796/.
Testo completoAbstract (sommario):
Research and development of applications utilising acoustic cavitation, particularly medical therapy, is often based on the spectrum of the scattered emissions collected during the cavitation occurrence. There is, however, limited understanding as to how driven bubble behaviour is related to the myriad of non-linear features that can exist within the cavitation noise spectrum, including those commonly reported. Moreover, there is an enduring tendency to classify cavitation activity as either stable or inertial, with no clear delineation between the two categories in terms of associated emissions. The work described in this thesis is dedicated to reconciling bubble dynamics driven by focused ultrasound, and resolved with ultra-high speed shadowgraphic imaging, to the acoustic emissions simultaneously detected via a broadband calibrated needle hydrophone system. Specifically, the role of periodic bubble collapse shock waves are experimentally investigated, supported by bubble oscillation models and spectral analysis. First, hydrophone-deconvolution for restoring an approximation to physical pressure data is demonstrated, through laser-plasma mediated bubble detection. Subsequent application to precision measurements of an acoustically driven cavitation bubble, verifies a contribution from periodic shock waves to all features within the emission spectrum, including the sub-harmonics. Moreover, complete spectral peak suppression at the sub-harmonic is demonstrated for a specific two-bubble configuration. Finally, the design of a bespoke passive cavitation detector, optimised for shock wave detection is described, and its performance evaluated against a comparable, commercially available device. Implications for cavitation detection and detector characterisation are discussed, as is the conventional classification of activity as stable or inertial, with reference to the literature.
4
Odeyemi, Babatunde O. "Hydrodynamic cavitation : effects of cavitation on inactivation of Escherichia coli (E.coli)". Thesis, Georgia Institute of Technology, 2003. http://hdl.handle.net/1853/11009.
Testo completo
5
Krahl, Dominik, Jürgen Weber e Maik Fuchs. "Visualization of cavitation and investigation of cavitation erosion in a valve". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-199616.
Testo completoAbstract (sommario):
Avoiding cavitation and especially cavitation erosion are tasks, which have to be considered when working with hydraulics. State of the art is the assessment of the risk of erosion by component testing or to completely avoid cavitation by means of CFD. Another reliable method to assess the risk of cavitation erosion is until now not available. This paper deals with this problem and delivers comparative values for a later method development. In a first step the cavitation of a poppet valve, which controls a methanol flow, is visualized. The resulting three cavitation appearances are deeply examined. After that the results of long-term tests at different operation conditions are presented. A poppet surface analysis following each experiment has shown different types of surface attacks. As a result of this work it is shown that both cavitation appearance and surface attack are strongly influenced by the temperature dependent air solubility of the liquid.
6
Jin, Yong-Hua. "Optical investigations of cavitation". Thesis, Loughborough University, 1995. https://dspace.lboro.ac.uk/2134/27390.
Testo completoAbstract (sommario):
This doctoral thesis describes the investigation carried out by the author in pursuit of a better understanding of the mechanism of cavitation. To create cavitation bubbles under laboratory conditions, an intense Q-switched Nd:YAG laser was used and the event was captured using a high-speed photography system. Three different aspects concerning the cavitation phenomenon were studied and they were the propagation of acoustic waves in a liquid, the resultant stress waves in a nearby solid medium and the interactions between a bubble and the nearby boundary. Optical measurement techniques, based on Mach-Zehnder interferometry, shadowgraphy, Schlieren photography and photoelasticity, were employed to assist the observation and analysis of a cavitation event.
7
Watson, Peter. "Cavitation in human joints". Thesis, Queen's University Belfast, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304536.
Testo completo
8
Hou, Hang-sheng. "Cavitation instability in solids". Thesis, Massachusetts Institute of Technology, 1990. http://hdl.handle.net/1721.1/13697.
Testo completo
9
Gerold, Bjoern. "Cavitation in focused ultrasound". Thesis, University of Dundee, 2013. https://discovery.dundee.ac.uk/en/studentTheses/f41bf6b9-ae59-4a41-ba29-d5873821418b.
Testo completoAbstract (sommario):
A novel experimental conguration is developed combining a highintensity focused ultrasound source and a pulsed-laser, for the study of cavitation in a eld typical of those used for therapeutic ultrasound. The sonoptic chamber is specically designed to avoid the formation of acoustic standing waves, known to have a critical in uence on cavitation behaviour. A new technique of laser-nucleated acous- tic cavitation is presented, whereby a laser-pulse of energy below the breakdown threshold for the host medium, acts to nucleate acoustic cavitation in a pre-established eld. This facilitates the incorporation of high-speed cameras for interrogation at unprecedented temporal and spatial resolution, combined with acoustic detection directly correlated to the observed cavitation activity. A number of cavitation phenomena are investigated, including bubble-ensemble oscillations at a very early stage of development, in response to the acoustic driving. The frequency of oscillation, which bifurcates with increasing intensity, is also detected in the acoustic emissions. The application of a single-bubble model predicts a source for the acoustic emissions of quiescent radius equivalent to the bubble-ensemble observed, for each intensity investigated. The physical translation of the ensemble, due to the radiation force imposed by the primary eld, is also analysed. For laser-pulses of energy above the breakdown threshold, applying focused ultrasound to the cavity promotes and actuates jet-formation. The characteristics of the so formed jets depend on the intensity and location of the cavity relative to the ultrasound focus.
10
Wilms, Jeffrey. "Flow visualization of cavitation". Thesis, Kansas State University, 2013. http://hdl.handle.net/2097/32158.
Testo completoAbstract (sommario):
Master of ScienceDepartment of Mechanical and Nuclear Engineering
Mohammad Hosni
A typical refrigeration loop is composed of an evaporator, compressor, condenser, and an expansion valve. There are many possible refrigerants that can be used, but the physical properties of water make it ineffective in the traditional refrigeration loop. But if water could be used it would have many advantages as it is abundant, cheap, and is safe for the environment. This research focuses on a different kind of refrigeration loop using water. This new refrigeration loop utilizes water flowing through a nozzle, initiating cavitation. Cavitation is generally defined as creating vapor from liquid, not through adding heat, but by decreasing the pressure. In a converging/ diverging nozzle, as the cross sectional area is constricted, the velocity of the flow will increase, decreasing the pressure. Therefore, by flowing water through the nozzle it will cavitate. Transforming liquid into gas requires a certain amount of energy, defined as the latent heat. When a liquid is turned to vapor by an increase in the temperature, the latent heat is provided by the heat transfer to the system. As no energy is being added to the nozzle to cause the cavitation, the energy transfer to create the vapor comes from the remaining liquid, effectively causing a temperature drop. This research focused on the flow visualization of water cavitating as it travelled through a converging/ diverging nozzle. Under different flow conditions and different nozzle geometries, the cavitation manifested itself in different formations. When gasses were entrained in the water they formed bubbles, which acted as nucleation sites as they moved through the nozzle. This was called travelling bubble cavitation. In venturi nozzles the cavitation nucleated off of the wall, forming attached wall cavitation. When water flowed out of an orifice, a turbulent mixture of liquid and vapor, orifice jet, was formed which caused vapor to form around it. This was known as shear cavitation. When the water was rotated prior to the throat of an orifice, the orifice jet expanded radially and formed swirl cavitation. In addition to studying how the cavitation was formed, the void fraction and velocity were measured for attached wall cavitation.
11
Letchford, Nicholas. "Cavitation in lubricating films". Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:17a49106-bb6d-443c-be6b-50398bbd4590.
Testo completoAbstract (sommario):
Wear and friction are signicant problems in machinery. A particularly harsh environment is the space between the piston ring and cylinder wall of a combustion engine. Lubricant lms are commonly used to mitigate these eects, separating metal to metal contact. The experiments of Zhu and Granick (Langmuir, 18, 2002) attempt to understand how surfactants change the behaviour of lubricating lms. They claim surfactants that have adsorbed onto a solid surface cause the uid to slip past the surface. We hypothesize that their observations could instead be caused by the liquid cavitating. We develop a mushy region model, based on the work of Ockendon, Howison, and Lacey (Q J Mech Appl Math, 56, 2003), to represent the cavitated region which we assume is composed of liquid, vapour, and gas. We show that the normal force predicted by the model gives a good t to experimental observations. We also model experiments of Professor J. Nouri, who investigated the formation of gaseous cavities in a slider bearing, again using a mushy region model. We present a detailed asymptotic analysis of the model and show how the solution changes from being quasi-steady to dynamic as the velocity of the bottom surface changes sign. We show that numerical solutions of the model are in good qualitative agreement with experimental measurements, yet several persistent discrepancies remain. We suggest possible causes of these discrepancies, and estimate the eects that various physical phenomena might have on the experiments. By modifying the mushy region model to study the impact of various additional processes on the lm, we attempt to explain specic dierences between the model and the experimental results. We also extend the model to include other eects so it is more representative of the operating conditions experienced by lubricants in engines.
12
Zuo, Zhigang. "Experimental and Numerical study of Laser-induced Cavitation Bubbles on Warwick Cavitation Rig". Thesis, University of Warwick, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488496.
Testo completoAbstract (sommario):
As part of the long-term effort for deepening our understanding of the physics of the damage powers of cavitation, a Markov stochastic model has been developed to describe the interaction of bubbles with nearby boundary walls. The objective of the PhD programme is to calibrate the newly designed and purposely constructed venturi-cavitation test rig at Warwick University, on which the Markov model will be studied; and to develop feasible and effective (experimental and numerical) approaches for implementating this stochastic study.
