On the combined effects of surface tension force calculation and interface advection on spurious currents within Volume of Fluid and Level Set frameworks

This paper deals with the comparison of Eulerian methods to take into account the capillary contribution in the vicinity of a fluid–fluid interface. Eulerian methods are well- known to produce additional vorticity close to the interface that leads to non-physical spurious currents. Numerical equilibrium between pressure gradient and capillary force for the static bubble test case within a VOF framework has been reached in [35] with the height-function technique [14,35]. However, once the bubble is translated in a uniform flow, spurious currents are maintained by slight errors induced by translation schemes. In this work, two main points are investigated: the ability of Volume of Fluid and Level Set methods to accurately calculate the curvature, and the magnitude of spurious currents due to errors in the calculation of the curvature after advection in both translating and rotating flows. The spurious currents source term is expressed from the vorticity equation and used to discuss and compare the methods. Simulations of gas–liquid Taylor flow at low capillary number show that the flow structure and the bubble velocity can be significantly affected by spurious currents

A Cost-Effective Method for Modelling Wave-OWSC Interaction

Bottom-hinged Oscillating Wave Surge Converters (OWSCs) are an efficient way of extracting power from ocean waves. In our previous studies, wave and OWSC interaction has been investigated via computational fluid dynamics (CFD) models. However, these models were highly time-consuming, and significant re-reflection was observed. The present work couples a Boussinesq wave model with a CFD model in order to extend the scope of the applications of the previous models. This model takes advantage of the Boussinesq wave model, which simulates the wave propagation effectively, and the CFD model, which provides the local flow details comprehensively. The model is validated by a comparison of the present results with those obtained with the pure CFD model and the experimental tank testing. The final objective of the present work is to simulate some events experienced and recorded by the full-scale prototype (Oyster 800 developed by Aquamarine Power) incorporating the real bathymetry at the Oyster 800 site.</p

Hydrodynamics of gas-liquid Taylor flow in microchannels

This thesis focuses on the hydrodynamics of gas-liquid Taylor flow (or slug flow) in microchannels. These flows, which are generally dominated by surface tension forces, have been investigated in rectangular channels of various cross-sectional aspect ratios by means of both experimental visualizations and numerical simulations. The first experimental part aims at characterizing the bubble generation process (bubble length and frequency of break-up) depending on the operating conditions, the fluid properties, as well as the junction where both fluids merge. Numerical simulations of fully developed Taylor flow have been carried out with the JADIM code. The computation of such surface tension dominated flows requires an accurate calculation of the surface tension force. Some limitations of the Volume of Fluid method have been highlighted and a Level Set method has been developed in order to improve the calculation of capillary effects. Both methods have been compared in detail in terms of spurious currents. 3D numerical simulations have been performed and the influence of the capillary number, as well as the effects of geometry have been highlighted. Inertial effects have been taken into account and their influence on the pressure drop has been shown to be non-negligible. Mixing in the liquid slug has also been studied

Hydrodynamics of gas-liquid Taylor flow in microchannels

Cette thèse porte sur l’étude des écoulements de Taylor (ou poche/bouchon) gazliquide en microcanal. Ces écoulements où les effets de tension de surface sont prépondérants ont été étudiées expérimentalement et numériquement pour des géométries rectangulaires avec divers rapports d’aspects. Une première partie expérimentale a consisté à caractériser la formation de bulles (taille, fréquence) en fonction des conditions opératoires, des propriétés des fluides (notamment à travers le nombre capillaire) et du mode de mise en contact des fluides. La dynamique de l’écoulement établi a par la suite été étudiée à l’aide du code JADIM. La simulation de ces écoulements dominés par la tension de surface a nécessité de lever les limitations liées à la prise en compte de la force capillaire. En effet des courants parasites numériques sont créés à proximité de l’interface lors de la simulation d’écoulements capillaires. Une méthode Level Set a été implémentée et comparée à la méthode Volume of Fluid d’origine en termes de courants parasites. Des simulations numériques 3D ont permis l’étude des effets du nombre capillaire et de la géométrie sur la dynamique des bulles de Taylor (vitesse, pression et formes de bulles). Les effets inertiels souvent négligés ont été considérés et leur influence, notamment sur les sauts de pression à l’interface, a été mise en évidence. Le mélange dans le bouchon liquide a également été étudié. ABSTRACT : This thesis focuses on the hydrodynamics of gas-liquid Taylor flow (or slug flow) in microchannels. These flows, which are generally dominated by surface tension forces, have been investigated in rectangular channels of various cross-sectional aspect ratios by means of both experimental visualizations and numerical simulations. The first experimental part aims at characterizing the bubble generation process (bubble length and frequency of break-up) depending on the operating conditions, the fluid properties, as well as the junction where both fluids merge. Numerical simulations of fully developed Taylor flow have been carried out with the JADIM code. The computation of such surface tension dominated flows requires an accurate calculation of the surface tension force. Some limitations of the Volume of Fluid method have been highlighted and a Level Set method has been developed in order to improve the calculation of capillary effects. Both methods have been compared in detail in terms of spurious currents. 3D numerical simulations have been performed and the influence of the capillary number, as well as the effects of geometry have been highlighted. Inertial effects have been taken into account and their influence on the pressure drop has been shown to be non-negligible. Mixing in the liquid slug has also been studied

