Transient Simulation of flow over a throttle body in CONVERGE

In this project, a trasient simulation of flow over a throttle body at different angles will be performed in CONVERGE.

Geometry Creation

The geometry can either be created manually using converge or it can also be imported from another CAD software, in my case I imported the simple geometry.

Do note that converge requires all geometry to be in metres, hence transformation may be required if the geometry was exported as millimetres (mm). This is the final geometry seen in converge.

Boundary

The inlet, outlet and body were given their separate boundaries as usual, and the throttle was given a separate boundary as well.

Case Setup

Now, on to the case setup tab:

Application Type

Materials: Gas Simulation, Global Transport Parameters and Reaction Mechanisms were set to default. In species, O2 and N2 were added.

Simulation Parameters

Calculation of Simulation End Time

The geometry is about 0.01m wide and 0.01m long, hence the furthest distance flow would have to travel in the domain once would be 0.02m. From the steady state analysis performed earlier, the inlet velocity is about 100m/s.

`Speed = (Distance)/(Time) or Time = (Distance)/(Speed)`

Plugging in the values, we get time ~= 0.002s for the fluid to traverse the domain once. To ensure all and any changes in flow must be recorded when the throttle moves, the time was multiplied by 5 to get the end time (0.01s) for the simulation.

Note: a maximum convection CFL limit is required else the solution will never converge.

Boundary Conditions

Inlet: 150000 Pa (1.5 bar)

Outlet: 100000 Pa (1 bar)

Body: No-Slip

Throttle:

The throttle is poised to rotate about the shaft in the geometry as follows:

• From 0 to 2 ms, the valve rotates by 25 degrees.
• From 2 to 4ms, the valve should stay at 25 degrees.
• By 8ms, the valve should get back to its original position.

This movement of the valve is acheived by setting the wall motion type as rotating and manually creating a file throttle_angular_position.in for the rotation rate.

The rotation center and axis is also determined from converge's Geometry -> Measure -> Direction

Initial Conditions

Physical Models

Turbulence was checked as vortices near the throttle are formed due to turbulence, also RNG k-epsilon was chosen as the model since this is a simple simulation.

Grid Control

This is the step were sizes of each element is provided.

Another option, fixed embedding was enabled for this simulation to ensure more cells are used for processing near the throttle.

Output/Post Processing

Because the runtime of the simulation is 0.01s and anywhere between 50 and 100 files are optimal, I chose the minimal file count (50), so interval between each 3D output will be: `0.01//50 = 0.0002`

Now, our case setup is complete. The files will be exported into a folder using the Files Export tool (File -> Export->Export input files)

In total 12 files were exported, these are:

These files contain all the necessary information for the simulation.

Running the Simulation

1. Open cygwin

2. Navigate to directory where case files were exported

3. Run the following command

mpiexec.exe -n 4 "C:Program FilesConvergent_ScienceCONVERGE3.0.16binintelmpiconverge.exe" restricted </dev/null> logfile.txt &

This will take a while, you can view the progress in task manager. CPU usage is usually maxed out.

Once CPU usage drops from 100%, the output files are generated. To view them in paraview, we must export them to a format which is supported by paraview.

Go to 3D-post processing in converge,

Post-Processing

In Paraview

Import these files into paraview

The required plots/animation are generated

In converge

Go to Line plotting, select the case folder and plots can be viewed