509 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

    Review of Computational Fluid Dynamics Analysis in Biomimetic Applications for Underwater Vehicles

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    Biomimetics, which draws inspiration from nature, has emerged as a key approach in the development of underwater vehicles. The integration of this approach with computational fluid dynamics (CFD) has further propelled research in this field. CFD, as an effective tool for dynamic analysis, contributes significantly to understanding and resolving complex fluid dynamic problems in underwater vehicles. Biomimetics seeks to harness innovative inspiration from the biological world. Through the imitation of the structure, behavior, and functions of organisms, biomimetics enables the creation of efficient and unique designs. These designs are aimed at enhancing the speed, reliability, and maneuverability of underwater vehicles, as well as reducing drag and noise. CFD technology, which is capable of precisely predicting and simulating fluid flow behaviors, plays a crucial role in optimizing the structural design of underwater vehicles, thereby significantly enhancing their hydrodynamic and kinematic performances. Combining biomimetics and CFD technology introduces a novel approach to underwater vehicle design and unveils broad prospects for research in natural science and engineering applications. Consequently, this paper aims to review the application of CFD technology in the biomimicry of underwater vehicles, with a primary focus on biomimetic propulsion, biomimetic drag reduction, and biomimetic noise reduction. Additionally, it explores the challenges faced in this field and anticipates future advancements

    Impact phenomena in multiphase flows

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    Models of polymer solutions in electrified jets and solution blowing

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    Fluid flows hosting electrical phenomena make the subject of a fascinating and highly interdisciplinary scientific field. In recent years, the extraordinary success of electrospinning and solution blowing technologies for the generation of polymer nanofibers has motivated vibrant research aiming at rationalizing the behavior of viscoelastic jets under applied electric fields or other stretching fields including gas streams. Theoretical models unveiled many original aspects in the underpinning physics of polymer solutions in jets, and provided useful information to improve experimental platforms. This article reviews advances in the theoretical description and numerical simulation of polymer solution jets in electrospinning and solution blowing. Instability phenomena of electrical and hydrodynamic origin are highlighted, which play a crucial role in the relevant flow physics. Specifications leading to accurate and computationally viable models are formulated. Electrohydrodynamic modeling, theories for the jet bending instability, recent advances in Lagrangian approaches to describe the jet flow, including strategies for dynamic refinement of simulations, and effects of strong elongational flow on polymer networks are reviewed. Finally, the current challenges and future perspectives of the field are outlined and discussed, including the task of correlating the physics of the jet flows with the properties of realized materials, as well as the development of multiscale techniques for modelling viscoelastic jets.Comment: 135 pages, 42 figure

    Inkjet printing digital image generation and compensation for surface chemistry effects

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    Additive manufacturing (AM) of electronic materials using digital inkjet printing (DIJP) is of research interests nowadays because of its potential benefits in the semiconductor industry. Current trends in manufacturing electronics feature DIJP as a key technology to enable the production of customised and microscale functional devices. However, the fabrication of microelectronic components at large scale demands fast printing of tight features with high dimensional accuracy on substrates with varied surface topography which push inkjet printing process to its limits. To understand the DIJP droplet deposition on such substrates, generally requires computational fluid dynamics modelling which is limited in its physics approximation of surface interactions. Otherwise, a kind of “trial and error” approach to determining how the ink spreads, coalesce and solidifies over the substrate is used, often a very time-consuming process. Consequently, this thesis aims to develop new modelling techniques to predict fast and accurately the surface morphology of inkjet-printed features, enabling the optimisation of DIJP control parameters and the compensation of images for better dimensional accuracy of printed electronics devices. This investigation explored three categories of modelling techniques to predict the surface morphology of inkjet-printed features: physics-based, data-driven and hybrid physics-based and data-driven. Two physics-based numerical models were developed to reproduce the inkjet printing droplet deposition and solidification processes using a lattice Boltzmann (LB) multiphase flow model and a finite element (FE) chemo-mechanical model, respectively. The LB model was limited to the simulation of single tracks and small square films and the FE model was mainly employed for the distortion prediction of multilayer objects. Alternatively, two data-driven models were implemented to reconstruct the surface morphology of single tracks and free-form films using images from experiments: image analysis (IA) and shape from shading (SFS). IA assumed volume conservation and minimal energy drop shape to reconstruct the surface while SFS resolved the height of the image using a reflection model. Finally, a hybrid physics-based and data-driven approach was generated which incorporates the uncertainty of droplet landing position and footprint, hydrostatic analytical models, empirical correlations derived from experiments, and relationships derived from physics-based models to predict fast and accurately any free-form layer pattern as a function of physical properties, printing parameters and wetting characteristics. Depending on the selection of the modelling technique to predict the deformed geometry, further considerations were required. For the purely physics-based and data-driven models, a surrogate model using response surface equations was employed to create a transfer function between printing parameters, substrate wetting characteristics and the resulting surface morphology. The development of a transfer function significantly decreased the computational time required by purely physics-based models and enabled the parameter optimisation using a multi-objective genetic algorithm approach to attain the best film dimensional accuracy. Additionally, for multilayer printing applications, a layer compensation approach was achieved utilizing a convolutional neural network trained by the predicted (deformed) geometry to reduce the out of plane error to target shape. The optimal combination of printing parameters and input image compensation helped with the generation of fine features that are traditionally difficult for inkjet, improved resolution of edges and corners by reducing the amount of overflow from material, accounted for varied topography and capillary effects thereof on the substrate surface and considered the effect of multiple layers built up on each other. This study revealed for the first time to the best of our knowledge the role of the droplet location and footprint diameter uncertainty in the stability and uniformity of printed features. Using a droplet overlap map which was proposed as a universal technique to assess the effect of printing parameters on pattern geometry, it was shown that reliable limits for break-up and bulging of printed features were obtained. Considering droplet position and diameter size uncertainties, predicted optimal printing parameters improved the quality of printed films on substrates with different wettability. Finally, a stability diagram illustrating the onset of bulging and separation for lines and films as well as the optimal drop spacing, printing frequency and stand-off distance was generated to inform visually the results. This investigation has developed a predictive physics-based model of the surface morphology of DIJP features on heterogeneous substrates and a methodology to find the printing parameters and compensate the layer geometry required for optimum part dimensional accuracy. The simplicity of the proposed technique makes it a promising tool for model driven inkjet printing process optimization, including real time process control and paves the way for better quality devices in the printed electronics industry

    A sharp interface approach for wetting dynamics of coated droplets and soft particles

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    The wetting dynamics of liquid particles, from coated droplets to soft capsules, holds significant technological interest. Motivated by the need to simulate liquid metal droplet with an oxidize surface layer, in this work we introduce a computational scheme that allows to simulate droplet dynamics with general surface properties and model different levels of interface stiffness, describing also cases that are intermediate between pure droplets and capsules. Our approach is based on a combination of the immersed boundary (IB) and the lattice Boltzmann (LB) methods. Here, we validate our approach against the theoretical predictions in the context of shear flow and static wetting properties and we show its effectiveness in accessing the wetting dynamics, exploring the ability of the scheme to address a broad phenomenology.Comment: 14 pages, 6 figure

    A CFD methodology for mass transfer of soluble species in incompressible two-phase flows: modelling and applications

