391 research outputs found

    Lattice Boltzmann Modelling of Droplet Dynamics on Fibres and Meshed Surfaces

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    Fibres and fibrous materials are ubiquitous in nature and industry, and their interactions with liquid droplets are often key for their use and functions. These structures can be employed as-is or combined to construct more complex mesh structures. Therefore, to optimise the effectiveness of these structures, the study of the wetting interactions between droplets and solids is essential. In this work, I use the numerical solver lattice Boltzmann method (LBM) to systematically study three different cases of droplet wetting, spreading, and moving across fibres, and droplets impacting mesh structures. First, I focus on partially wetting droplets moving along a fibre. For the so-called clamshell morphology, I find three possible dynamic regimes upon varying the droplet Bond number and the fibre radius: compact, breakup, and oscillation. For small Bond numbers, in the compact regime, the droplet reaches a steady state, and its velocity scales linearly with the driving body force. For higher Bond numbers, in the breakup regime, satellite droplets are formed trailing the initial moving droplet, which is easier with smaller fibre radii. Finally, in the oscillation regime (favoured in the midrange of fibre radius), the droplet shape periodically extends and contracts along the fibre. Outside of the commonly known fully wetting and partial wetting states, there exists the pseudo-partial wetting state (where both the spherical cap and the thin film can coexist together), which few numerical methods are able to simulate. I implement long-range interactions between the fluid and solid in LBM to realise this wetting state. The robustness of this approach is shown by simulating a number of scenarios. I start by simulating droplets in fully, partial, and pseudo-partial wetting states on flat surfaces, followed by pseudo-partially wetting droplets spreading on grooved surfaces and fibre structures. I also explore the effects of key parameters in long-range interactions. For the dynamics demonstration, I simulate droplets in the pseudo-partial wetting state moving along a fibre in both the barrel and clamshell morphologies at different droplet volumes and fibre radii. Finally, I focus on the dynamics of droplets impacting square mesh structures. I systematically vary the impact point, trajectory, and velocity. To rationalise the results, I find it useful to consider whether the droplet trajectory is dominated by orthogonal or diagonal movement. The former leads to a lower incident rate and a more uniform interaction time distribution, while the latter is typically characterised by more complex droplet trajectories with less predictability. Then, focussing on an impact point, I compare the droplet dynamics impacting a single-layer structure and equivalent double-layer structures. From a water-capturing capability perspective (given the same effective pore size), a double-layer structure performs slightly worse. A double-layer structure also generally leads to shorter interaction time compared to a single-layer structure

    Study of supersonic nozzle flows in low-pressure environments: starting jets and lunar plume-surface interactions

