Week 1: Channel flow simulation using CONVERGE CFD

Aim:

Simulate the flow of air through a channel of given dimension using CONVERGE CFD.

Objectives

• Simulate the flow using three different mesh sizes
• Velocity and pressure contours for all the 3 base mesh sizes
• Plots for velocity, pressure, mass flow rate, and total cell count for all 3 base mesh sizes.

Methodology

Here flow properties when air is flowing through a channel is studied using CONVERGE CFD with the help of CYGWIN and ParaView. The simulation is carried out for different grid sizes as follows

	dx	dy	dz
Case 1	0.0004	0.0002	0.0002
Case 2	0.00015	0.00015	0.00015
Case 3	0.0001	0.0001	0.0001

The dimension of the channel is 0.1x0.01x0.01m. The dimensions are in meters.

To create the geometry and to set up the problem, Converge studio is used.

To simulate the problem, CYGWIN is used.

To post-process the results ParaView and converge are used.

Throughout the simulation, the flow is considered to be steady and laminar.

• Pre-processing

Model Creation

Open converge studio and go to geometry. Under geometry click on create and create a box with the following dimensions as shown in the figure. Click on create to generate the geometry.

The next step is to create boundaries by grouping the triangles. Go to boundaries, click on the flag and create the boundaries as required.

Here 5 boundaries are created. Inlet, Outlet, Top_Bottom_wall, Front_2D and back_2D

Enable the normal vectors to see whether the normals are pointing towards the fluid region. To run the simulation, the normals should point towards the fluid region.

If it is not, click on transform, go to normal, select triangle, and click on apply. Now the normals are pointed towards the fluid region.

To check for any errors in the geometry, go to diagnosis and click on find. If all the items are green then the geometry is clean and we are ready to proceed with case setup.

Case setup

Click on begin case setup under case setup. Since we are not dealing with the IC engine problems, select Time-based and click Apply.

Now converge will give access to edit all the simulation setups. Now click on materials to assign material. Select Air as material. Since we are not doing any reactions, uncheck the reaction mechanism and check on species.

Validate gas simulation and global transport parameters and move on to species. Under species add O2 and N2 as species.

Click on the run parameter and select steady-state solver as the solver. Under the misc tab, check the steady-state monitor and uncheck the use shared memory. Click on ok and for load recommended defaults click cancel.

The following changes were made to the steady-state monitor. The solver will run at least for 8000 cycles even if the solution gets converged before.

The simulation is set to run for 15000 cycles with the following time parameters.

Under solver parameters, change solver type to pressure-based solver

Next, we will define the fluid region and initialization parameters by clicking on regions and initialization. Click on add and create a region. Select air as species in that region and change the mass fractions of O2 and N2 as shown in the figure

After defining the regions go to boundary to define the boundary conditions. The following boundary conditions are used and select the above defined region as region for all BC.

Inlet – INFLOW

Specified value = 100001 Pa

Temperature = 300 k

Species = Air

Outlet – OUTFLOW

Specified value = 100000 Pa

Temperature = 300 K

Backflow species = Air

Top_bottom_wall – WALL

No-slip

Temperature = 300 K

Front_2D & back_2D - 2D

Since we are dealing with laminar flow, uncheck the turbulence model from the physical models

Grid sizes are changed as per the case setup defined in the methodology.

Select all the required variables required from the Post variable selection. Since we are dealing with laminar flow, uncheck all turbulence variables from the post variable selection.

Another important thing to note is the time interval for writing the 3D output data files. There should be at least 100 output files available by the end of the simulation. So the number of intervals should carefully be given. We will be keeping 100 as the interval. So for 15000 cycles, there will be 150 output files. For this, click on output files and navigate to writing time intervals. Give 100 cycles at time interval for writing 3D output data files.

Click on final validate to validate all the setup. If all are fine, converge will say that no issues found.

Now go to file then click on export and click on export input files to export the setup files. Save these files to Cygwin run folder to make the simulation run through CYGWIN.

The simulation is run using the CYGWIN interface and the results were then converted to binary form using CONVERGE so that ParaView can understand the result data and post-process it.

Results and Discussion

1. Mesh
   
   
   Case 1 Case 2 Case 3
   
   
2. Velocity Contour
   
   Case 1
   Case 2
   case 3
   
3. Pressure contour
   
   Case 1
   Case 2
   Case 3
   
4. Velocity profile
   
   Case 1
   
   Case 2
   Case 3
   
5. Mass flow rate
   
   Case 1
   
   Case 2
   
   Case 3
   
6. Cell count
   
   Case 1
   
   Case 2
   
   Case 3
   
   
   
   
7. Velocity plot
   
   Case 1
   
   Case 2
   
   Case 3
   
8. Total Pressure plot
   
   Case 1
   
   Case 2
   
   Case 3
9. Animated video
   
   Case 1: https://drive.google.com/file/d/1IeIDGg8dqZf8NT4h6rBxkDDsQOFm8_eX/view?usp=sharing
   
   Case 2: https://drive.google.com/file/d/1yDAVT8Ql02hvUCS3kJF8uINMFLb3E0jd/view?usp=sharing
   
   Case 3: https://drive.google.com/file/d/15SUMK1qtIz04Np0Ad0FgcLFz8Fr_aaBC/view?usp=sharing

Conclusions

• The velocity contour plot shows a clear picture of boundary layer formation for all mesh sizes. Even though the contour looks similar for all cases, the velocity profile gives clarity to the result for different grid sizes.
• The velocity profile at various x distances indicated the formation of a boundary layer in the flow clearly. The profile is almost flat at the inlet and it shows a fully developed profile near the exit. From the plot, the velocity near the wall is Zero and reaches the maximum at the center.
• As the mesh size reduces the simulation time also increases and more accurate results are obtained.
• It is clear from the mass flow rate plot that the mass flow rate at the inlet is the same as at the outlet.
• The total pressure plot indicates that the outlet pressure is increasing to reach up to the inlet pressure.
• The attached animations give a clear visualization of the flow field throughout the simulation. It clearly shows the boundary layer formation.
• From the cell count plot, we can observe that the cell count increases as the grid size reduces. Case 3 has a large number of cells compared to case 1. This leads to more refined results in case 3.