13
Herbert, Éric. "Cavitation acoustique dans l'eau pure". Phd thesis, Université Paris-Diderot - Paris VII, 2006. http://tel.archives-ouvertes.fr/tel-00139435.
Testo completoAbstract (sommario):
Tout liquide peut être surchauffé ou détendu au delà de sa courbe d'ébullition. Il est dans un état métastable, jusqu'à ce qu'une bulle de vapeur apparaisse, c'est le phénomène de cavitation. L'étude de la limité de métastabilité renseigne sur la cohésion du liquide et sur son équation d'état. Le cas de l'eau est spécialement intéressant : des théories concurrentes prédisent des variations en température de cette limite qualitativement différentes : monotone (la pression de cavitation augmente avec la température), ou avec un minimum. Nous mettons l'eau sous tension à l'aide d'une onde ultrasonore, émise par une céramique piézo-électrique hémisphérique. L'onde est focalisée pendant une courte durée et dans un petit volume loin de toute paroi, ce qui minimise l'influence d'éventuelles impuretés. Nous obtenons des résultats très reproductibles, permettant de mesurer la statistique de cavitation et de définir précisément son seuil dans différents types d'eaux. La céramique est calibrée de deux manières : avec des hydrophones à aiguille, et avec une méthode basée sur la variation de la pression statique du liquide. Les deux méthodes sont en accord et donnent une pression de cavitation monotone de -26 MPa à 0 Celsius C à -16 MPa à 80 Celsius C. Cela fait partie des pressions les plus négatives observées dans l'eau mais reste loin de la valeur théorique attendue (environ -120 MPa) et observée dans une seule expérience (Zheng etal , 1991, Science 254, 829). Nous discutons les causes possibles de ce désaccord : il peut être du soit à la présence d'impuretés, dont nous discutons la nature et la concentration, soit à une courbure inattendue de l'équation d'état aux pressions très négatives.
14
Ewunkem, Asoumu Emmanuel. "Cavitation erosion of engineering materials". Thesis, City University London, 1993. http://openaccess.city.ac.uk/8358/.
Testo completoAbstract (sommario):
The work presented in this thesis is concerned with evaluating the erosion resistance or behaviour of various engineering materials to cavitation erosion in a through flow device. A wide range of engineering materials have been utilized, from metal alloys, plastics, ceramics to composiles. These were procured from various industrial and research establishments. The metal alloys which were supplied in various condition of heat treatments, ranged from aluminium alloy to nitrided and tool steels. The plastic employed were epoxy resins. These were supplied in two different formulations the "Novalac" and the Bisphenol" systems. They were produced in as cast and machined conditions. Thus the influence of both the formulation and the production processes on their cavitation erosion behaviour have been analysed. Silicon carbide arid silicon nitride were the ceramic materials employed in this project. Both were produced under three surface finish conditions. These were fired or sintered, ground and lapped respectively. The performances of all three surface finishes have been elucidated and the cavitation erosion resistance of both silicon carbide and silicon nitride have been ascertained. Glass reinforced plastic (GRP) and Fybroc were the composites utilized. They both employed glass fibre as the reinforcing element. Their cavitation erosion behaviour and resistance have been evaluated. Detailed observation of damage progression in the above three classes of materials have been made. A comprehensive cavitation erosion test data base has been obtained. An appraisal on a comparative basis of the different erosion rates of the various material tested is presented.
15
Pennathur, Sumita 1978. "Micro-scale turbopump blade cavitation". Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/8719.
Testo completoAbstract (sommario):
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2001.Includes bibliographical references (p. 195-196).
The possibility of a silicon micro-fabricated turbopump for the use in a micro-fabricated bipropellant liquid rocket engine is of interest. Such a pump might have airfoils on the order of 1mm chord and 0.2mm span and operate at a Reynolds number of 6000. Cavitation is a major technical issue in such a pump, but there is little work in the literature at this length scale. This work documents analytical and experimental investigations of cavitation on millimeter long pump blading. Cavitation inception and bubble growth are analyzed on a micro-scale and deviations from macro-scale theory are discussed. The analysis suggests that residence time, surface roughness, surface tension, and passage area constraints are significant factors in cavitation inception and growth. A non-rotating microfabricated cascade has been designed, fabricated, and tested to quantify the behavior of micro-scale cavitation. An experimental rig has been constructed, and 18 micro-cascades have been tested. Visual observations confirm the existence of cavitation, and illustrate the phenomena of hysteresis and time lag. Comparisons of test results with analysis indicate that cavitation inception is adequately modeled by macro-scale theory. Test repeatability is established and the experimental data is found to be in agreement with 3D numerical results. Performance impacts of cavitation on micro-scale bade rows are discussed and several useful correlations are included. No apparent surface damage has been observed in these experiments. The experimental and analytical results are compiled in the form of design criteria for micro-scale turbopumps, and are used to evaluate the performance impacts due to cavitation. It is estimated that for a micro-turbopump operating at the most severe expected cavitating conditions, the performance loss in terms of pressure recovery is not greater than 20%.
by Sumita Pennathur.
S.M.
16
Kotopoulis, Spiros. "Biomedical ultrasonics, cavitation, and sonoporation". Thesis, University of Hull, 2011. http://hydra.hull.ac.uk/resources/hull:4746.
Testo completoAbstract (sommario):
This thesis treats biomedical ultrasonics, cavitation and sonoporation. Focussed ultrasound surgery can heat tissue to a temperature that causes protein denaturation and coagulative necrosis. For high-resolution focused ultrasound microsurgery, high working frequencies are necessary. We manufactured a highfrequency, high-intensity focussed ultrasound transducer, using lithium niobate as the active element. The transducer was capable of creating 2.5×3.4 (mm)2 lesions without affecting surrounding tissue. Such disruptive effects of ultrasound also have applications outside medicine. Since cyanobacteria contain gas vesicles, we hypothesised that these can be disrupted with the aid of ultrasound. During 1-hour sonication in the clinical diagnostic range, we forced blue-green algae to sink, thus promoting natural decay. In medical diagnostics, ultrasound contrast agents are added to the blood stream to differentiate between blood and other tissue types. We injected such lipid-shelled microbubbles into a synthetic capillary and sonicated using continuous ultrasound. The microbubbles formed clusters at a quarter wavelength apart owing to radiation forces. We observed cluster coalescence and translation towards the capillary wall. To study acoustic cavitation, we designed and built a scientific instrument combining a pulsed laser and a high-intensity focussed ultrasound transducer, capable of nucleating at precise locations. The cavitation dynamics were recorded using highspeed cameras. At high acoustic intensities, interacting cavitation clouds were formed. Microbubbles under sonication have been observed to create transient pores in adjacent cell membranes. This so called sonoporation has been associated with highly non-linear bubble phenomena. We observed lipid-shelled microbubbles near cancer cells under quasi-continuous low-amplitude sonication. Typically within a second of sonication, microbubbles were seen to enter the cells and dissolve. This new explanation of sonoporation was verified using high-speed photography and confocal fluorescence microscopy. If drug and genes can be successfully coupled to acoustically active vehicles, sonoporation might revolutionise non-invasive therapy as we know it.
17
Lisle-Taylor, S. C. "Cavitation performance of pumped hydrocarbons". Thesis, Cranfield University, 1997. http://dspace.lib.cranfield.ac.uk/handle/1826/9676.
Testo completo
18
Herbert, Eric. "Cavitation acoustique dans l'eau pure". Paris 7, 2006. http://www.theses.fr/2006PA077185.
Testo completoAbstract (sommario):
Tout liquide peut être surchauffé ou détendu au delà de sa courbe d'ébullition. Il est dans un état métastable, jusqu'à ce qu'une bulle de vapeur apparaisse, c'est le phénomène de cavitation. L'étude de la limité de métastabilité renseigne sur la cohésion du liquide et sur son équation d'état. Le cas de l'eau est spécialement intéressant : des théories concurrentes prédisent des variations en température de cette limite qualitativement différentes : monotone (la pression de cavitation augmente avec la température), ou avec un minimum. Nous mettons l'eau sous tension à l'aide d'uni onde ultrasonore, émise par une céramique piézo-électrique hémisphérique. L'onde est focalisée pendant une courte durée et dans un petit volume loin de toute paroi, ce qui minimise l'influence d'éventuelles impuretés. Nous obtenons des résultats très reproductibles, permettant de mesurer la statistique de cavitation et de définir précisément son seuil dans différents types d'eaux. La céramique est calibrée de deux manières : avec des hydrophones à aiguille, et avec une méthode basée sur la variation de la pression statique du liquide. Les deux méthodes sont en accord et donnent une pression de cavitation monotone de -26 MPa à 0°C à -16 MPa à 80°C. Cela fait partie des pressions les plus négatives observées dans l'eau mais reste loin de la valeur théorique attendue (environ -120 MPa) et observée dans une seule expérience (Zheng et al. , 1991, Science 254, 829). Nous discutons les causes possibles de ce désaccord : il peut être du soit à la présence d'impuretés, dont nous discutons la nature et la concentration, soit à une courbure inattendue de l'équation d'état aux pressions très négativesAny liquid can be superheated or stretched beyond its boiling curve. It will then remain in a metastable state, until a vapor bubble appears : this is the cavitation phenomenon. The study of the metastability limit gives information about the cohesion of the liquid and its equation of state. The case of water is of special interest ; indeed competiting theories predict qualitatively different temperature variations of the stability limit : either monotonic (the cavitation pressure increase with temperature), or with a minimum. We have stretched water with an ultrasonic wave, generated by a hemispherical piezoelectric ceramic, duting a short time and a small volume, far from any wall, which reduces the effect of impurities. We obtain very accurate and reproducible results, allowing u to study in details the statistic of cavitation and to define a cavitation thresold. The ceramic was calibrated using two independent methods : needle hydrophones, and a method based on the variation of the static pressure. Both of them lead to the same result. We find a monotically increasing cavitation pressure, from -26 MPa at 0°C to -16 MPa at 80°C. These values are among the most negative reported, but far away from the values expected theoritically (-120 Mpa), and observed only once (Zheng et al. , 1991, Science 254, 829). We discuss the possible origin of this discrepancy : it can be due either to the presence of impurities in the liquid, whose nature and concentration we discuss, or to an unexpected change in the equation of state of water at large negative pressures
19
Hankey, S. E. "Cavitation erosion of WC-Co". Master's thesis, University of Cape Town, 1987. http://hdl.handle.net/11427/21135.
