Fluent-Adv-multiphase 13.0 L02 Vof

Customer Training Material

Lecture 2Lecture 2

Volume of Fluid Model in Fluent

Multiphase FlowMultiphase Flow Modeling using ANSYS FLUENT

L2-1ANSYS, Inc. Proprietary 2011 ANSYS, Inc. All rights reserved.

Release 13.0June 2011

FLUENT


Customer Training MaterialWelcome! Welcome to the ANSYS Advanced Multiphase training course! This training course covers the details of modeling multiphase using the

Volume of fluid (VOF) model of ANSYS Fluent. It is intended for all advanced ANSYS Fluent users who wish to use VOF

multiphase model Course Objectives:

General understanding of the applicability and the equations of the VOF model General understanding of the applicability and the equations of the VOF model Understanding the sub-models available with VOF model including surface

tension, coupled level set and turbulence dampening Suggestions on the boundary conditions Suggestions on solver options available for modeling implicit and explicit

and their scope Suggestions on the discretization schemes available for tracking interface and

their scopetheir scope Examples from industrial applications showcasing the capability of the model




Customer Training MaterialApplicability of the Volume of Fluid Model VOF model is used to model immiscible fluids

with clearly defined interface. Two gases cannot be modeled since they mix at the Interface

length larger

VOF Applicable

molecular level. Liquid/liquid interfaces can be modeled as long as

the two liquids are immiscible.VOF i t i t if i t f l th i ll

length larger than grid

VOF is not appropriate if interface length is small compared to a computational grid

Accuracy of VOF decreases with interface length scale getting closer to the computational grid scale

Interface length scale larger than

computational idscale getting closer to the computational grid scale

Typical problems: Jet breakup

M ti f l b bbl i li idVOF Not Applicable

grid

Motion of large bubbles in a liquid Motion of liquid after a dam break Steady or transient tracking of any liquid-gas

interface

Interface length scale is smaller than

grid



interface grid


Customer Training MaterialVolume of Fluid Model Inputs Arbitrary number of phases are

allowed Phases are defined through phases

panel




Customer Training MaterialSolver compatibility for VOF model Solver

Only pressure-based solver is available.

VOF model can be run in both Steady and Unsteady mode (unsteady mode is most often used)most often used).

Gravity should be enabled for most VOF cases.

Non-Iterative TimeNon Iterative Time Advancement (NITA) may be used with VOF model for unsteady mode.




Customer Training MaterialVolume of Fluid Model Schemes VOF Scheme controls how phase

continuity (volume fraction through which interface is tracked) equation is solved.

Explicit VOF Default and used only with unsteady simulation which is also default

Begin time step

Solve VOF

Solve Momentumand Pressure

Solve VOF

Iterations



Explicit VOFIterations


Customer Training MaterialNumerical schemes for VOF model Explicit Scheme

Explicit scheme solves the volume fraction in sub time steps.

Number of sub time steps is dictated by the value of the Courant number.The default value 0 25 is robust The default value 0.25 is robust and should not be changed.

1+nTT

Interface location update

nT

nT



Interface location update during one time step


Customer Training MaterialNumerical schemes for VOF model Implicit Scheme

Implicit scheme solves phase continuity equation (volume fraction) it ti l t th ith titeratively together with momentum and pressure.

Available with both steady and unsteady simulationunsteady simulation




Customer Training MaterialExplicit Vs Implicit schemesTradeoff between explicit and implicit VOF schemes Explicit Scheme

Advantages Allows use of Geo-Reconstruct discretization scheme

Georeconstruct

for VOF. This scheme renders a clear, crisp interface without numerical diffusion.

Should be used in simulation of flows where surface tension is important because of highly accurate curvature calculation.

Disadvantages Poor convergence for skewed meshes. Poor convergence if phases are compressible.

Implicit Scheme Advantages

Does not have Courant number limitation can be run with large time steps or in steady state mode.

Can be used with poor mesh quality and for complex flows (e.g. compressible).

Disadvantages Numerical diffusion of interface does not allow

accurate prediction of interface curvature so



accurate prediction of interface curvature so accurate prediction of flows where surface tension is important is not feasible.

Modified HRIC


Customer Training MaterialVolume of Fluid Model Inputs Implicit body force

Designed for flows with large body forces.

Gravity acting on phases with large density difference.

Flows with large rotational accelerations (such as centrifugal ( gseparators and/or rotating machinery).

