Korea 2014ugm Multiphase Flow Modeling

1 © 2014 ANSYS, Inc. May 13, 2014 ANSYS Confidential

Multiphase Flow Modeling with Free Surfaces Flow

Jinwon Seo TAESUNG Software and Engineering, INC


Outline

• Overview of Multiphase Flow

• Multiphase Models in ANSYS CFD

• Separated / Free Surface Flows

• Volume of Fluid (VOF) Model

• Key Concepts

• VOF Model Inputs & Requirements

• Best Practices

• VOF Model Examples


Multiphase flow involves the simultaneous flow of two or more immiscible interacting phases.

Introduction of Multiphase Flow


Features of Multiphase Flows

Multiple Length Scales

Several Flow Regimes

Multiple Physics


Flow Always Accompanied by Other Physics!

Fluid Dynamics

Heat transfer

Heterogeneous and

homogeneous reactions

Phase change

Size change


Multiphase Models in ANSYS CFD

Separated flows

VOF model

Dispersed flows

Eulerian Models

Lagrangian models


A Solution for Every Multiphase Problem


Separated / Free Surface Flows

Fluids are separated by a distinct resolvable interface

• Separated Flows • Both phases are continuous and both are of interest • Interface length scale is large • Stratified flows

• Free Surface Flows • Only liquid phase is of interest • Open channel flows


Applications • Inkjets

• Coating

• Tank Filling and Sloshing

• Jet breakup

• Open channel flows

• Offshore transport

• Gear lubrication

• Piston cooling

• Ship Hull

• Wave Loading

Courtesy Speedo


Volume of Fluid (VOF) Method

• Method to track/capture the sharp interfaces

between immiscible fluids

• Shape of the interface is of interest

Fluid-1

Fluid-2

Volume Fraction : Scalar indicator function between 0 and 1, for each fluid represented as

V

Vf

f

f = 1 : Fluid-1 f = 0 : Fluid-2 0 < f < 1 : Interface


Applicability of VOF Model

• VOF model is used to model immiscible fluids with clearly defined interface

• Two gases cannot be modeled since they mix at the molecular level

• Liquid/liquid interfaces can be modeled as long as the two liquids are immiscible

• 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 larger

than grid

VOF applicable VOF not applicable

Interface length scale

is smaller than grid


VOF Scheme Comparisons

Advantages Disadvantages

Explicit VOF Sharper interface Accurate solution

Poor convergence for skewed meshes

Poor convergence if phases are compressible

Implicit VOF 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 flows)

Numerical diffusion of interface does not allow accurate prediction of interface curvature

Implicit Compressive scheme along with Bounded Second Order time discretization scheme give sharp interface and accurate solution

(with uniform mesh size or gradual cell jumps)

Take Away


Interface scheme comparisons for VOF Scheme

Interface scheme Implicit Explicit Accuracy Speed

First order Not recommende

d Not

recommended

Second order Not

recommended Not

recommended

QUICK Low High

Modified HRIC Medium High

CICSAM High Medium

Compressive High Medium to High

Georeconstruct Very high Low to medium

BGM Very high Low to medium


VOF Implicit, Second order time

Compressive HRIC

VOF Explicit, First order time

Compressive CICSAM First Order Geo-Recon

Implicit Compressive scheme along with Bounded Second Order time discretization scheme give sharp interface which is comparable to the most

accurate Geo-Reconstruct

Take Away



Implicit(or Steady State) Schemes Comparison

Speed HRIC > Compressive > BGM

Sharpness BGM > Compressive > HRIC

Stability HRIC > Compressive > BGM

Explicit Schemes Comparison

Accuracy Geo-Reconstruct > Compressive > CICSAM > HRIC

Speed HRIC > CICSAM > Compressive > Geo-Reconstruct

Sharpness Geo-Reconstruct > CICSAM > Compressive > HRIC

Transient Formulation Comparison

Accuracy Bounded Second Order > Second Order > First Order

Speed First order > Second Order > Bounded Second Order

Stability First order > Bounded Second Order > Second Order



Zonal Discretization Schemes

• This option enables you to set diffusive or sharp interface modeling in different cell zones based on the value of zone dependent slope limiter. Extension of compressive scheme.