Testo completoAbstract (sommario):
Bibliography: pages 68-70.An investigation involving the vibratory cavitation erosion of WC-Co alloys was undertaken in order to determine the mechanisms of material removal. Nineteen grades of WC-Co alloys were studied. These alloys had been previously characterised according to microstructural and mechanical properties. Further characterisation by way of Young's modulus and density of the materials was undertaken. An investigation of the i nfluence of various parameters on cavitation erosion established a binder content dependence on erosion. For two grain sizes, erosion was found to increase to a maximum at 12 vo1-% binder content (1.8 μm grain size) and 23 vol-% binder (2.8 μm grain size). The main mode of material removal was found to be cobalt removal followed by WC grain pull-out. In high binder content alloys, cobalt removal was predominant with little loss of WC grains. X-ray diffraction showed that the allotropic phase transformation of the binder under cavitational attack was beneficial to the erosion resistance of these alloys. The erosion of low binder content alloys was controlled by the contiguity of the WC skeleton. Maximum erosion occurred at binder contents which corresponded to the combination of a fragile WC skeleton and a small volume of available cobalt for strain induced transformation.
20
Lesko, Timothy Michael McKoy Vincent. "Chemical effects of acoustic cavitation /". Diss., Pasadena, Calif. : California Institute of Technology, 2004. http://resolver.caltech.edu/CaltechETD:etd-04262004-184449.
Testo completo
21
He, Bingrong. "An investigation of cavitation thresholds /". Search for this dissertation online, 2004. http://wwwlib.umi.com/cr/ksu/main.
Testo completo
22
Li, Shengcai. "Cavitation associated low-frequency fluctuation". Thesis, University of Warwick, 1990. http://wrap.warwick.ac.uk/108599/.
Testo completoAbstract (sommario):
This self-guided thesis presents the research results of the Cavitation Associated Low-frequency Fluctuation, i. e. one part of the long-term research project on Statistical Characteristics of Cavitation Bubble Collapse Pulses. Associated Low-frequency Fluctuations and Flow Noise which was initiated and carried out by the author in USA. China and UK successively over last 9 years. The background and the objectives of the long-term project is introduced in the thesis through a broad review of the development of the cavitation research in the fields relevant to the project. Then, the observed phenomenon of Cavitation Associated Low-frequency Fluctuation in the venturi cavitating flow of the University of Michigan ( USA ) is explained in detail. A one-dimensional linear physical model based on the theorem of the interaction between the cavitation cloud of the homogeneous liquid-vapour/gas mixture and the surrounding liquid portion in the flow system is proposed to describe the mechanism of the phenomenon. The appearance of this low-frequency fluctuation is an association nature of the cavitating flow, which could be regarded as an alternatively indirect indicator of cavitation inception especially in such sort of cavitating flow systems. When the natural frequencies for both the cavitation cloud and the surrounding liquid portion coincide, the low- frequency pressure fluctuation component reaches its maximum, which is defined as Cavitation Resonance. A numerical verification of the frequency response characteristics, using the hydraulic impedance approach, of this venturi system is presented as well. The knowledge of the cavitation associated low-frequency fluctuation acquired from the study of the venturi cavitating flow has been used to analyze the phenomenon of the pressure fluctuations associated with the cavitating flows in the hydraulic machinery systems such as the HL-160-25 ( Francis ) Hydraulic Turbine System (draft-tube cavitating flow). The results are also briefly presented.
23
Pineda, Rondon Saira Freda. "Numerical prediction of cavitation erosion". Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEC031/document.
Testo completoAbstract (sommario):
La cavitation peut avoir lieu dans les turbines hydrauliques. Ce phénomène se produit lorsque les bulles de vapeur s’effondrent à proximité de la surface de la machine. Ceci entraîne des conséquences négatives, telles que l’érosion, affectant ainsi les performances de la machine. L’effondrement d’une bulle de gaz non-condensable dans l’eau est simulé en utilisant la méthode sans maillage SPH-ALE, qui intègre un modèle pour simuler les écoulements compressibles et multiphases. Le modèle résout les équations de conservation de masse, de quantité de mouvement et d’énergie du système d’Euler, en utilisant l’équation d’état de Stiffened Gas pour l’eau et l’équation d’état de gaz parfait pour le gaz non-condensable à l´ıntérieur de la bulle. Les deux phases sont modélisées comme compressibles et le changement de phase n’est pas considéré. La caractéristique sans maillage de la méthode SPH-ALE permet le calcul des écoulements diphasiques où l’interface est nettement définie. Pour les applications de cavitation, où le nombre de Mach atteint des valeurs de 0.5, la distribution de particules doit être corrigée. Cela est réalisé grâce à la fonctionnalité ALE. Le modèle compressible a été validé à l’aide de configurations monodimensionnelles, comme le cas du tube à choc pour des écoulements monophase et multiphases. L’effondrement de la bulle près d´une paroi a été abordé comme le mécanisme fondamental qui produit des dégâts. Son comportement général se caractérise par la formation d’un micro jet d’eau et par l’effondrement de la bulle sur elle-même. Le phénomène est analysé en tenant compte des principaux paramètres qui le régissent, comme la distance initiale entre le centre de la bulle et la paroi (H0), la taille de la bulle (R0) et le taux de pression qui entraîne l’effondrement (pw/pb). Il est démontré que l’intensité de l’effondrement dépend principalement du rapport de pression entre le liquide et la bulle (pw/pb). De plus, quatre indicateurs, comme la pression en paroi, l’impulsion, la pression du coup de bélier et la vitesse du micro jet d’eau, servent à déterminer le chargement. Cette analyse indique qu’une bulle initialement située à une distance inférieure à H0/R0 = 2 présente un haut potentiel d’endommagement. Afin de prédire cet endommagement, la mécanique du solide est analysée à l’aide de simulations d’interaction fluide-structure. On obtient que le matériau réagit aux charges hydrauliques en ayant des zones de compression et de traction. Ceci suggère qu’un mécanisme de fatigue entraîne le phénomène d’endommagement. En plus, on constate que les contraintes les plus importantes sont situées sous la surface du matériau, indiquant que cette zone peut être sujette à une déformation plastiqueHydraulic turbines can experience cavitation, which is a phenomenon occurring when vapor bubbles collapse in the vicinity of the machine’s surface. This phenomenon can lead to negative consequences, such as erosion, that affect the machine’s performance. The compression of a non-condensable gas bubble in water is simulated with the Smoothed Particle Hydrodynamics method following the Arbitrary Lagrange Euler approach (SPHALE), where a compressible and multiphase model has been developed. The model solves the mass, momentum and energy conservation equations of the Euler system using the Stiffened Gas EOS for water and the ideal gas EOS for the non-condensable gas inside the bubble. Both phases are modeled as compressible and the phase change is not considered. The meshless feature of the SPH-ALE method allows the calculation of multiphase flows where the interface is sharply defined. For cavitation applications, where the Mach number reaches values of 0.5, the distribution of particles must be corrected, which is achieved by the ALE feature. The compressible model was validated through monodimensional configurations, such as shock tube test cases for monophase and multiphase flows. The bubble compression close to the wall has been addressed as the fundamental mechanism producing damage. Its general behavior is characterized by the formation of a water jet and by the collapse of the bubble by itself. The phenomenon is analyzed by considering the major parameters that govern the bubble collapse dynamics, such as the initial distance between the bubble center and the wall (H0), the bubble size (R0), and the collapse driven pressure ratio (pw/pb). It is shown that the intensity of the collapse depends mainly on the pressure ratio between the liquid and the bubble (pw/pb). As well, four indicators, such as the pressure at the wall, the impulse, the water-hammer pressure and the water jet velocity, are used to determine the loading. This analysis gives that the bubble initially located at a distance lower than H0/R0 = 2 presents high potential to cause damage. In order to predict the damage due to the bubble collapse, the solid mechanics is analyzed through fluid-structure interaction simulations. It is obtained that the material reacts to the hydraulic loads by having compression and traction zones, suggesting that a fatigue mechanism drives the damage phenomenon. Additionally, it is found that the highest stresses are located below the material surface, indicating that this zone may reach plastic deformation
24
Chebli, Rezki. "Simulation 2D et 3D des écoulements cavitants : développement d'un algorithme original dans Code_Saturne et étude de l'influence de la modélisation de la turbulence". Thesis, Paris, ENSAM, 2014. http://www.theses.fr/2014ENAM0040/document.
