Showing papers on "Volume of fluid method published in 2023"

Numerical investigation of air cushioning in the impact of micro-droplet under electrostatic fields

Abstract: Air cushioning widely occurs when a droplet impacts onto a solid or fluid surface at low velocity, which is mediated by the lubrication pressure of a thin air layer. Such air cushioning phenomena for micro-sized droplets bear important implications for precision coating and inkjet printing. In this study, we investigate numerically the air cushioning in the micro-sized droplets of various sizes impacting on a solid surface based on the volume of fluid method as implemented in the OpenFOAM framework. We find that the critical impact speed for bouncing on the air cushion increases as the droplet radius decreases, while the Weber number remains in a narrow range from 1 to 4. The scaling law of the critical impact speed for bouncing is derived by balancing the lubrication pressure of the air cushion with the capillary pressure and droplet inertia. The impact mode transforms from bouncing to wetting with an electric field. A group of phase diagrams of the electric Bond number vs. the Weber number are presented for various droplet sizes. The diagrams are consistent with the scaling law of the critical electric field for the Wetting-without-bubble mode. The findings provide insights for applications based on micro-droplet deposition, such as inkjet/electrohydrodynamic printing and spray coating, to avoid the adverse effect of air cushioning or air entrapment.

Numerical studies of bubble formation dynamics in gas-liquid interaction using Volume of Fluid (VOF) method

Abstract: This work aims to investigate the bubble generation and its behavior during two-phase interaction using the Volume of Fluid (VOF) method for interface capturing and reconstruction. The conservation equations of mass and momentum are computed in real time taking into account surface tension and gravity. Pressure-velocity coupling is achieved through the PISO scheme and the interface was reconstructed using the geo-reconstruct PLIC scheme. The obtained results of bubble behaviors are in solidarity with the available literature. Gas (air) was injected in the liquid water through an orifice of 1 mm diameter located in the bottm wall. Influence of surface tension and orifice properties including inlet orifice velocity, orifice diameter on bubble generation and dynamics were analyzed considering the orifice on the bottom wall. The increase in surface tension is directly proportional to bubble diameter and the time taken for detachment. Detachment time behaves inversely with increasing gas velocity. The presence of two orifices and the influence of the spacing between them on bubble dynamics was also investigated.

Investigation of the Splashing Characteristics of Lead Slag in Side-Blown Bath Melting Process

Abstract: Aiming at the melt splashing behavior in the smelting process of an oxygen-enriched side-blowing furnace, the volume of fluid model and the realizable k−ε turbulence model are coupled and simulated. The effects of different operating parameters (injection velocity, immersion depth, liquid level) on splash height are explored, and the simulation results are verified by water model experiments. The results show that the bubbles with residual kinetic energy escape to the slag surface and cause slag splashing. The slag splashing height gradually increases with the increase in injection velocity, and the time-averaged splashing height reaches 1.01 m when the injection speed is 160 m/s. Increasing the immersion depth of the lance, and the slag splashing height gradually decreases. When the immersion depth is 0.12 m, the time-averaged splashing height is 0.85 m. Increasing the liquid level is beneficial to reduce the splash height, when the liquid level is 2.7 m, the splash height reduces to 0.77 m. With the increase in the liquid level, the slag splashing height gradually decreases, and the time-averaged splashing height is 0.77 m when the initial liquid level is 2.7 m.

Hydrodynamics of an OWC Device in Irregular Incident Waves Using RANS Model

Abstract: This research examines the hydrodynamic performance of an oscillating water column device placed over a sloping seabed under the influence of irregular incident waves. The numerical model is based on the Reynolds-veraged Navier–Stokes (RANS) equations with a modified k−ω turbulence model and uses the volume-of-fluid (VOF) approach to monitor the air–water interface. To explore the hydrodynamic performance of the OWC device in actual ocean conditions, the Pierson–Moskowitz (P-M) spectrum was used as the incident wave spectrum, together with the four distinct sea states which occur most often along the western coast of Portugal. The numerical simulation offers a comprehensive velocity vector and streamline profiles inside the OWC device’s chamber during an entire cycle of pressure fluctuation. In addition, the impact of the irregular wave conditions on the free-surface elevation at various places, the pressure drop between the chamber and the outside, and the airflow rate via the orifice per unit width of the OWC device are investigated in detail. The results demonstrate that the amplitudes of the inward and outward velocities via the orifice, free-surface elevations, and flow characteristics are greater for more significant wave heights. Further, it is noticed that the power generation and capture efficiency are higher for a seabed having moderate slopes.

