Week 3: Flow over a backward facing step

Aim:

Simulate the flow over a backward-facing step 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.
• Explain where and why separation is taking place.

Methodology

Here, flow properties when air flows through a channel are 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	0.002	0.002	0.0002
Case 2	0.0015	0.0015	0.0015
Case 3	0.001	0.001	0.001

The geometry is imported from a 3d file. 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 turbulent.

• Pre-processing

Model Creation

After opening converge studio import the 3D part file of backward-facing step 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.

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 the 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 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 interested in species uncheck species and click on apply.

Validate gas simulation and global transport parameters and move on to run parameters. Change the solver to a steady-state solver. Under the misc tab, check the steady-state monitor and uncheck use shared memory. Click on ok and for load recommended defaults click cancel.

Next, we will define the fluid region and initialization parameters by clicking on regions and initialization. Click add and create a region. Select air as species in that region and click ok.

After defining the regions go to the 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 = 110325 Pa

Temperature = 300 k

Species = Air

Outlet – OUTFLOW

Specified value = 101325 Pa

Temperature = 300 K

Backflow species = Air

Wall – WALL

Law of wall

Temperature = 300 K

Zero normal gradient (NE)

Front & back - 2D

Since we are dealing with turbulent flow, select RNG k-epsilon from turbulence.

Grid sizes are changed as per the case setup defined in the methodology. We go for fixed embedding in grid size options. Click on add, select BOUNDARY as entity type, select wall, and select permanent as mode.

The following changes were made to the steady-state monitor. The solver will run at least for 5000 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

Select all the required variables required from the Post variable selection and click on ok.

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 an 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 as 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 were 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
   Case1
   Case 2Case 3
   
2. Velocity contour
   Case 1
   Case 2
   Case 3
   
3. Pressure contour
   Case 1
   Case 2Case 3
   
4. Separation
   Case 1
   Case 2
   Case 3
   
   
5. Mass Flow rate
   Case 1
   Case 2
   Case 3
   
   
   
   
6. Velocity plot
   Case 1
   Case 2
   Case 3
   
   
7. Total Pressure
   Case 1
   Case 2
   Case 3
   

9. Cell count
   

   Case 1
   
   Case 2
   
   Case 3

   Separation

   A low static pressure region is created just after the step due to the sudden deceleration of flow called as wake region. The plot under separation shows that the flow gets separated from the boundary and recirculates within the wake region. This phenomenon is called separation. So in the flow field, due to the sudden geometry variation, an adverse pressure gradient is created. It is clearly visible in the plots.

   Conclusion

   • The velocity contour shows the velocity distribution along the length for different mesh sizes. It is clear from the plots that the higher the refinement, the higher clarity of the flow field.
   • The velocity is maximum at the inlet and it got reduced as the flow progressed.
   • 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 Separation also.