Testo completoAbstract (sommario):
La cavitation est l'un des phénomènes physiques les plus contraignants influençant les performances des machines hydrauliques. Il est donc primordial de savoir prédire son apparition et son développement, et de quantifier les pertes de performances qui lui sont associées. L'objectif de ce travail est de développer un algorithme 3D instationnaire pour la simulation numérique de la cavitation dans le code industriel « Code_Saturne ». Il est basé sur la méthode à pas fractionnaires et préserve le principe du minimum/maximum sur le taux de vide. Un solveur implicite, basé sur l'équation de transport du taux de vide couplée avec les équations Navier-Stokes est proposé. Un traitement numérique spécifique des termes sources de cavitation permet d'obtenir des valeurs physiques du taux de vide (entre 0 et 1) sans aucune limitation artificielle. L'influence des modèles de turbulence RANS sur la simulation de la cavitation est étudiée sur deux types de géométries 2D (Venturi et Hydrofoil). Cela confirme que la modification de Reboud et al. (1998) appliquée aux modèles à viscosité turbulente à deux équations, k-epsilon et k-omega-SST, permet de reproduire les principales caractéristiques du comportement instationnaire de la poche de cavitation. Le modèle du second ordre RSM-SSG, basé sur le transport des contraintes de Reynolds, se révèle capable de reproduire le comportement instationnaire de l'écoulement sans aucune modification arbitraire. Les effets tridimensionnels intervenant dans les mécanismes d'instabilité de la poche sont également analysés. Ce travail nous permet d'aboutir à un outil numérique, validé sur des configurations d'écoulements cavitants complexes, afin d'améliorer la compréhension des mécanismes physiques qui contrôlent les effets instationnaires tridimensionnels intervenants dans les mécanismes d'instabilitéCavitation is one of the most demanding physical phenomena influencing the performance of hydraulic machines. It is therefore important to predict correctly its inception and development, in order to quantify the performance drop it induces, and also to characterize the resulting flow instabilities. The aim of this work is to develop an unsteady 3D algorithm for the numerical simulation of cavitation in an industrial CFD solver « Code_saturne ». It is based on a fractional step method and preserves the minimum/maximum principle of the void fraction. An implicit solver, based on a transport equation of the void fraction coupled with the Navier-Stokes equations is proposed. A specific numerical treatment of the cavitation source terms provides physical values of the void fraction (between 0 and 1) without including any artificial numerical limitation. The influence of RANS turbulence models on the simulation of cavitation on 2D geometries (Venturi and Hydrofoil) is then studied. It confirms the capability of the two-equation eddy viscosity models, k-epsilon and k-omega-SST, with the modification proposed by Reboud et al. (1998) to reproduce the main features of the unsteady sheet cavity behavior. The second order model RSM-SSG, based on the Reynolds stress transport, appears able to reproduce the highly unsteady flow behavior without including any arbitrary modification. The three-dimensional effects involved in the instability mechanisms are also analyzed. This work allows us to achieve a numerical tool, validated on complex configurations of cavitating flows, to improve the understanding of the physical mechanisms that control the three-dimensional unsteady effects involved in the mechanisms of instability
25
Lafond, Maxime. "Confocal Ultrasound for the Potentiation of Chemotherapy by Ultrasonic Cavitation without External Nucleation Agents". Thesis, Lyon, 2016. http://www.theses.fr/2016LYSE1243/document.
Testo completoAbstract (sommario):
Le cancer est reconnu comme l'un des principaux enjeux de santé actuels. Même si de grands progrès ont été réalisés dans ce domaine, les effets systémiques des chimiothérapies et le caractère invasif des procédures actuelles de potentialisation (agents physiques) sont autant d'éléments limitants. Les ultrasons se démarquent néanmoins par leur faible morbidité. Appliqués de façon extracorporelle, ils peuvent pénétrer en profondeur dans les tissus et y induire effets thermiques et mécaniques, incluant entre autres la cavitation. La cavitation peut se définir comme la formation et l'oscillation de bulles dans le milieu de propagation. Il a été montré de potentiels bénéfices de ce mécanisme dans la potentialisation d'agents thérapeutiques. Bien que la génération de cavitation puisse être aidée par l'ajout d'agents de nucléation extérieurs, le travail présenté ici s'en affranchit afin de rendre la procédure plus versatile. Des simulations ont montré qu'un dispositif ultrasonore basé sur deux faisceaux confocaux permettait des conditions favorables à l'obtention de cavitation dans ces conditions. De plus, études in vivo ont montré l'innocuité du phénomène en regard de la stabilité de la doxorubicine, des effets histologiques sur tissus sains ainsi que sur l'éventuelle diffusion métastatique. L'efficacité du traitement combiné n'a en revanche pas pu être démontrée. Pour investiguer la combinaison de chimiothérapie avec de la cavitation stable, une stratégie de régulation est mise en place. Bien que la synergie ait pu être démontrée in vitro, l'étude préclinique ne permet pas de conclure sur l'effet in vivo. Dans l'hypothèse d'un défaut de localisation du nuage de cavitation, une méthode de localisation spatiale est mise en place et validéeCancer is recognized as one of the major health issues of this beginning century. Even if great achievements have been performed, chemotherapies induce systemic toxicity and combinable physical agents are invasive. Ultrasound has shown a great potential as an external physical agent. Applied extracorporeally, it can penetrate in depth in tissue and induce various biological effects, mechanical of thermal. Notably, cavitation, which is the formation and oscillatory motion of bubbles in a media, has effects providing the possibility to enhance the delivery of chemotherapeutic agents. This effect can be induced in biological tissues by using external nucleation agents such as ultrasound contrast agents. However, to avoid diffusion issues, this work focuses on cavitation without external nucleation agents. For this purpose, a particular setup based on two confocal transducers was designed. Simulations showed its advantages for cavitation applications. A developed preclinical device demonstrated the safety of using unseeded inertial cavitation for the potentiation of doxorubicin (DOX) regarding the drug stability, the effect on healthy tissues and the metastatic spreading. Unfortunately, no effect of combining inertial cavitation with DOX in could have been demonstrated in vivo. To investigate stable cavitation phenomenon, a control process was developed. It permitted to evidence in vitro the synergistic interaction between DOX and stable cavitation. Again, preclinical studies were not able to prove this synergy in vivo. To assess the correct tissue exposures to stable cavitation, a localization method was developed and validated
26
Ahmed, Zayed. "Quantitative flow measurement and visualization of cavitation initiation and cavitating flows in a converging-diverging nozzle". Thesis, Kansas State University, 2017. http://hdl.handle.net/2097/35522.
Testo completoAbstract (sommario):
Master of ScienceDepartment of Mechanical and Nuclear Engineering
B. Terry Beck
Mohammad H. Hosni
Cavitation is the formation of vapor phase from the liquid phase by reduction in its absolute pressure below the saturation pressure. Unlike boiling, where the temperature of the liquid is increased to cause vaporization, the reduction in the pressure alone can cause the liquid to turn into vapor. Cavitation is undesirable in many engineering applications as it is associated with reduction in efficiency and is known to cause damage to pump and propeller components. However, the endothermic nature of cavitation could be utilized to create a region of low temperature that could be utilized to develop a new refrigeration cycle. The work presented in this thesis is part of ongoing research into the potential cooling capacity of cavitation phenomena, where the cavitation in a converging-diverging nozzle is being investigated. Due to the constricting nature of the throat of the converging-diverging nozzle, the liquid velocity at the throat is increased, obeying the continuity law. With an increase in velocity, a reduction in absolute pressure is accompanied at the throat of the nozzle according to the Bernoulli’s principle. The local absolute pressure at the throat can go lower than the saturation vapor pressure, thereby causing the fluid to cavitate. The effect of water temperature on the flowrates, the onset of cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle are the subject of this thesis. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can go beyond the choked flowrate, causing the local pressure in the throat to go well below zero for an extended amount of time in the metastable state, before nucleating (cavitating) into a stable state. Flow visualization using a high speed digital camera under different operating conditions was aimed at investigating the region of cavitation onset, which appears to be associated with boundary layer separation just downstream of the nozzle throat. In order to delay the boundary layer separation point in the downstream section of the nozzle, the diffuser region of the nozzle was modified to enable two flow paths, where one path would suck the flow near the inner walls of the nozzle and the other would allow the bulk of the flow to pass through. This was achieved with the use of inserts. Various inserts were tested in an attempt to capture the effect of inserts on the cavitation phenomena. Their effect on the flowrates, length of two phase region, and cavitation onset are presented in this thesis.
27
Guilleumas, Montserrat. "Nucleation and cavitation in liquid helium". Doctoral thesis, Universitat de Barcelona, 1995. http://hdl.handle.net/10803/667593.
Testo completoAbstract (sommario):
Las transiciones de fase no siempre ocurren bajo condiciones de equilibrio. Un fluido puede ser conducido a un estado metastable. A pesar de ser internamente estable, existe otra configuración con un potencial termodinámico menor, por tanto energéticamente más favorable y a la cual tenderá el sistema. Esta transición está bloqueada por una barrera termodinámica que puede ser superada con resultado de la formación y crecimiento de pequeños nucleos de la nueva fase estable en la fase metastable. Este proceso se llama nucleación (formación de gotas en un vapor sub-enfriado) o cavitacion (burbujas en un líquido sobre-calentado).
