In this set of instructions, the steps for modeling fluid flow in complex 3D structures are described. Many of the steps are similar to one shown above with several important differences. I will model a simple microfluidic channel that has large interconnect reservoirs on the ends of the channels. This will demonstrate what happens to the fluid when needles and reservoirs are used in conjunction with small microfluidic channels. Start a new ANSYS filename as shown above.
Set preferences.
You will now set preferences in order to filter quantities that pertain to this discipline only. 1. Main Menu > Preferences 2. Turn on FLOTRAN CFD filtering 3. OK.
Define element type.
4. Main Menu > Preprocessor > Element Type > Add/Edit/Delete
5. Add an element type.
6. Choose 2D FLOTRAN element (FLUID141). 7. Choose 3D FLOTRAN element (FLUID142). 8. OK.
9. Close.
Units.
Note: All units shown in this tutorial are in cgs units.
Main Menu > Preprocessor > Materials > temperature unit
12. Choose the Celsius scale
Create Keypoints:
Keypoints for the design will be entered here. What will be entered are the coordinate for the channel, reservoirs and needle inputs. 2D areas will first be made then extruded into the 3D design.
IMPORTANT!! When doing 3D modeling, the areas have to overlap slightly. Areas
that butt up against each other WILL NOT work with this simulation. Be aware of this when inputting your points.
To add keypoints to a coordinate system:
Preprocessor -> -Modeling- Create -> Keypoints -> In Active CS…
Fill in the fields as shown below, then click “APPLY”. When you click on “Apply”, the command is issued to create keypoint number 1 at (x,y)=(-0.06, 0.00625). Note that when the Z field is left blank, in this case, the blank space defaults to zero, which is desired. Since you clicked on “Apply”, instead of “OK”, then the keypoint creation box remains open.
When the final keypoint is entered, click on “OK” instead of “APPLY”. “OK” issues the command and also closes the keypoint creation box.
Before moving on, it is probably a good idea to check the keypoint locations. Along the top toolbar:
Choose: List -> Keypoints -> Coordinates Only. A box should open up with the keypoint location information. If any keypoint is not in the correct location, at this point, you can just re-issue the keypoint creation command for that particular keypoint. To do this, choose: Preprocessor -> -Modeling- Create -> Keypoints -> In Active CS…
Fill in the correct information for that particular keypoint in the box, and click “OK”. The keypoint will be moved to the correct location. If you have some keypoint incorrectly numbered above number 12, this will not cause a problem. Just be sure you have keypoint numbers 1 thru 12 located correctly.
You can close the box listing the keypoint locations, by clicking, in that listing box, on “File-> Close”.
The keypoints should look like the image below.
Keypoint # X Coordinate Y Coordinate Keypoint # X Coordinate Y Coordinate
2 0 0.075 17 0.95 -0.075 3 -0.15 0.075 18 0.95 0.075 4 -0.15 -0.075 19 0.8 0.075 5 0 -0.075 20 0.86 0.00625 6 -0.06 -0.00625 21 -0.075 -0.01 7 0.3 -0.00625 22 -0.075 0.01 8 0.29375 -0.36 23 -0.095 0.01 9 0.225 -0.3 24 -0.095 -0.01 10 0.225 -0.45 25 0.29 -0.375 11 0.375 -0.45 26 0.29 -0.395 12 0.375 -0.3 27 0.31 -0.375 13 0.30625 -0.36 28 0.31 -0.395 14 0.3125 -0.00625 29 0.895 -0.01 15 0.8 -0.00625 30 0.875 -0.01 16 0.8 -0.075 31 0.875 0.01 32 0.895 0.01
Create areas.
In this design, all lines connecting the keypoints are straight lines so the following command is used.
Main Menu > Preprocessor > -Modeling- Create > -Areas- Arbitrary > By Lines
There are seven separate regions that have to be made into areas. There are 3 small squares that represent the needle inputs, 3 larger squares that are the reservoirs and the microfluidic channel. So make three small square using the above command, three larger squares and the microfluidic channel so it looks like the image below.
Save your work Toolbar: SAVE_DB
Extrude the meshed area into a 3D meshed volume.
In this step, first changing the element type to Fluid 142, which is defined as element type 2, and then extruding the area into a volume generates the 3-D volume.
1. Main Menu > Preprocessor >
-Modeling- Operate > Extrude > Elem Ext Opts
2. Choose 2 (FLOTRAN 142) for Element type number.
3. Enter 20 for the No. of element divisions.
4. OK.
5. Main Menu > Preprocessor >
-Modeling- Operate > Extrude > -Areas- By XYZ Offset
6. Choose the small squares that
represent the needles and extrude then 0.21 cm in the Z direction. 7. Apply
8. Choose the 3 larger squares that represent the reservoirs and extrude then 0.20 cm in the Z direction.
9. Apply
10. Choose the microfluidic channel and extrude it 0.01 cm in the Z direction.
11. OK 12. Close
The resulting 3D model should
Inlet
Inlet
Unselect 2-D elements.
Before applying boundary values to the microfluidic channels, unselect all FLOTRAN 141 elements used in the 2-D area mesh since they will not be used for the analysis.
