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 DT (Figure 2a). Four equally spaced baffles
with width wbf  0.1DT and thickness thbf  DI / 40 were mounted on the tank wall. The
tank was agitated by a Rushton turbine (disk with six perpendicular blades) with
diameter DI  DT / 3 , disk diameter DD  0.75DI , blade width wbl  DI / 4 , blade height
hbl  0.2 DI blade thickness thbl  0.01DI . (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/ .
Figure 1. 3D representation of the Tank agitated by the Rushton from Inventor 10
b.
a.
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:
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 appear.
1. Import the mesh
File Import
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. First, of all we need to know how many volumes we have
and what 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
following five volumes.
ronment 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:
that
the rotational zone and everything that is inside it must stop where the interface stops,
in order of the model to be functional for simulation 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 volumes 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 constructed up in the
CAD program. The next step 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
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:
File Export Mesh
(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:
File Read
In this step we read the .msh file which we export from GAMBIT.
Step 2:
Grid Check Grid
FLUENT performs various checks to analyze the quality of the mesh and report
everything in the console window.
Step3:
Display Grid
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:
Define Models Solver
Here we choose if we want steady or unsteady calculations as well as the velocity
formulation (System of reference)
Step 5:
Define Models Viscous
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:
Define Operating 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:
Define Materials
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: 992Kg / m3
Viscosity: 0.001155Kg / m sec
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:
Define Boundary 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)
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:
Re 
N  DI2

N
  Re
DI2
Where N is the rotational speed in revolutions/sec, DI 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 1.155924 106 m2 / sec .
Therefore N  2.933 revolutions/sec. But in FLUENT menu we should put it in
rad/sec so we multiply it by 2 and we take: 18.43 rad / sec
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:
Solve Monitors Residuals
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 104 and for the rest 105 .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:
File Write Autosave
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:
Solve Controls Solution
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
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:
Solve Initialize Initialize Apply
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:
File Save Case
We save the entire case file which contains what we did until this point.
Step 14:
Solve Iterate
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:
Display Contours
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 Display Vectors 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:
File Export Tecplot
At this step we export the data to Tecplot. From the File Type we choose Tecplot,
from the Surfaces
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.