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In order to elucidate the progression mechanism of cavitation erosion, the behaviors of a single cavitation bubble collapse near a pit on a wall and both the resulting pressure wave in fluid and stress wave in material are investigated in detail. To find out the mechanism of cavitation erosion, many experimental studies on the bubble collapse behavior near a flat rigid wall and the resulting material damage have been conducted so far. A lot of numerical studies using only fluid analysis have been also carried out. In recent years, a few studies on the bubble collapse near a more complex geometry were made and it is reported that more complex geometry has an effect on the bubble collapse behavior, jet formation and subsequent wave dynamics. It is, however, very challenging to introduce a material analysis and investigate detailed stress wave propagation in the material and its effect on the material damage i.e. cavitation erosion. This study tackles this problem using an in-house fluid/material two-way coupled numerical analysis method which considers reflection and transmission of plane waves with acoustic impedance at the fluid/material boundary. In the fluid domain, the locally homogeneous model of compressible gas-liquid two-phase medium is used for capturing the gas-liquid interface. The compressibility of two-phase flow is also considered in this model so that the propagation of pressure wave can be also be taken into account. The governing equations are the 3D compressible gas-liquid two-phase Navier-Stokes equations. In the material domain, the governing equations are composed of the motion equations and the time-differential constitutive equations assuming that the material is a homogeneous isotropic elastic medium, which can simulate the stress wave propagation in the material. Results show that the stress waves are concentrated at the bottom of the pit regardless of the initial bubble position. It is also found that the surface pressure in the fluid side does not necessarily correlate with the stresses in the material, suggesting the importance of material analysis. Moreover, under high pressure conditions, a rapid bubble collapse causes a gas phase generation at the bottom of the pit and its gas phase is contracted and collapsed by the pressure wave, which leads to pressure and stress peaks at the bottom of the pit. Furthermore, through the study of the effect of initial bubble position on its collapse behavior, it is confirmed that, when the initial bubble position is shifted horizontally, bubble collapses asymmetrically and the pressure waves tend to be directed away from a pit. This research numerically reveals that a single bubble collapse near a pit on a wall results in high strain energy concentration at the bottom of the pit, which gives rise to deeper erosion progression at the bottom of the pit. / För att klargöra framstegsmekanismen för kavitationserosion kollapsar beteendet hos en enda kavitationsbubbla nära en grop på en vägg och både den resulterande tryckvågen i vätska och stressvåg i material undersöks i detalj. För att ta reda på mekanismen för kavitationserosion har många experimentella studier av bubblans kollapsbeteende nära en platt styv vägg och den resulterande materialskada genomförts hittills. Många numeriska studier med endast vätskeanalys har också genomförts. Under de senaste åren gjordes några studier om bubblans kollaps nära en mer komplex geometri och det rapporteras att mer komplex geometri har en effekt på bubblans kollapsbeteende, strålbildning och efterföljande vågdynamik. Det är dock mycket utmanande att införa en materialanalys och undersöka detaljerad spänningsvågförökning i materialet och dess inverkan på materialskadorna, dvs. kavitationserosion. Denna studie hanterar detta problem med hjälp av en inbyggd tvåvägs kopplad numerisk analysmetod som tar hänsyn till reflektion och överföring av plana vågor med akustisk impedans vid vätska / materialgränsen. I fluiddomänen används den lokalt homogena modellen av tvåfasmedium för komprimerbar gas-vätska för att fånga gas-vätskegränssnittet. Komprimerbarheten av tvåfasflöde beaktas också i denna modell så att utbredningen av tryckvågen också kan beaktas. De styrande ekvationerna är de 3D-komprimerbara tvåfasiga gasvätska Navier-Stokes-ekvationerna. I materialdomänen är de styrande ekvationerna sammansatta av rörelseekvationer och tidsdifferentialkonstitutiva ekvationer förutsatt att materialet är ett homogent isotropiskt elastiskt medium, vilket kan simulera spänningsvågutbredningen i materialet. Resultaten visar att stressvågorna är koncentrerade längst ner i gropen oavsett den ursprungliga bubbelpositionen. Man har också funnit att yttrycket i vätskesidan inte nödvändigtvis korrelerar med spänningarna i materialet, vilket tyder på vikten av materialanalys. Vidare orsakar en snabb bubbelskollaps under högtrycksförhållanden en gasfasgenerering vid botten av gropen och dess gasfas dras samman och kollapsas av tryckvågen, vilket leder till tryck och spänningstoppar vid botten av gropen. Vidare bekräftas det genom studien av effekten av den ursprungliga bubbelpositionen på dess kollapsbeteende att när den ursprungliga bubbelpositionen förskjuts horisontellt kollapsar bubblan asymmetriskt och tryckvågorna tenderar att riktas bort från en grop. Denna undersökning avslöjar numeriskt att en enda bubbla kollapsar nära en grop på en vägg resulterar i hög spänningsenergikoncentration längst ner i gropen, vilket ger upphov till djupare erosionsprogression längst ner i gropen.