En esta tesis hemos estudiado los fenómenos de cavitación y nucleación en helio dentro del marco de la teoria del funcional de la densidad. Primero hemos estudiado la activación térmica en 3He y 4He puros y en las mezclas 3He-4He, estimando la presión a partir de la cual tiene lugar la transición de fase. Para los sistemas puros hemos estudiado también la activación cuántica a muy bajas temperaturas, estimando la temperatura de transición entre ambos regímenes.
28
Zhou, Zhi-ang. "Gas nucleation and cavitation in flotation". Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=40477.
Testo completoAbstract (sommario):
The role of gas nucleation and cavitation in flotation has been systemically studied. It was shown theoretically that about a 10 m high recovery zone is required to ensure at least one collision between a particle less than 10 $ mu$m and a bubble, under conventional column flotation conditions, indicating that the collision is the rate limiting step for fine particle flotation. Direct formation of small bubbles on the hydrophobic particle surfaces and in slurry by gas nucleation and cavitation has, therefore, been exploited as a way to accelerate the fine particle collection rate.Experimentally, the existence of gas nuclei in water was demonstrated from coagulation, sedimentation and filtration tests of fine coal (d$ sb{50} approx 5 mu$m) and silica (d$ sb{50} approx 3.5 mu$m) particles. Small bubbles are generated, from the expansion of the pre-existing gas nuclei, by hydrodynamic cavitation induced by flow of liquid through a nozzle at a velocity of 8-15 m/s, depending on nozzle diameter and length. This velocity of bubble initiation is reduced 5-7 m/s when the liquid is gas-supersaturated. Surfactants do not affect the on-set of bubble formation by cavitation, but increase the amount of bubbles formed. The addition of a small amount of hydrophobic particles in gas-supersaturated systems increases the quantity of bubbles generated, while the presence of hydrophilic particles reduces bubble formation.
Dissolved air (or carbon dioxide) flotation of fine silica (d$ sb{50} approx 1.6 mu$m) has shown that recovery increases with the slurry flow velocity, regardless of the saturation pressures applied (102-310 kPA). Releasing the gas-supersaturated slurry into a solution gives a higher recovery than releasing the gas-supersaturated slurry into a slurry. These observations suggest that bubble nucleation is a better mechanism for enhancing particle collection compared with the direct particle-bubble contact.
Flotation results using a newly designed flotation reactor indicate that adding a cavitation tube and a small amount of air in the feed stream increases recovery significantly, and that bubbles formed by hydrodynamic cavitation play a role in improving flotation kinetics, even in the absence of added gas in the stream.
29
Moule, R. T. "Cavitation erosion in a corrosive environment". Thesis, Coventry University, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.374256.
Testo completo
30
Lloyd, James G. "The cavitation phenomenon in nonlinear elasticity". Thesis, University of Bath, 2013. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.589654.
Testo completoAbstract (sommario):
This thesis studies the phenomenon of cavitation in nonlinear elasticity. In Chapter 2 we study the problem of cavitation in a ball of elastic material which is subjected to a radial dead-load on its boundary. We show that the rotationally symmetric equilibrium solution, in which a spherical cavity forms at the centre of the deformed ball, cannot be a global energy minimiser. This is achieved by proving the existence of a related axisymmetric homogeneous equilibrium with less energy (our arguments are related to the Rivlin instability in incompressible materials). In Chapter 3 we develop new necessary conditions for radially symmetric equilibria to be strong local minimisers of the energy. In Chapters 4 and 5 we study cavitation at, or near, the boundary of an elastic body. In Chapter 4 we consider cavitation occurring near the boundary of an elastic body occupying a half-space in its reference configuration. We show that the energy of such an equilibrium can be further lowered by moving the point of cavitation further away from the boundary. In Chapter 5, we prove existence of energy minimisers in classes of deformations which allow cavitation to occur at a boundary point.
31
Villeneuve, Randall A. (Randall Alan). "Effects of viscosity on hydrofoil cavitation". Thesis, Massachusetts Institute of Technology, 1993. http://hdl.handle.net/1721.1/17325.
Testo completoAbstract (sommario):
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1993, and Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1993.Includes bibliographical references (leaves 118-120).
by Randall A. Villeneuve.
M.S.
32
Lambaré, Hadrien. "Cavitation quantique dans l'helium-4 superfluide". Paris 6, 1998. http://www.theses.fr/1998PA066539.
Testo completoAbstract (sommario):
Nous avons etudie experimentalement la cavitation (c'est-a-dire la nucleation de bulles) dans l'helium-4 superfluide. En utilisant un cryostat a dilution, nous avons obtenu les premieres mesures au-dessous de 800 mk, et jusqu'a 65 mk. La nucleation est declenchee par des impulsions ultrasonores focalisees par un transducteur piezo-electrique hemispherique. Nous avons ainsi obtenu des oscillations de pression negative proches de la limite spinodale de l'helium-4 superfluide (estimee a 9. 5 bars a temperature nulle), ou la compressibilite du liquide devient infinie, et ou la barriere d'energie de nucleation tend vers zero. Les bulles sont detectees par la diffusion de la lumiere d'un laser. Nous avons trouve que la cavitation est un phenomene stochastique jusqu'aux plus basses temperatures, et qu'au-dessous de 600 mk la probabilite de nucleation est independante de la temperature. Ces resultats sont en accord avec l'hypothese qu'a basse temperature la nucleation se fait par effet tunnel quantique macroscopique. De recents modeles de fonctionnelle de densite predisent une transition entre un regime d'activation thermique et un regime quantique (independant de la temperature) vers 200 mk. La difference avec la temperature de transition observee est compatible avec le refroidissement adiabatique local lors de l'oscillation de pression negative. La largeur de la courbe de probabilite de cavitation en fonction de la tension electrique appliquee au transducteur est egalement compatible avec la theorie, une fois tenu compte de la non-linearite de l'onde acoustique. Nous avons egalement etudie la dynamique des bulles en fonction de la pression statique. Nos resultats sont en tres bon accord avec le modele classique de rayleigh-plesset. Nous avons ainsi pu determiner l'energie cinetique initiale des bulles (de 2 a 9 nj), ainsi que le temps experimental (83 ns).
33
Sjöholm, Henrik. "Pressure Effects in Orifice Cavitation Modeling". Thesis, Umeå universitet, Institutionen för fysik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-176885.
Testo completoAbstract (sommario):
In this thesis computational models for cavitating flows around orifice plates has been studied and compared. The goal was to fit a model with experimental data and this was done with some success, although problems with numerical stability, long calculation times and geometry overfitting remain. Cavitation is a complex fluid phenomenon that can occur in pressurized liquid flows. It starts when the liquid pressure is lowered below the boiling pressure and water that undergoes cavitation forms vapor which later implodes violently. This process can cause problems such as noise, vibrations and corrosion in piping systems. Loud noise is a nuisance, however powerful vibrations and corrosion can have serious consequences for the structural integrity of pipes. The for example lessened performance, leakages or even failure. Therefore the minimization of cavitation is often a goal in orifice and piping design. Vattenfall AB, together with Forsmark and Ringhals nuclear plants have studied cavitating flows around orifice plates used for flow limitation. A set of data from laboratory tests made by Vattenfall was used as the basis of analysis. Existing computational models in OpenFOAM were tested and evaluated based on their ability to model the experimental data accurately, as well as their computational performance and stability. The cavitation phenomenon was difficult to simulate using established methods so a new method was created and verified. It is based on the Kunz cavitation model together with Large Eddy Simulations, but with turbulence as a predictor of cavitation. The new computational model will serve as a tool for knowing how to design orifices in the future, so that laboratory experiments will not have to be conducted for each new piping design.
34
Kozák, Jiří. "Cavitation Induced by Rotation of Liquid". Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2020. http://www.nusl.cz/ntk/nusl-409529.
Testo completoAbstract (sommario):
Tato disertační práce se zabývá experimentálním a numerickým výzkumem kavitace vyvolané rotací. Pro potřeby tohoto výzkumu byla využita transparentní osově symetrická Venturiho dýza, díky čemuž bylo možné zkoumat dynamiku kavitujícího proudění pomocí analýzy vysokorychlostních nahrávek.
35
Pamidi, Taraka Rama Krishna. "Process Intensification by Ultrasound Controlled Cavitation". Licentiate thesis, Luleå tekniska universitet, Drift, underhåll och akustik, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-73856.
Testo completoAbstract (sommario):
Process industries are cornerstones in today’s industrialized society. They contribute significantly in the manufacturing of various goods and products that are used in our day-to-day life. Our society’s paradigm of consumerism accompanied by a rise in global population drives an ever increasing demand for goods. One of many strategies developed to satisfy these demands and at the same time improve production capabilities is known as process intensification. As an example, this can be accomplished by implementation of devices using the principle of hydrodynamic and acoustic cavitation. High-intensity cavitation in the ultrasonic range can change the physical and chemical properties of a wide range of substances and hence, improve the production rate or quality. Despite the generally accepted benefits of hydrodynamic and acoustic cavitation, applications in the process industry are yet limited. The reasons are that the method requires extensive optimization, which depends on multiple process parameters and encounters problem in the implementation on a larger scale. Scalable cavitation reactor concepts for industrial applications need to meet challenges like stability and robustness, energy efficiency and high flow rates. This thesis focuses on the methodology for the design and optimization of a flow through cavitation reactor. An ultrasound reactor concept has been developed and tested for two different applications: i) Fibrillation processes typical for paper and pulp industry; ii) Metal leaching of mineral concentrates. Simulations were carried out using a commercially available software for multiphysics modeling which combines acoustics, structural dynamics, fluid dynamics and piezoelectrics. However, the optimization procedure requires extensive experimental work in parallel with multi-physical simulations. In general, the application leads to hydrodynamic initiation of small gas bubbles in the fluid to be excited and collapsed by high-intensity ultrasound. This transient collapse of the cavitation bubbles provides both mechanical and chemical effect on materials. The developed reactor has a power conversion efficiency of 36% in batch mode and is well suited for a scale-up. In flow-through mode, the cavitation effect improves extensively and provides stable results. Energy efficiency requires hydrodynamic initiation of cavitation bubbles, high acoustic cavitation intensity by multiple excitation frequencies adapted to the optimized reactor geometry, as well as optimal process pressure and temperature with respect to the materials to be treated. The impact of flow conditions and hydrodynamic cavitation is significant and almost doubles the yield at the same ultrasonic power input. In the case of fibrillation of cellulose fibers, results obtained indicate that generated cavitation intensity changes the mechanical properties of the fiber wall. In the case of leaching, experiments show that six hours of exposure gave a 57% recovery of tungsten from the scheelite concentrate at 80°C and atmospheric pressure. Future research will focus on different types of excitation signals, extended reactor volume, increased flow rates and use of a higher process temperature.