On the effects of hybrid surface wettability on the structure of cavitating flow using implicit large eddy simulation

Abstract: Supercavitation flow is characterized by the presence of a supercavity, which can be unstable and cause unwanted motion or even loss of control of an object. To address this issue, flow control techniques are often employed. These techniques aim to manipulate the flow of liquid around the object and improve stability and control. Some of these methods include using control surfaces, injecting fluids, or more advanced techniques such as active flow control or smart materials. The aim of this study is to use computational methods to investigate the behaviour of supercavitation flow over a Clark-Y hydrofoil with different contact angle patterns (hybrid wettability) inspired by the Namib desert beetle. The hybrid wettability consists of six distinct patterns with a 160-degree contact angle for superhydrophobic surfaces and a five-degree contact angle for superhydrophilic surfaces. To predict the dynamic and unsteady behaviours of the supercavitation flow with a cavity number of 0.4, the implicit large eddy simulation (ILES) technique and Kunz mass transfer model are used. The compressive volume of fluid technique is used in conjunction with dynamic adaptive mesh refinement to track the interface between the vapour and liquid phases. The simulations are conducted using the interPhaseChangeFoam two-phase flow solver within the OpenFOAM framework. This study analyses the time-averaged and instantaneous fluid dynamic properties of the supercavitation flow of the hydrofoil with a hybrid surface, including pressure, velocity, vorticity fields, wake flow, recirculation zone, and liquid volume fraction. In addition, it examines the instantaneous cavity leading edge and flow separation, the vortical flow structure, vorticity stretching and dilatation, spanwise flow details, the creation of a low-pressure zone behind the hybrid hydrofoil, streamwise velocity fluctuations, and the cavity dynamics over an entire cycle. The results show that the contact angle has a significant effect on the structure of the supercavitation flow. Specifically, the use of a superhydrophobic surface on the pressure side and trailing edge of a hydrofoil, along with a superhydrophilic surface on the leading edge, can reduce the thickness of the wake zone, the length of the cloud cavity, and flow instability. Additionally, this approach can delay the onset of cavitation.

Numerical investigation on the impact pressure induced by a cavitation bubble collapsing near a solid wall

Abstract: Cavitation erosion often occurs on the surface of many underwater applications, which can cause severe damage to materials and reduce their performance. Since the cause of erosion is the impact pressure induced by the collapse of an individual cavitation bubble near the wall, to make a better prediction and prevent the damage potential, in this paper, we carry out systematic investigations on the impact characteristics by direct numerical simulation using a vapor bubble model. The volume of fluid (VOF) method is adopted to capture the interface between the two phases. The numerical results show that pressure wave and jet are two primary inducements of the impacts on the wall. The reason for the pressure wave impacts is the pressure wave emission after the collapse of the bubble's main part. And the reason for the jet impact is the stagnation pressure in front of the jet. After a parametric study of the two impacts with respect to the initial radius, driving pressure, and stand-off distance, the predicting equations for the pressure wave impact and jet impact are proposed at γ ≥ 1.74. When γ < 1.74, the impact pattern becomes complex due to the arrival time of the two impacts and the collapse of the vapor fragments right on the wall.

Thermocapillary Bubble Oscillations and Migration in a Vibrating Cylinder in a Zero-Gravity Environment

Abstract: Bubble migration in a vibrating zero gravity environment is numerically investigated using ANSYS-FLUENT software. A 3D CFD model is developed describing the two-phase flow of a nitrogen bubble immersed in a container full of ethanol. The Volume of Fluid (VOF) method and the geometric reconstruction scheme are used to track the gas–liquid interface. The liquid in the container is vibrated horizontally in the x momentum with different frequencies from 0 to 1 Hz, and amplitudes from 0.005 to 0.1 m/s2. The vibration impact on the bubble arrival time to the top and its ensuing dynamic is analyzed. Different bubble trajectory shapes are observed, other than the conventional vertical translation induced by the temperature difference. Compared to the no vibration case, the bubble motion is slightly either accelerated or decelerated for very low vibration amplitudes. For a fixed frequency f = 1 Hz, the bubble takes more time to reach the top with the vibration amplitude increment relatively to the no vibration case. The vibration effect becomes more intense with the Marangoni number decrease when f = 0.2 Hz and Ab = 0.005 m/s2. Those results are difficult to obtain experimentally, signifying the importance of this numerical study to understand bubble motion and migration in space.

Numerical simulation of hydrodynamics in wet cooling tower packings

Abstract: Wet cooling towers are used in many industrial processes but hydrodynamic behavior of air-water counter flows in towers packing remains unknown. Hydrodynamics is directly linked with main operating issues (e.g. packing fouling reducing cooling efficiency). The objective of this work is to use Computational Fluid Dynamics (CFD) simulations to characterize local hydrodynamic parameters such as water film thickness, velocity or wall shear stress and system scale parameters such as wetting rate or interfacial area. After a theoretical study of fluid mechanics, Volume of Fluid (VOF) simulations, with dedicated models for air/water interface capture and solid/liquid contact angle, successfully reproduced water falling films on packing surface. Simulations allowed the calculation of main water flow local and system parameters. The results have pointed out trends of increasing liquid film thicknesses and wetting rates with increasing water flowrate in agreement with experimental observations. A correlation of the interfacial area has been proposed as a function of the hydrodynamic adimensional numbers to characterize transfer phenomena in the considered packing.

