fluent modeling

8/8/2019 Fluent Modeling

1/5

CHEG 341 CFD MODELING USING FLUENTFluent consists of two separate programs Gambit and Fluent. Gambit is used to

construct the flow geometry, along with the mesh for solving the equations of

motion and continuity at fixed points. Fluent 6.2.16 is the program which actually

solves the equations for the geometries constructed using Gambit.

GAMBIT OPERATION1. Before using either Fluent or Gambit in eCALC or the Dell Lab, you must set the

environment. Using the Windows drop-down menus, enter: Start Programs

Fluent Inc. Products Fluent 6.2.16 Set Environment. Answer Yes and OK

when prompted.

2. Create a new geometry file using Gambit. Go to Start Run and type

cmd

At the Z: prompt, type C:

At the C: prompt, type gambit

This will create a new gambit file. After about 10 seconds, a large black

screen with a Cartesian coordinate system will appear. MINIMIZE (but DO

NOT CLOSE) the smaller black command window.

3. In Gambint, click on the grey cube (the Geometry button). Then, click on the

fourth button, which looks like a white cube. A new button menu, called Volume,

should appear.

4. RIGHT click on the icon that looks like a white shoebox (the second button). This

will reveal a drop down menu of different 3D shapes. Select Frustrum for this

practice session. Create a frustrum with a height of 27, with Radius 1 and Radius 2

both 1.55, and Radius 3 = 1.0 (we will insert units later, in Fluent rather than

Gambit). Make the Axis Location Centered X. Label the tube LAD. When you have

entered all this info, click the Apply button. If you want your tube to fill the entire

screen, Click the button with the triangle near the bottom of the screen, under the

word Active.

5. Right click on the frustrum symbol, and select Torus as the next geometry tobuild. Choose Radius 1 as 1.275 and Radius 2 as 0.5. Label this Stenosis and center

it on the X axis. Click Apply.

6. Right click on the icon of the two intersecting circles, and select Subtract from the

drop down menu.

Pick LAD as the Volume, and Stenosis as the Subtract Volumes. Click Apply. Your

tube will now have a toroidal chunk missing from its center.


2/5

If you mess up, the Undo button is the curved blue arrow at the bottom right hand

of the screen.

7. Choose Solver Fluent 5/6. DO NOT SKIP THIS STEP.

8. From the Operation menu at the top, select Zones (third button from the left, a

blue-green divided cube). You will now label the boundaries of your LAD. Click the

left cube (white one with blue side) to specify boundary types Click the dark black

arrow next to the yellow box to bring up the list of boundaries. Select face.1 from

the Available list, and transfer it to the Picked list using the arrow. Face.1 will then

be highlighted in red. (You can also pick an entity by left-clicking on it while you

hold down the Shift key.) If this is your exit face, name it Outlet, and select

Outlet_Vent as the Type from the dropdown menu.

9. Repeat step 6 for the inlet (Type is Mass_Flow_Inlet). The remaining faces are

considered Walls as default.

10. Next, create the mesh. Click on the Yellow cube in the Operation menu. Click

the White cube (4th button) under Mesh, and then the first button (Cube with Pencil)

under Volume. Pick LAD as your Volumes:, and choose Tet/Hybrid as your

Elements: Enter 0.4 as your Spacing, then click the Apply button. Note that 0.4 mm

will be too coarse of a mesh for your actual study, but we will use a coarse mesh

here so that the calculations take less time.

11. Finally, Export your mesh. Choose File Export Mesh from the top menu

bar. Browse for the location on the scratch drive where you store your personal files

(e.g., your thumb drive), and save the file with filename.msh (be sure to include the

.msh). You may now exit Gambit.

FLUENT OPERATION(1) Before using either Fluent or Gambit in eCALC or the Dell Lab, you must set the

environment. Using the Windows drop-down menus, enter: Start All Programs

Fluent Inc. Set Environment. Answer Yes and OK when prompted. You do not

need to repeat this step if you already did it once since sitting down at the computer.

(2) Start up Fluent: Start Programs Fluent Inc. Fluent 6.2.16. Select 3d to

RUN.

(3) Read in the desired geometry . From the Fluent drop down menu, choose: File

Read Case Look in: Location where you stored your mesh (C: is default).

Fluent will then open the mesh file which you created.


3/5

(4) Check the grid: Grid Check

(5) Set the Scale of the grid to be mm. This is important, because the default is

meters, and larger diameters will result in larger Reynolds numbers. Select Grid

Scale and enter mm as the unit the grid was created in, then click SCALE (and closethe window).

(5) Display the grid: Diplay Grid. Be sure the surfaces are highlighted, and click

Display. The grid should come up in a black window on your screen.

(6) Tell Fluent that your fluid is blood. Select DefineMaterials. Change the name

from air to Blood and replace the density and viscosity of air with those of blood.

Click Change/Create, and agree when Fluent asks if its OK to overwrite air. Close

the dialog box.

(7) Tell Fluent this will be a laminar flow problem. Choose Define Models

Viscous Laminar OK.

(8) Input the correct inlet flow rate by selecting Define Boundary Conditions

Inlet Mass Flow Inlet Set. Enter 0.00167 kg/sec as the mass flow rate. Click

OK.