36
Gong, Cuiling 1964. "Ultrasound induced cavitation and sonochemical effects". Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9443.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1999.Includes bibliographical references (p. 132-137).
The introduction of a strong acoustic field to an aqueous solution results in the generation of cavitation microbubbles. The non-linear motion of these microbubbles focuses energy from the macro-scale acoustic waves to the micro-scale vapor inside the bubbles. As a result, extremely high localized pressures on the order of hundreds of atmospheres and temperatures on the order of thousands of degrees Kelvin are generated. Under such extreme conditions molecular dissociation occurs and produces highly reactive free radicals. This phenomenon provides a means of "burning" substances in liquids and enhancing reactions that cannot be achieved by conventional means. Sonochemistry, the chemistry associated with this phenomenon, has found application in drug delivery, waste decomposition, water treatment, chemical reaction enhancement and numerous novel material processes. A theoretical framework that directly couples the dynamics of bubble motion and the associated kinetics of gas phase reactions is established for the first time in an attempt to understand the fundamental mechanisms of the sonochemical phenomenon. Several fundamental mechanisms, which are believed to be critical in understanding the unusual experimentally observed sonoluminescence and sonochemical behavior, are revealed. First, not all chemical reactions associated with bubble oscillation in a sound field have reached thermodynamic equilibrium. Second, chemical kinetics couples closely with the bubble motion and has significant impact on the dynamics of bubble motion when a bubble contains a combustible gas mixture. Third, the dissolved gases affect the activities of a sonochemical event through both thermal effect by changing the peak collapse temperatures in the bubble and chemical effect by directly participating in reactions. In addition, a laboratory scale sonochemical experiment is conducted to demonstrate the sonochemical effects as a result of ultrasonic irradiation in a Fricke solution. Effects of the dissolved gases on sonochemical activities are experimentally quantified and compared with the predicted results using the model developed in this thesis.
by Cuiling Gong.
Ph.D.
37
Davis, R. Ted. "Aerating Butterfly Valves to Suppress Cavitation". DigitalCommons@USU, 1986. https://digitalcommons.usu.edu/etd/3952.
Testo completoAbstract (sommario):
Proper aeration of cavitating hydraulic equipment can greatly reduce cavitation intensity, noise, and damage. This thesis quantifies the benefit, in terms of damage and noise, from aerating six inch butterfly valve. The incipient damage level of cavitation was obtained for both aerated and non -ae ra ted conditions. The level is defined as one pit per square inch of a soft aluminum test specimen per one minute of operation. A description of the cavitation pits that occurred plus where they appeared is presented. A graph showing the aerated and non-aerated limits of incipient damage is given along with a table showing the percent reduct ion of damage from aeration. A graph and table are also given depicting the reduction in noise. The proper location of aeration ports to allow natural aeration is outlined .
38
Williams, Scott C. "Cavitation noise in a model spool valve". Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/20230.
Testo completo
39
Whaley, Erica Lee. "The Interaction Between Cavitation and Wear in Enclosed Spaces with Oscillating Boundaries". Miami University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=miami1558435589443526.
Testo completo
40
Leclercq, Christophe. "Simulation numérique du chargement mécanique en paroi généré par les écoulements cavitants, pour application à l'usure par cavitation des pompes centrifuges". Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAI119/document.
Testo completoAbstract (sommario):
Lors du fonctionnement des machines hydrauliques, le développement de structures de cavitation peut entraîner une chute de performance. De plus, la cavitation peut être responsable de l'usure des pièces mécaniques par érosion. La taille et la vitesse de rotation des pompes étant fortement dépendantes du niveau de cavitation acceptable, la présence de cavitation dans une installation hydraulique est liée à un compromis technico-économique. La prévision de l'érosion de cavitation serait utile à la fois pour améliorer la conception des matériels en projet mais également pour optimiser les périodes de maintenance de ceux existants. Bien que des essais, permettant de caractériser le comportement à l'érosion d'une machine, soient possibles, ceux-ci restent coûteux. Bénéficiant des progrès de la simulation des écoulements, une prévision par voie numérique de l'érosion de cavitation parait accessible.L'érosion est un phénomène multi-physique et multi-échelle. Multi-physique car elle fait intervenir la mécanique des fluides et la réponse du solide ; multi-échelle car les échelles en temps et en espace vont de celles caractérisant l'écoulement autour du composant hydraulique (0.1 m - 1 ms) jusqu'à celles de la déformation plastique observée sur le matériau (1 µm - 10 ns) . Dans cette thèse, seule la partie fluide est considérée, plus particulièrement, le chargement d'un écoulement cavitant sur une paroi solide, appelé "intensité de cavitation". L'objectif est d'estimer cette intensité à partir de la simulation d'un écoulement cavitant.Des écoulements instationnaires 3D en régime cavitant sont simulés en utilisant un modèle homogène, implémenté dans le Code_Saturne cavitant. Une description du modèle numérique et de l'approche physique considérée est présentée dans le mémoire. Le modèle de prédiction de l'endommagement, basé sur une approche énergétique, est développé et appliqué sur un hydrofoil NACA 65012 testé au LMH-EPFL (École Polytechnique Fédérale de Lausanne) et sur la pompe centrifuge SHF testée au sein d'EDF R&D. Des comparaisons entre des simulations 3D sur différents maillages sont analysées et une bonne estimation qualitative de l'érosion est obtenue via ce modèle à différentes vitesses d'écoulement. Une tentative de validation quantitative pour le cas de l'hydrofoil est mise en place et semble prometteuse.Dans le but d'enrichir le modèle de prévision de l'intensité de cavitation, des simulations à l'échelle d'une bulle sont également menées. Ces simulations permettent une meilleure compréhension des interactions entre une onde incidente et l'implosion d'une bulle en proche paroi. Des phénomènes d'amplification des collapsus de bulles ont pu être simulés, pouvant être à l'origine de fortes ondes de pression dont l'impact serait responsable de l'endommagement des matériaux avoisinantsThe development of cavitation structures can lead to efficiency losses during hydraulic machinery duty. Moreover, cavitation can be responsible for wear of mechanical parts through cavitation erosion. The presence of cavitation in a hydraulic machine is also linked to a technical and economical trade-off, because the size and rotational speed of pumps are highly dependent on the acceptable cavitation level. Therefore, it seems likely that cavitation will remain present in current and future pump designs and its consequences must be dealt with. The prediction of the erosion is important both for the improvement of hydraulic components at the design stage but also for the optimization of maintenance periods of existing machinery. Experimental tests can be carried out to characterize the erosion behaviour of a machine, but are still expensive. With the recent advances in Computational Fluid Dynamics, the numerical prediction of cavitation erosion appears as a reachable and cost-effective alternative.The erosion is a multi-physical and multi-scale phenomenon. Multi-physical because it deals with both fluid and solid mechanics and multi-scale because the length and time scales of the flow around the hydraulic component (0.1 m - 1 ms) and of the plastic deformation observed on the material (1 µm - 10 ns) are not of the same order of magnitude. In this thesis, only the fluid part is considered, more particularly, the mechanical load imposed by a cavitating flow on a material, called "cavitation intensity". The objective is to estimate this intensity from cavitating flow simulations.In this work, 3D unsteady simulations of cavitation regimes are carried out using a homogeneous model, implemented in the CFD code Code_Saturne cavitant. The prediction damage model, based on an energy approach, is developed and applied to a NACA 65012 hydrofoil tested at the LMH-EPFL (École Polytechnique Fédérale de Lausanne) and on the SHF centrifugal pump tested at EDF R&D. Comparisons between 3D simulations on different meshes show that the model provides good qualitative predictions of erosion at different flow velocities. An attempt is made to propose a quantitative validation for the case of the hydrofoil, with promising results.In order to enrich the cavitation intensity prediction model, simulations at the bubble scale are also performed. These simulations allow for a better understanding of the interaction between an incident pressure wave and the implosion of a near-wall bubble. The mechanism of bubble collapse amplification is simulated, and is shown to be associated with high magnitude pressure waves. This amplification phenomenon is suspected to be a strong contributor to the damage of neighbouring materials
41
Sabraoui, Abbas. "Régulation de la cavitation acoustique appliquée à la transfection cellulaire". Thesis, Lyon 1, 2012. http://www.theses.fr/2012LYO10007.
