The motion of microorganisms presents interesting and diffcult problems ranging from mechanisms of propulsion to collective effects. Experimentally, some of the complicating factors, such as death, reproduction, chemotaxis, etc., can be suppressed through genetic manipulation or environmental control. Nonequilibrium statistical mechanics has been used to study simple models, however proceeding analytically is extremely challenging. Thus simulations, where one has total control over and knowledge of the system, are a compelling method for examining models of their behaviour. In this work I present simulations of minimal, self-propelled particles, while ensuring realistic hydrodynamic behaviour using the lattice Boltzmann method (LBM), a well-studied method for simulating fluid flows that scales linearly in computational effort with the system volume. The derivation of the LBM is reviewed, including the addition of forces in a consistent, accurate manner as well as thermal fluctuations that satisfy the fluctuation-dissipation theorem. It is extended to include singular forces via a regularization of the Dirac δ-function. This is implemented and extensively tested for agreement with low Reynolds number hydrodynamics. The regularized singularities are used to develop an effcient algorithm for pointlike particles which move under the influence of an external force, such as gravity, or thermal fluctuations of the fluid. The method is compared to theoretical results and simulations using a well-studied algorithm that resolves the particle, finding good agreement in the dilute limit and significantly reduced computational requirements. Using the singular forces, we then construct a minimal model for self-propelled particles, that may also experience forces or undergo random changes of orientation (modelling the “run-and-tumble” dynamics observed in swimming bacteria such as E. coli). The collective behaviour of these model swimmers is studied in three situations: sedimentation under gravity; in a central, harmonic trap; and in a Poiseuille flow between parallel plates. For sedimentation, the behaviour is not very different from that expected of non-interacting run-and-tumble particles, except that total collapse to the container bottomwhen the weight of the particles equals the propelling force is prevented by the velocity fluctuations caused by the particles’ activity. The trapped particles, for runlengths comparable to the trap size, self-assemble into a pump-like structure, while for short run-lengths an approximately Gaussian distribution seenwithout hydrodynamic interactions, is maintained. In Poiseuille flows we find the particles orient upstream; forweak flows this results in a net upstreamcurrent. We find significant hydrodynamic effects, in the dilute limit, only when there is some mechanism that causes alignment of the particles.

Liquid water management in the cathode of polymer electrolyte membrane fuel cells (PEMFC) is crucial to efficient transport of gases and to maintaining electrochemical activity in the catalyst layer. Cracks and interfacial voids are typical of catalyst layers in operating cells, and are thought to affect water management and other transport properties such as gas diffusion and conductivity. This thesis investigates the effect of such morphological imperfections on liquid water transport using a combination of numerical techniques. Both the catalyst layer and microporous layer parts of the cathode are considered. The layers are first numerically reconstructed using data from advanced microscopy, and cracks, perforations and interfacial voids are created. Lattice Boltzmann simulations of the dynamics liquid water imbibition process are performed to study the effect of characterizing features of the cracks and interfacial voids such as aperture area, degree of protrusion, and tortuosity. The resulting liquid water distributions were then input into a pore scale model to characterize the effect of the morphological features on other transport properties, such as effective diffusivities and conductivities. Larger crack apertures were found to increase liquid water uptake, and elongated cracks allowed for faster breakthrough at lower saturation levels. A notable observation is that short and large interfacial cracks have a higher liquid water uptake potential due to the lower effective capillary pressures. It was also found that elongated cracks aligned with the pressure gradient provide preferential pathway, and a capillary pressure increase that favours liquid water transport towards the membrane and mitigates flooding. The effective diffusivity increased for all crack protrusion depths, even for the wet catalyst layer, likely due to low liquid water saturation. The geometry with the most elongated crack showed a significant increase in gas diffusion under wet conditions, indicating that enhanced gas transport is achievable when liquid water removal is effective. Protonic and electrical conductivities decreased for all crack shapes due to higher contact resistance. / Graduate / 0548 / vlosier@uvic.ca

Thesis (Ph. D.)--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 64-65).