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    Supersonic nozzle flows play an important role in aerospace engineering, e.g. controlling motions, attitudes, and orbits of space vehicles using various propulsion systems. Supersonic nozzle flows include free nozzle flows and restricted nozzle flows, such as plume-surface interactions if a surface obstructs the flow propagation. When compressed gas is discharged from a nozzle into a low-pressure environment in the case of free nozzle flows, the shock wave diffracts around the nozzle lip and a vortex loop forms. These phenomena have attracted much attention in the continuum flow regime, but how the shock diffraction and vortex behave under rarefied flow conditions has received less attention. Understanding transient flow in rarefied conditions is helpful for increasing thrust vector control and avoiding potential contamination and erosion of spacecraft surfaces. Furthermore, comprehending plume-surface interactions is critical for the design of lander modules and future bases on bodies such as the moon, as it is necessary to anticipate surface erosion patterns and the transport of displaced regolith material. Extraterrestrial conditions are difficult to recreate experimentally (e.g. the effects of low gravity, strong radiation and extreme temperature difference). Available numerical techniques for modelling regolith entrainment and subsequent movement suffer from limited accessibility and different levels of sophistication. In this thesis, a design for an open-ended shock tube connected to a vacuum chamber is presented. This is used to release a shockwave into a low-pressure environment and study the subsequent vortex ring formation as the gas diffracts around the shocktube exit. Schlieren visualisation and pressure measurements of the vortex ring formation are conducted. The flow structure degenerates through a decrease in the strength of the embedded shock waves and an increase in their thickness, and the counter-rotating vortex ring when the environmental pressure decreases. The existence of the vortex ring is confirmed through spectral analysis when the environmental pressure is as low as 1.0kPa. Due to limitations with experimental measurement equipment and techniques, the shock wave diffraction problem should be complemented with numerical techniques. A program to generate ensemble-averaged direct simulation Monte Carlo (DSMC) results is designed. Computational fluid dynamics (CFD) and ensemble-averaged DSMC methods are implemented to simulate the formation of a two-dimensional vortex loop due to shock wave diffraction around a 90◦ corner. The influence of the Mach number and rarefaction on the development and growth of the vortex loop are studied. A concept, called rorticity, was used to investigate the transient structures of vortex loops. The simplification of the internal structure of vortex loops and postponement of the vortex loop formation due to the increase of the rarefaction level are confirmed. Two properties from the decomposition equation of vorticity to quantify the vortex strength; rorticity flux (i.e. representing the vortex rotational strength), and the shear vector flux (i.e. representing the vortex shear movement strength), are derived. A mutual transformation relationship between the rorticity and shear vectors has been identified, suggesting that this concept can be employed to better explain vortex flow phenomena. It is found that the increase of the Knudsen number thickens the Knudsen layer, causing the failure of the generation of the vortex sheet and the subsequent formation of vortex loops. A new solver based on dsmcFoamPlus – rarefiedMultiphaseFoam, is developed for solving rarefied multiphase flows. The solver is extended to include a two-way coupling model and a particle phase change model. Additionally, the solid stochastic collison model and the multiphase nparticle-in-cell (MPPIC) method for solving dilute and dense granular flows, respectively, have been implemented in the new solver. The models mentioned are rigorously benchmarked against analytical solutions and previous results in the literature. The benchmarking results of the two-way coupling method show excellent agreement with analytical results. The results of a reproduced uniform gas-solid flow and a purely gravity-controlled granular flow sedimentation agree well with previous numerical results in the literature. A solid particle is allowed to experience a physical and continuous phase change and diameter variation using the updated phase change model. Finally, the rarefiedMultiphaseFoam solver is used to simulate two lunar plume-surface interaction (PSI) cases using the stochastic collision model and the MPPIC method, respectively. Both methods are applied to a scaled down version of the Apollo era lunar module descent engine and comparisons are made between the two simulation results. The results show that the transient effects are essential to both the gas and solid phase evolution and the entrained dust particles significantly influence the evolution of the gas flow. In the PSI simulations, the MPPIC method is more reliable than the stochastic collision method because it takes enduring contacts and the close-packing limit into account. Furthermore, it is identified that the breakdown of the locally free-molecular flow assumption has a significant impact on the solid particle temperatures

