TUTORIAL:

TUTORIAL: PERFORMING FLUENT

SIMULATIONS IN A STIRRING

VESSELSMIXING VESSEL

Report

By

Pavlos Vlachos, Vasileios N. Vlachakis and Demetri Telionis

May 5th, 2006

Blacksburg, Virginia

Introduction

This tutorial illustrates the step- by- step setup and solution of the three -

dimensional turbulent flow in agitated vessels. The Mixing Tank configuration is

encountered in many industries such as in chemical, mining, pharmaceutical and

biotechnological. Therefore, accurate prediction of the flow field and the turbulent

characteristics (Turbulent Kinetic Energy, Dissipation Rate, Reynolds Stresses, and

Vorticity) in the whole tank and especially in the vicinity of the discharge impeller

region are of great importance. The reason is the presence of tThe complex

phenomena that take place in a tankand have directly infulence cause in the

production quality and the maintenance cost.

This tutorial is prepared after the authors conducted numerical simulations for

a tank with the specific parameters of a Dorr Olive flotation system. The commercial

code Fluent was used. The aim here is to provide guidance to Dorr Oliver engineers

on how to use the software in order to explore the effect of different values of the

parameters the shape and size of the tank and its impeller/stator system, the input

power, speed, material properties and others.

More specifically, tThis tutorial will guide you how to:

Export the right format from a CAD program

Read the CAD file in Gambit

Make changes in the geometry (add, split, subtract faces and volumes)

before undertaking in order to get prepared for the meshing process

Mesh the geometry, i.e. create a computational grid

Set the boundary cConditions for the solid and fluid

Import the mesh in FLUENT

Set the physical problem in FLUENT

Specify different the framess of reference : Multi Reference Frame

(MRF)

Perform the calculation using turbulent modeling

Judge Convergence

Display the Graphics

Export data for importing them to a Graphics program like Tecplot or

other format for further post processing.

Problem Description

The two mixing tank configurations are considered in this study. The first one is a

baffled cylindrical vessel with diameter (Figure 2a). Four equally spaced baffles

with width and thickness were mounted on the tank wall. The

tank was agitated by a Rushton turbine (disk with six perpendicular blades) with

diameter , disk diameter , blade width , blade height

blade thickness . (Figure 2b, 2c) The working fluid was water and

its height was equal to the height of the tank. This model is identical to the model

employed in the experimental work carried out by the present team. The second one is

a conical tank agitated by a six curved blade impeller. A stator mounted in the bottom

of the tank house the impeller. The whole design is patent of the Dorr-Oliver

company. (Figure 3). In both sets of calculations, the origin of the coordinate system

was fixed in the center of the impeller. The Reynolds number was based on the

impeller diameter, Re=NDI2/ν.

Figure 1. 3D representation of the Tank agitated by the Rushton from Inventor 10

a.

b.

c.

Figure 2. 2D representation of the Tank agitated by the Rushton impeller

a. b.

c.

Figure 3. 3D representation of the Stator, Impeller and Tank of the Dorr-Oliver configuration

In the CAD drawing in addition to the real geometry we have to add a cylindrical

zone around the impeller to account for the rotational zone which is needed for both

the MRF and the SG. After finalizing the geometry in a CAD program (In this study

Inventor 10 was used) we save the file as a family of the IGES format which includes

the following files: .igs, .ige, .iges. Then we open GAMBIT. A start up menu will

come up with the following format:

Working Directory: …………………… (Browse)

Session Id: new session

Options: -r.2.2.30

We continue and automatically Exceed will run on the background as well as a new

window of GAMBIT will be appeared.

1. Import the mesh

FileImportIGES….

Under the Filename we will browse to find our file (from now on we will refer to it

as test with the appropriate ending every time).

Import Options:

Translator: Native Spatial (Choose Spatial)

Import Sources: (Generic, AutoCAD, SolidWorks, Jama)

(Choose Generic if the 3D geometry has been made with other programs than the

three listed above)

Make Tolerant (Choose this)

Heal Geometry (Choose this option if you have stand alone vertices or faces)

2. Make changes to the geometry (add, subtract, and split volumes)

In this section we will make some changes in the geometry in order to make the

meshing more convenient. not to have problems with the meshing technique. First, of

all we need to know how many volumes we have and what do they represent. Let’ us

take for example the Dorr-Oliver configuration, without adding the stator at this first

step of this study. We propose to define the Therefore we are going to have the

following five vVolumes.

