Aim:

1. To setting up a backward step flow tutorial by using the converge.

2. To run the case for three different base mesh sizes by Cygwin

3. To post process using the Paraview.

4. To compare the results between all the simulation

Geometry

For simulate airflow in the backward-facing step closed passage.

The backward-facing step is widely used for its application of turbulence in internal flows. The backward-facing step is widely used for its application of turbulence in internal flows. The flow separation is caused due to the sudden changes in the geometry and the inlet and outlet pressure values. The flow separation causes a point of reattachment and a point of recirculation.

The backward-facing step represents separation flows, which can be widely seen in aerodynamic flows, heat transfer, engine flows, condensers, vehicles, and flow around the building.

Mesh Size

dx = dy = dz = 2e-3m

dx = dy = dz = 1.0e-3m

dx = dy = dz = 1.0e-4m

Geometry/Model

Drag and drop predefined model (in .stl format) to the Converge studio. Make sure all the normal towards inside because we are analyses inside of the backward step channel step.

To create the boundaries, select the boundary tab, create new boundary by using the” +” icon. Define the ID, Name and color of the surface. Then ok

Select the surface in the model and the select the boundary, then apply and we can observe that surface color will change according to Boundary ID and name.

After assign the boundary, Diagnosis the geometry by diagnosis option.

Case setup

Time based simulation has been selected for the simulation

Gas Simulation

Here we need to select reaction mechanism even though there is no reaction mechanism

Global transport Parameters

Reaction Mechanism

Simulation Parameters,

Run Parameter

Total number cycle will be 15000, Initial time step 1e-06.

The CFL limit for maximum convection has been set as 1 to control the simulation stability.

Density based solver chosen

Preconditioner used to be none

Boundary Conditions

consider 1.10325 bar pressure at inlet. Temperature is 300K and zero normal gradient for velocity boundary condition.

Outlet

consider 1.01325 bar pressure at outlet

Top and bottom wall

Front and back 2D

Initial conditions

Physical Model

Grid Control
selected adaptive mesh refinement and fixed embedding.

Base Grid

Change the grid size according to mesh size

Adaptive mesh refinement

Fixed Embedding

Post processing

Mass fraction is defined

In Output files, the time interval for writing 3D output files & writing restarting output files has been decreased from 1000 to 150 to maintain the 100 output files to post process the simulation results.

Export the input files

File >export >export input files

These files will be used in the Cygwin to perform the simulation as Converge CFD does not have solver in built.

Simulation,

Go to the Converge installed location

C:\Program Files\Convergent_Science\CONVERGE\3.1.9\bin\intelmpi

Copy the converge.exe files to The input folder.

Open the Cygwin64 terminal, navigate to the Input files folder.

“mpiexec -n 4 converge --license explore” use this command for parallel computing.

Analysis of all the cases

Solution converged at 599 cycles. Time taken 10.115 seconds

Convert the output files to Paraview VTK file. Navigate the directry to outputfolder.

Select all files. Click on convert.

Open Para view, Navigate to output folder. Find the .vtm , group type file. Open it

Click on the slice dataset with plane, select the z-normal, uncheck planes, uncheck triangulate the slice. Then apply.

Mesh view

Results

Case 1

For the Grid size 2e-3m

Mesh view

Recirculation region

Velocity contour

Pressure contour

Mass flow rate

Total pressure Plot

Velocity plot

Cell Count

Case 2

For the Grid size 1e-3m

Mesh view

Velocity Contour

Pressure Contour

Mass flow rate plot

Velocity plot

Pressure Plot

Cell count

Case 3

For the Grid size 1.0e-4m

Mesh View

Velocity Contour

Pressure Contour

Recirculation Region

Mass Flow rate

Total Pressure plot

Velocity Plot

Total cell

All the animation upload in the uploaded in YouTube, link available below

https://youtu.be/qOevhYD2lr0 (Velocity)

https://youtu.be/8IuzboAUxcY (Pressure)

https://youtu.be/5JuzmeHeEZU (Density)

Conclusion

From the above analysis, we can conclude that

1. When mesh is refined, simulation run time increases and also accuracy increases

2. By observing Velocity plot (paraview), Velocity magnitude is smoother in case 3 than case1 Hence, as the grid size increases the results obtained are much clear and accurate.

3. As air enters the inlet of the backwards-facing step experiences turbulent flow, the turbulent flow is characterized by choosing the proper turbulence model to visualize the flow.

4. The turbulent flow initializes the formation of eddies near the wall and away from it. The size of the eddies is restricted near the wall and as we move away from the wall the size gets bigger.

5. The flow gets separated from the boundary layer after the step due to a change in the geometry and after the step, there are a recirculation zone and reattachment zones leading to the deacceleration of the fluid and thus reduction in the static pressure.

6. The average velocity at the inlet is higher than that of outlet because of sudden expansion region.