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The filling stage in injection/infusion moulding processes plays a key role in composite manufacturing that can be influenced by the inlet and vent ports. This will affect the production of void-free parts and the desirable process time. Flow control is usually required in experiments to optimise such a stage; however, numerical simulations can be alternatively used to predict manufacturing-induced deficiencies and potentially remove them in the actual experiments. This study uses ANSYS Fluent software to model flow-front advancement during the impregnation of woven fabrics. A developed technique is applied by creating tracking points (e.g., on-line monitor) in the direction of the flow to report/collect data for flow-front positions as a function of time. The study adopts the FVM-VOF-based two-phase flow model together with an implicit time-stepping scheme, i.e., a dual-time formulation solution method with a preconditioned pseudo-time derivative. Initially, three time-step sizes, 5 s (small), 25 s, and 50 s (large), are evaluated to examine their impact on numerical saturation lines at various fabric porosities, 40%, 50%, and 60%, for a two-dimensional (2D) rectangular mould, and predictions are then compared with the well-known analytical Darcy. This is followed by a three-dimensional (3D) curved mould for a fillet L-shaped structure, wherein the degree-of-curvature of fibre preforms is incorporated using a User-Defined Function (UDF) to tailor the impregnation process. The developed approach shows its validation (1–5.7%) with theoretical calculations and experimental data for 2D and 3D cases, respectively. The results also stress that a shorter computational time can be achieved with a large time-step size while maintaining the same level of accuracy. [ABSTRACT FROM AUTHOR]

This study predicts how the Free Surface Level (FSL) variations around finite length vegetation affect flow structure by using a numerical simulation. The volume of fluid (VOF) technique with the Reynolds stress model (RSM) was used for the simulation. Multizone Hexahedral meshing was adopted to accurately track the free surface level with minimum numerical diffusion at the water–air interface. After the validation, finite length emergent vegetation patches were selected based on the aspect ratio (AR = vegetation width-length ratio) under constant subcritical flow conditions for an inland tsunami flow. The results showed that the generation of large vortices was predominated in wider vegetation patches (AR > 1) due to the increase and decrease in the FSL at the front and back of the vegetation compared to longer vegetation patches (AR ≤ 1), as this offered more resistance against the approaching flow. The wider vegetation patches (AR > 1) are favorable in terms of generating a large area of low velocity compared to the longer vegetation patch (AR < 1) directly downstream of the vegetation patch. On the other hand, it has a negative impact on the adjacent downstream gap region, where a 14.3–34.9% increase in velocity was observed. The longer vegetation patches (AR < 1) generate optimal conditions within the vegetation region due to great velocity reduction. Moreover, in all the AR vegetation cases, the water turbulent intensity was maximum in the vegetation region compared to the adjacent gap region and air turbulent intensity above the FSL, suggesting strong air entrainment over this region. The results of this study are important in constructing vegetation layouts based on the AR of the vegetation for tsunami mitigation. [ABSTRACT FROM AUTHOR]

Wire-mesh sensors (WMSs) have been used to measure various characteristics of bubbles, such as volume fraction, size and velocity. The WMS used in this study consists of a grid of nine by nine wires arranged in an orthogonal shape. Transmitter wires are activated sequentially and receiver wires pick up the electric signals and convert them to a current. Multiphase flow, the electric conductivity of each phase, allows characteristic electrical signals to flow and from this information the volume fraction can be calculated. However, deformation of bubbles and the intrusive effects of wires have yet to be considered. To address these issues, we conducted numerical simulations using commercial software. The actual behavior of a single bubble was considered to calculate the electrostatic and two-phase flow simultaneously. First, a bubble was considered stationary in a spherical shape at the center of the WMS. A user-defined function was used to calculate the governing equation of the electric field and then activate electric signals on the transmitter wires. The numerical results were compared with reference values, and the error of a volume fraction was quantified. After that, we considered a bubble rising without a change of shape in the pseudo-dynamic process. However, for the dynamic process, a volume of fluid model was added and a bubble interface that reflects its actual movement was considered. A comparison between the pseudo-dynamic and dynamic processes was conducted using the same positional relations between the wire and bubble surface. When the bubble contacted with and then separated from the wire, differences in the received signal and an overshooting phenomenon were observed owing to deformation of the bubble. Also, the range of locations where the bubble would begin to affect the electric field because of the flattened shape of the bubble was observed. [ABSTRACT FROM AUTHOR]

Autonomous Underwater Helicopter (AUH) is a novel dish-shaped multi-functional submersible vehicle in the autonomous underwater vehicle (AUV) family. Numerical study of launch and recovery system for AUVs from a research vessel is significant in the overall design. Single-arm crane and air-launch way are the important ways of launching AUVs with the high expertise and security. However, AUVs are still easily subject to a huge impact load which can cause the body damage, malfunction of electronic components, and other serious accidents when AUVs are launching into water. In this paper, unsteady, incompressible, two-phase flow with identified boundary conditions was modeled in the computational domain using Volume of Fluid (VOF) technology, and different overset-grid sizes were set and analyzed to ascertain the accuracy of computational fluid dynamics (CFD) simulation first due to the setting of the grid which will directly affect the accuracy of the calculation results, especially the overset-grid convergence validation. Then, the change of water-entry impact force and velocity of AUH with different water-entry velocities and angles were calculated by CFD analysis software STAR-CCM + solver. The proper immersion angle of the AUH should be 75° under comprehensive analysis and comparison. Besides, the variations of pressure distribution of the AUH in the whole water-entry process were also obtained, which provides reference for the shape optimization in the future work. [ABSTRACT FROM AUTHOR]

• The waxy crude oil/water core-annular flows were simulated across inclined pipes with a gradual expansion. • A special viscoplastic model was used to model the non-Newtonian rheological behavior of waxy crudes. • Volume of fluid model was utilized for the capture of the oil/water interface. • Up to 80% reduction in overall pressure drop was reported as a result of core-annular flow formation inside horizontal pipes. • The effect of the inclination angle and the expansion angle was comprehensively investigated. In the present work, a 3D numerical study has been performed to investigate the core-annular flow of waxy crude oil/water in inclined pipes with a gradual expansion. Waxy crudes are highly viscous crude oils that exhibit non-Newtonian flow behavior, and their efficient transportation is still a technical challenge. In the current work, the use of the core-annular method for the transportation of waxy crudes, which are modeled as viscoplastic fluids, is examined comprehensively where the core oil flows in the laminar flow regime, and the water flow field is turbulent. The volume of fluid (VOF) multiphase flow model is used to capture the oil/water interface, and the SST k- ω turbulence model has been employed to predict the turbulent features of the water flow field. The governing equations are discretized and solved using the finite volume method on a structured numerical grid. The effects of several parameters, such as the wax content of the crude oil, inlet velocities, the expansion angle, and the inclination angle of the pipe have been investigated on the non-Newtonian core-annular flow comprehensively. The results revealed that as the wax content of the crude oil increased, the use of the core-annular flow became more economically lucrative for the transportation of waxy crude oils in comparison to the conventional single-phase oil transportation. Moreover, The simulation results indicated that increasing the expansion angle in the core-annular regime from 3.7° to 45° elevated the overall pressure drop more than fourfold. Finally, it is shown that for downward flows, by increasing the inclination angle, the overall pressure drop monotonically decreased. However, in upward flows, the overall pressure drop profile as a function of the inclination angle had a local maximum around 45°. [ABSTRACT FROM AUTHOR]

This research presents a numerical investigation of two-phase flow during the expulsion of entrapped air in a non-confined pipe. A modified version of the volume of fluid (VOF) approach is employed considering the effect of compressibility in the liquid. A modification is introduced to the original approach relating the density changes in the liquid to the pressure changes using the fluid bulk modulus. The bulk modulus is also modified to consider the pipe elasticity, the air bubble entrainment and the two-phase flow regime in a pipe. Fluid–structure interaction (FSI) code is developed and used to calculate the motion of the downstream orifice wall during the impact of the water column on the pipe end wall. The numerical results of the pressure variation agree well with experimental data. The two-phase flow structure and physics behind the pressure waves are investigated. The numerical results show that to capture the amplitude and time interval of the pressure surges, the effect of FSI should be considered during the expulsion of the entrapped air. Additionally, the effects of initial air pocket size, supply pressure and orifice size on the pressure increase are investigated. The developed VOF-FSI approach can be employed as a numerical tool to investigate the transient flow during pipe filling. [ABSTRACT FROM AUTHOR]

When a bubble detaches from a nozzle immersed in water, a sound is emitted owing to the detachment. The bubble deformation and sound emission generated after detachment has been investigated in many studies, in which the breathing mode with a natural frequency was discussed based on the dynamics of the interface between the air and water. In this study, the deformation of a bubble was observed, and the sound emitted upon detachment was measured experimentally. To analyze the bubble deformation process, a computational fluid dynamics (CFD) simulation was conducted using the volume of fluid (VOF) method to predict the sound emission. In the analysis, the deformation behavior, the oscillation frequencies, sound pressure, and radius variation were discussed by comparing the numerical and experimental data. Furthermore, the natural frequency and low frequency vibrations were discussed based on the interference between the detached bubbles and the air column vibrations.