Testo completoAbstract (sommario):
Le travail présenté ici porte sur l’étude et le contrôle de la cavitation acoustique dans le but de développer un système de sonoporation efficace pour les cellules en suspension et les cellules adhérentes. Le manuscrit est composé de trois chapitres. Tout d’abord, une revue de la littérature sur les différentes techniques physiques utilisées en transfection cellulaire, et plus particulièrement la sonoporation. Il a été démontré que le principal mécanisme de la sonoporation est étroitement lié au phénomène de cavitation acoustique. Un contrôle de ce phénomène aléatoire apparaît alors intéressant afin d’augmenter le taux de transfection tout en gardant une forte viabilité cellulaire. Dans le second chapitre, un système de régulation de cavitation ultrasonore basé sur un indice acoustique de cavitation a été étudié. Cet indice, est basé sur la mesure de bruit large bande émis lors de l’implosion des bulles de cavitation. Les avantages d’un tel système sont : un suivi en temps réel du niveau de cavitation durant l’irradiation, des informations quasi-instantanées sur les composantes spectrales caractéristiques de la cavitation, une meilleure reproductibilité et stabilité du niveau de cavitation surtout pour les intensités modérés. Dans le troisième chapitre, pour comprendre les mécanismes de la sonoporation, un deuxième système de cavitation contrôlé a été conçu dans le but de permettre une visualisation du milieu en cours d’insonification. Ce nouveau dispositif est adapté à un fonctionnement sous microscope photonique à transmission et à fluorescence. Des essais de transfection de siRNAs, sur les cellules en suspension (RL du lymphome folliculaire) et sur les cellules adhérentes (cancer du sein ; MDA-MB 231) ont permis de valider in vitro l’efficacité de ce système en atteignant un taux de 40 % de transfection pour ces deux types de cellules, avec un très faible taux de mortalité (< 10 %)The aim of the present work, which is based on the study and the control of acoustic cavitation, is to develop an efficient sonoporation system to transfect the cells in suspension and the adherent cells. The manuscript is composed of three chapters. The first one takes a glance on the state of art of different physical techniques used in cells transfection, and more precisely on sonoporation. It has been shown that the principal mechanism of sonoporation is closely linked to acoustic cavitation. Thus, a control of this random phenomenon is important to increase the rate of transfection while keeping strong cell viability. In the second chapter, a regulated cavitation generator based on an acoustic index was studied. This index is based on the measure of broad band noise emitted during the implosion of the cavitation bubbles. The advantage of such a system is: a control in real time of the level cavitation during sonication, leading to a better reproducibility and stability of the cavitation level, especially for the moderate intensities. In the third chapter, in order further study the sonoporation mechanisms, a second regulated cavitation generator was studied; its aim is to be able to visualize the medium during sonication. This new device is adapted to the performance under a fluorescencemicroscope with fluorescence transmission. SiRNAs transfection, was validated in vitro by attending a rate of 40 % of transfection for the two types of cells, with a very low rate of mortality (< 10%), for both suspended cells (RL of follicular lymphoma) and adherent cells (Cancer of breast; MDA-MB 231)
42
Haese, Peter Michael. "Interior source methods for planar and axisymmetric supercavitating flows". Title page, contents and abstract only, 2003. http://web4.library.adelaide.edu.au/theses/09PH/09phh136.pdf.
Testo completoAbstract (sommario):
"May 2003." Bibliography: leaves 125-130. This study considers use of an interior source method for modelling supercavitation in 2-dimensional (planar) and 3-dimensional axisymmetric flows. Aspects considered include the determination of where the fluid separates from the body, the shapes of the cavities formed, the pressure distribution on the body and the pressure on the cavity surface, and the resulting drag and lift forces on the body.
43
Pereira, Francisco. "Prédiction de l'érosion de cavitation : approche énergétique /". [S.l.] : [s.n.], 1997. http://library.epfl.ch/theses/?display=detail&nr=1592.
Testo completo
44
Cristiano, Antonella. "Fracture by cavitation of model polyurethane elastomers". Phd thesis, Université Pierre et Marie Curie - Paris VI, 2009. http://pastel.archives-ouvertes.fr/pastel-00578499.
Testo completoAbstract (sommario):
L'objectif principal de cette thèse a été de déterminer le rôle joué par l'architecture macromoléculaire du réseau sur les propriétés élastiques non linéaires, la résistance à la rupture et la résistance à la cavitation sous chargement hydrostatique. Nous avons synthétisé, dans des conditions contrôlées, trois réseaux élastomères dits « modèles » de Polyuréthanne (PU), à partir d'un triisocyanate et de polyether diols isomoléculaires (PPG). Une caractérisation physico-chimique fine des réactifs et des réseaux a été réalisée en utilisant des techniques telles que : RMN, FTIR et fractions solubles. Les propriétés élastiques non linéaires, viscoélastiques linéaires et la résistance à la rupture en mode I des trois réseaux modèles ont été caractérisées. Les essais de cavitation ont été effectués sur un dispositif expérimental développé pour cette étude, permettant de suivre les mécanismes de formation de cavités, à la résolution optique près, en temps réel. En menant une analyse systématique des conditions de cavitation, en fonction de la vitesse de déformation et de la température, il est apparu que, contrairement au modèle d'instabilité élastique communément utilisé, l'expansion critique de la cavité n'est pas uniquement pilotée par le module élastique; mais dépend fortement de l'énergie de rupture, GC et de l'extensibilité limite du réseau. Par ailleurs, nous avons observé l'apparition de cavités pré-critiques avant la fracture catastrophique ; ce qui met en évidence l'existence de deux critères : l'un, propre au processus de nucléation, principalement piloté par des mécanismes statistiques et activés thermiquement (distribution de défauts, temps, température, etc.) ; et l'autre, lié à la croissance de la cavité en milieu confiné contrôlé par GIC, et par le comportement aux grandes déformations. Enfin, la présence d'enchevêtrements dans l'architecture du réseau macromoléculaire s'est avérée clairement bénéfique pour stabiliser la croissance de cavités et donc pour renforcer la résistance à la cavitation.
45
Eulaerts, Olivier. "Etude physico-chimique de la cavitation acoustique". Doctoral thesis, Universite Libre de Bruxelles, 2001. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/211619.
Testo completo
46
Chanda, Suranjit Kumar. "Disintegration of sludge using ozone-hydrodynamic cavitation". Thesis, University of British Columbia, 2012. http://hdl.handle.net/2429/43105.
Testo completoAbstract (sommario):
In our study we applied hydrodynamic cavitation and ozonation both separately and combined, to determine the synergistic effect of these two technologies for sludge disintegration. A 2 mm orifice was used as a cavitation device. Ozone was injected after cavitation at a concentration of 35 mg/L at a flow of 3 Lpm. The sludge was subjected to treatment for 90 minutes in all three cases. In a final step, combined ozone and cavitation was applied to disintegrate the sludge for 10 hours. It was found that the combination of ozonation and cavitation exhibited higher sludge disintegration capacity than the individual technologies applied alone. About 31% of VSS was reduced by the combined system, whereas 19% was reduced by ozone and only 4% was reduced by cavitation alone after 90 minutes of treatment. Soluble chemical oxygen demand (SCOD) was increased by approximately 1600 mg/L in the combined system, which was much higher than the individual effect observed with either ozone or cavitation alone. About 75% of this SCOD was found to be biodegradable. In the combined system, soluble TOC increased by approximately 1.5 times the amount released by ozone alone, indicating higher release of organic matter from biomass. Only cavitation did not show any significant release of soluble total organic carbon (TOC) compared to the other treatments. In combined application, soluble biochemical oxygen demand (BOD) increased to 420 mg/L, which was significantly higher than the increase of BOD due to the application of ozone alone which was 260 mg/L. Based on the SCOD, it was found that a maximum of 50% of the sludge could be solubilised in 10 hours of combined treatment. It was also found that with ozone-cavitation, approximately 26% of the total sludge phosphorus was released to the solution after 10 hours, although most of the release occured within the first 2 hours. Within the operating conditions applied, cavitation itself was not sufficient for the disintegration of sludge, although better disintegration was found in combined application at higher inlet pressure. In the case of ozonation alone, higher disintegration was obtained at longer ozonation time.
47
Gyöngy, Miklós. "Passive cavitation mapping for monitoring ultrasound therapy". Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:af6f3c5a-bec5-4378-a617-c89d2b16d95d.