(8) Initialize the grid with numerical values. You only have to do this the FIRST time

you run a calculation with a new Fluent geometry. For all subsequent times, you can

just start from the last calculated values, which is usually faster. Choose: Solve

Initialize Initialize Init. You can accept the default values. Close the window

when finished initializing.

(9) Solve the problem. If you want to monitor the progress of the solver as it works,

first choose: Solve Monitors Residual and check both the Print and Plot boxes,

and click OK. This is useful in order to catch any mistakes you may have made in

solving a problem. Then, choose Solve Iterate and select the number of iterations

to be 200, then click Iterate. If it takes more than 200 iterations, simply click the

Iterate button again until you get a Solution is converged message on the screen.

(10) Display the pressure profiles. Choose: Display Contours Pressure. Select

default-interior from the Surfaces list. Choose the options you prefer Filled is quiteimpressive. Click Display. A color image of the pressure contours will be displayed.

You can rotate this image to see inside the pipe by drigging any point on the screen.

(11) View the outlet velocity profile by rotating the pipe to look at it end-on, or by

returning to the Contours window and selecting outlet as the surface (remember to

de-select default-interior).


4/5

(12) You may wish to save your case and data to your own folder by choosing: File

Write Case and Data.

(13) You can copy pictures for your report as a JPEG or other formats. Choose: File

Hard Copy JPEG. Choose the Color option, and the Reverse

Foreground/Background option (to avoid wasting the expensive color toner on the

black background).

(14) You can save numerical data from a fixed surface by choosing File Write

Profile and then selecting the surface and value you wish to record (e.g. Outlet and

Velocity). Save the file as filename.csv; this format can be opened in Excel. In your

.csv file, you will get a single column listing first all the x-coordinates for your

chosen surface, followed by the y-coordinates, the z-coordinates and the value (e.g.,

velocity) associated with the respective (x,y,z) coordinates. You will probably want to

convert this data into a 4-column listing, so that each row contains the x, y, and z

coordinates, followed by the velocity.

MODIFYING AN EXISTING GEOMETRY (BEFORE MESHING)You should use this procedure when modeling the bifurcation in the left main

coronary artery. Because geometry construction in Gambit is tedious, we have built

for you a model of the healthy (no stenosis) bifurcation. For your project, you will

have to mesh this geometry, as well as add stenoses and bypass grafts. These

instructions explain how to open the file in Gambit.

(1) The geometry file of the healthy bifurcation is located on the classfiles Y:drive in eCALC and the Dell Lab, as

Y:\CHEG341\2005Fluent\artery_healthy.dbs However, because students do

not have write privileges to this drive, you must first copy the file onto your

own accessible media (e.g., your C: drive). Otherwise, you will NOT be able to

open the file.

(2) Start up Gambit using the instructions 1. and 2. under Gambit Operation,above..

(3) Open the geometry file: File Open. In the ID: box, type the address ofyour file, e.g., F:\artery-healthy. DO NOT append the .dbs to the artery-

healthy name. Agree to save the current session.

(4) Continue with step (10) under Gambit Operation.

SOME COMMENTS ON BOUNDARY CONDITIONSWhen you have a single inlet and a single outlet, as we had in the example above,

you can either specify the inlet flow rate (or velocity), and Fluent will calculate the

pressure at the inlet, or else you can specify the inlet pressure, and Fluent will


5/5

calculate the velocity. Note that specifying an Outlent Vent as an exit BC sets the

pressure there to 0 (gage).

When you model a geometry in Fluent which has more than one outlet, you must

provide boundary conditions for each inlet and outlet. Unfortunately, you can

provide a mass flow BC only for inlet boundaries; outlet BCs must be specified aspressure BCs. Hence, a trial-and-error procedure is necessary, even for a stenosis-

free bifurcation. Using the known pressure at the LM inlet, you must guess the

pressures at the LCX and the LAD exits (these will likely be different) until your

Fluent simulation reports the correct flow rates for the healthy LCX and LAD.

A note about numerical simulations: Fluent solves the flow equations to within a

certain tolerance the default value of the residual is 0.001. So, if you are solving

for the pressure drop across a vessel where the entrance pressure is 10,000 Pa,

Fluent will consider the problem solved when the exit pressure is calculated to

within +/- 10 Pa (10 Pa / 10,000 Pa = 0.001). However, if the total pressure drop

across your vessel is supposed to be 20 Pa, note that Fluent will only be able to

calculate the pressure drop to within 50%, i.e., to 20 Pa +/- 10 Pa. Although you can

fix this problem by lowering the tolerance (say, to 0.00001), this will dramatically

increase the computation time, and may even run up against the limit of single

precision computations. So, a trick for avoiding this problem is to set the initial

pressure to a value that is lower than what you know the true value is, say, to 100 Pa

instead of 10,000 Pa. Then Fluent will calculate your exit pressure precise to +/-

0.1 Pa (at a tolerance of 0.001), so your P will be 20 +/- 0.1 Pa, which is correct to

within +/- 0.5%.

ONLINE USERS MANUAL FOR FLUENTIf you wish additional information about using Fluent, you can access the Users

Manual on the X: Drive in eCALC or the Dell Lab.

In eCALC, go to

X: (applications on archimedes) Fluent.IncFluent6.0helpindex.htm and click

on the Users Guide.

In the Dell Lab, go to

ChEApplicationsFluentIncFluent6.2Documentation and click Users Guide.

fluent modeling

Documents