A two-phase bubbly flow is often found in the process industry. For the efficient operation of such devices, it is important to know the details of the flow. The paper presents a numerical simulation of the rising bubble in a stagnant liquid column. The interFOAM solver from the open source Computational Fluid Dynamics (CFD) toolbox OpenFOAM was used to obtain the necessary data. The constant and dynamic computational grids were used in the numerical simulation. The results of the calculation were compared with the measured values. As expected, by using the dynamic mesh, the bubble trajectory was closer to the experimental results due to the more detailed description of the gas⁻liquid interface.

We investigated flow characteristics in a hydraulic coupling at different charged water conditions and speed ratios. Hence, simulations were performed for three-dimensional two-phase flow by using the VOF method. The realizable k-ε turbulence model was adopted. To resolve the interaction of passing blades of the primary and secondary wheels, simulations were conducted in the unsteady framework using a sliding grid technique. The results show that the water-air distribution inside the wheel is strongly dependent upon both amount of charged water and speed ratio. Generally, air is accumulated in the center of the wheel, forming a toroidal shape wrapped by the circulating water. The results also show that at high speed ratios, the solid-body-like rotation causes dry areas on the periphery of the wheels and, hence, considerably decreases the circulating flow rate and the transmitted torque. Furthermore, the momentum transfer was investigated through the concept of a mass flux triangle based on the local velocity multiplied by the local mixture density instead of the velocity triangle commonly used in a single-phase turbomachine analysis. Also, the mass fluxes along the radius of the coupling in the partially charged and fully charged cases were found to be completely different. It is shown that the flow rate at the interfacial plane and also the transmitted torque are closely related and are strongly dependent upon both the amount of charged water and speed ratio. Finally, a conceptual categorization together with two comprehensive maps was provided for the torque transmission and also circulating flow rates. These two maps in turn exhibit valuable engineering information and can serve as bases for an optimal design of a hydraulic coupling. [ABSTRACT FROM AUTHOR]

The Peregrine breather, today widely regarded as a prototype for spatio-temporally localized rogue waves on the ocean caused by nonlinear focusing, is analyzed by direct numerical simulations based on two-phase Navier–Stokes equations. A finite-volume approach with a volume of fluid method is applied to study the Peregrine breather dynamics up to the initial stages of wave breaking. The comparison of the numerical results with laboratory experiments to validate the numerical approach shows very good agreement and suggests that the chosen method is an effective tool to study modulation instability and breather dynamics in water waves with high accuracy even up to the onset of wave breaking. The numerical results also indicate some previously unnoticed characteristics of the flow fields below the water surface of breathers, which might be of significance for short-term prediction of rogue waves. Recurrent wave breaking is also observed. [ABSTRACT FROM AUTHOR]

Abstract: In the present study, the path followed by the oil and the rotation of the crankshaft in a lubrication system of a hermetic reciprocating compressor have been visualized by the means of a high speed camera. Then, the climbing time and the time variation in the speed of the crankshaft have been computed by the means of FLUENT software. The variation of the crankshaft speed with respect to time has been used as an input of the simulation. The predicted climbing time, on the other hand, was the output of the computations, evaluated as a tool to validate the model. Both the accelerated and constant start-up conditions have been considered for the modeling. The change of the crankshaft speed at the start-up has been correlated and used as the initial condition, by writing a step function in the Fluent software. For the sake of applicability and simplicity, no refrigerant is used in the experiments and the computations. Due to the Reynolds number calculated based on the channel diameter, laminar flow conditions have been considered. The two-phase flow of the air–oil pair has been studied by both VOF and Eulerian models. [Copyright &y& Elsevier]

For a gas-liquid two-phase flow, the interface changes its position with respect to time and space. Flow and thermal field in each phase in the vicinity of the interface influence the interface location. The movement of interface in turn influences the flow field. Various two-phase flow regimes are thus observed as the flow parameters of the two phases are varied. The phenomena of bubble coalescence, growth and breakup, and the mass, momentum and energy exchange between the two phases add up to the numerical complexities towards capturing these flow patterns using Computational Fluid Dynamics (CFD) models. The numerical models for the two-phase flow are thus still in the developmental phase. A comprehensive comparison with experiment can go a long way in establishing the strength and reliability of the existing CFD models. Towards this objective, an experimental setup is developed to visualize the temporal and spatial organization of air-water two-phase flow patterns. The flow patterns captured with a high speed camera are compared with the numerically visualized flow patterns obtained using the Volume of Fluid (VOF) model for the two-phase flow. The development of stratified, wavy, plug, slug and annular flow patterns are discussed in detail, and interfacial and wall shear stresses for all these patterns are compared. [ABSTRACT FROM AUTHOR]

