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Tutorial: Dam-Break Simulation Using Volume of Fluid

Model of ANSYS FLUENT

Introduction

This tutorial examines the dam-break problem using the volume of fluid (VOF) multiphasemodel.

This tutorial demonstrates how to do the following:

Set up a dam-break problem. Choose the time step by estimating the maximum possible velocity of the interface

and the mesh cell dimension.

Solve the problem using the VOF model.

Manipulate the solution parameters.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 fromANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly.

For more information on VOF multiphase model, refer to Section 26.3, Setting Up the VOFModelin ANSYS FLUENT 13.0 Users Guide.

Problem Description

The initial setup of the dam-break problem is shown in Figure1.

In this problem, a rectangular column of water, in hydrostatic equilibrium, is confinedbetween two walls. Gravity is acting downwards with a magnitude of -9.81 m/s2. At thebeginning of the calculation, the right wall is removed and the water is allowed to flow outto the horizontal wall.

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Figure 1: Initial Setup of the Problem

Setup and Solution

Preparation

1. Copy the mesh file (dambreak.msh.gz) to your working folder.

2. Use FLUENT Launcher to start the 2DDP version ofANSYS FLUENT.

For more information aboutFLUENT Launcher see Section 1.1.2, Starting ANSYSFLUENT Using FLUENT Launcher inANSYS FLUENT 13.0 Users Guide.

TheDisplay Options are enabled by default. Therefore, after you read in the mesh, itwill be displayed in the embedded graphics window.

Step 1: Mesh

1. Read the mesh file (dambreak.msh).

File Read Mesh...

As the mesh file is read, ANSYS FLUENT will report the progress in the console.

Step 2: General Settings

1. Define the solver settings.

General Transient

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2. Check the mesh.

General Check

3. Display the mesh.

General Display...

(a) Click the Colors... to open the Mesh Colors dialog box.

i. Select Color By ID.

This assigns a different color to each zone in the domain, rather than to each

type of zone.

ii. Close the Mesh Colors dialog box.

(b) Click Display(see Figure2).

(c) Close theMesh Displaydialog box.

Figure 2: Mesh Display

Step 3: Models

1. Define the multiphase model.

Models

Multiphase

Edit...(a) Select the Volume of Fluid multiphase model.

(b) Enable Implicit Body Force.

(c) Click OK to close the Multiphase Modeldialog box.

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Step 3: Materials

Materials Create/Edit...

1. Retain the default settings forair.

2. Define the new material by copyingwater-liquid (h2o) fromFLUENT Database....

3. Click Change/Create and close the Create/Edit Materials dialog box.

Step 4: Phases

1. Define air as primary phase.

Phases phase-1-Primary Phase Edit...

(a) Selectair in the Phase Material drop-down list.

(b) Enter air forName.

(c) Click OK to close the Primary Phase dialog box.

2. Define water-liquid as the secondary phase.

Phases phase-2-Secondary Phase Edit...

(a) Selectwater-liquid in the Phase Material drop-down list.

(b) Enter water-liquidforName.

(c) Click OK to close the Secondary Phase dialog box.

Step 5: Boundary Conditions

1. Set the boundary conditions for poutlet.

Boundary Conditions poutlet

2. Select water-liquid from the Phase drop-down list.

3. Click Edit... to open Pressure Outletdialog box.

(a) Click Multiphase tab and ensure that Backflow Volume Fraction is 0.

(b) Click OK to close the Pressure Outletdialog box.

Step 6: Operating Conditions

Boundary Conditions Operating Conditions...

1. Enable Gravity.

2. Enter -9.81 m/s2 for Gravitational Acceleration in the Y direction.

3. EnableSpecified Operating Densityand retain the default value for Operating Density.

4. Click OKto close the Operating Conditions dialog box.

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Step 7: Solution

1. Set the solution method parameters.

Solution Methods

(a) SelectPRESTO! from the Pressure drop-down list.

(b) SelectFirst Order Upwind from the Momentumdrop-down list in theSpatial Discretization group box.

(c) SelectPISO from the Scheme drop-down list in Pressure-Velocity Coupling groupbox.

PISO is recommended for transient flow simulations.

2. Set the solution control parameters.

Solution Controls

(a) Ensure that theUnder-Relaxation Factors for the parameters is as follows:

Parameter Value

Pressure 0.9

Density 1

Body Forces 1

Momentum 0.7

(b) Set the termination criteria forPressure.

Solution Controls Advanced...

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i. Enter 0.001forTermination for the Pressure.

ii. Click OKto close the Advanced Solution Controls dialog box.

3. Retain the default values for all parameters and click Initialize.

Solution Initialization

4. Patch the initial distribution of thewater-liquid.

Solution Initialization Patch...

(a) Selectwater-liquid from the Phase drop-down list.

(b) Select Volume Fraction from the Variable list.

(c) Enter 1 for the Value.

(d) Select water from the Zones to Patch list.

(e) Click Patch and close the Patch dialog box.

5. Set the time step parameters.

For the calculation ofTime Step Size, refer to the Appendix.

Run Calculation

(a) Enter 0.01 (s) for Time Step Size.

(b) Enter 20 forNumber of Time Steps.

(c) Enter 40 forMax Iterations/Time Step.

(d) Save the initial case and data files (dambreak.cas/dat.gz).

(e) Click Calculate.

6. Save the case and data files (dambreak-20.cas/dat.gz).

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Step 8: Postprocessing

1. Display velocity vectors after 20 time steps.

Graphics and Animations Vectors Set Up...

(a) SelectVelocityfrom the Vectors ofdrop-down list.

(b) Selectmixture from the Phase drop-down list.

(c) SelectVelocity... and Velocity Magnitude from the Color by drop-down lists.

(d) Click Display(see Figure3).

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Figure 3: Velocity Vectors Colored by Velocity Magnitude After 20 Time Steps

2. Display filled contours ofVolume fraction ofwater-liquid after 20 time steps.

Graphics and Animations Contours Set Up...

(a) SelectPhases... and Volume fraction from the Contours ofdrop-down lists.

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Figure 8: Contours of Volume Fraction After 80 Time Steps

Figure 9: Velocity Vectors Colored by Velocity Magnitude After 100 Time Steps

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Figure 10: Contours of Volume Fraction After 100 Time Steps

Step 8: Solution Parameters Manipulation

1. Solve the problem by manipulating different solution parameters.

Use interface tracking scheme.

Enable/Disable PISO.

Use pressure interpolation scheme.

Use discretization scheme for momentum, volume fraction. Change the time step size.

Appendix

Calculate the time step by estimating the maximum possible velocity of the interface andthe mesh cell dimension using the following equations:

Courant= t

xcell

vfluid

gh=

2v2fluid

vfluid =

2gh 10m/s

t= xcell/vfluid 0.01sec

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