The force is handled in robust numerical manner.




Customer Training MaterialOptions for modeling open channel flows Open Channel Flow option

Applicable to flows where both inertia and gravity are dominant with known depths of the liquid at the inlets or outletsp q

Example Ship moving through the sea at depth yin and speed Vin

Prescribe yin and Vin at inlet and yout at the outlet. Charectarised by Froude Number , Fr =

V/ sqrt(g y)V/ sqrt(g y) Upsteam boundary conditions

Pressure inlet Mass flow inlet

Downstream boundary conditions Pressure outlet Outflow

y

i

n

y

o

u

t

inVr



in


Customer Training MaterialWave bc for open channel flow Open Channel Wave BC Option

First order or linear wave theory is applicable for shallow to deep liquid depth ranges

Higher order or non-linear wave theories are applicable for intermediate to deep liquid

Wave Input Analysis Through TUI

applicable for intermediate to deep liquid depth range.

Choice of Wave theory (within wave breaking limit) :

Wave theories should be chosen in accordance with wave steepness (wave h i ht/ l th)height/ wave length)




Customer Training MaterialControlling interface sharpness Zonal Discretization Option This option provides diffusive

or sharp interface modeling inor sharp interface modeling in different fluid (cell)zones based on the value of zone dependent slope limiter.dependent slope limiter.

Slope Limiter (Beta) Scheme

Beta = 0 First Order Upwind

Beta = 1 Second order upwind

Beta = 2 Compressive



(Zone 1) (Zone 2) (Zone 3) 0 < Beta < 1 , 1 < Beta < 2

Blended scheme


Customer Training MaterialBounded Gradient Maximization (BGM)

BGM scheme is introduced to obtain sharp interfaces with the VOF model, comparable to that obtained by the Geometric Reconstruction scheme.

Currently this scheme is available only with the steady state solver.

Steady state schemes comparison

Speed HRIC ~ Compressive > BGM

Sharpness BGM > Compressive > HRIC

Stability HRIC > Compressive > BGM

Best Practices : Lower under-relaxation for vof might be needed for gbetter stability. Switching to BGM at later stage for sharp interface.




Customer Training MaterialTurbulence Damping

Turbulence damping at the interface The mesh at the free surface is not

fi h t t th l itfine enough to capture the velocity gradients correctly. The turbulent quantities are over predicted due to this higher velocity gradients.this higher velocity gradients.

Turbulence damping helps in getting correct profiles with coarse meshes

This treatment is available only for k-yomega turbulence model

Source term is added for the omega equation in the interfacial cells which qenforces the high value of omega and thus produces turbulence damping.




Customer Training MaterialVolume of Fluid Model Inputs

Definition of phases Any phase can be primary or secondary not important in VOF

d lmodel. Usual practice is to have secondary phase which has less

presence in the domain Three ways phases Three ways phases may interact in VOF

Mass exchange Heterogeneous g

reactions Surface tension with

optional wall adhesioneffecteffect




Customer Training MaterialModeling using surface tension Surface Tension

Surface tension coefficient can be constant or function of variables (often temperature)

Pressure difference is equal to surface tension coefficient times sum of inverse curvature radii

significance of surface tension

+=

2112

11RR

PPFor 3D:depends on the Reynolds number:

For Re > 1, evaluate the Weber number:

Surface tension is important

21= UCa

2We UL= Surface tension is important

when We >>1 or Ca


Customer Training MaterialWorking with wall adhesion Wall adhesion

Wall adhesion force is a measure of the cohesive forces acting between the fluid and walls.Adh i i i t t h d li Adhesion is important when modeling meniscus shapes and/or wettability.

Jump Adhesion contact angle specification at porous

jump boundaryjump boundary

Gas



Liquido7=w


Customer Training MaterialWorking with surface tension In standard VOF methods, the curvature calculations are based on volume

fraction fields Interface sharpening schemes provide discontinuous or sharply changing

l f ti l t i t fvolume fractions close to interface Calculation of curvature based on volume fractions can be inaccurate and

cause convergence issues in problems dominated by surface tension Two options can be used to address them Two options can be used to address them

Use node based smoothening of VOF field for curvature calculations (Default option )

