Week 3: Flow over a backward facing step

Aim:

• To simulate the airflow over a backward-facing step using converge studio.
• To study the turbulent flow characteristics downstream to backward-facing step.
• To compare the flow over backward-facing step for different mesh grading of 2e-3, 1.5e-3, and 1.0e-3.

Theory:

Flow separation:

Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into awake. Separation occurs inflow that is slowing down, with pressure increasing, after passing the thickest part of a streamlined body. or It is a point at which the velocity gradient at the surface in a direction normal to the wall decreases to zero.

This separation happens due to the presence of either viscosity or pressure gradient or due to abrupt change in geometry. In most times it occurs due to the change in the geometry at sharp edges. From the below figure the flow was separated due to a change in the geometry. It shows the recirculation zone which happens due to flow separation. In this type of zones, zone eddies form i.e flow flows in a circular fashion. After that the recirculation zone is reattached and flow again happens over the surface.

Procedure:

1. Load the geometry and assign the boundaries and check the direction of the normals. Actually, the normals should be pointed towards the flow of fluid or air in the channel. Enable the normals by clicking the normal toggle icon which is located on the bottom side in the left panel. If the normals are pointed reverse then select any triangle on the face whose normal is pointed away from the flow and select transform in the geometry toolbox > normal > click on propagate change from single triangle and press apply.

2. Case setup:

2.1 select application type as time-based. In materials select premixed mixture as air.

2.2 Run Parameters: Select steady-state solver in the solver and under Misc. uncheck the use shared memory and check the steady-state monitor and click on ok.

2.3 Simulation Time parameters:

2.4 Solver parameters:

3. Regions and Initialization:

4. Physical modeling:

5. Grid control: Enable the fixed embedding check and click on ok. By using fixed embedding, we can get finer mesh around the top and bottom walls. This case study using 3 different mesh grid sizes. So, the different mesh grids are dx = dy = dz = 2e-3, 1.5e-3, and 1e-3.

6. Output/Post-processing:

6.1 Post variable selection: Keep all default settings and press ok.

6.2 Output files:

7. Boundary Conditions:

And for back-2d and front-2d select boundary type as TWO-D.

8. Now export all input files: Click on files > Export > Export input files > choose the path and select all files and press ok.

9. Run the simulation and post-convert the files: Copy and paste the mpiexec.exe file and converge.exe file to the folder in which the input files are exported to run the simulation in Cygwin. Copy the post_convert_30_msmpi_64.exe file and paste it into the output folder. Now open Cygwin and go to the output folder and type mpiexec.exe -n 4 post_convert_30_msmpi_64.exe > enter the case name(as case1) > select file type as Paraview VTK in-line binary format > type all for files and cell variables and click enter (or) we can convert those files in converge studio.

10. Post-processing the files:

To post-process the files lanch paraview and open the file location as case1 which is saved in output > case1 .. vtm (this is a group file which is formed during post-converting the files) > click on apply > create a slice along the z-axis.

Results:

For 2e-3:

Pressure counter:

Velocity counter:

For 1.5e-3:

Pressure counter:

Velocity counter:

For 1e-3:

In this case of the backward-facing step the flow gets separated at the step due to a change in geometry. Here the flow flows in a circular fashion.

Pressure counter:

Velocity counter:

Plots:

For 2e-3:

Mass flow rate:

Pressure:

Velocity:

Total cells:

We can observe that the velocity is high where the cross-sectional area is low. The inlet mass flow rate is almost equal to the outlet mass flow.

For 1.5e-3:

Mass flow rate:

Pressure:

Velocity:

Total cells:

We can observe that the velocity is high where the cross-sectional area is low. From the mass flow rate plot the inflow and outflow mass flow rates are equal.

For 1e-3:

Mass flow rate:

Pressure:

Velocity:

Total cells:

The inlet and outlet mass flow rate is almost equal. For 1e-3 we can see that we have enabled fixed embedding, due to the mesh grid size. We can observe that the velocity is high where the cross-sectional area is low.

where and why separation is taking place?

In a backward-facing step, the flow separation occurs due to unexpected changes in the geometry. It happens after the step, near the corner formed by the bottom wall and the step. The above images show where the flow separation happens.

Animation link for 2e-3:

https://www.youtube.com/watch?v=gBhSYlT3ZfU&ab_channel=Harsha

Conclusion:

• If the mesh size decreases, the results become more accurate and produce smooth plots due to increasing in the computation time.
• If the mesh size decreases, the converge decreases the time step to maintain the CFL number.