Fluid Method Research Articles

Finite Element Analysis and Electrohydrodynamic Multiphysics Modeling of a Corona-Streamer Discharge in a Two-Phase Flow Medium

This study proposes an electrohydrodynamic multiphysics modeling and finite element analysis technique to accurately simulate corona-streamer discharges in a two-phase flow medium. The discharge phenomenon is modeled as a multiphysics system, coupling the Poisson equation for the electric field with a charge dynamics model based on fluid methods and a thermofluid field for temperature effects. To optimize the numerical simulation, the tip-flat plate electrode model was simplified to two-dimensional axisymmetry, and an unordered lattice network was used to reduce computational time while maintaining high resolution in the region of interest. A high DC voltage was applied to the model to generate a local non-uniform electric field exceeding 10 MV/m, allowing the numerical simulations of ionization, recombination, and charge attachment in the streamer channel. The numerical results were compared with voltage and current measurements from full-scale experiments under identical geometry and initial conditions to verify the effectiveness of the proposed method. The results of this study enhance the understanding of the multiphysical mechanisms behind electrical discharge phenomena and can enable the prediction of insulation failure through simple simulations, eliminating insulation experiments on devices.

Influence of Bubble Size on Hydrodynamics in Shear–thinning Liquids

Introduction: Bubble rising behavior in shear-thinning liquids was simulated using a volume of fluid (VOF) method. Method: The authors conducted a detailed analysis of the effects of bubble diameter D, inelastic relaxation time (λ), and rheological index (n) on bubble deformation and flow fields. objective: It can be seen from the above review that the overwhelming majority of the present investigations on the rising behaviour of bubbles in the shear-thinning liquids focus on the effect of liquid-phase rheological parameters such as the flow index and the inelastic time constant on the bubble hydrodynamics. As a matter of fact, the bubble size is a very important parameter for the bubbly system, which determines the residence time of bubbles and the void fraction in a bubbly system. Therefore, it is essential to investigate the dependence of hydrodynamics on the bubble size in the shear-thinning liquids. In this paper, the authors investigated the rising behavior of the bubbles with different diameter in the shear-thinning liquids in detail. Result: The results indicated that bubble diameter significantly affects bubble dynamics. Bubble terminal velocity tends to stabilize within a certain diameter range rather than continuously increasing with an increase in bubble diameter, and this trend is particularly noticeable when the shearthinning effect of liquid is strong— with an increase in λ, the region and degree of a decrease in apparent viscosity increase, causing bubble deformation and terminal velocity to increase. A reduction in n leads to an increase in bubble terminal velocity and deformation degree. Conclusion: These findings have potential implications for patent applications in industrial fields involving shear-thinning liquids containing bubbles.

Investigating air compressibility effects in numerical modeling of oscillating water column wave energy converters: a case study on performance and accuracy

ABSTRACT Wave energy, as a clean and abundant resource, offers significant potential for sustainable power. Oscillating Water Column (OWC) is a key technology in this domain. In studies related to OWC, air compressibility can be a significant parameter. This research examines the error caused by neglecting the effects of air compressibility in numerical modeling. To this end, a numerical solution for a 1:10 scaled OWC model was developed based on RANS equations and the Volume of Fluid (VoF) method for free surface water. The wave conditions studied correspond to those in the Caspian Sea. For validation, the pressure within the chamber was compared with experimental results. The results indicated that the error for the incompressible model is 9.89% and for the compressible model is 8.43%. To account for air compressibility effects, a User-Defined Function (UDF) was added to the numerical solution, which adjusts the air density according to isentropic pressure equations. The results showed that considering air compressibility can lead to up to a 5.71% overestimation of the converter’s output power at certain incident wave frequencies. Additionally, at resonance conditions where the incident wave frequency matches the natural frequency of the converter, the effect of air compressibility is minimized.

Corresponding-states framework for classical and quantum fluids-Beyond Feynman-Hibbs.

Effective potential methods, obtained by applying a quantum correction to a classical pair potential, are widely used for describing the thermophysical properties of fluids with mild nuclear quantum effects. In case of strong nuclear quantum effects, such as for liquid hydrogen and helium, the accuracy of these quantum corrections deteriorates significantly, but at present no simple alternatives are available. In this work, we solve this issue by developing a new, three-parameter corresponding-states principle that remains applicable in the regions of the phase diagram where quantum effects become significant. The new principle emerges from a mapping procedure, which shows that quantum-corrected pair potentials can be made conformal to their underlying classical pair potential by modifying the latter's repulsive range. This mapping enables an accurate description of fluids with quantum-corrected interactions based on off-the-shelf methods for classical fluids (e.g., equations of state, classical density functional theory, and entropy scaling) using effective, mapped intermolecular-potential parameters. These effective parameters depend on temperature and molecular mass; simple analytic equations in case of a classical Mie potential with Feynman-Hibbs quantum corrections are presented. Using Mie Feynman-Hibbs force fields from the literature, we show that this procedure provides accurate predictions for the properties of fluids with mild nuclear quantum effects, such as neon or hydrogen at moderate temperatures. Moreover, by adjusting the functional form of the effective intermolecular-potential parameters to experimental data for helium and hydrogen, we are able to apply the corresponding-states principle for optimal quantum-corrected pair potentials that far surpass the accuracy of the Feynman-Hibbs correction.

Two-Dimensional Analysis of Air–Water Interaction in Actual Water Pipe-Filling Processes

This paper investigates air–water interactions during a controlled filling process of an actual water pipeline using a two-dimensional Computational Fluid Dynamics (CFD) model. The main objectives are to understand the dynamic interaction of these fluids through water inflow patterns, pressure pulses, and air-pocket dynamics based on contours. This study uses an existing cast iron pipeline 485 m in length, a nominal diameter of 400 mm, and an air valve with a nominal diameter of 50 mm. The methodology of this CFD model includes the Partial Volume of Fluid (pVoF) method for air–water interface tracking, a turbulence model, mesh sensitivity and numerical validation with pressure and velocity measurements. Results highlight the gradual pressurization of pipelines and air pocket behavior at critical points and show the thermodynamic interaction concerning heat transfer between gas and liquid. This study advances the application of CFD in actual water pipelines, offering a novel approach to air pocket management.

RANS simulation of supercritical open channel flows around obstacles

The URANS equations are solved with the SST turbulence closure model, and the free surface is modelled using the Volume-of-Fluid method. Numerical results are verified through a detailed comparison with existing laboratory measurements for the supercritical flow patterns: detached hydraulic jump and wall-jet-like bow wave. Results demonstrate that the model accurately captures key flow features in both cases. A quasi-periodic oscillation of the bow wave with a Strouhal number of 0.08 is observed. The horseshoe vortex is composed of four vortices, similar to cases of cylinders in subcritical flows. The bottom shear stress beneath the main horseshoe vortex shows an amplification comparable to that observed in subcritical cases, suggesting that scour at bridge piers should be of comparable magnitude in both supercritical and in subcritical flows. Obtained results illustrate the applicability, capacities, and limitations of RANS models with the Volume-of Fluid method for the simulation of supercritical flows around obstacles.

A systematic global recharacterization method for reservoir fluids in compositional simulations

A systematic global recharacterization method for reservoir fluids in compositional simulations

ConvNet-based prediction of droplet collision dynamics in microchannels

The dynamics of droplet collisions in microchannels are inherently complex, governed by multiple interdependent physical and geometric factors. Understanding and predicting the outcomes of these collisions (whether coalescence, reverse-back, or pass-over) pose significant challenges, particularly due to the deformability of droplets and the influence of key parameters such as the density and viscosity of immiscible fluids, the initial offset between droplets, and the confined geometry of microchannels. Traditional methods for analyzing these collisions, including computational and experimental techniques, are time-consuming and resource-intensive, limiting their scalability for real-time applications. In this work, we explore a novel data-driven approach to predict droplet collision outcomes using convolutional neural network (CNN). CNN-based approaches present a significant advantage over traditional methods, offering faster, scalable solutions for analyzing large datasets with varying physical parameters. Using a lattice Boltzmann method for binary immiscible fluids, we numerically generated droplet collision data under confined shear flow. These data, represented as droplet shapes, serve as input to the CNN model, which automatically learns hierarchical features from the images, allowing for accurate and efficient collision outcome predictions based on deformation and orientation. The model achieves a prediction accuracy of 0.972, even on test datasets with density and viscosity ratios not included in the training. Our findings suggest that the CNN-based models offer improved accuracy in predicting collision outcomes while drastically reducing computational and time constraints. This work highlights the potential of machine learning to advance droplet dynamics studies, providing a valuable tool for researchers in fluid dynamics and soft matter.

Simulation and experimental investigation on kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall

Liquid nitrogen droplets impacting superheated wall is an essential phenomenon in cryogenic phase-change spray cooling. In this study, the Volume of Fluid (VOF) method was used to develop a numerical model to investigate the kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall. The experiment for the impact of liquid nitrogen droplets was conducted to validate the simulation. The findings indicate that liquid nitrogen droplets impacting superheated wall exhibit three boiling regimes: contact boiling, transition boiling, and film boiling. During contact boiling, as We increases, droplets undergo three modes sequentially: spreading, forming fingering-like structures, and fragmentation. During transition boiling and film boiling, as We increases, droplets exhibit spreading, splashing, and fragmentation. Increasing the wall temperature leads to the formation of vapor pockets and vapor film, which results in deteriorated heat transfer at the solid–liquid contact area, and meanwhile reducing the extent of droplet spreading. Increasing We promotes droplet spreading, reducing vapor pockets and vapor film thickness, increasing wetting area, and postpones the onset of heat transfer deterioration. Increasing the wall temperature and We both lead to a higher decreasing rate of liquid volume of droplet which indicates an intensified vaporization rate. The larger the contact angle of the wall, the less the droplet spreads, and the lower the heat transfer between the droplets and the wall.

Phase-Controlled Construction of Copper-Leaf-like 2D NiS for Enhanced Supercapacitor Performance: A Supercritical Fluid Process

This study investigates a rapid and straightforward approach for constructing phase-controlled two-dimensional NiS nanosheets using the supercritical fluid (SCF) method. Through structural and morphological analyses, it is observed that the single-phase NiS nanosheets exhibit a rough and hierarchical surface, potentially offering a versatile electroactive area for energy storage. The electrochemical performance of fabricated single-phase NiS-based supercapacitors are measured in both three-electrode and two-electrode systems. In asymmetric supercapacitor (ASC) configuration with H2SO4 electrolyte, α-NiS demonstrated a remarkably high specific areal capacitance of 1664.61 mF.cm-2 at a current density of 1mA.cm-2. In asymmetric supercapacitor mode, NiS nanostructures achieved an exceptionally high volumetric energy density of 1953.61 mWh.cm-3 at a highest power density of 45.5W.cm-3. Furthermore, it showed an outstanding cycling performance of 95.77% retention after 5000 cycles at a high current density of 10mA.cm-2.

A fully-integrated lattice Boltzmann method for fluid–structure interaction

A fully-integrated lattice Boltzmann method for fluid–structure interaction

Investigation of the two-phase flow and heat transfer characteristics with different inlet and outlet arrangements in the ionic liquid compressor for the hydrogen refuelling station

The ionic liquid compressor is a prospective device for hydrogen pressurisation in the hydrogen refuelling station. Ionic liquids are introduced into the compressor chamber between hydrogen and the free piston for lubrication, sealing and cooling. Therefore, it is essential to investigate the two-phase flow and heat transfer since they impact the operational efficiency of the compressor. In this research, numerical simulation using the volume of fluid method was taken out to study the two-phase flow and heat exchange characteristics of hydrogen and ionic liquids in one compression process with four different inlet and outlet arrangements. The results showed that the inlet flow shock caused severe deformation of the phase interface, producing a large number of droplets and bubbles, which significantly enhanced the heat transfer. This deformation was greatly influenced by inlet and outlet arrangements. In general, the axial inlet design caused more obvious interface distortion than the radial one, enlarging the heat transfer area while also raising the risk of the piston being exposed. The recommended structure was the axial decentralised inlet form, which avoided piston exposure while maximizing the heat transfer area, with a hydrogen temperature rise of 16.24 K and a polytropic index of 1.11.

Kinetic study of head-on collisions of unequal-sized compound droplets

In this study, the head-on collision process of compound droplets of unequal sizes in a liquid environment is investigated using the volume of fluid method. The investigation reveals four main collision mechanisms: coalescence stabilization, coalescence release, rupture entrapment, and rupture coalescence. The transition between these collision mechanisms is analyzed in detail according to We. The effects of various initial collision parameters on the relative offset velocity CSrov, axial thicknesses l* and radial thicknesses h*, deformation coefficients, and core droplet release time Crt of compound droplet core shells were quantitatively analyzed. Additionally, this study examines the collision process of multi-layer compound droplets, revealing a more complex dynamic evolution of the interface, including the coalescence-release-entrapment phase and changes in the release direction. This study not only provides theoretical support for understanding the stability of compound droplets but also provides new insight into understanding multi-phase interactions in complex fluid systems.