A representative snapshot from the Gambit’s environment is presented in Figure 4.

Figure 4. Representation of the GAMBIT’S environment when the Dorr-Oliver Tank

Configuration was imported from Inventor 10

Volume 1: Impeller: The reason for which we “break” the shaft into two parts is

because we have that the rotational zone and everything that is inside itthis zone must

stop where the interface stops, in order of the model to be functional for simulationng

it in FLUENT. As a result in the CAD program we should add this piece of the shaft.

Volume 2: Rotational Zone (RZ): Not in actual geometry but needed for the

simulation in FLUENT. In the MRF system the three- dimensional Navier – Stokes

equations are solved unsteady while in the outside system the equations are solved

steady state.

Volume 3: Piece of Shaft (small piece) which is inside the RZ: This is the small piece

of the actual shaft that needs to be inside the rotational zone

Volume 4: The rest of the shaft (the large piece)

Volume 5: The rest of the tank

Steps for creating vVolumes that can be meshed without GAMBIT reporting the error

“Can not be meshed because there is only adjacent cell”

a. Split Volume 5 using Volume 2 : In the

i. Operation menu in the left Column choose the first box (1st )

ii. Geometry menu choose the fourth box (4th)

iii. Volume menu choose the second box (2nd) from the second row

(2nd ) , right click on it and choose: Split Volume

iv. Split Volume menu choose Volume 5 and Split it with (Volume

Real) Volume 2 and delete the old one. From the choices you have:

Retain

Bidirectional

Connected

Choose the last one (Connected) and unclick the rest (they will

become grey)

b. Subtract Volume 4 from Volume 5 : In the

i. The same as before (a. i.)

ii. The same as before (a. ii.)

iii. Volume menu choose the second box (3rd) from the first row (1st ) ,

right click on it and choose: Subtract

iv. Subtract Real Volume menu choose Volume 4 to be subtracted

from Volume 5. Do not click any of the retain buttons.

At the end of this procedure we will have just two volumes, one for the rotational

zone and one for the rest of the tank. In other cases we may end up with more than

two volumes. This depends on how the initial geometry was constructedbuild up in

the CAD program.

The next step after let’s say this “healing of the geometry is to set the Boundary

Conditions in GAMBIT.

2. Setting the Boundary Conditions (BC) in Gambit.

From the Operation menu choose the third box (3rd) and from the Zone menu

choose the first box (1st). This box is referred as the Boundary type command where

one can specify different types of Boundary Conditions such as: Wall, Axis, Outflow,

Symmetry, Periodic and others (In the Type menu). In continuation press in the

Action menu the first choice on the left which is Add and type a name for the first

BC. Then from the Face Menu choose all the faces from which the boundary consists

of and press Apply. In case of mistakes there are in the Action Menu other choices to

modify or delete the BC that are not valid. In our case we set everything as a Wall

except from the faces that include the rotational zone which we set as an Interior.

After setting the above BC select the second box (2nd) of the Zone menu and set

the two volumes (Volume 2 and Volume 5 in this example) as Fluid. This means that

inside and outside of the rotational there is the working fluid (in our case is water but

we will talk later on how we set this up in FLUENT).

3. Meshing the model in Gambit.

For the meshing choose the second box (2nd) of the Operation menu, the forth (4th)

from the Mesh menu (this is for Volume meshing) and lastly choose one by one the

existing volumes. In the next boxes there are some choices about the mesh elements.

The menu includes the following choices:

a. Hex

b. Hex/Wedge

c. Tet/Hybrid

From which we select the last one (hybrid grid with tetrahedral and triangular

elements).

The Type menu includes the following choices from which we choose the first (Map)

and from the Smoothing menu choose None

a. Map

b. Submap

c. Tet Primitive

d. Cooper

e. Stairstep

Under Spacing there are three choices from which we choose the second (Interval

size):

a. Interval count

b. Interval size

c. Shortest edge %

For every volume we change the spacing depending on how detailed we want to be

the mesh. For example for the shaft we don’t need too much detail because there is

nothing that we want to capture. On the other hand, the rotational zone and the tank is

where we need a fine mesh because all the fluid phenomena happen there.

