Showing papers on "Volume of fluid method published in 2007"

Conservative Wetting and Drying Methodology for Quadrilateral Grid Finite-Volume Models

Abstract: Algebraic equations relating fluid volume and the free surface elevation in partially wetted quadrilateral computational cells are derived and incorporated into a Godunov-type, finite-volume, shallow-water model. These equations make it straightforward to reconstruct the free surface elevation based on the volume of fluid in a computational cell, the dependent variable tracked by finite volume models for conservation purposes, regardless of whether the cell is fully or partially wetted. Improvements to the variable reconstruction process streamline the computation of mass and momentum fluxes with approximate Riemann solvers, yielding a model that simulates sub-, super-, and transcritical flows over irregular topography with wetting and drying fronts. Furthermore, the model is free from fluid and scalar mass conservation errors and it eliminates nonphysical distributions of scalars by avoiding artificial concentration and/or dilution at wet/dry interfaces. Use of this wetting and drying methodology adds roughly 10% to the execution time of flow simulations.

Stabilized edge‐based finite element simulation of free‐surface flows

Abstract: Free-surface flows occur in several problems in hydrodynamics, such as fuel or water sloshing in tanks, waves breaking in ships, offshore platforms, harbours and coastal areas. The computation of such highly nonlinear flows is challenging since free-surfaces commonly present merging, fragmentation and breaking parts, leading to the use of interface-capturing Eulerian approaches. In such methods the surface between two fluids is captured by the use of a marking function which is transported in a flow field. In this work we present a three-dimensional parallel edge-based incompressible SUPG/PSPG finite element method to cope with free-surface problems with volume-of-fluid (VOF) extensions to track the evolving free surface. The pure advection equation for the scalar marking function was solved by a fully implicit parallel edge-based SUPG finite element formulation. We studied variants of this formulation, considering the effects of discontinuity capturing and a particular tangent transformation designed to increase interface sharpness. Global mass conservation is enforced adding or removing mass proportionally to the absolute value of the normal velocity of the interface. We introduce a parallel dynamic deactivation algorithm to solve the marking function equation only in a small region around the interface. The implementation is targeted to distributed memory systems with cache-based processors. The performance and accuracy of the proposed solution method were tested with several validation problems. Copyright © 2007 John Wiley & Sons, Ltd.

Formation of bubbles in a simple co-flowing micro-channel

Abstract: Bubble generation in a simple co-flowing micro-channel with a cross-sectional area of 169 × 007 mm2 was experimentally and numerically investigated Air and water were used as the gas and liquid, respectively Mixtures of water–glycerol and water–Tween 20 were also used to obtain the effects of viscosity and surface tension The experimental data show that the break-up process is periodic under certain operating conditions The break-up dynamics are also examined using three-dimensional incompressible two-phase flow numerical simulation based on the volume of fluid (VOF) method The simulation successfully predicts the flow behavior and provides a more detailed examination of the bubble shape The physics can be further explained by the detailed micro-PIV measurements, which show that the bubble is formed due to the velocity component perpendicular to the gas flow created by the sudden change of the liquid velocity distribution around the barrier The bubble length L is dependent on the liquid flow rate Ql and the gas flow rate Qg, and the ratio of L to the channel width w is a function of the ratio of gas and liquid flow rates Qg/Ql which is similar to that previously used in the T-junction case The formulation of bubble frequency f is derived under current conditions and it shows a good agreement with the experimental data at the low frequency region Different bubble shapes can be obtained at different liquid viscosities and surface tensions The ratio L/w can still be predicted by a modified equation which uses the real bubble width wb or an equivalent bubble length Le

Simulation of flows with violent free surface motion and moving objects using unstructured grids

Abstract: A volume of fluid (VOF) technique has been developed and coupled with an incompressible Euler/Navier-Stokes solver operating on adaptive, unstructured grids to simulate the interactions of extreme waves and three-dimensional structures. The present implementation follows the classic VOF implementation for the liquid-gas system, considering only the liquid phase. Extrapolation algorithms are used to obtain velocities and pressure in the gas region near the free surface. The VOF technique is validated against the classic dam-break problem, as well as series of 2D sloshing experiments and results from SPH calculations. These and a series of other examples demonstrate that the ability of the present approach to simulate violent free surface flows with strong nonlinear behaviour.

Validation of the volume of fluid method for free surface calculation: the broad-crested weir

Abstract: This paper describes the validation of Computational Fluid Dynamics (CFD) for modelling free surface flows over common hydraulic structures. A series of CFD simulations are compared against an existing set of experimental data for the free surface flow over a broad-crested weir. By fixing the upstream and downstream water depths in the CFD model, it was possible to reproduce the experimental free surface profiles, pressure and velocity profiles and discharges over the weir for a range of discharge rates. The sensitivity of the results to the choice of turbulence model is presented. Gaining confidence in a modeling technique in this way, allows for the future modeling of more complex hydraulic structures and the introduction of the more complex physics required for modeling processes such as scour.