• The usage in porous medium application: • Diffusive interface modeling in porous medium zone

• Sharp interface modeling outside the porous zone

(Zone 1) (Zone 2) (Zone 3)

Slope Limiter (Beta) Scheme

Beta = 0 First Order Upwind

Beta = 1 Second order upwind

Beta = 2 Compressive

0 < Beta < 1 , 1 < Beta < 2

Blended scheme

ddf


Surface Tension

• Attractive forces between molecules in a fluid

– VOF model can include the effects of surface tension along the interface between each pair of phases, through source term in momentum equation

• Surface tension force made of two components:

– Normal component (due to interface curvature): σκδ

– Tangential component (due to variations in the surface tension coefficient): (sσ)δ

• Importance of surface tension effects:

– For Re >> 1, Weber number - droplet formation

– For Re<<1, Capillary number - coating flows

force tension Surface

force InertialWe

2

LU

force tension Surface

force Viscous WeReCa

U

Surface tension effects can be neglected if Ca>>1 or We>>1.

Continuum Surface Force Model (CSF) and Continuum Surface Stress Model (CSS) are available in ANSYS Fluent

Take Away


• Resolving Velocity Gradient in the vicinity of interface • High velocity gradients at the free surface

results in high turbulence generation • Important to resolve interfacial instability • Numerical damping of turbulence by adding

source term for turbulent dissipation in interfacial cells.

• This treatment is available only for k-omega turbulence model

t = 8.3s

t = 8.1s

t = 8.5s

t = 9s

Interfacial instability

Slug formation

Slug growth

Reference : Experimental investigation and CFD simulation of horizontal stratified two-phase flow phenomena, Christophe Vall ee , Thomas H¨ohne, Horst-Michael Prasser, Tobias S¨uhnel Nuclear Engineering and Design 238 (2008) 637–646

No Damping

With Damping

Turbulence Damping


Open Channel Flows

• Characterized by Froude Number ,

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

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

• Prescribe yin and Vin at inlet and yout at the outlet.

y in

y ou

t

inV

forceGravity

force Inertia

gL

VFr


•First order Airy wave theory •Linear •Small amplitude •Shallow to deep liquid depth

•Stokes wave theories

•Non linear •Finite amplitude •Intermediate to deep water range. ( h/L > 0.1)

•Cnoidal & Solitary •Non linear •Finite amplitude •Shallow water

H - Wave height h - Water depth L - Wave length

Modeling Surface Gravity Waves ANSYS CFD (Fluent) has the inbuilt capability for simulating complete wave regime.


Using a TUI command

/define/boundary-conditions/

open-channel-wave-settings

Open Channel Wave BC Checking


Modeling Oblique Waves

• User can specify the Reference Wave Direction as Averaged Flow Direction, Direction Vector or Normal to Boundary

• Now user can specify different velocity magnitude and directions for the flow current, wave and ship .


Wave Spectrum for Random Sea ( Beta Feature) Wave spectrum is used for simulating irregular waves

(Short and long crested waves) – Wave spectrum available in 15.0

• Pierson-Moskowitz (Fully developed seas)

• Jonswap (Fetch limited seas)

• TMA ( Fetch limited finite depth seas)


Multi-Fluid/Inhomogeneous VOF

• Adds interfacial sharpening schemes in Eulerian Model Framework

– Different Velocities and Temperatures at the interphase

• Capable for modeling both dispersed and separated flow regimes

– Physics in the stratified region: surface tension, no-slip at the interface

– Physics in the dispersed region: wall lubrication, sub-grid scale drag models based on predicted diameter

• Anisotropic drag

– Higher drag in the interfacial normal direction for the velocity continuity

– Lower drag in the tangential direction to allow different shear stresses


VOF Model Compatibility with Other Models

•Compatible – Solidification and Melting Model

– Moving Dynamic Mesh

– Six Degrees of Freedom (6DOF) Model

– System Coupling ( FSI)

– Phase Change / Cavitation Model

•Not Compatible – Turbulent Combustion Models

– Boiling Models

VOF + Cavitation Model

NACA 66 hydrofoil

Air entrapment during mold filling and solidification in casting process

VOF + Solidification & Melting


VOF + Solidification/Melting

Casting Applications : • Droplet solidification during

impingement • Casting, air entrapment • Effect of air convection on

solidification rate • Shrinkage/expansion • Welding of different metals • Effect of arc pressure on molten pool • Impingement of filler droplets in

welding

Air entrapment during mold filling and solidification in casting process

Welding


Free Surface Mass Transfer

Gas

(air + vapor species)

Liquid (water only)

Evaporation occurring at free surface

Wall condensation Equilibrium

After heating

Evaporation: Bubble growth (pressure contours)

Using UDFs for mass & heat transfer • Free surface evaporation

and condensation • Direct contact

condensation • Film boiling • Wall condensation


• Phases

• Arbitrary number of phases are allowed

• Any phase can be primary or secondary – not important in VOF model.