In this work, the lattice Boltzmann method (LBM) was validated for direct numerical simulation (DNS) of wall-bounded turbulent flows. The LBM is a discrete-particle-based method that numerically solves the Boltzmann equation as opposed to conventional DNS methods that are based on the Navier-Stokes (NS) equations. The advantages of the LBM are its simple implementation, its ability to handle complex geometries, and its scalability on modern high-performance computers. An LBM code was developed and used to simulate fully-developed turbulent channel flow. In order to validate the results, the turbulence statistics were compared to those calculated from a conventional NS-based finite difference (FD) simulation. In the present study, special care was taken to make sure the computational domains for LBM and FD simulations were the same. Similar validation studies in the literature have used LBM simulations with smaller computational domains in order to reduce the computational cost. However, reducing the size of the computational domain affects the turbulence statistics and confounds the results of the validation. The turbulence statistics calculated from the LBM and FD simulations were found to agree qualitatively; however, there were several significant deviations, particularly in the variance profiles. The largest discrepancy was in the variance of the pressure fluctuations, which differed by approximately 7%. Given that both the LBM and FD simulations resolved the full range of turbulent scales and no models were used, this error was deemed to be significant. The cause of the discrepancy in the pressure variance was found to be the compressibility of the LBM. The LBM allows the density to vary, while the FD method does not since it solves the incompressible form of the NS equations. The effect of the compressibility could be reduced by lowering the Mach number, but this would come at the cost of significantly increasing the computational cost. Therefore, the conclusion of this work is that, while the LBM is capable of producing accurate solutions for incompressible turbulent flows, it is significantly more expensive than conventional methods for simple wall-bounded turbulent flows. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2011-09-15 23:24:09.968

No description available.

<div> Underlying physics in bubble coalescence is critical for understanding bubble transportation. It is one of the major mechanisms of microfluidics. Understanding the mechanism has benefits in the design, development, and optimization of microfluidics for various applications. The underlying physics in bubble coalescence is investigated numerically using the free energy-based lattice Boltzmann method by massive parametrization and classification.</div><div><br></div><div> Firstly, comprehensive GPU (Graphics Processing Unit) parallelization, convergence check, and validation are carried out to ensure the computational efficiency and physical accuracy for the numerical simulations.</div><div><br></div><div> Then, the liquid-gas system is characterized by an Ohnesorge number (Oh). Two distinct coalescence phenomena with and without oscillation, are separated by a critical Oh (~0.477)number. For the oscillation cases(Oh<0.477), the mechanism of damped oscillation in microbubble coalescence is explored in terms of the competition between driving and resisting forces. Through an analogy to the conventional damped harmonic oscillator, the saddle-point trajectory over the entire oscillation can be well predicted analytically. Without oscillation in the range of 0.50r^-n </div><div><br></div><div> After that, the liquid-gas-solid interface is taken into consideration in the liquid-gas system. Six cases based on the experiment set-ups are simulated first for validation of the computational results. Based on these, a hypothesis is established about critical factors to determine if coalescence-induced microbubble detachment (CIMD) will occur. From the eighteen experimental and computational cases, we conclude that when the radius ratio is close to 1 and the father bubble is larger, then it will lead to CIMD.</div><div><br></div><div> Lastly, the effects of initial conditions on the coalescence of two equal-sized air microbubbles (R₀) in water are investigated. In both initial scenarios, the neck bridge evolution exhibits a half power-law scaling, r/R₀=A₀(t/t_i)^1/2 after development time. The development time is caused by the significant bias between the capillary forces contributed by the meniscus curvature and the neck bridge curvature. Meanwhile, the physical mechanism behind each behavior has been explored.</div>