<p>Many practical fluid flows involve liquid-particle systems and so there is a need to better understand the mechanism of particle deposition, adhesion, and agglomeration in suspensions, especially in complex geometries with moving boundaries and free surfaces. In this thesis, the nature of the particle-solid interactions and particle-fluid interactions is studied where the above complexities are present, taking into account particle collision, colloidal, and hydrodynamic forces, and two way coupling between the fluid flow and particles. The research is motivated by the industrial examples of: flow of dross particles near the sink roll surface in a galvanizing bath (moving surface), and the flow of particles in slot coating dies (free and moving surfaces). Particle motion and agglomeration play important roles in the example systems chosen for this fundamental 3-D study. Numerical studies of flow of dispersed suspensions makes it possible to understand the effects of flow conditions, particle characteristics, and flow geometry specifications that lead to agglomeration of particles in complex systems, especially where experimental studies are difficult to perform. Often the effects of these conditions are discovered due to process or product failures, rather than through insight into the processing steps.</p> <p>The modeling methodology used in this work is that micron sized spherical particles are tracked in the fluid phase by solution of Newton`s second law of motion for each particle. Fluid phase applies hydrodynamic forces on particles (drag, lift). Body forces, (soft sphere) particle-particle collisions and particle-wall collisions are considered. Particle concentrations are in the dilute regime between 0.01-5%vol. Flow of particles with the fluid phase is a fully coupled formulation in systems with particle concentrations > 1%vol.</p> <p>The thesis is organized around three example problems taken from industry that pose challenging modeling issues. The first involves particle collisions with a moving wall (dross particles in a zinc bath). The second problem includes particle-particle and particle-wall collisions in a turning flow geometry. The third problem, particle dispersion flows in a slot coating die, has the most complexity and includes particle-particle, particle-wall and free surfaces.</p> <p>Dross particle build up on the sink roll inside the zinc bath is an industrial problem that causes significant down time, and where an experimental study of the molten zinc in a bath running at C is difficult to perform. With the aid of computational fluid dynamics, turbulent flow of molten zinc in galvanizing bath is simulated, compared with previous cold model experiments, and coupled with the motion of dross particles around the sink roll. The presence of fixed position hardware and moving sink roll and guide rolls in a bath with dimensions in the orders of meters, and micron sized (20-100 ) dross particles makes this case a complex study. Drag, buoyancy, lift force and soft sphere nonlinear collision is considered in solution of Newton`s law of motion for each particle. Turbulent flow is simulated using a standard model. Simulations show regions on the sink roll where particles are dragged toward the surface of sink roll and have long residence times. These regions have been reported to experience large particle build-ups in the hot-dip galvanizing process.</p> <p>In another study, formation and breakage of agglomerates in a turning flow is studied. Neutrally buoyant particles with concentration of 5%vol are tracked in a fully coupled flow. Particles form agglomerates at the corner, where drag and lift force from the fluid breaks a number of agglomerates. The presence of a moving wall in the turning flow shifts the suspended particle formations toward the inside of channel. Location of particles agglomerates shifts toward the free surface with the presence of free surface at the turning flow.