Use Coupled Level set + VOF method Node based smoothing done using TUI

Number of smoothings can be increased to 2 or 3

For higher smoothings the relaxation factor can be set to



0.9Better curvature calculations


Customer Training MaterialCoupled Level set + VOF method Coupled Level set + VOF method The method uses a level set function defined as

distance from the interface The function is smoothly varying compared to

volume fraction fields The level set function is used for calculating

curvaturecurvature Better calculations obtained for curvature in

surface tension dominated flows Advantages Advantages Works better for surface tension dominated flows No requirement for VOF smoothing

Di d t Disadvantages Recommended only for geo-reconstruct scheme Requires finer meshes compared to just VOF runs




Customer Training MaterialModeling Heterogeneous reactions Heterogeneous reactions Example of heterogeneous reaction simulating evaporation of water into

vapor-air atmosphere Gas phase is mixture of vapor and air species Liquid phase is mixture of water and species

Gas(air + vapor species)

Liquid(water species)

Evaporation rate at free surface



at free surface



Pressure velocity coupling

Solver Settings Explicit VOF Formulation Pressure-velocity coupling

PISO recommended for incompressible flow

SIMPLE recommended for compressible flows or flows in closed domainsdomains.

Under-relaxation factors Pressure discretization

Pressure Body Force Weighted for high body force (rotation). Otherwise, use PRESTO!

Volume frac discretization Geo-Reconstruct (default) no

numerical diffusion, high accuracy g ycurvature, needs high quality grid.

Compressive for large jobs Compressive for medium

quality grids




Customer Training MaterialSolver Settings Explicit VOF Formulation Variable time stepping Scheme for explicit VOF Automatically adjusts the

time step based on Global Courant number

Controls can be provided ffor

Max and min time steps used

Change factor for time gsteps

It is useful for explicit problems as the time step determines the stability for speed of the solution




Customer Training MaterialSolver Settings Implicit VOF Formulation

When Implicit is chosen as the VOF formulation, an under-relaxation factor for the volume fraction equation is addedequation is added

Compressive is the recommended discretization




Customer Training MaterialBounded Second Order Time Formulation Second order accuracy with better stability

compared to existing second order time formulation.

Larger time step size compared to first order Larger time step size compared to first order and second order implicit (Adams Bashforth) scheme.

It is available with all models (single phase/multiphase) using pressure based solverphase/multiphase) using pressure based solver except for MDM.

Bounded by the lower and upper bounds for any variables based on availability of bounds. (For eg volume fraction species mass(For eg. , volume fraction, species mass fraction, turbulent K.E, turbulent dissipation)

Schemes comparisonAccuracy Bounded Second order ~ Second order > First orderAccuracy Bounded Second order Second order > First order

Speed First order > Second order > Bounded second order

Stability First order > Bounded second order > second order




Customer Training MaterialComparison of schemes Comparison of schemes : Pure advection of shapes

Need to point out what to compare, 1st order versus 2nd order implicit? Then both need to be implicit?

Compressive CICSAM Modified HRIC Compressive Modified HRIC



VOF Explicit, First order time VOF Implicit, Second order time

p



Operating conditions Specify the Reference

P L tiPressure Location Specified Operating Density

Density of the lightest phasey g pIf a phase is compressible,

provide Zero for operating densitydensity

3kg/m2251= 3kg/m2998=ref kg/m225.1= ref kg/m2.998= Choice of large density as reference will result in incorrect display of pressure distribution



Pressure distribution in the dam break examplepressure distribution



How to prescribe boundary conditions Velocity inlet Only one phase can enter; therefore, only one phase can

have inlet volume fraction set to 1 and nonzero inlet velocity. All inputs for other phases are irrelevant.

Mass flow inlet Same as above. Only one phase can have nonzero mass flow rate Inputs for all phases are irrelevantmass flow rate. Inputs for all phases are irrelevant.

Pressure inlet Same as above. Only one phase can have inlet volume fraction of 1. Inputs for all other phases are irrelevant.

In order to facilitate convergence for pressure inlet, it is advisable to patchIn order to facilitate convergence for pressure inlet, it is advisable to patch VOF of incoming to 1 for the cell layer adjacent to the pressure inlet.




Customer Training MaterialVolume of Fluid Model Inputs How to prescribe boundary conditions

Pressure outlet Be careful which VOF you specify as backflow VOF! Consider ink jet injected into atmosphere

Liquid ink(secondary phase)

Air(primary phase)

Pressure outlet

Ink velocity inlet

Pressure outlet(atmosphere)

Backflow VOF of ink phase at atmosphere is zero




Customer Training MaterialSolution Strategies Time Dependence How to choose transient time step for VOF calculations

Time step in Solve panel can be estimated as

V 3/1

where Vcell,min can be obtained from the Grid Check panel and U is the velocity scale of the problem (e g inlet velocity)

UV

t mincell,=

velocity scale of the problem (e.g. inlet velocity). If divergence occurs at first time step, decrease time step by factor of 10

and see if it converges. Ideally, you should converge each time step in around 10-15 iterations.around 10 15 iterations.