Numerical Simulation of Droplet Impact on Patterned Surfaces

Abstract: This work presents numerical simulation results for molten nickel and zirconia (YZS) droplets impacting on different microscale-patterned surfaces of silicon. The numerical simulation clearly showed the effect of surface roughness and solidification on the shape of the final splat, as well as the pore creation beneath the sprayed material. Simulations were performed using computational fluid dynamic software, SimDrop. The code uses a three-dimensional finite-difference algorithm solving the full Navier-Stokes equation, including heat transfer and phase change. A volume of fluid (VOF) tracking algorithm is used to track the droplet-free surface. Thermal contact resistance at the droplet-substrate interface is also included in the model. Specific attention is paid to the simulation of droplet impact under plasma spraying conditions. Droplet sizes ranged from 15 to 60 microns with initial velocities of 70-250 m/s. Substrate surfaces were patterned with regular arrays of cubes 1-3 μm high, spaced either 1 μm or 5 μm from each other. Different splat morphologies produced by simulations are compared with those obtained from the experiment conducted under the same impact and surface conditions.

Three-dimensional numerical simulation of metal flow and solidification in the multi-cavity casting moulds of automotive components

Abstract: The liquid metal flow and the solidification behaviours in a multi-cavity casting mould of two automotive cast parts were simulated in three dimensions. The commercial code, FLOW-3D® was used because it can track the front of the molten metal by a Volume of Fluid (VOF) method and allows complicated parts to be modeled by the Fractional Area/Volume Obstacle Representation (FAVOR) method. The grey iron automotive components including a brake disc and a flywheel were cast using an automatic sand casting production line. For simulation analysis, the solid models of the casting, the gating system and the ceramic filter were spatially discretised in a multi-block pattern. The surface roughness and the contact angle of the mould were taken into account in the model, based on the properties of the sand mould used. The turbulent flow was simulated using the k-e turbulence model. The Darcy's law was used to analyse the fluid flow throughout the ceramic filter designed in the gating system. Proper boundary conditions were assigned for the model so that both the simulated filling time and the solidification time were achieved in the range of real experimental measurements. The predicted hot spot of the castings were in agreement with experiments. The verified simulation model showed that the four-cavity mould used for the flywheel part is more suitable than the three-cavity one of the brake disc, in getting a more uniform fluid flow and heat transfer conditions which causes similar cast parts in each mould. The simulated flow pattern during the mould filling of the castings showed that the first gate of the gating system was not working properly as it remains partially-filled (not pressurised) throughout the half of the filling stage, causing a possible air absorption by the melt. A smaller cross sectional area for the first gate was suggested. The present simulation model is able to analyse different casting parameters of the automatic multi-cavity sand casting process.

Ventilation system for seat

Abstract: A climate controlled seat assembly comprises a chamber defined by a substantially fluid impermeable layer, the fluid impermeable layer having a first side and second side, the first side comprising a plurality of openings, a support structure positioned within the chamber, the support structure being configured to substantially maintain the shape of the chamber, a fluid transfer device configured to provide a volume of fluid to the chamber, a fluid inlet in fluid communication with both the chamber and the fluid transfer device, a fluid distribution layer positioned adjacent to the first side of the fluid impermeable layer and a seat covering positioned along the fluid distribution layer. In some embodiments, the fluid distribution layer is configured to generally distribute fluid from the openings toward the seat covering.

A Numerical Study of Combined Heat and Mass Transfer in an Inclined Channel Using the VOF Multiphase Model

Abstract: A numerical study has been performed to determine heat and mass transfer from the surface of liquid ethanol flowing in an inclined channel. The Volume-of-fluid (VOF) multiphase model was used to track the liquid and gas phases. An algorithm has been implemented to determine interfacial heat and mass transfer characteristics. A parametric study was done to determine the effect of gas-phase inlet velocity, temperature, and vapor mass fraction.

Computational Methods for Multiphase Flow: Immersed boundary methods for fluid interfaces

Abstract: Nearly half a century of computational fluid dynamics has shown that it is very hard to beat uniform structured grids in terms of ease of implementation and computational efficiency. It is therefore not surprising that a large fraction of the most popular methods for finite Reynolds number multiphase flows today are methods where the governing equations are solved on such grids. The possibility of writing one set of governing equations for the whole flow field, frequently referred to as the “one-fluid” formulation, has been known since the beginning of large-scale computational studies of multiphase flows. It was, in particular, used by researchers at the Los Alamos National Laboratory in the early 1960s for the marker-and-cell (MAC) method, which permitted the first successful simulation of the finite Reynolds number motion of free surfaces and fluid interfaces. This approach was based on using marker particles distributed uniformly in each fluid to identify the different fluids. The material properties were reconstructed from the marker particles and sometimes separate surface markers were also introduced to facilitate the computation of the surface tension. While the historical importance of the MAC method for multiphase flow simulations cannot be overstated, it is now obsolete. In current usage, the term “MAC method” usually refers to a projection method using a staggered grid. When the governing equations are solved on a fixed grid, the different fluids must be identified by a marker function that is advected by the flow. Several methods have been developed for that purpose. The volume-of-fluid (VOF) method is the oldest and, after many improvements and innovations, continues to be widely used. Other marker function methods include the level-set method, the phase-field method, and the constrained interpolated propagation (CIP) method.

Adaptive vof with curvature-based refinement

Abstract: Adaptive refinement is implemented in the context of the volume-of-fluid (VOF) methodology in order to study the efficacy of resolving interfaces adaptively based on the local value of curvature. The usual uniform mesh VOF implementation is modified slightly to ensure accurate advection of fluxes between cells at different resolutions. Normals and curvatures are calculated accurately via height functions. Results of a series of tests indicate that in most instances the use of adaptive refinement (when compared to uniform refinement with a similar number of cells) leads to more accurate VOF advection. The results also clearly show that curvature-based adaptive refinement leads to a distribution of errors along an interface that is nearly independent of curvature. Copyright © 2007 John Wiley & Sons, Ltd.