Testo completoAbstract (sommario):
Cavitation is a phenomenon present during many ultrasound therapies, including the thermal ablation of malignant tissue using high intensity focused ultrasound (HIFU). Inertial cavitation, in particular, has been previously shown to result in increased heat deposition and to be associated with broadband noise emissions that can be readily monitored using a passive receiver without interference from the main ultrasound signal. The present work demonstrates how an array of passive receivers can be used to generate maps of cavitation distribution during HIFU exposure, uncovering a new potential method of monitoring HIFU treatment. Using a commercially available ultrasound system (z.one, Zonare, USA), pulse transmission can be switched off and data from 64 elements of an array can be simultaneously acquired to generate passive maps of acoustic source power. For the present work, a 38 mm aperture 5-10 MHz linear array was used, with the 64 elements chosen to span the entire aperture. Theory and simulations were used to show the spatial resolution of the system, the latter showing that the broadband nature of inertial cavitation makes passive maps robust to interference between cavitating bubbles. Passive source mapping was first applied to wire scatterers, demonstrating the ability of the system to resolve broadband sources. With the array transversely placed to the HIFU axis, high-resolution passive maps are generated, and emissions from several cavitating bubbles are resolved. The sensitivity of passive mapping during HIFU exposure is compared with that of an active cavitation detector following exposure. The array was then placed within a rectangular opening in the centre of the HIFU transducer, providing a geometric setup that could be used clinically to monitor HIFU treatment. Cavitation was instigated in continuous and disjoint regions in agar tissue mimicking gel, with the expected regions of cavitation validating the passive maps obtained. Finally, passive maps were generated for samples of ox liver exposed to HIFU. The onset of inertial cavitation as detected by the passive mapping approach was found to provide a much more robust indicator of lesioning than post-exposure B-mode hyperecho, which is in current clinical use. Passive maps based on the broadband component of the received signal were able to localize the lesions both transversely and axially, however cavitation is generally indicated 5 mm prefocal to the lesions. Further work is needed to establish the source of this discrepancy. It is believed that with use of an appropriately designed cavitation detection array, passive mapping will represent a major advance in ultrasound-guided HIFU therapy. Not only can it be utilized in real-time during HIFU exposure, without the need to turn the therapeutic ultrasound field off, but it has also been shown in the context of the present work to provide a strong indicator of successful lesioning and high signal-to-noise compared to conventional B-mode ultrasound techniques.
48
Liverani, Luca. "Cavitation in Real-Size Diesel Injector Nozzles". Thesis, City University London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.525149.
Testo completo
49
Joshi, Shrey. "Modélisation de l'érosion de cavitation par SPH". Thesis, Université Grenoble Alpes (ComUE), 2018. http://www.theses.fr/2018GREAI080/document.
Testo completoAbstract (sommario):
La thèse est organisée autour du développement d'un modèle numérique d’interaction fluide - structure pour simuler la déformation induite dans un matériau solide lors de l’implosion de bulles de cavitation. Le solveur est développé à partir du code open source SPHYSICS_2D utilisant la méthode des Smoothed Particles Hydrodynamics (SPH). Dans cette thèse, le code 2D a été modifié pour traiter le cas de fluides en conditions 2D-axisymétrique. Le solveur solide SPH a été complètement développé en interne en 2D-axisymétrique avec un nouveau schéma pour résoudre les problèmes apparaissant à proximité de l'axe de symétrie. Une loi de comportement élasto-visco-plastique de type Johnson Cook est implémentée dans le solveur solide afin de prendre en compte l’effet de la vitesse de déformation sur l’écrouissage du matériau.Les applications du solveur FSI traitent le cas d’une bulle unique implosant au voisinage d’une surface solide. Deux cas sont envisagés : celui d’une bulle détachée de la surface solide pour laquelle l’effondrement génère une onde de choc ; et celui d’une bulle au contact de la surface pour lequel un micro-jet de liquide vient impacter la surface solide. Pour une taille de bulle donnée, les résultats montrent que le micro-jet peut produire deux fois solide pour laquelle l’effondrement génère une onde de choc ; et celui d’une bulle au contact de la surface pour lequel un micro-jet de liquide vient impacter la surface solide. Pour une taille de bulle donnée, les résultats montrent que le micro-jet peut produire deux fois plus de déformation plastique que l'onde de choc, réduisant ainsi le temps d'incubation. Par contre, le volume de matière déformée plastiquement dans le cas du micro-jet (cavité attachée) est 800 fois plus petit que celui déformé par l’impact d'une onde de choc (cavité détachée). Par conséquent, la capacité d'érosion d'une cavité détachée est beaucoup plus élevée que celle d'une cavité attachée. Un important résultat de cette étude concerne les cavités détachées où il est montré que la déformation plastique ne se produit pas au centre de l'effondrement mais à un décalage par rapport à l’axe de symétrie. Les résultats montrent également que même si la pression subie par le matériau est la plus élevée au niveau de l’axe de symétrie, la déformation plastique ne sera pas maximale à cet endroit mais dans une zone éloignée du centre. Nous Une étude paramétrique est menée pour quantifier les effets de la distance bulle/paroi, de la pression d’effondrement et du rayon de la bulle. Les résultats montrent que l'énergie totale absorbée et le taux d'érosion devraient être plus élevés pour une cavité détachée que pour une cavité attachée. La densité d'énergie absorbée (d'où le temps d'incubation) et l'énergie totale absorbée (d'où le taux d'érosion) augmentent avec la pression d’effondrement. Le changement du rayon de la bulle tout en gardant les autres paramètres constants n'affecte pas beaucoup l'amplitude de la déformation plastique ni la densité d'énergie absorbée, ce qui suggère que quelle que soit la taille de la bulle de cavitation, le temps d'incubation devrait rester similaire. Cependant, comme le volume de la zone déformée plastiquement varie presque linéairement avec la taille de la bulle, l'énergie totale absorbée ou le taux d'érosion augmente significativement avec la taille de la bulle.Dans le passé, les études similaires n'ont jamais pris en compte la sensibilité à la vitesse de déformation dans le modèle de plasticité. Nos simulations montrent que l'ampleur de la déformation plastique est alors surestimée d'environ 60% pour les cavités détachées présentées dans ce document et d'environ 200% pour les cavités attachées. Nous montrons ainsi que de telles études réductrices fondées sur des modèles de plasticité insensibles à la vitesse de déformation conduisent à une sous-estimation du temps d'incubation et à une surestimation du taux d'érosionThe thesis is focused on development of a Smoothed Particle Hydrodynamics (SPH) Fluid-Structure Interaction (FSI) cavitation solver to understand the phenomenon of material deformation under cavitation load better. This summary presents a brief overview of the methodology used to solve a fluid-structure interaction simulation of a bubble collapse over a deformable solid medium. The fluid solver and the solid solver are validated against Rayleigh-Plesset spherical bubble collapse case and FEM solver respectively. The fluid solver is developed using an open source SPH code SPHYSICS_2D, the code is changed from 2D to 2D axisymmetric. The solid SPH solver is developed in-house in 2D axisymmetric, a novel scheme is derived to solve typical issues near symmetry axis in the solid axisymmetric SPH solver. The solid solver has the capability to solve for non-linear isotropic hardening with strain rate effects (commonly known as Johnson-Cook plasticity model).A case each for a detached and an attached cavity is simulated using the FSI solver, the results show that for the same magnitude of pressure wave initiating the collapse and the same size of the bubble, the micro-jet can produce twice the maximum plastic deformation compared to a shock wave, hence a micro-jet dominated impact would exhibit a smaller incubation time compared to the detached cavity. It is also observed that the volume of material that is plastically deformed in case of a micro-jet is miniscule compared to a shock wave impact (almost 800 times smaller). This would imply that even though the incubation time for material erosion might be lower for a micro jet collapse, the shock wave can plastify a much larger volume of material and so the erosion rate should be higher for a shock wave impact. Hence it could be inferred that the material erosion ability of a shock wave is much higher than a micro-jet.An important and novel finding in the present study is the response of the material for a detached cavity where plastic deformation does not occur at the center of collapse but at an offset from the center. The results show that even though the pressure experienced by the material is the highest at the center, it does not produce the maximum plastic deformation. This is for the first time that such a phenomenon is reported in cavitation studies, we find that the phenomenon is linked to inertial effects where the material does not respond to the load as the rate of loading and unloading is extremely high. The effect is linked to the high loading and unloading rate near the center of the collapse due to the flat geometry of the solid medium. The study clearly demonstrate that maximum pressure does not always correspond to the location of maximum plastic deformation or material erosion.Fluid-Structure Interaction simulations for different stand-off ratios, driving pressure and bubble radius have been computed. Results show that for varying stand-off ratio while keeping the bubble radius and driving pressure constant, the attached cavities (SR<=1) show a higher plastic strain magnitude and a higher absorbed energy density which would suggest a quicker incubation time. However, the volume of plastic defamation zone is much lower in attached cavities thus the total absorbed energy and the erosion rate would be higher for a detached cavity compared to an attached one.The strain rate effects suggest that the magnitude of plastic strain is over predicted while using plasticity models that do not use strain rate sensitivity. The over prediction of the magnitude of plastic strain of around 60% for detached cavities presented in the paper and around 200% for attached cavities presented in the paper is observed. This would lead to an under prediction of incubation time and over prediction of erosion rate while using strain rate insensitive plasticity models
50
Vyas, Nina. "Imaging dental ultrasonic cavitation and its effects". Thesis, University of Birmingham, 2017. http://etheses.bham.ac.uk//id/eprint/7338/.
Testo completoAbstract (sommario):
Current methods of dental biofilm removal are predominantly mechanical and are not effective in removing it from irregular surfaces in the mouth. Cavitation occurs around dental ultrasonic scalers and may be a more efficient and less damaging technique. Previous work has failed to quantify the cavitation bubble dynamics around ultrasonic scalers and its effects. The aim was to develop imaging and analysis protocols to analyse the cavitation and to investigate its ability to disrupt biofilms and deliver sub-micron particles into dentine. High speed imaging was used to characterise cavitation. Its effect on biofilm removal and dentinal tubule occlusion was studied using electron microscopy and x-ray micro computed tomography. We are able to demonstrate that cavitation occurs at the free end of scaler tips and increases with power and vibration amplitude. Biofilm can effectively be removed from dental implant surfaces using this cavitation. It can also be used to transport sub-micron particles further into dentinal tubules. The results show that ultrasonic scalers could be optimised for non-contact use and improved removal of plaque from the teeth. The protocols established in this study can be applied to future studies for quantitative investigation of biofilm growth and removal and analysis of cavitation dynamics.