Hydrodynamics of gas-liquid Taylor flow in rectangular microchannels

The effect of fluid properties and operating conditions on the generation of gas–liquid Taylor flow in microchannels has been investigated experimentally and numerically. Visualisation experiments and 2D numerical simulations have been performed to study bubble and slug lengths, liquid film hold-up and bubble velocities. The results show that the bubble and slug lengths increase as a function of the gas and liquid flow rate ratios. The bubble and slug lengths follow the model developed by Garstecki et al. (Lab chip 6:437-446, 2006) and van Steijn et al. (Chem Eng Sci 62:7505-7514, 2007), however, the model coefficients appear to be dependent on the liquid properties and flow conditions in some cases. The ratio of the bubble velocity to superficial two-phase velocity is close to unity, which confirms a thin liquid film under the assumption of a stagnant liquid film. Numerical simulations confirm the hypothesis of a stagnant liquid film and provide information on the thickness of the liquid film

Data-driven modelling for drop size distributions

The prediction of the drop size distribution (DSD) resulting from liquid atomization is key to the optimization of multi-phase flows, from gas-turbine propulsion, through agriculture, to healthcare. Obtaining high-fidelity data of liquid atomization, either experimentally or numerically, is expensive, which makes the exploration of the design space difficult. First, to tackle these challenges, we propose a framework to predict the DSD of a liquid spray based on data as a function of the spray angle, the Reynolds number, and the Weber number. Second, to guide the design of liquid atomizers, the model accurately predicts the volume of fluid contained in drops of specific sizes whilst providing uncertainty estimation. To do so, we propose a Gaussian process regression (GPR) model, which infers the DSD and its uncertainty form the knowledge of its integrals, and of its first moment, i.e., the mean drop diameter. Third, we deploy multiple GPR models to estimate these quantities at arbitrary points of the design space from data obtained from a large number of numerical simulations of a flat fan spray. The kernel used for reconstructing the DSD incorporates prior physical knowledge, which enables the prediction of sharply peaked and heavy-tailed distributions. Fourth, we compare our method with a benchmark approach, which estimates the DSD by interpolating the frequency polygon of the binned drops with a GPR. We show that our integral approach is significantly more accurate, especially in the tail of the distribution (i.e., large, rare drops), and it reduces the bias of the density estimator by up to ten times. Finally, we discuss physical aspects of the model's predictions and interpret them against experimental results from the literature. This work opens opportunities for modelling drop size distribution in multiphase flows from data.Comment: 18 pages, 9 figure

Transition to elasto-capillary thinning dynamics in viscoelastic jets

We perform simulations of an impulsively-started, axisymmetric viscoelastic jet exiting a nozzle and entering a stagnant gas phase using the open-source code Basilisk. This code allows for efficient computations through an adaptively-refined volume-of-fluid technique that can accurately capture the deformation of the liquid-gas interface. We use the FENE-P constitutive equation to describe the viscoelasticity of the liquid and employ the log-conformation transformation, which provides stable solutions for the evolution of the conformation tensor as the jet thins down under the action of interfacial tension. For the first time, the entire jetting and breakup process of a viscoelastic fluid is simulated, including the pre-shearing flow through the nozzle, which results in an inhomogeneous initial radial stress distribution in the fluid thread that affects the subsequent breakup dynamics. The evolution of the velocity field and the elastic stresses in the nozzle are validated against analytical solutions where possible, and the early-stage dynamics of the jet evolution are compared favourably to the predictions of linear stability theory. We study the effect of the flow inside the nozzle on the thinning dynamics of the viscoelastic jet (which develops distinctive "beads-on-a-string" structures) and on the spatio-temporal evolution of the polymeric stresses in order to systematically explore the dependence of the filament thinning and breakup characteristics on the initial axial momentum of the jet and the extensibility of the dissolved polymer chains

Gas-liquid Taylor flow in microchannels

Cette étude, comportant un volet expérimental et un volet numérique, vise à mieux comprendre l'hydrodynamique des écoulements de Taylor gaz-liquide en microcanaux rectangulaires (dh ~ 500 µm). L'étape de génération des bulles lors de la mise en contact des fluides est étudiée à travers la taille des poches de gaz et bouchons liquides ainsi que la fréquence de formation. La dynamique de l'écoulement établi est également analysée (forme des bulles, épaisseur de film, vitesse)

First demonstration of hydrophobic membrane contactors for removal of ammonia from condensate wastewater

Hydrophobic membrane contactors represent a promising solution to the problem of recovering ammoniacal nitrogen from wastewater. The process has been shown to work best with wastewater streams that present high ammonia concentrations, low buffering capacities and low total suspended solids. The removal of ammonia from rendering condensate, produced during heat treatment of waste animal tissue, was assessed in this research using a hydrophobic membrane contactor. The main objective was to test the ammonia stripping technology using two types of hydrophobic membrane materials, polypropylene and polytetrafluoroethylene, at pilot scale and carry out process modification for ammonia removal. The results demonstrate that polypropylene membranes are not compatible with the condensate waste as it caused wetting. The polytetrafluoroethylene membranes showed potential and had a longer lifetime than the polypropylene membranes, removing up to 64% of ammonia from the condensate waste. The product formed contained a 30% concentrated ammonium sulphate salt which has a potential application as a fertilizer. This is the first demonstration of hydrophobic membrane contactors for treatment of condensate wastewater