Abstract: Carbon dioxide bubble removal at the anode of a direct methanol fuel cell (DMFC) is an important technique especially for applications in the portable power sources. This paper presents numerical investigations of the two-phase flow, CO2 bubbles in a liquid methanol solution, in the anode microchannels from the aspect of microfluidics using a thermal lattice Boltzmann model (TLBM). The main purpose is to derive an efficient and effective computational scheme to deal with this technical problem. It is then examined by a commercially available software using Navier–Stokes plus volume of fluid (VOF) method. The latter approach is normally employed by most researchers. A simplified microchannel simulation domain with the dimension of 1.5μm in height (or width) and 16.0μm in length has been setup for both cases to mimic the actual flow path of a CO2 bubble inside an anodic diffusion layer in the DMFC. This paper compares both numerical schemes and results under the same operation conditions from the viewpoint of fuel cell engineering. [Copyright &y& Elsevier]

Abstract: Understanding the physics of microscale two-phase flow is important for a broad variety of engineering applications including compact PEM fuel cells and heat exchangers. The low Bond number and confined geometry make it critical to consider both the surface tension at the liquid–gas interfaces and the surface forces acting at the channel boundaries. Within the framework of a numerical volume of fluid (VOF) approach, the present work proposes a model to account for surface adhesion forces by considering the effects of contact angle hysteresis. A transient model is developed by correcting boundary force balances through specification of the local contact angle and instantaneously updating the local angle values based on the variation of the volume fraction from previous time steps. The model compares very well with new data provided here for droplets on a rotating disk and liquid slug flow in microchannel. The simulation reveals that the contact angle distribution along the slug profile in the microchannel flow can be approximated using a piecewise linear function. This study indicates that the asymmetric distribution of the contact angle might be responsible for several phenomena observed in the microchannel experiments, including slug instability. [Copyright &y& Elsevier]

A new scheme is proposed for the approximation of the curvature field in the context of two-phase flows. Direct computation of the curvature from the volume fraction distribution produces so-called spurious currents, which are numerical non-physical velocities especially in the vicinity of the interface. The proposed method computes the curvature from the signed distance function. We have implemented the method inside Open FOAM® framework (an open source C++ toolbox for computational fluid dynamics simulations based on finite volume discretization). The surface tension force has been formulated into the sharp surface force (SSF). The implementation is applicable for general mesh topologies (two or three dimensional polyhedra). Different validation test cases have been performed; static air bubble in water, droplet undergoing linear shear, rising bubble into viscous fluid, and the impact of a viscous droplet on a thin cylindrical fiber. The proposed method shows an accurate calculation of the curvature and a significant reduction of spurious currents by several orders of magnitude. Such reduction of spurious currents allows accurate simulations of surface tension dominated flows such as micro-droplets, or liquid–liquid separation. • A new scheme for the computation of curvature field is proposed. • The magnitude of spurious currents is reduced by several orders. • Validations with published data for free surface and contact line dynamics is performed. [ABSTRACT FROM AUTHOR]

The sequential casting of slabs is a major trend in the steel industry where steel quality is the most important factor. The operating parameters have the most influence on mixing phenomenon apart from the design and shapes of the tundish and its furniture. Moreover, in industrial practice, the bath height in tundish varied with time when the ladle is changed. In the present work, the numerical simulation has been carried out to study the effect of residual volume and outflow (throughput) rate on the mixing phenomenon inside the tundish. A transient, three-dimensional two-phase model using the Volume of Fluid (VOF) method and Level Set interface tracking method has been used to investigate the intermixed grade steel formation. A comparison of the two interface tracking schemes, i.e., Geo-reconstruct and Modified HRIC (High-Resolution Interface Capturing Scheme) has also been presented. The results obtained through numerical simulation has been compared with experimental results. In a later section, the results showed that residual volume has a significant effect on the grade mixing. The mixing phenomenon in tundish is considerably influenced by the advance-pouring box (APB). Further, the outflow rate of tundish has little impact on the grade intermixing phenomenon. [ABSTRACT FROM AUTHOR]

Abstract: Computational fluid dynamics (CFD) simulations were conducted to study the liquid water transport behaviors in flow channels of polymer electrolyte membrane fuel cells (PEMFCs). The effects of geometries and surface properties of flow channels on the liquid water exhaust capabilities were quantitatively analyzed based on the exhaust time and stagnant volume data obtained by the CFD simulations. Two different liquid morphologies were found to develop during the two-phase (water/air) transport in flow channels: liquid film morphology for hydrophobic channels and droplet morphology for hydrophilic channels. It was also observed that hydrophobic channels exhausted liquid water much faster than hydrophilic channels, but the stagnant liquid volume in corners of hydrophobic channels was larger than that of hydrophilic channels. The exhaust capability results determined by the quantitative analysis were explained by referring to the different morphologies of liquid water in flow channels. [Copyright &y& Elsevier]