Numerical analysis of bubble dynamics in different gravity environments
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The separation of phases in a multiphase fluid mixture is significant in aerospace applications. The controlling of multiphase flows is therefore investigated numerically in this thesis. It is assumed that a gas bubble immersed in a liquid is subject to an acoustic field at different gravity levels. In the computation, the equations for radial oscillations and translational motion will be solved simultaneously with a numerical solver. While most of the numerical investigations of bubble phenomena deal with the radial bubble behavior and thus, the heat exchange, only a few studies were conducted on the translational motion. While the behavior of air bubbles immersed in water in an acoustic field parallel to gravity is well evaluated, the translational motion of vapor bubbles in liquid hydrogen has not been investigated so far, as well as the behavior of bubbles in different gravity environments. Here the rise of bubbles in micro and hypergravity conditions are discussed. Finally, the bubble motion is investigated in an acoustic field perpendicular to gravity. The results obtained in this thesis will be the first steps towards a numerical code with the capability of fully controlling the vertical and horizontal position of a bubble within an acoustic field.Keywords:
Zero gravity
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Bubbles in confined geometries serve an important role for industrial operations involving bubble-liquid interactions. However, high Reynolds number bubble dynamics in confined flows are still not well understood due to experimental challenges. In the present paper, combined experimental and numerical methods are used to provide a comprehensive insight into these dynamics. The bubble behaviour in a vertical Hele-Shaw cell is investigated experimentally with a fully wetting liquid for a variety of gap thicknesses. A numerical model is developed using the volume of fluid method coupled with a continuum surface force model and a wall friction model. The developed model successfully simulates the dynamics of a bubble under the present experimental conditions and shows good agreement between experimental and simulation results. It is found that with an increased spacing between the cell walls, the bubble shape changes from oblate ellipsoid and spherical-cap to more complicated shapes, while the bubble path changes from only rectilinear to a combination of oscillating and rectilinear; the bubble drag coefficient decreases and this results in a higher bubble velocity caused by a lower pressure exerted on the bubble; the wake boundary and wake length evolve gradually accompanied by vortex formation and shedding.
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The accurate description of the growth or dissolution dynamics of a soluble gas bubble in a super- or undersaturated solution requires taking into account a number of physical effects that contribute to the instantaneous mass transfer rate. One of these effects is the so-called history effect. It refers to the contribution of the local concentration boundary layer around the bubble that has developed from past mass transfer events between the bubble and liquid surroundings. In Part 1 of this work (Peñas-López et al. , J. Fluid Mech. , vol. 800, 2016 b , pp. 180–212), a theoretical treatment of this effect was given for a spherical, isolated bubble. Here, Part 2 provides an experimental and numerical study of the history effect regarding a spherical bubble attached to a horizontal flat plate and in the presence of gravity. The simulation technique developed in this paper is based on a streamfunction–vorticity formulation that may be applied to other flows where bubbles or drops exchange mass in the presence of a gravity field. Using this numerical tool, simulations are performed for the same conditions used in the experiments, in which the bubble is exposed to subsequent growth and dissolution stages, using stepwise variations in the ambient pressure. Besides proving the relevance of the history effect, the simulations highlight the importance that boundary-induced advection has to accurately describe bubble growth and shrinkage, i.e. the bubble radius evolution. In addition, natural convection has a significant influence that shows up in the velocity field even at short times, although given the supersaturation conditions studied here, the bubble evolution is expected to be mainly diffusive.
Supersaturation
Bubble point
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Dynamics
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Nucleate boiling is an efficient mechanism of heat transfer. The rate of bubble growth and the subsequent bubble motion has a tremendous influence on heat transfer. The study of bubble dynamics is a coupled problem. The rate of evaporation controls the interface speed. One approach to study bubble dynamics is to decouple the problem from energy conservation equation and use an input value of rate of evaporation. The objective is to observe how irregular evaporation rate controls bubble dynamics and the shape of bubble and to study the local over-pressure. The level set method is used to track the liquid-vapor interface. The model consists of the Navier-Stokes equations which govern the momentum and mass balances and the level set equation which governs the interface motion due to phase change. The dynamics of a single bubble under different rates of evaporation and varying levels of gravity have been studied. The results of the numerical simulation show that this model adequately describes bubble dynamics in nucleate boiling, including conditions of microgravity.
Bubble point
Momentum (technical analysis)
Level set method
Conservation of mass
Dynamics
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Dynamics
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The rise of gaseous bubbles in viscous liquids is a fundamental problem in fluid physics, and it is also a common phenomenon in many industrial applications such as materials processing, food processing, and fusion reactor cooling. In this work, the motion of a single argon gas bubble rising in quiescent liquid steel under an external magnetic field is studied numerically using a Volume-of-Fluid method. To mitigate spurious velocities normally generated during numerical simulation of multiphase flows with large density differences, an improved algorithm for surface tension modeling, originally proposed by Wang and Tong [“Deformation and oscillations of a single gas bubble rising in a narrow vertical tube,” Int. J. Therm. Sci. 47, 221–228 (2008)] is implemented, validated and used in the present computations. The governing equations are integrated by a second-order space and time accurate numerical scheme, and implemented on multiple Graphics Processing Units with high parallel efficiency. The motion and terminal velocities of the rising bubble under different magnetic fields are compared and a reduction in rise velocity is seen in cases with the magnetic field applied. The shape deformation and the path of the bubble are discussed. An elongation of the bubble along the field direction is seen, and the physics behind these phenomena is discussed. The wake structures behind the bubble are visualized and effects of the magnetic field on the wake structures are presented. A modified drag coefficient is obtained to include the additional resistance force caused by adding a transverse magnetic field.
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Abstract : Contents: Bubble Dynamics and Cavitation Inception in Non-Uniform Flow Fields; Bubble Interactions with Vortices; Cavitation Dynamics at Microscale Level; Viscous Interaction Between Bubble and Line Vortex; The Motion of a Spherical Body Below a Free Surface; Study of Jet Instability Formation on Free Surfaces; The Final Stage of the Collapse of a Cavitation Bubble Near a Rigid Walls Study of the Interaction Between a Bubble and a Vortical Structure; Asymptotic Study of Bubble Dynamics in a Nonuniform Potential Flow; Analytical study of the interaction a Gas Bubble and a Line Vortex; Analytical and Numerical Study of Large Bubble/Bubble and Bubble/Flow Interaction; Asymptotic Study of Bubble Dynamics in a Slightly Compressible Flow; Asymptotic Study of Bubble Cloud Dynamics in the Proximity of a Body in Potential Flow; Dynamical Interactions in a Bubble Cloud; and, Dynamics of the Interaction of Non- Spherical Cavities.
Microscale chemistry
Free surface
Dynamics
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Rest (music)
Dynamics
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Hypergravity
Inviscid flow
Underwater explosion
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Study of bubble dynamics near a rigid boundary has a great significance to many areas such as underwater explosion and erosion of propeller. Based on compressible two-phase flow theory, bubble dynamics near a rigid are numerically modeled with the finite volume method using the open source software OpenFOAM. Volume of fluid (VoF) method is used to capture the bubble-water interface and PIMPLE algorithm is used to solve the velocity-pressure coupling. The formation and collapse of a single bubble in a free field is simulated, compared with Rayleigh-Plesset model, to show the reliability of the numerical method. Moreover, simulation of bubble near a wall is performed to capture the bubble expansion, collapse and high speed liquid jet. The pressure within the bubble and around the surrounding liquid at different times during the formation and collapse of the bubble is also obtained to estimate the impact on ambient structure. Influence of bubble-structure distance on bubble behavior is studied.
Underwater explosion
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