    Multiphase Flow in Porous Media: Dewatering and Consolidation

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    Multiphase flow in porous media represents one of the most complicated processes that occur in a range of applications across science and engineering disciplines. The complexity escalates when the flows are driven under a negative pressure as often encountered in geotechnical engineering, petroleum exploration, and underground water resource cycling. The proposed study aims to model gas−liquid flows in porous media, calculate deformation of porous media induced by the two-phase flows, and capture pressure variation in vertical drains. This thesis comprises four article publications (Chapters 2 to 5) which are either published or submitted to journals for possible publication by the time of thesis lodgement. Chapter 1 is the Introduction. This chapter provides an overview of the research, highlights the research gaps, presents the research aims and objectives, and outlines the thesis structure. Chapter 2 comprises a paper “Large Strain Consolidation of Unsaturated Soil: Model Formulation and Numerical Analysis”. This paper has been published in the ACSE International Journal of Geomechanics. The content presents a novel numerical model to simulate the unsaturated soil consolidation utilising the Lagrangian–Convective coordinate system. The proposed model was solved via the explicit finite difference method and was verified against the conventional analytical solution. The developed model enabled the nonlinear soil properties including soil water characteristic curve, shrinkage curve, compressibility curve and permeability curve. A parametric study was conducted to focus on the effect of the initial soil degree of saturation on the consolidation degree. The results indicated that soil with a higher initial degree of saturation has a greater consolidation settlement, and vice versa. Chapter 3 presents the second paper manuscript, entitled “Vertical drain aided consolidation and solute transport”, which is under review by Computers and Geotechnics. This study coupled the consolidation model in Chapter 2 with the solute transport model and studied the dewatering and solute discharge efficiency under different consolidation conditions. The coupled equations were solved via the alternative direction finite difference time domain method. The model was experimentally verified and then applied to examine the effects of soil saturation conditions, solute transport conditions, and consolidation efforts on solute transport. The results showed that the dispersion process contributes to the solute discharge, whereas the contribution becomes less noticeable in unsaturated conditions. The solute sorption process counteracts the solute transport and delays the clean-up. The consolidation accelerates the transport of reactive chemicals but shows limited effects on the transport of non-reactive chemicals. Chapter 4 presents the third paper manuscript, entitled “Large strain consolidation model of vacuum and air-booster combined dewatering”. This manuscript was submitted to Computers and Geotechnics. The work presented a finite strain model for solving the vacuum and air-booster combined consolidation problem. The model also took account of soil desaturation due to air injection. The model was solved numerically via the alternative direction finite difference time domain method, and the solution was verified against the field test and laboratory measurement. Chapter 5 contains the fourth paper manuscript, entitled “Modelling air-water flow in the vacuum-aided vertical drain”, which was submitted to Geosynthetic International. This modelling work presented a numerical method used to estimate the vacuum-induced two-phase flow pressure distribution along the vertical drain. The soil medium was modelled