En raison de la tendance positives pour le développement durable, les systèmes solaires(capteurs solaires, concentrateur solaire, etc.) Intègrent (et demandent d'intégrer encore plus)intensivement les résidences et les industries. Dans ce contexte, les systèmes diphasiques comme le caloduc semblent être très efficaces en raison de leurs capacités élevées de transport de chaleur et de leur fonctionnement passif appliqués aux capteurs. Compte-tenu de la complexité des caloducs à structure poreuse dans ce type d'application, la plupart des systèmes existants sur le marché utilisent des thermosiphons. Ainsi, le besoin croissant de solutions de contrôle thermique fiables et plus efficaces croit rapidement pour de tels systèmes.Ce travail de thèse porte sur la caractérisation des performances des caloducs à structure poreuse utilisés dans les applications solaires. Une étude numérique a été réalisée pour modéliser et simuler le comportement d'un caloduc typique à l'aide de la méthode Lattice Boltzmann. Une étude expérimentale a également été réalisée pour caractériser les performances de trois prototypes testes dans différentes conditions (température du condenseur, puissance introduite et angle d'inclinaison). Les effets induits par plusieurs paramètres incluant le taux de remplissage, le fluide de travail et la symétrie de la puissance appliquée sur les performances de ces dispositifs ont également été étudiés. En particulier, l'asymétrie du chauffage induit un assèchement plus précoce, toutes choses étant égales par ailleurs. L'inclinaison optimale est également déterminée là où est équilibrée la chaleur solaire maximale disponible et reçue par le caloduc et l'écoulement de liquide assisté par gravité à l'intérieur de ce dispositif. / Owing to the trend to development sustainability, solar systems (solar collector, solar concentrator, etc.) Are integrating (and asked to integrate even more) intensively residences and industries. In this context, two-phase systems like heat pipe seem highly effective because of their high heat transport capabilities and their passive operation in collectors’ technology. In view of the complexity of the heat pipes with a porous structure in this kind of application,most of the existing systems on the market use thermosyphons. Thus, the growing need of reliable and more efficient thermal control solutions is increasing for such systems. This thesis work focuses on the performance characterization of heat pipes with porous structure used in solar applications. A numerical study has been performed to model and simulate the behavior of a typical heat pipe using the Lattice Boltzmann method. An experimental study has also been done to characterize the performance of three prototypes tested under different conditions (condenser temperature, heat input and inclination angle). The effects induced by several parameters including the filling rate, working fluid and symmetry of the applied heat on the performance of these devices has also been investigated. In particular, heating asymmetry is found to induce dry-out earlier, all other things being equal. Optimal inclination is also determined where is balanced the maximum solar heat available and received by the heat pipe and the gravity-assisted liquid flow inside that device.

As the operating conditions of the pressurized water reactor (PWR) have been increased towards the thermal limits of the core for economics, the subcooled boiling heat transfer performance of the rod bundles under normal operating conditions has become an increasingly important design focus. Effective field models such as two-fluid model, on which most previous numerical studies in the nuclear fields have focused, cannot predict detailed phenomenon of subcooled boiling because it involves complex multiphase dynamics, such as nucleation, growth, detachment bubbles from a wall, deformation, break-up, coalescence, and condensation. It also requires numerous, additional closure relations. On the other hand, direct numerical simulations with interfacial tracking enable us to capture specific two-phase flow and do not require additional empirical closure relations. In this thesis, we simulate isothermal, two-dimensional bubble dynamics as a starting point toward direct simulation of the subcooled boiling. We adopt a lattice Boltzmann method with the phase-field model. The lattice Boltzmann method is a mesoscopic approach well-adapted to the simulation of complex fluids and is simple to implement. The phase field model can capture complex topological deformation, such as coalescence and break-up, with better numerical stability than other interfacial tracking methods like Volume of Fluid (VOF) and level set methods. We validate the present method for stationary and moving two-phase interfaces by comparing with theoretical solutions for a single static bubble in a stationary liquid and a capillary wave, respectively. In addition, the capability of the current method to simulate the coalescence of two bubbles and droplets is validated by comparing with experimental data. To see the applicability of the method to problems involving complex bubble behaviors and interactions with a high-density ratio as in subcooled boiling water, we simulate rising single and double bubbles in a viscous fluid. For a single bubble problem, the bubble shapes and terminal velocity agreed well with the experimental results for different fluid dynamic conditions. For a double bubble case, the current method can capture the interaction and dynamics of the bubbles. Thus, it is expected that this study can serve as a stepping-stone extension to convective subcooled boiling heat transfer in the nuclear reactor core.