Two-phase MPM modelling of debris flow impact against dual rigid barriers

Abstract: Multiple barriers are usually installed to effectively mitigate large-volume debris flows. Existing multiple-barrier design recommendations ignore the effects of solid-fluid interaction and may cause uncertainties in estimating the flow impact force on subsequent barriers. In this study, a fully coupled, two-layer, two-phase material point method (MPM) with an incompressible fluid phase is implemented and validated using the experimental results of dry sand, water, and sand-water mixture flows impacting on rigid barriers. Numerical parametric study is carried out using the validated model to investigate the effects of Froude number (Fr) and barrier spacing on second barrier impact force. Debris flow volume of 500 m3 is modelled with the Fr ranging from 2 to 6, which are relevant to gentle and steep terrains in Hong Kong. Simulation results show the importance of changes in fluidisation ratio, which is the ratio of basal pore fluid pressure and total stress. The debris flow after impacting on the first barrier, overflows, and lands on the slope between the two barriers. The fluidisation ratio of debris flow after landing increases by up to 30%, leading to an increase of impact velocity at the second barrier by up to 80%. Consequently, the impact force of debris flows on second barrier is underestimated by up to 50% compared with existing design guidelines. This implies neglecting fluidisation ratio may lead to non-conservative design of multiple barriers.

Numerical Modeling of the Cavitation Flow in Throttle Geometry

Abstract: The modern fuel injectors work with ultra-high injection pressure with a micro-size nozzle, which inevitably triggers the cavitation flow inside the nozzle. The formation of vapor bubbles and their development inside the nozzle is difficult to characterize due to its highly fluctuating spatial and temporal parameters. The numerical models can predict the temporal behavior of cavitating flow with the real-size nozzle geometry, which is fairly expensive with the experiments. A systematic study has been carried out using throttle geometry to characterize the cavitation flow. The different turbulence, multiphase, and cavitation models are extensively evaluated and validated with experimental data. A combination of numerical models has been proposed to predict the cavitation flow more accurately with low computational time. The results obtained with the k-ω SST (Shear Stress Transport) turbulence model and the ZGB (Zwart-Gerber-Belamri) cavitation model are more consistent with the experimental results. The overall structure of cavitation is well captured with both the VOF (Volume of Fluid) and the Mixture multiphase models. Although, the smaller structures like bubble formation and ligament breakup are only captured with the VOF (Volume of Fluid) tuned with the sharp interface method. The effect of pressure difference on the cavitation flow has been estimated with diesel and bio-diesel fuel. The effect of nozzle conicity on cavitation phenomena has also been reported.

Digital Twin of Degradable Fluid Loss Additive Supported by Advanced Slurry Flow Model

Abstract: This study is a part of larger scheme of reservoir strategy for use of degradable fluid loss additives. Components of this framework have been presented in two previous papers and this paper specifically describes the novel modeling approach. The hydraulic fracturing model accounts for the effect of fluid loss additive on fluid leakoff into the rock during the treatment. It is implemented with slurry flow model based on the lubrication theory. An empirical closure relation for Carter leakoff coefficient represents the effect of leaked fluid volume and the volume of fluid loss additive deposited in the filter cake. The numerical algorithm is based on the combined Eulerian-Lagrangian approach. A simulation example showing the effect of fluid loss additives on pad volume reduction and potential production performance enhancement is presented. The presented approach is the first slurry flow model which allows to accurately simulate the benefits of fluid loss additive and aims to create a digital twin of these additives eliminating the need to conduct laboratory experiments and design scenarios, revamping the fracturing design strategy.

Research on ship motion characteristics in a cross sea based on computational fluid dynamics and potential flow theory

Abstract: The motion response of a ship in a cross sea is studied based on computational fluid dynamics. Firstly, according to the established numerical pool, and based on the Reynolds-averaged Navier–Stokes equations and the re-normalisation group K–ϵ turbulence model, the free surface is treated by the volume of fluid(VOF) method, and a numerical simulation method is established. The wave results obtained by the numerical simulation are compared with theoretical waves to verify the reliability of the method. Then, considering the DTMB5415 ship model, a prediction method for the ship’s motion in a cross sea is established by using an overlapping mesh and VOF technology. The influence of a cross sea on ship motion performance under different wave direction angles, different wave heights and for different periods is analyzed. In addition, a method is also established for studying the ship’s motion response based on potential flow theory, and it is found that ships sailing under superimposed waves will exhibit the phenomenon of beat vibration. Finally, by comparing the results obtained by the two methods, the consistency of the two methods is verified, which provides a strong basis for the safety assessment of ships in cross seas.