If there is a need to remove a part of the mesh first of all we choose the volume that

we want to unmesh then we unclick the mesh button in the Mesh Volume menu and

we enable the two other boxes with the names: Remove old mesh and Remove lower

mesh. A snapshot of the meshed grid of the Dorr-Oliver Tank can be seen in Figure 5.

Figure 5. Representation of the Mesh made by GAMBIT for the Dorr-Oliver Tank

Configuration

After finishing the meshing of the model we go to the Solver dropdown menu in

GAMBIT and we choose FLUENT 5/6. Now we are ready to export the mesh by

following the steps:

FileExportMesh (Choose Filename and Folder)Accept

Now we are ready to load it in FLUENT

When we double click the FLUENT icon it will open another one asking which

version of FLUENT we want to run. The available versions are:

2d

2ddp

3d

3ddp

From which we choose the last one. The dp in both two and three dimensional means

double precision for the results (accuracy of 16 digits behind the number)

Step 1:

FileReadCase…

In this step we read the .msh file which we export from GAMBIT.

Step 2:

GridCheck Grid

FLUENT performs various checks to analyze the quality of the mesh and report

everything in the console window.

Step3:

DisplayGrid

Here we can display every surface of the model (impeller, shaft, tank wall and etc.)

Figure 6. Display panel showing the grid in FLUENT

Step 4:

DefineModelsSolver

Here we choose if we want steady or unsteady calculations as well as the velocity

formulation (System of reference)

Step 5:

DefineModelsViscous

Here we choose the turbulent model and some other aspects of them. The three

turbulent models that we used were:

a. The standard k-e model with standard wall functions and without changing

anything in the model constants

b. The RNG k-e with enabled the option of Swirl dominated flow and

changing the swirl factor at the value of 0.02 as well as choosing the

enhanced wall treatment in the Near Wall Treatment menu

c. The Reynolds Stress model with Standard wall treatment and enabled the

options of Wall Boundary conditions from the k equation and the Wall

Reflection effects from the Reynolds stresses menu

Step 6:

DefineOperating Conditions

Here we set the operating condition such as the gravity and condition for the pressure

It is important here to know how the axes have been set in the CAD program in order

to know in which direction we should apply the gravity. In our example the z-axis is

the perpendicular axis therefore in the Operating Condition menu we set as an

operating pressure 101325 Pascal= 1atm at z=0.448 which is the top of the tank where

the liquid stops and the gravity acceleration as -9.81 again in the z-axis because is

pointing downward.

Step 7:

DefineMaterials

At this point we will choose the working fluid which in our case is water. By default

FLUENT uses air so we need to change it. In the following figure the materials menu

can be seen.

The next step here is to change as we said the working fluid. This can be done by

choosing the FLUENT Database menu on the top right. A new window like the one

below will appear:

From the FLUENT Fluid Materials we will choose water-liquid [h2o<l>] and then

Copy. After that we are going to change the density and viscosity of the water with

the values that have been found from tables for water with temperature 20 degrees.

Density:

Viscosity:

Then press Change/Create and the new material with the properties that we want is

ready for use. Now we are ready to apply the boundary conditions.

Step 8:

DefineBoundary Conditions

In this step we will set the type of the boundary conditions for every zone. In our

problem the model will consist of the following BC as they can be seen from the

following figure.

The continuum_tank is the fluid inside the tank which is water so the type is fluid. In

continuation we press set and the following menu will appear in which we keep

everything as it is in the figure below.(when finish press OK)

The next three zones are set as interiors (inside of the tank, interface, inside of the

rotational zone) so we don’t have to press set. But the next zone is a rotational zone

where we have to press set and make the following changes:

Motion Type: Moving Reference Frame (MRF)

Speed (rad/sec):

Depending on the Reynolds number we set the rotational speed. For example

if we want Re=35000 we should solve the following equation with respect to N:

Where N is the rotational speed in revolutions/sec, is the impeller diameter

in meters (m) which in the case of the Dorr-Oliver impeller is 0.01016m and is the

kinematic viscosity which in our case for water of 20 degrees is .

Therefore revolutions/sec. But in FLUENT menu we should put it in

rad/sec so we multiply it by and we take:

The type off boundary conditions for the last two surfaces is wall and we select the

momentum menu where we make the following choices:

Wall Motion: Stationary

Shear Condition: Non-Slip

Roughness Height: 0

Roughness Constant: 0

Step 9:

SolveMonitorsResiduals

From this option we watch the progress of the residues and based on that we judge the

convergence of the simulation. Under Options we tick the box of the Plot and we set

the number of iterations and plotting. We can set a large number to make sure that we

will not loose any data. (eg. Iterations: 20000 or more). In addition, in the

convergence criterion box we put for the continuity and for the rest .That

means when all the residues of every variable reach the above numbers the simulation

will converge. Usually when we will observe that the residuals do not change as the

iterations increase and the lines are almost flatten out we can say that the model has

converged.