Advancements in electronic performance lead to a decrease in device size and an increase in power density. Because of these changes, current cooling mechanisms for electronic devices are beginning to be ineffective. Microchannels, with their large heat transfer surface area to volume ratio, cooled with either gas or liquid coolant, have shown some potential in adequately maintaining a safe surface temperature. By modifying the walls of the microchannel with fins, the cooling performance can be improved. Using computational fluid dynamics software, microfins placed in a staggered array on the bottom surface of a rectangular minichannel are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Fin geometry, dimensions, spacing, height, and material are analyzed. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to recent studies. These correlations only apply to short fins in the laminar regime. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better thermal performance. Once the effects of microfins were found, an experiment with multi-walled carbon nanotubes (MWNTs) grown on the surface were tested using both water and Al2O3/H2O nanofluid as the working medium. Minichannel devices containing two different MWNT structures – one fully coated surface of MWNTs and the other with a circular staggered fin array of MWNTs - were tested and compared to a minichannel device with no MWNTs. It was observed that the sedimentation of Al2O3 nanoparticles on a channel surface with no MWNTs increases the surface roughness and the thermal performance. Finally, using the lattice Boltzmann method, a two dimensional channel with suspended particles is modeled in order to get an accurate characterization of the fluid/particle motion in nanofluid. Using the analysis based on an ideal fin, approximate results for nanofluids with increase surface roughness was obtained. Microchannels have proven to be effective cooling systems and understanding how to achieve the maximum performance is vital for the innovation of electronics. Implementation of these modified channel devices can allow for longer lasting electronic systems.

In the recent years, researchers have discovered a multitude of applications using nanofibers in fields like composites, biotechnology, environmental engineering, defense, optics and electronics. This increase in nanofiber applications needs a higher rate of nanofiber production. Electrospinning has proven to be the best nanofiber manufacturing process because of simplicity and material compatibility. Study of effects of various electrospinning parameters is important to improve the rate of nanofiber processing. In addition, several applications demand well-oriented nanofibers. Researchers have experimentally tried to control the nanofibers using secondary external electric field. In the first study, the electrospinning process is modeled and the bending instability of a viscoelastic jet is simulated. For this, the existing discrete bead model is modified and the results are compared, qualitatively, with previous works in literature. In this study, an attempt is also made to simulate the effect of secondary electric field on electrospinning process and whipping instability. It is observed that the external secondary field unwinds the jet spirals, reduces the whipping instability and increases the tension in the fiber. Lattice Boltzmann method (LBM) has gained popularity in the past decade as the method is easy implement and can also be parallelized. In the second part of this thesis, a hybrid numerical scheme which couples lattice Boltzmann method with finite difference method for a Oldroyd-B viscoelastic solution is proposed. In this scheme, the polymer viscoelastic stress tensor is included in the equilibrium distribution function and the distribution function is updated using SRT-LBE model. Then, the local velocities from the distribution function are evaluated. These local velocities are used to evaluate local velocity gradients using a central difference method in space. Next, a forward difference scheme in time is used on the Maxwell Upper Convected model and the viscoelastic stress tensor is updated. Finally, using the proposed numerical method start-up Couette flow problem for Re = 0.5 and We = 1.1, is simulated. The velocity and stress results from these simulations agree very well with the analytical solutions.