Numerical investigation on the hydrodynamic wave forces on the three barges in proximity

Two-dimensional nonlinear hydrodynamic wave forces acting on three side-by-side barges are numerically investigated in this work. A finite volume viscous flow model, incorporating the volume of fluid method for free-surface capturing, is implemented based on OpenFOAM® platform to simulate interactions between nonlinear free-surface waves and multiple barges. The study explores the impact of fluid resonances in the gaps between three barges on the hydrodynamic wave forces. The initial transient and final quasi-steady wave forces are first examined to highlight the significance of transient evolution stage. The steady-state wave forces are subsequently concerned with the amplitude-frequency response of wave forces, along with the pressure and viscous force components. Relationship between wave forces and fluid oscillations within the gaps is then clarified to explain the mechanism of wave forces on the barges associated with gap resonance. Significant nonlinear higher-order harmonics are further demonstrated, with their origins closely linked to fluid oscillations within gaps between the barges. Contributions of higher-order harmonics to the overall wave force amplitudes are explored through harmonic analysis utilizing Fourier transformations. Finally, effects of the normalized incident wave amplitudes on the amplitudes of wave forces and higher-order harmonics are found to be highly dependent on frequency bands.

Investigation of film thickness uniformity in inkjet printing of liquid optically clear adhesive

The inkjet printing process of liquid optically clear adhesive film often leads to the phenomenon of end protrusions, which cause bubbles during subsequent bonding and encapsulation processes or localized Mura defects. To gain a deeper understanding of the mechanism behind the formation of protrusions during film retracts, we derived a dynamic model for the formation of end protrusions in the film. This research also simulated the coalescence of multiple droplets into a film line, including protrusion formation, using the volume of fluid method. The simulation results were compared with theoretical calculations, revealing that the theoretical values were approximately twice as large as the simulated ones. We discovered that the formation of protrusions is the result of the interaction between surface tension and internal forces within the liquid film. During this process, the maximum height of the film line exhibits a positive correlation with the power of time. Finally, the influence of contact angle on the uniformity of film thickness was also explored. It was found that a smaller contact angle can suppress the occurrence of protrusions, leading to a significant improvement in the uniformity of film thickness.

A numerical study of two identical CO2 bubbles rising side by side in water coupled with mass transfer

ABSTRACT In contrast to the relatively straightforward rising dynamics of single bubbles, the behavior of multiple bubbles involves a more intricate interplay of flow field dynamics, trajectory patterns, morphological changes, and mass transfer phenomena. To reveal the coalescence characteristics and mass transfer effects of CO2 bubbles during the rising process, this study developed a coupled mass transfer numerical model for bubbles. The Volume of Fluid (VOF) method was used in combination with a mass transfer code for solving the model. The effects of bubble pair initial spacing and liquid viscosity on the coalescence and mass transfer were discussed. The numerical simulation results show that as the initial distance between the two bubbles decreases, the interaction force between the bubbles increases, and the possibility of bubble coalescence increases, the coalescence behavior reduces the contact area and thus decreases the amount of CO2 dissolved about 24.07%. When the initial viscosity of the liquid phase is low, there is an oscillation in the bubble velocity, and the two bubbles tend to merge. As the initial viscosity of the liquid phase increases, resistance increases, causing a decrease in bubble velocity. Consequently, the path of bubble ascent shifts from “approach-repulsion-approach” to “continuous repulsion.” The viscous resistance reduces the gas–liquid contact area, resulting in a corresponding decrease in the mass transfer rate, the dissolution amount of carbon dioxide decreased by approximately 24.42% in this case. The exploration of coupled fluid dynamics and mass transfer phenomena at the multi-bubble scale holds paramount importance in guiding the design and exploration of gas–liquid two-phase flow systems.