Start with smaller time step size for a few time steps, and then increase the time step size.

If divergence persists, try switching off Skewness-Neighbor Coupling in g p y g g p gthe Solution Controls panel.

Non-iterative time advancement (NITA) reduces computational effort per time step in comparison with iterative schemes (SIMPLE, PISO)



Use variable time stepping for explicit schemes to optimize time steps


Customer Training MaterialSolution Strategies Common Mistakes Outflow boundary condition is not recommended for multiphase

flows. Outflow can be used only when you select open channel flow bc.

At any inlet or outlet, only one phase must enter or exit. This means that on any flow boundary, the volume fraction must be either 0 or 1. No intermediate values should be used.

Operating density should be of the lightest phase.p g y g p

Back flow vof at the outlet boundary.

Interface update scheme is most sensitive to grid quality. It is advisable to put highest quality mesh in areas where the interface is expected



expected.


Customer Training MaterialSolution Strategies Common Mistakes Smooth, good quality mesh is essential to good convergence of VOF

model.

Cells which have large skewness, high aspect ratio, or large size variation are detrimental to convergence rate. Good Bad



Good Bad


Customer Training MaterialComparison of interface construction schemes

Interfacescheme

Implicit Explicit Accuracy Speedscheme

First order R K Not recommended Not recommendedSecond order R K Not recommended Not recommended

QUICK R R Low HighQUICK R R Low HighModified HRIC R R Medium High

CICSAM K R High MediumCompressive R R High Medium to High

Georeconstruct K R Very high Low to mediumBGM R K Very high Low to mediumBGM R K y g



VOF M d l E lVOF Model Examples

Tank FillingSlug Flow in a PipeSlug Flow in a Pipe3D Falling BoxSpinning Gear

2-34ANSYS, Inc. Proprietary 2009 ANSYS, Inc. All rights

d

April 30, 2009Inventory #002683

p g



Simple filling of a vessel




Customer Training MaterialVOF Example Automobile Fuel Tank Sloshing

Sloshing of liquid in an automotive fuel tank under various acceleratingvarious accelerating conditions is simulated by the VOF model in FLUENT.

Simulation shows the tank Simulation shows the tank with internal baffles will keep the fuel intake orifice fully submerged at all times, while the intake orifice is out of the Fuel Tank Without Bafflest = 1.05 sec

t = 2.05 secthe intake orifice is out of the fuel at certain times for the tank without internal baffles.



Fuel Tank With Baffles


Customer Training MaterialVOF Example Slugging in a horizontal pipe Turbulence Damping treatment was used at the free surface




Customer Training MaterialVOF Example Wave Interaction with a Floating Structure MDM (Moving Deforming Mesh), 6DOF (6 Degrees of Freedom)and

Open channel Wave BC along with VOF model was used




Customer Training MaterialVOF Example 3D Falling Box MDM and 6DOF with VOF model was used to simulate a box falling

into liquid.




Customer Training MaterialFree Surface Flow around a Spinning Gear Sliding mesh model with VOF




Customer Training MaterialDroplet formation in cross-flow emulsification process

Comparison of coupled level set with VOF

Geometry Channel ( t )(water)

x 130 m y 45 m z 90 mz 90 m Geometry Pore (oil) Diameter 10 m Height 10 m Max droplet size: 40 m Downstream length 100

m, 2.5 times covers Height 90 m 2 25 times Height 90 m, 2.25 times

covers




Customer Training MaterialComparison of interface: BGM and mod HRIC

HRIC HRIC



VOF contours (velocity along 140 )

BGM BGM

VOF contours (Zoomed near the interface)Flow over a bump


Customer Training MaterialSummary VOF is an Eulerian fixed-grid technique.

VOF is numerically robust and accurate.VOF is numerically robust and accurate.

Available in conjunction with most other ANSYS FLUENT models.Not available with the following reacting flow models: Not available with the following reacting flow models:

Eddy dissipation concept Premixed, non-premixed, partially premixed Composition PDFp NOx and soot



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