as the orifice along the vertical drain. The proposed model was validated against the experiment and computational fluid dynamic results. A parametric study was conducted, and the results indicated that the nonlinear pressure distributions occurred in the drainpipe, and the pressure dropped more noticeably in the presence of air. The modelling suggests under one-atmosphere vacuum pressure, the drain lift depth is approximately 6.3 to 7.5m depending on the orifice size. Chapter 6 is the Summary of this thesis, concluding research contributions achieved and suggestions for further work.Thesis (Ph.D.) -- University of Adelaide, School of Architecture and Civil Engineering, 202

    Hybrid numerical methods for multiphysics simulation of flow in coal

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    Coalbeds are important resources for natural gas, referred to as coalbed methane. They can also be used for storage of carbon dioxide or hydrogen. Pore/cleat-scale modelling is a valuable tool for capturing flow physics in these multi-scale fractured media. Direct numerical simulations (DNS) and pore-network modelling (PNM) have been used for predicting multi-scale multi-physics flow on images of the porous media obtained from micro-computed tomography (micro-CT) scanning. Both approaches have advantages and disadvantages, while the development of hybrid methods can produce more efficient simulation tools. A fracture pore-network (PN) model is coupled with a diffusion flow solver to capture the transient gas flow physics in coal. The Finite Volume Method (FVM) is used for the discretisation of Fick’s second law and results are used to update fluxes in the coal matrix and the adjacent fractures. The Langmuir equation is solved to account for sorption and to correlate between the gas pressure and concentration when coupling PN convection and FVM diffusion models. To simulate two-phase flow in coal fractures, a hybrid model using PNM and the volume of fluid (VOF) advection scheme is proposed. The model is validated against the conventional VOF realisation by conducting a comprehensive sensitivity analysis. The developed VOF-PNM model allows for dynamic tracking of fluid interfaces and for predicting relative permeability and capillary pressure curves. The developed solver can be one to two orders of magnitude faster than the conventional VOF implementation while its accuracy is within 5%. A two-phase Darcy-Brinkman-Stokes (DBS) framework is employed for multi-scale multiphysics fluid flow in coal. Diffusion, sorption, gas and rock compressibility are embedded into the numerical scheme. Variation of permeability due to coal deformation is accounted for via the Palmer-Mansoori analytical model, while surface diffusion is introduced by Fick’s second law. The Langmuir equation is used to introduce sorption via its implicit coupling with pressure equations within a DBS framework. A hybrid of VOF, PN, and the continuum Darcy model is applied for computationally efficient multi-scale multiphysics flow simulations in coal. The model applies the Hagen–Poiseuille analytical solution and VOF advection scheme for two-phase flow in fractures coupled with the continuum Darcy biphasic flow in the coal matrix. The model has similar functionality to the previously developed DBS framework while the computational cost is more than 90 times lower. Each stage of the conducted research introduces a novel hybrid approach for multi-scale multiphysics flow simulations in coal seams by an efficient numerical combination of PNM, DNS, and continuum models. The developed hybrid models capture several complex physical mechanisms occurring in coalbed methane reservoirs at different stages of gas production and storage and can be used as a predictive tool for optimisation of gas recovery and monitoring possible environmental impacts