• Usual practice is to have secondary phase which has less presence in the domain

• Compressible phase as primary phase

• Implicit body force (Designed for flows with large body forces)

• The force is handled in robust numerical manner.

• Gravity acting on phases with large density difference.

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

VOF Inputs


Mesh Requirement

•Uniform mesh

•Gradual cell growth in case of non uniform mesh

•Same mesh type in the interface region – For speed up with Explicit VOF

– For less numerical diffusion with Implicit VOF

Tet is better than Tet+hex in this case


Best Practice: VOF Schemes

• Steady ( Only Implicit VOF available)

– Compressive – recommended for most of the problems

– BGM – for sharper interface

– P-V Coupling: Coupled VOF ( for faster convergence)

• Transient

– Explicit Compressive / Implicit Compressive with Bounded second order time discretization - recommended for most of the problems

– Geo-reconstruct (available only with Explicit) - for sharper interface


Best Practice: VOF Schemes & Solver Settings Explicit VOF

Turn off for surface tension dominated flows

Operating Conditions must set properly for most of the VOF cases

Generic conservative settings


• Use PISO algorithm • Use lower URF for Pressure and Momentum if any divergence ( Pressure-0.2, Momentum-0.3) • If the liquid interface mesh is not uniform or the velocity is varying

• Use Variable Time stepping Method • Use best suited courant calculation method

• Solve > set > vof-explicit-controls 0 = velocity based , 1 = flux based (default), 2 = flux averaged , 3= hybrid

Generic conservative settings

New in R 15.0

Best Practice: VOF Schemes & Solver Settings Explicit VOF


Best Practice: VOF Schemes & Solver Settings Implicit VOF Generic conservative settings


Best Practice: For Speed Up

1. Use Implicit VOF, Compressive and Bounded Second Order Time Discretization scheme

– This allows to use a larger time step size

– Use higher URFs for pressure and momentum( up to 0.8)

– NITA can be tried along with this if the phases are modeled as incompressible

2. If the solution is not accurate with Implicit VOF – Check the solution with a smaller time step size

– Use Explicit Compressive or Geo-Reconstruct

3. Explicit VOF – Use uniform mesh in the liquid interface regions

• Use Variable Time Stepping for non uniform mesh in the interface region

– Try with different courant calculation methods

• Solve > set > vof-explicit-controls

– NITA can be tried if the phases are modeled as incompressible


NITA(Non Iterative Transient Advancement) for Transient Speed-up

Sloshing in a Tank with baffles

ITA PISO CPU-15,794

NITA PISO CPU-3,450

• NITA can be used when the phases are modeled as incompressible

ITA vs NITA

NITA is 3 to 5 times faster and does not compromise on accuracy

Water loading on a structure

Computational Time in 8 CPU, Mesh count-264K: ITA- 9.5 hr, NITA- 2.67 hr

Take Away


VOF Model Examples


Tank Filling


Sliding mesh model with VOF

Free Surface Flow around a Spinning Gear


Box falling

MDM (Moving Deforming Mesh) Remeshing & 6DOF (6 Degrees of Freedom)


Air Inlet

Water Inlet

Splitter plate

Diameter: 0.078m Length: 37m

Slug frequency

ANSYS FLUENT

Experiment (Reference)

Reference : Slug initiation and evolution in two-phase horizontal flow Priscilla M. Ujang, Christopher J. Lawrence, Colin P. Hale, Geoffrey F. Hewitt , International Journal of Multiphase Flow 32 (2006)

Slug Flow


Solids Gas

Slurry (Water + Solids)

Gas bubble

Gas Solids

HRIC Phase localized Compressive Slope limiters : Gas-Solid = 2 Gas-Fluid = 2 , Fluid-Solid = 0

t = 0.2 s

Bubble Rise in Slurry


Wave interaction with a floating structure Wave slamming on submarine

MDM (Moving Deforming Mesh), 6DOF (6 Degrees of Freedom)and Open channel Wave BC along with VOF model

Wave Slamming


Wave Slamming MDM (Moving Deforming Mesh), 6DOF (6 Degrees of Freedom)and Open channel Wave BC along with VOF model


Wave Impact Loading on an Offshore Oil Rig Open Channel Wave BC with Solitary Wave


Documents

Korea 2014ugm Multiphase Flow Modeling