1. Utility Menu > Select > Entities 2. Choose Elements.
3. Choose By Attributes. 4. Choose Elem type num.
5. Enter 1 for the element type number. 6. Choose Unselect.
7. Apply.
Overlap 3D volumes.
This step you overlap the seven different volumes. I tried adding together the volumes but that does not seem to work.
1. Main Menu > Preprocessor > -Modeling- Operate > Overlap 2. Pick all seven volumes.
3. OK.
The 3D modeling design should now be constructed.
Meshing
Of all the steps in this process, meshing is by far the most important step to get an accurate modeling of your system. Meshing breaks up the areas of your design into user defined shapes. The smaller the shape the more accurate the analysis will be but the downside being the finer the mesh, the longer the processing time. A good balance is needed between processing time and resolution of the solution.
The next step is to specify mesh controls in order to obtain a particular mesh density.
2. Set global size controls. 3. Enter 0.005 for element edge length.
4. OK. 5. Mesh.
6. Pick All (in picking menu). 7. Close.
8. Close Mesh Tool. 9. SAVE_DB
Boundary Values
Now comes the fun part, adding velocities and pressures to the areas.
IMPORTANT!! Do not add velocities to the areas inside the volumes. Only the areas
that are exposed to the outside get boundary values.
A velocity of -0.1 cm/s is applied in the Z direction (VZ) at the inlet, and a zero velocity is applied in the transverse direction at the inlet (VX, VY). Zero velocities in all three dimensions are applied on all exterior areas, and a zero pressure is applied at the outlet.
Apply the inlet boundary condition.
1. Main Menu > Preprocessor > Loads > -Loads- Apply > -Fluid/CFD- Velocity > On Areas.
2. Pick the two needle inlets areas. 3. OK.
4. Enter 0.0 for VX. 5. Enter 0.0 for VY.
7. OK
Now, doing a couple of areas at a time, select all other outside areas (except for the needle outlet) and apply a 0.0 to VX, VY and VZ.
8. OK
Apply the outlet condition.
1. Main Menu > Preprocessor > Loads > -Loads- Apply > -Fluid/CFD- Pressure DOF > On Areas.
2. Pick the outlet needle area. 3. OK.
4. Enter 0 for the pressure value. 5. Set endpoints to
yes. 6. OK.
7. Toolbar: SAVE_DB. After all the pressures and velocities have been added to the model, the following picture will appear. This is the most difficult step so be careful and take your time.
If you miss an area, the simulation will not work.
Establish fluid properties.
Fluid properties will be established for water in the cgs system. 1. Main Menu > Solution > FLOTRAN Set Up > Fluid Properties
2. Choose liquid for density and viscosity. Leave conductivity and specific heat as constant.
4. Enter 1.0 for density, 0.01 for viscosity and 0.04 for conductivity. Leave specific heat at –1.
5. OK.
Set execution controls.
Choose the execution control from the FLOTRAN Set Up Menu.
1. Main Menu > Solution > FLOTRAN Set Up >
Execution Ctrl
2. Enter 40 Global iterations (Note: 40 global iterations is arbitrary with no guarantee of convergence.)
3. OK to apply and close.
Change reference conditions.
At the end of this tutorial there is a table of constants and reference conditions for water. The reference conditions will have to be changed to suit your design and units used.
1. Main Menu > Solution > FLOTRAN Set
Up > Flow Environment > Ref Conditions
2. Change the reference pressure to 101350 (cgs units, equivalent to 1 atmosphere). 3. Change the nominal, stagnation, and reference temperatures to 20oC.
4. Change bulk modulus to 0.21x1011
6. OK.
7. Toolbar: SAVE_DB.
The modeling should be ready to be run.
Execute FLOTRAN solution.
1. Main Menu > Solution > Run FLOTRAN 2. Close the information
window when the solution is done.
While running the FLOTRAN solution, ANSYS will plot the "Normalized Rate of Change" as a function of the "Cumulative Iteration Number." This is the Graphical Solution Tracker, which allows visual monitoring of the solution for convergence.
This step might take some
time so be patient, the output should look similar to the following.
Post processing (Laminar Analysis)
Read in the results for post processing.
Enter the general postprocessor and read in the latest set of solution results, and then create a vector plot.
1. Main Menu > General Postproc > -Read Results- Last set
1. Main Menu > General Postproc > Plot Results > -Vector Plot- Predefined 2. Choose DOF solution.
3. Choose velocity v. 4. OK.
1. Main Menu > General Postproc > Plot Results > -Vector Plot- Predefined 2. Choose Nodal solution.
3. Choose Velocity Vx or Vy. 4. OK.
Vy
This gives the velocity profiles for the x and y direction in the channels. Red is the highest pressure. These images show that the output is turbulent.
These images show a generalized velocity profile in the microfluidic system. If using 3D you can rotate the design to get a better angle.
Density Coeff 1 Coeff 2 Molecular wt Viscosity Micro Pa Mass Diffusion Conductivity N2 gas 1.1381 300 101325 28.018 .00001786 .0000160 1 0.02598 H2 gas 0.0819 300 101325 2.016 .00000894 .0000496 0.1815 H2O 1.0 300 101325 18 10 0.00001 0.04 O2 gas 1.2998 300 101325 31.99 .00001206 7 .0000214 9 0.02674
For other constants check out the CRC handbook online at: