Steady state simulation of Flow of Air over a Throttle Body

Objective:

The objective of this project is to simulate the steady-state airflow through a throttle body by using Converge CFD.

Introduction:

A throttle is a mechanism by which fluid flow is managed by constriction or obstruction. The term throttle has come to refer, informally, to any mechanism by which the power or speed of an engine is regulated, such as a car's accelerator pedal etc. In an internal combustion engine, the throttle is a means of controlling an engine's power by regulating the amount of fuel or air entering the engine. The throttle body is the part of the air intake system. In this case, we will simulate the airflow through a fully open & steady-state valve.

Steps for Simulation:

1. Pre-processing
2. Running the simulation.
3. Post-processing.

1) Pre-processing:

This includes the creation of geometry, cleaning of geometry (i.e. solving the geometry issues like transforming normal towards the flow of fluid, solving the gaps between the edges, vertex etc.), setting up the case with various simulation parameters as shown below.

Creating Geometry:

CONVERGE CFD does not provide separate CAD software; instead, it provides simple CAD tools through which some basic geometry can be constructed. We can import geometry or any other design from different software’s.

In this case, we will import the CAD model from Solid Works. The file needs to be saved in ‘.stl’ format and by dragging the CAD file directly from its folder into the Converge CFD we can open the file.

As seen in the above image, the model has a thickness to it. Thickness can be used under conjugate simulations. As we are performing a flow simulation, the thickness is not a major contributor for this simulation; therefore this thickness can be deleted.

Steps: Geometry>Boundary> Find/clean>find>from the drop-down menu 'by angle'.

As soon as we delete, a yellow line appears at the edges of each end, which states that the marked lines are open edges and they are not suitable for the simulation. For joining the open edges following steps should be followed.

Steps: Geometry dock>Repair>Delete>edge>from drop-down menu, select ‘by open edges’>select the edges from geometry>Apply.

Once the geometry is ready, always run the Diagnosis to check whether there are no issues in it for ex. open edges/vertexes, isolated triangles etc.

Steps: select the diagnosis dock> find> in the dialog box all the parameters will appear. If the parameters are checked with a green tick then the geometry is clean.

As the file is been imported from the CAD software, the default design measurements in the CAD software’s are in millimetres. Converge CFD by default designs/considers the geometry to be in meters. So we need to convert the CAD file imported into metres for proper orientation of the geometry.

Steps: Geometry dock> transform>scale>under entity type ‘entire surface’>under scale factor ‘uniform’>enter the conversion scale 0.001.

The conversion rate is 0.001 as we convert the geometry to metres.

Creating a Boundary:

Creating a boundary is one of the most important steps in analytical simulations. Almost every CFD problem is defined under the limits of initial and boundary conditions. We assign boundary conditions to define how the system operates.

Boundary conditions include inlet boundary conditions, outlet boundary conditions, wall boundary conditions, pressure boundary conditions, axisymmetric boundary conditions, symmetric boundary conditions, and periodic boundary conditions.

Steps: Geometry docks>Boundary>Flag>Select 'Create multiple boundaries'> Ok.

For this project, we have set four different boundaries:

1) Walls

2) Inlet

3) Outlet

4) Throttle_valve.

Boundaries are created by selecting a group of triangles.

Case Setup:

In this, we set up the parameters on which the simulation is going to run.

Application> General Flow>Done.

Materials> select Air.

Global transport parameters>Select Ok without changing any parameters.

In Simulation Parameters, Run parameters>Solver> select Pressure-based steady solver from the drop-down menu.

Under simulation time parameters:

Start time> 0 cycles.

End time> 15000 cycles.

Initial time step> 1e-9sec.

Minimum time step> 1e-9 sec.

Maximum time step> 1 sec.

Select Ok.

Solver parameters> select OK.

Under Initial Conditions & Events,

Regions and Initialization> select Add> rename 'Volumetric region user defined'> Select 'Air'>OK.