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    Continuous flow chemistry is an interesting technology that allows to overcome many of the limitations in terms of scalability of classical batch reactor designs. This approach is particularly relevant for both photochemistry and electrochemistry as new optimal solutions can be designed to limit, for example, the issues related to light penetration, reactor fouling, excessive distance between electrodes and management of hazardous compounds, whilst keeping the productivity high. Such devices operate often in a two-phase regime, where the appearance of a gas in the form of a disperse bubbly flow can be either a desirable feature (e.g. when the gas is needed for the reaction) or the result of a spontaneous reaction (e.g. electrochemistry). Such systems are very complicated flows where many bubbles populate the reactor at the same time and deform under the effect of several forces, such as surface tension, buoyancy and pressure and viscosity terms. Due to the solubility of gas in the liquid solvent, the disperse phase exchanges mass with the liquid (where the reactions generally occur) and the volume of the bubbles changes accordingly. Such physics is mainly a convection-dominated process that occurs at very small length scales (within the concentration boundary layer, which is generally thinner than the hydrodynamic one) and numerical tools for routine design are based on simplifying assumptions (reduced order methods) for the modelling of this region. However, such approaches often lead to errors in the prediction of the mass transfer rate and a fully-resolved method is generally needed to capture the physics at the interface. This last approach comes with a high computational cost (which makes it non suitable for common design processes) but can be employed in simplified scenarios to explore fundamental physics and derive correlation formulae to be used in reduced order models. For the above reasons, this work aims at developing a high-fidelity numerical simulation framework for the study of mass transfer of soluble species in two-phase systems. The numerical modelling of these processes has several challenges, such as the small characteristic spatial scales and the discontinuities in both concentration and velocity profiles at the interface. All these points need to be properly taken into account to obtain an accurate solution at the gas-liquid interface. In this thesis, a new methodology, based on a two scalar approach for the transport of species, is combined with a geometric Volume of Fluid method in the open source software Basilisk (http://basilisk.fr/). A new algorithm is proposed for the treatment of the interfacial velocity jump, which consists of the redistribution of the mass transfer term from the interfacial cells to the neighbouring pure gas ones, in order to ensure the conservation of mass during the advection of the interface. This step is a crucial point of the methodology, since it allows to accurately describe the velocity field near the interface and, consequently, to capture the distribution of species within the concentration boundary layer. The solver is extensively validated against analytical, experimental and numerical benchmarks, which include suspended bubbles in both super- and under-saturated solutions, the Stefan problem for a planar interface, dissolving rising bubbles and competing mass transfer of mixtures in mixed super- and under-saturated liquids. Finally, the methodology is used for the study of real applications, namely the growth of electrochemically generated bubbles on a planar electrode and the mass transfer of a single bubble in a Taylor-Couette device. The effects of the main parameters that characterise the systems (e.g. contact angle, current density and rotor speed) on the growth/dissolution rate of bubbles are investigated. Although these systems need to be necessarily simplified to allow for direct numerical simulations, these examples show that the insight gained into the fundamental physics is valuable information that can be used to develop reduced order models

    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

    Modelling the Evaporation of a Binary Droplet in a Well

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    While the drying behaviour of sessile droplets has been extensively studied over the last 25 years, the evaporation of droplets from wells (DiWs) has largely been neglected, especially from a mathematical modelling standpoint. Understanding a drying DiW is both important for industrial processes (such as inkjet printing and, increasingly, the manufacture of organic displays) and an interesting problem in its own right as a natural progression from sessile droplets, and we still do not have a thorough theoretical description of their evaporation. The main aim of this project was to build an understanding of pure and binary DiWs under the lubrication approximation by constructing a simple mathematical model for the evolution of their shape. We solved the resulting partial differential equations for droplet height and composition profile numerically using the Method of Lines. In the case of a pure droplet, we found that we could control the interface shape using a single parameter (C) based on the capillary number; the more complex binary system required two new parameters governing evaporation and surface tension differences. Comparison to experimental data was improved with the inclusion of a dynamic evaporative flux for each component that depended on the their volume fraction distribution. These simulations offer insight into the deposit that evaporating DiWs leave behind. We have shown that the smaller C, the more likely the DiW will cause an undesirable ring stain, but this is suppressed in binary droplets in which the more volatile component has the lower surface tension
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