Optimization of a monochromatic clapper-yule spectral prediction model based on the single ink drop spreading behavior in inkjet printing

In recent years, inkjet digital printing technology has become a popular research area. This paper focuses on the spreading behavior of single ink drops on coated paper in digital inkjet printing. It explores the impact of ink drop spreading on monochromatic spectral reflectance, providing new insights for the theoretical development of spectral prediction models. The study involves constructing a spreading behavior model through theoretical modeling. Numerical simulations were conducted using the Volume of Fluid (VOF) method in Fluent software to assess the feasibility of the theoretical model. Finally, experimental fitting was used to validate the accuracy of the theoretical model. Considering the feathering effect in practical printing, a comprehensive monochromatic extended Clapper-Yule spectral prediction model was developed based on the spreading behavior of single ink drops. The prediction results of this model showed significant accuracy improvements in monochromatic spectral prediction compared to the traditional segmented Clapper-Yule model. This research offers new perspectives for spectral prediction and provides theoretical guidance for enhancing print image quality.

Mathematical modeling of a perforated continuous steel-smelting unit

Relevance. The volume of steel production in Russia and in the world has doubled over the past 20 years, the cost of steel in Russia in the period from October 2018 to March 2020 increased from 45 thousand rubles to 105 thousand rubles. This determines the urgency of developing energy-efficient steel production technologies that will reduce the cost of production. The most common technology for the producing steel of the full metallurgical cycle involves iron reduction in blast furnaces and characterized by significant emissions of pollutants into the environment. One of the most promising areas of environmentally friendly and energy-efficient steel production is non-straw production. At the moment, there are about a hundred different iron recovery processes, some of them have been brought to industrial use. Aim. To develop a fuel supply system in a perforated hearth, eliminating heat losses in the steelmaking unit by organizing a perforated hearth, which allows heat to be returned to the working space of the furnace by heating the reducing agent. Methods. Numerical modeling by Volume of Fluid (VOF) and Euler-Euler (EE) methods. Results. The authors have determined the rate of supply of reducing gas, which ensures its conversion to carbon and hydrogen at the entrance to the working area of the furnace. It was found that the surface temperature of the perforated hearth on the gas side is 380°C, on the melt side does not exceed 1313°C, which is significantly lower than the melting point of the refractory material.

Prediction of hydrodynamic behavior of impinging Glycerin hollow droplet on a concave surface using jet counter and dynamic contact angle

PurposeThis study aims to study hollow droplet collisions for their hydrodynamic behavior and jet properties.Design/methodology/approachThe volume of fluid (VOF) method was used to simulate a hollow impact using OpenFoam software (VOF).FindingsThe height of the edge-jet decreased as the air diameter (d) and length of the concave surface (L) increased. Height is specific for case 1 at t = 4 ms and its value is 3 mm. The minimum height is 0.585 mm in case 5. Also, the length of the edge-jet changed with time and decreased with the increasing length of concave and air diameter. The maximum length observed in case 1 was 9.23 mm, and the minimum appeared in case 5, in which the length was 0.68 mm.Originality/valueThe impact of a hollow droplet on a solid concave surface was numerically analyzed in this paper at various lengths of surface and shell thicknesses.

Numerical and Experimental Study on the Hydrodynamic Performance of a Sloping OWC Wave Energy Converter Device Integrated into Breakwater

This study presents numerical and experimental investigations on an oscillating water column (OWC) wave energy device integrated into a sloping breakwater. Regular waves were generated in a physical wave tank to investigate the hydrodynamic performance and extraction efficiency of the small-scale nested OWC device. Simultaneously, to complement various scenarios, numerical simulations were conducted using the open-source computational fluid dynamics platform OpenFOAM. The volume of fluid (VOF) method was employed to capture the complex evolution of the air–water interface, and an artificial source term (Forchheimer flow region) was introduced into the Navier–Stokes equations to replace the power take-off (PTO) system. By analyzing wave reflection properties, energy absorption efficiency, and wave run-up, the hydrodynamic characteristics of the inclined OWC device were explored. The comparison between the numerical and experimental results indicate a good consistence. A smaller front wall draft broadens the high-efficiency frequency bandwidth. For relatively long waves, increasing the air chamber width enhances energy conversion efficiency and reduces wave run-up. The optimal configuration was achieved with the following dimensionless parameters: front wall draft a/h=1/3, air chamber width d1/h=2/9, and slope i=2. Due to the sloped structure, when compared with a vertical OWC, long waves can more easily enter the chamber. This causes the efficient frequency bandwidth to shift towards the low frequency range, allowing more wave energy to be converted into pneumatic energy. As a result, wave run-up is reduced, enhancing the protective function of the breakwater.