    Numerical simulations of gas transport in argillaceous rocks: A molecular dynamics and pore-scale simulation study

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    This dissertation investigates the gas transport and clay behavior within the context of deep geological disposal of nuclear waste. The repository for spent fuel and high-level waste can generate substantial amounts of gas through processes such as anaerobic corrosion of carbon steel, radiolysis of water, and radioactive decay in the waste. Likewise, gas production can occur in low and intermediate-level waste repositories due to chemical degradation of organic waste materials and corrosion of metals. If these gases cannot sufficiently escape from the vicinity of the repository, a localized build-up of gas pressure could compromise the integrity of the barriers and the safety design of the repository. Therefore, a thorough understanding of gas transport mechanisms and processes is crucial for assessing the repository’s performance. Diffusion is the primary mechanism governing solute and fluid transport in these clays due to their low permeability. While experiments can provide valuable transport parameters for de- signing the barrier materials, they may not fully capture the long-term evolution of transport processes and specific subsurface conditions. Consequently, numerical and computer simu- lations become indispensable for determining the transport mechanisms and exploring the behavior of the system beyond the limits of experimental detection. These simulations offer the opportunity to explain experimental results, probe scales, and processes that are below the detection limit of experiments, and enhance our understanding of the transport mechanisms involved. Gas diffusion simulation in fully saturated Na-montmorillonite (Na-MMT) was performed and the effects of pore size, gas species, and temperature were investigated. Classical molecular dy- namics simulations were utilized to study the diffusion coefficients of various gases (CO2, H2, CH4, He, Ar). The findings indicate that the diffusion coefficients are influenced by the pore size, with H2 and He demonstrating higher mobility compared to Ar, CO2, and CH4. The be- havior of gases is affected by the confinement and the structuring of water molecules near the clay surface, as evidenced by density profiles and radial distribution functions. The obtained diffusion coefficients for different gases and slit pore sizes were parameterized using a single empirical relationship, enabling their application in macroscopic simulations of gas transport. Considering the long-term desaturation and resaturation process, the study extends to simulate gas diffusion in partially saturated Na-MMT and investigates the partitioning of gas molecules between the gas-rich and water-rich phases. Classical molecular dynamics simulations were employed to explore the impact of gas-filled pore widths, temperature, gas mean free path, gas size, and gas molecular weights on diffusion coefficients and partitioning coefficients. The re- sults demonstrate that the diffusion coefficient in the gas phase increases with larger gas-filled pore widths and eventually converges asymptotically towards the diffusion coefficient in the bulk state. Partitioning coefficients were found to be strongly dependent on temperature and gas molecular weights. Furthermore, non-equilibrium molecular dynamics simulations were conducted to investigate the mobility of gases in a pressure-driven flow within a partially sat- urated Na-MMT mesopore. The results reveal the presence of slip boundary conditions at the microscale, which challenges the assumptions made in continuum models. To predict the dif- fusion coefficient and dynamic viscosity of the gas, a Bosanquet-type equation was developed as a function of the average pore width, gas mean free path, geometric factor, and thickness of the adsorbed water film. Na-montmorillonite, being a swelling clay, undergoes changes in its swelling behavior when exposed to different chemical species like gas due to variations in chemical potential. These alterations can subsequently impact the hydraulic properties and transport mechanism of the clay. Consequently, we investigated the influence of gas presence on the swelling pressure of Na-MMT. To achieve this, classical molecular dynamics simulations were employed as a methodology to examine the effect of gas on swelling pressure. The findings indicate that gas molecules cause an increase in the swelling pressure of Na-montmorillonite, with an approx- imate rise of 3 MPa. The specific behavior observed is influenced by factors such as the dry density and the characteristics of the gas species. Additionally, the analysis includes a com- prehensive exploration of structural transformations occurring within the clay interlayer, pro- viding insights into the discrepancies observed between experimental and simulated curves, particularly at high levels of compaction. The thesis delves into pore-scale modeling to determine diffusion coefficients of water in com- pacted porous smectite clay structures. This exploration is motivated by the limitations inher- ent in conventional approaches used to obtain transport parameters, which tend to oversim- plify the intricate porous nature of clay media by treating them as a continuum. This oversim- plification neglects the behaviors occurring at smaller scales. To overcome this limitation, the thesis employs various techniques such as random walk simulations, lattice Boltzmann mod- eling, and large-scale molecular dynamics simulations to investigate transport mechanisms. These advanced modeling techniques take into account local diffusivities within the represen- tative elementary volume, allowing for a more accurate understanding of transport phenom- ena. By considering local diffusivities, particularly near chemically reactive clay surfaces, this approach sheds light on the significance of accurately comprehending transport phenomena in porous materials. By overcoming the limitations of conventional approaches, the thesis provides valuable insights into the diffusion coefficients of water within compacted porous smectite clay structures. This thesis offers a comprehensive exploration of gas transport and clay behavior, focusing on their relevance to deep geological disposal of nuclear waste and energy storage. By establishing connections between simulations conducted under fully saturated and partially saturated con- ditions, examining the influence of gases on swelling pressure, and incorporating pore-scale modeling, this research provides valuable insights into diffusion, swelling, and pore-scale pro- cesses. These findings contribute to the development of effective barrier materials and enhance our understanding of waste management strategies in complex geological environments. The knowledge gained from this study has practical implications for improving the safety and effi- ciency of deep geological disposal systems and advancing energy storage technologies

