CFX-Intro 14.5 WS09 Tank-Flush

8/13/2019 CFX-Intro 14.5 WS09 Tank-Flush

1/20

2012 ANSYS, Inc. December 17, 2012 1 Release 14.5

14.5 Release

Introduction to ANSYS

CFX

Workshop 09

Tank Flush


2/20


Workshop Description:

This workshop models a water tank filling and then

emptying through a siphon. The problem is transient and

solved as a two-fluid, multiphase case (air + water).

An initial water level is set in the tank. The water supply is

turned on for the first second of the simulation and then

shut off for the rest of the simulation. The water level rises

until water flows out the U-tube generating a siphoning

effect which effectively empties the tank.

Learning Aims:

This workshop introduces several new skills:

Setting up and post-processing a transient

simulation

Setting up a multiphase simulation

Using ifstatements to initialize physics

Introduction

Introduction Setup Solution Results Summary


3/20


Mesh Import

1. Start Workbench, add a CFX Component Systemand edit Setup to

start CFX-Pre

2. Right-click on Mesh> Import Mesh > ICEM CFD

3. Set the Mesh Unitsto cm

For some mesh formats it is important to know the units used to generatethe mesh

4. Import the mesh calledflush.cfx5



4/20


Define Simulation Type

1. Edit theAnalysis Type object in the Outline tree

2. Set the OptionforAnalysis Type to Transient

3. Set the Total Timeto 2.5 [s]

4. Set the Timestepsto 0.01 [s]and click OK

The simulation will have 250 timesteps

The first step is to change theAnalysis Typeto Transient:



5/20


Edit Default Domain

1. Edit Default Domainfrom the Outlinetree

2. Delete Fluid 1 under Fluid and Particle

Definition

3. Click on the New icon

4. Name the new fluidAir

5. Set the MaterialtoAir at 25Cand the

Morphology to Continuous Fluid

6. Create another fluid namedWater

7. Set the Materialto Waterand the Morphology

to Continuous Fluid



6/20


Edit Default Domain8. Turn on Buoyancyand set the (X, Y, Z) gravity

components to (0, -g, 0)

Use the expression icon to enter -g( gis a built-in

constant )

9. Set the Buoy. Ref. Densityto 1.185 [kg m^-3]

This is the density ofAir at 25 C. In this simulationthere is a distinct interface between the two fluids,

rather than one being dispersed in the other. For a

free-surface model such as this, the buoyancy

reference density is set to that of the less dense fluid.

The hydrostatic head then appears in the pressure field

of the more dense fluid, which is more natural. Also themomentum source is added to the fluid with the

greater inertia, making the solution more stable.

Search the help for Buoyancy in Multiphase Flow

(including the quotes in the search) for more details.



7/20 2012 ANSYS, Inc. December 17, 2012 7 Release 14.5

Edit Default Domain

10. Switch to the Fluid Models tab

11. Under Multiphase enable the HomogeneousModel

This makes the simplifying assumption that bothphases share the same velocity field

12. Set the Free Surface ModelOptionto

Standard

This changes some solver numerics to resolve the freesurface interface better

13. Under Heat Transferenable theHomogeneous Modeltoggle and set theOption to None

14. Set the Turbulence Model Optionto k-Epsilon




Edit Default Domain

15. Switch to the Fluid Pair Models tab

16. Enable the Surface Tension Coefficienttoggle

and set the coefficient to 0.072 [N m^1]

17. Under Surface Tension Forceset the Optionto

Continuum Surface Force

18. Set the Primary Fluidto Water For liquid-gas, free-surface flows the primary

fluid should be the liquid

19. Under Interphase Transferset the Optionto

Free Surface

20. Click OKto complete the changes to the

domain




Create Boundary Conditions

1. Insert a new boundary namedAmbient

2. Set the Boundary Type to Openingand the LocationtoAMBIENT

3. On the Boundary Details tab set the Mass and Momentum Optionto Opening Pres. And Dirnwith a Relative Pressureof 0 [Pa]

4. On the Fluid Values tab set the Volume FractionofAir to 1and the

Volume Fractionof Waterto 0

5. Click OKto create the boundary

Start by creating an Openingboundary at the top of the tank to allow

air to escape as the tank is filled:




Create Boundary Conditions

1. Insert a new boundary named Outletof the Boundary Type,Opening,and

the Locationset to OUTLET

2. In the Boundary Detailsuse Opening Pres. And Dirnwith a RelativePressureof 0 [Pa]

3. In the Fluid Valuesset the Volume FractionofAir to 1and the Volume

Fractionof Waterto 0

4. Click OKto create the boundary

5. Insert a Symmetryboundary named Sym1on the Location SYM1

6. Insert a Symmetryboundary named Sym2on the Location SYM2

Now create the outlet and symmetry boundaries. Since recirculation may occur at

the outlet this boundary will be specified as an Openingto allow flow both into andout of the domain:



11/20


Inlet Water Flow Function

1. Right-click on Expressionsin the Outlinetree and select Insert > Expression

2. Enter the NameasflowProfile

3. Enter the Definitionas: if(t


12/20


Define Expressions

1. Insert the following expressions:

waterHt = 6 [cm] waterVF= if(y-0.01 [m],1,0)* if(x>-0.028 [m],1,0)

waterDen = 998 [kg m^-3]

HydroP = waterDen * g * (waterHt - y) * waterVF

Next you will create expressions to define the initial water height and

hydrostatic pressure field. These expressions must define the correct initialflow field because the transient simulation is started cold (it is not started

from a converged steady-state simulation).

waterHtis the initial height of the water in the tank. waterVFprovides theinitial volume fraction distribution in the tank (see next slide). waterDenis the

density of water. HydroPprovides the initial pressure distribution due to the

hydrostatic pressure of water.



13/20


Define Expressions

The expression for waterVFcontains three if()function terms multiplied together. The first

function, if(y -

0.01[m]. The third function returns 1 whenx > -

0.028 [m].

The result is that the volume fraction of water is

equal to 1 only in the shaded area shown to the

right. This defines the initial water volume

fraction.

x = - 0.028

y = waterHt

y = - 0.01



14/20


Define Initial Conditions

1. Right-click on Flow Analysis 1in the Outlinetree and select Insert > Global

Initialisation

2. Set all Cartesian Velocities Componentsto 0 [m s^-1]

3. Set the Relative Pressureto the expression HydroP

4. On the Fluid Settingstab set the Volume Fractionfor Waterto the

expression waterVF. Set the Volume FractionforAir to the expression 1 -

waterVF

5. Click OKto set the initial conditions

Now set the initial conditions using these expressions:



15/20


Define Transient Results

1. Edit the Output Control object in the Outline tree

2. On the Trn Resultstab create a new Transient Results object, accepting thedefault Name

3. Set the Optionto Selected Variables

This reduces the file size by only writing out selected variables

4. In the Output Variables List, use the icon and the Ctrlkey to pickAir.Volume Fraction, Velocity,and Water.Volume Fraction

5. Under Output Frequency, set the Timestep Intervalto 2and click OK Transient results will be written every second timestep, thus creating a total of

125 Transient Results files

By default results are only written at the end of the simulation. You must

define transient results to view the intermediate solution:



16/20


Create Monitor Point

1. Insert a new expression named waterVolwith the Definition set to:volumeInt(Water.Volume Fraction)@Default Domain

This is the volume integral the water volume fraction in the domain

2. Edit the Output Control object in the Outline tree3. On the Monitortab, toggle Monitor Options,insert a new Monitor Point

named Water Volume

4. Set the Optionto Expressionand enter the Expression Valueas waterVol,then click OK

Next create a monitor point to track the volume of water in the domain during

the solution:



17/20


Run Solver

1. Close CFX-Pre and save the project as TankFlush.wbpj

2. In the Project Schematic, Editthe Solutionobject to start the CFX-SolverManager

3. Start the run from the CFX-Solver Manager

You can monitor the volume of water in the domain during the simulation on the

User Points tab

The simulation will take about 30 minutes to complete. Therefore results files

have been provided with this workshop

4. After a few timesteps stop your run from the Project Schematic by right-

clicking on the Solutioncell and selecting Interrupt Update

5. Select File > Monitor Finished Runin the CFX-Solver Manager6. Browse to the results file provided with the workshop

Note the shape of the Water Volume curve (User Points) and see that less water is in the

domain at the end of the run than at the beginning



18/20


Post-Process Results

1. Using Windows Explorer, locate the

results file supplied, TankFlush_001.res,

and drag it into an empty region of the

Project Schematic

2. A new CFX Solutionand Resultscell will

appear. Double-click on the Results

object to open the file in CFD-Post



19/20


Post-Process Results

1. Click on Z-axis to align view and turn on Visibility for

Sym1

2. On the Colortab set the Variable to Water.Volume

Fractionand the Color Mapto White to Blue

3. Use the Timestep Selector to load results from

different points in the simulation

4. With the first Timesteploaded open theAnimationtool

5. Select the Quick Animationtoggle and select

Timestepsas the object to animate

6. Turn off the Repeat Foreverbutton

7. Enable the Save Movie toggle and then click the

Playicon to animate the results and generate an

MPEG


Note thatAir(white) becomes

entrained in Water(blue)


20/20


Summary

A tank flush has been simulated using a transient simulation with multiphaseflow. Due to the nature of transient simulations, they tend to take longer to

solve and post-process.

The results show that a significant amount of air became entrained in the water.

As the Homogeneousmultiphase model was used and the fluids shared the

same velocity field, it was not possible for the air to separate out from the water

as a result of buoyancy. In order for this to have happened, the Inhomogeneousmultiphase model would have been required. Each phase would then have had

its own velocity field and the entrained air bubble could have risen relative to the

water.

When running the Inhomogeneousmodel the entrained phase should be set as

a Dispersed Phase in CFX-Pre.


Documents

CFX-Intro 14.5 WS09 Tank-Flush