</p> <p>Motion of micron sized spherical particles with 1-4%vol through a slot die coating system is elucidated in a separate study. The system is complex with presence of moving web and free surface. Discrete element method (DEM) for motion of dispersed phase and volume of fluid (VOF) method for solution of continuous phase are integrated in a simulation study. Particles are 2-4 and the flow dimensions of the system are in the order of 100 . Particles experience collision, colloidal and hydrodynamic forces. Coupling between flow of particles and fluid phase is conducted. The results of this study show particle positions on the coating film can be predefined and depends on their initial positions within the feed slot. Particles agglomerate in recirculating regions of the coating gap and follow the streamlines of flow on the moving web. Regions in the coating gap where particles have high residence times (inside the die and near the feed slot edges) have particle agglomerations in the slot die coating system.</p> / Doctor of Philosophy (PhD)

This dissertation contains a comprehensive analysis of both non-coupled and mutually coupled switched reluctance motors with concentrated windings for an electric bicycle traction application. Multiple pole configurations are analyzed and compared for each motor type. Includes magnetic design, thermal analysis, and structural analysis. A prototype is designed, manufactured, and validated. / This thesis discusses the design of both a conventional non-coupled switched reluctance motor (CSRM) and a mutually-coupled SRM (MCSRM) for an exterior rotor e-bike application. Several novel pole configurations were analyzed for each machine type, and the performance of the final CSRM and MCSRM designs were compared for this application. A commercially available e-bike permanent magnet synchronous motor (PMSM) was purchased, reverse engineered, and validated to define the geometry constraints and performance targets for the designs. Since switched reluctance motors do not use rare-earth permanent magnets, they are often seen as a potential low-cost alternative to permanent magnet machines. The goal of this research is to explain the relative advantages of CSRMs and MCSRMs when compared to PMSM machines for a direct-drive e-bike application. The final CSRM and MCSRM designs are analyzed in detail; electromagnetic, controls, thermal, and structural considerations are all studied. A prototype of the final CSRM design was manufactured and validated experimentally, using a dynamometer setup. The finalized CSRM design is shown to be competitive with the PMSM machine when considering torque output, and is superior in terms of peak efficiency, and high speed torque performance. However, the CSRM noise output and torque ripple were not compared to the PMSM, and a less-common asymmetric-bridge converter is required for the CSRM, which may hinder the ability for the machine to be implemented into existing e-bike packages. The high speed torque performance of the MCSRM is shown to be inferior to both the CSRM and PMSM, as is the torque quality and efficiency. The MCSRM is shown to be highly resistant to saturation which gives it the potential for high torque output at low speed (if thermal limits are not breached), though low saturation levels also contribute to low machine power factor. The MCSRM may be better suited to lower speed, high torque applications, for this reason. / Thesis / Doctor of Philosophy (PhD) / This thesis studies the design process and analysis of two different motor types, for an electric bicycle application. They are designed to replace a commercially available permanent magnet synchronous motor (PMSM). This type of motor is typically expensive due to the rare-earth magnet material it requires. The two motors discussed in this thesis are switched reluctance motors (SRMs), which do not require magnet material, and thus have the potential to save cost (in addition to other benefits). One of the SRMs has magnetic fields that are independently controlled (CSRM), and one has fields that are controlled together to produce torque (MCSRM). The magnetics, control, thermal, and structural aspects of the CSRM and MCSRM are studied in detail. Novel geometry considerations (i.e. novel pole configurations) which impact the magnetics of each machine are compared to find the best-performing configuration for each machine type.