    Generalized equilibria for color-gradient lattice Boltzmann model based on higher-order Hermite polynomials: A simplified implementation with central moments

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    We propose generalized equilibria of a three-dimensional color-gradient lattice Boltzmann model for two-component two-phase flows using higher-order Hermite polynomials. Although the resulting equilibrium distribution function, which includes a sixth-order term on the velocity, is computationally cumbersome, its equilibrium central moments (CMs) are velocity-independent and have a simplified form. Numerical experiments show that our approach, as in Wen et al. [{Phys. Rev. E \textbf{100}, 023301 (2019)}] who consider terms up to third order, improves the Galilean invariance compared to that of the conventional approach. Dynamic problems can be solved with high accuracy at a density ratio of 10; however, the accuracy is still limited to a density ratio of 1000. For lower density ratios, the generalized equilibria benefit from the CM-based multiple-relaxation-time model, especially at very high Reynolds numbers, significantly improving the numerical stability.Comment: 22 pages, 8 figure

    Lattice Boltzmann Methods for Partial Differential Equations

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    Lattice Boltzmann methods provide a robust and highly scalable numerical technique in modern computational fluid dynamics. Besides the discretization procedure, the relaxation principles form the basis of any lattice Boltzmann scheme and render the method a bottom-up approach, which obstructs its development for approximating broad classes of partial differential equations. This work introduces a novel coherent mathematical path to jointly approach the topics of constructability, stability, and limit consistency for lattice Boltzmann methods. A new constructive ansatz for lattice Boltzmann equations is introduced, which highlights the concept of relaxation in a top-down procedure starting at the targeted partial differential equation. Modular convergence proofs are used at each step to identify the key ingredients of relaxation frequencies, equilibria, and moment bases in the ansatz, which determine linear and nonlinear stability as well as consistency orders of relaxation and space-time discretization. For the latter, conventional techniques are employed and extended to determine the impact of the kinetic limit at the very foundation of lattice Boltzmann methods. To computationally analyze nonlinear stability, extensive numerical tests are enabled by combining the intrinsic parallelizability of lattice Boltzmann methods with the platform-agnostic and scalable open-source framework OpenLB. Through upscaling the number and quality of computations, large variations in the parameter spaces of classical benchmark problems are considered for the exploratory indication of methodological insights. Finally, the introduced mathematical and computational techniques are applied for the proposal and analysis of new lattice Boltzmann methods. Based on stabilized relaxation, limit consistent discretizations, and consistent temporal filters, novel numerical schemes are developed for approximating initial value problems and initial boundary value problems as well as coupled systems thereof. In particular, lattice Boltzmann methods are proposed and analyzed for temporal large eddy simulation, for simulating homogenized nonstationary fluid flow through porous media, for binary fluid flow simulations with higher order free energy models, and for the combination with Monte Carlo sampling to approximate statistical solutions of the incompressible Euler equations in three dimensions

    Changing Priorities. 3rd VIBRArch

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    In order to warrant a good present and future for people around the planet and to safe the care of the planet itself, research in architecture has to release all its potential. Therefore, the aims of the 3rd Valencia International Biennial of Research in Architecture are: - To focus on the most relevant needs of humanity and the planet and what architectural research can do for solving them. - To assess the evolution of architectural research in traditionally matters of interest and the current state of these popular and widespread topics. - To deepen in the current state and findings of architectural research on subjects akin to post-capitalism and frequently related to equal opportunities and the universal right to personal development and happiness. - To showcase all kinds of research related to the new and holistic concept of sustainability and to climate emergency. - To place in the spotlight those ongoing works or available proposals developed by architectural researchers in order to combat the effects of the COVID-19 pandemic. - To underline the capacity of architectural research to develop resiliency and abilities to adapt itself to changing priorities. - To highlight architecture's multidisciplinarity as a melting pot of multiple approaches, points of view and expertise. - To open new perspectives for architectural research by promoting the development of multidisciplinary and inter-university networks and research groups. For all that, the 3rd Valencia International Biennial of Research in Architecture is open not only to architects, but also for any academic, practitioner, professional or student with a determination to develop research in architecture or neighboring fields.Cabrera Fausto, I. (2023). Changing Priorities. 3rd VIBRArch. Editorial Universitat Politècnica de València. https://doi.org/10.4995/VIBRArch2022.2022.1686

    Efficient parallel solver for high-speed rarefied gas flow using GSIS

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    Recently, the general synthetic iterative scheme (GSIS) has been proposed to find the steady-state solution of the Boltzmann equation in the whole range of gas rarefaction, where its fast-converging and asymptotic-preserving properties lead to the significant reduction of iteration numbers and spatial cells in the near-continuum flow regime. However, the efficiency and accuracy of GSIS has only been demonstrated in two-dimensional problems with small numbers of spatial cell and discrete velocities. Here, a large-scale parallel computing strategy is designed to extend the GSIS to three-dimensional high-speed flow problems. Since the GSIS involves the calculation of the mesoscopic kinetic equation which is defined in six-dimensional phase-space, and the macroscopic high-temperature Navier-Stokes-Fourier equations in three-dimensional physical space, the proper partition of the spatial and velocity spaces, and the allocation of CPU cores to the mesoscopic and macroscopic solvers, are the keys to improving the overall computational efficiency. These factors are systematically tested to achieve optimal performance, up to 100 billion spatial and velocity grids. For hypersonic flows around the Apollo reentry capsule, the X38-like vehicle, and the space station, our parallel solver can get the converged solution within one hour
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