Step 10:

FileWriteAutosave

Here we set the frequency of which FLUENT will save the case and the data file. The

case saves the grid and all the other options and the data the values of all the variables

(velocities, kinetic energy, dissipation and etc.). Under the filename we choose the

how we want to name the file and then -% i. The i at the end of the % means iterations

while t means time. So in our case where we solve the steady state model we need to

save the case and data file after a number of iterations (as many as they are in the

boxes).If we solve the unsteady then we will have put t. (Save every ..number of time

steps).

Step 11:

SolveControlsSolution

Through this panel we select the discretization scheme for the:

a. Pressure

b. Momentum

c. Turbulent Kinetic Energy

d. Turbulent Dissipation Rate

e. Reynolds Stresses (If we have chosen the Reynolds Stresses turbulent

model)

Usually for the Pressure-Velocity Coupling we choose the SIMPLE algorithm but

FLUENT gives us another two choices: SIMPLEC and PISO. The latter one is usually

used in the unsteady simulations. For steady state simulations the available

discretization schemes for the pressure are:

a. Standard

b. PRESTO

c. Linear

d. Second order

e. Body force weighted

from which we select the a or b and for the other variables are:

a. First Order

b. Second Order

c. Power Law

d. QUICK

e. Third order MUSCL

A general rule is that we first start with the first order or the power law until the

residues show to be stable (Not oscillations: rapid changes) and then we can continue

the simulation chosing second order or the third order schemes for better accuracy.

Although a higher order discretization scheme increases the accuracy of the

simulation it can also cause larger errors. Therefore this is not always the best

solution. Furthermore, the high order schemes at the beginning show more unstable

behavior but at the end they converge faster. Again, this is in not always true. The

QUICK scheme is usually used when the mesh consists of hexahedral elements.

Because of its nature it converges faster with this type of meshes.

As far as for the under-relaxation factors, at the beginning we should set them low and

if a stable behavior is observed then we can increase them in order the simulation to

converge faster. Sometimes though they can drive the system to point where it

“blows” (The residues take very high values and instead of decreasing they are always

increasing). Hence, there are no special rules of what value should one set to the

under-relaxation parameters. This is a matter of experience.

Step 12:

SolveInitializeInitializeApply

In this step we predict some initial values for the variables. Although a bad prediction

of the values will not affect too much the simulation a good prediction can speed it

up. Under the reference frame we can choose either absolute or relative to Cell Zone

values. In our case we have selected the absolute. We can start by putting the pressure

101325Pa the velocities zero and the TKE and TDR a very small value (0.01 for

example)

Step 13:

FileSaveCase

We save the entire case file which contains what we did until this point.

Step 14:

SolveIterate

At this very last point we choose the number of iterations and the reporting interval.

In the number of iteration it is good to put a large number, for example 30000 in order

to be sure that it will not stop especially if the model runs overnight and if it

converges faster then it will stop. In case that it will need more iterations than the

number we have originally set, we can update it and set a bigger number. The

reporting interval controls the number of iterations after which FLUENT will check

for convergence. Therefore it is good to be set to 1 because in that case after every

iteration FLUENT will check if the values of the variables are less than the number

we set in the residue menu in order for the model to get converged.

Step 15:

DisplayContours

In this step we can display the contours or vectors of any variables available in the

two first boxes on the right of the contour menu. We can choose between filled and

unfilled as well as the number of the contour levels. For displaying the vectors we go

to DisplayVectors and a similar menu will come up. The procedure for displaying

the vectors is similar with the one displaying the contours. A better way to plot

contours, slices, isosurfaces and etc is to export the data to TECPLOT (a graphics

program with many options designed for plotting CFD and experimental data).

Step 16:

FileExportTecplot

At this step we export the data to Tecplot. From the File Type we choose Tecplot,

from the Surfaces we don’t choose anything and from the Functions to Write we

select which ones we want to export. If we want all of them we press the button with

the three bold horizontal bars and then Write.