Boundary Conditions:

During setting up the case, this parameter is always before the Initialization & Events. But it is necessary to understand that Boundary conditions should be set up after Initialization & Events because while setting up the Boundary Conditions it needs to define its regions and therefore we need to set up regions first.

• For Inlet> select 'Inflow' from dropdown menu> Enter pressure '1.5e5' Pa> Select 'Air'
• For Outlet> select 'outflow' from dropdown menu> Enter pressure '1e5' Pa> Select Air.
• For walls> select 'walls' from dropdown menu>No changes in the default setting.
• For throttle_valve> select ‘walls’ from the drop-down menu.

Under Physical Modeling,

Turbulence Modeling, select OK without changing any default setting.

Grid control, select fixed embedding>done

Base grid> enter the mesh dimensions.

1) (dx,dy,dz): (0.002,0.002,0.002)

Fixed embedding>Add>select ‘Boundary’ from dropdown of Entity type>select ‘throttle_valve’ from dropdown of Boundary ID> select ‘permanent’ from Mode> enter in scale: 3 & embedded layers: 2>Ok.

Post Variable Section>Ok.

Output files>Time interval to write 3D output files> 1000 cycles.

As the simulation is set to run till 15000 cycles. The above changes state we will get 3D output data file after every 1000 cycles and in total 15 data files will be collected.

After setting up the case, the input files are exported in a saved case directory from where it is simulated with the help of executor.

2) Running the simulation:

After setting up the case, the input files are exported in a saved case directory from where it is simulated with the help of executor. The executor used in this project is Cygwin64.

The code for running the simulation is:

mpiexec.exe -n 2 converge-2.3.exe </dev/null> logfile-1 &

mpiexec.exe- Run an MPI program

-n and number indicate the number of cores to run the simulation.

converge-2.3.exe is the name of

</dev/null> logfile-1 indicates that the results obtained by running/solving the simulation should be saved in a dedicated file. It is useful as during large simulations results can be saved and retrieved in Notepad.

Computational Time:

The computational time recorded was 2952.169 seconds. The time recorded can be different for others who have different or better computer systems.

3) Post-processing:

• Meshing: Mesh is generated as shown below.

The grid scaling or embedding provided near the throttle valve is shown below:

As seen, the mesh around the throttle is fine than the rest of the geometry. This fine mesh helps us to obtain accurate flow results near the throttle valve.

• Cell counts: The cell count would be high as we have inserted embedding layers around the throttle valve.

Total cells count is 41,454.

Cells in individual cores 1,2,3,4 are 11066, 10236, 10076, 10076 respectively.

• Mass flow rate:

The plot shows that the flow converges around 2000 cycles. The plots of inlet and outlet showcase conservation of mass. Symmetric plots can be seen for both inlet and outlet except at the beginning of the simulation which has some disturbance due to the initial conditions.

• Velocity Plot:

The velocity plot shows that the flow converges at around 2000 cycles.

• Velocity Contour:

Velocity Animation:

https://youtu.be/2bhxmPg3SL4

• Pressure Contour:

Pressure Animation:

https://youtu.be/0gx-bvj6kGo

• Pressure Plot:

The static pressure at the start drops due to the increase in velocity, as per conservation law and then gets stable as the simulation proceeds and converges around 4500 cycles. And total pressure increases at the outlet and converges around 4500 cycles.

The total pressure is the pressure a fluid exerts as it is brought to a stop. Total pressure is what acts on you as you face into the wind and the air collides with your body.

Whereas, Static pressure is the pressure you have if the fluid isn't moving or if you are moving with the fluid.

• Velocity and pressure near Throttle:

Pressure

Velocity

The velocity at the gap between the throttle and wall is high as the pressure at that region is least. Whereas the pressure is high at the front of the throttle from the inlet side which results in low velocity. This is due to the phenomenon of Bernoulli's Effect i.e. the velocity of the fluid increases with a decrease in pressure and vice versa.

• Conclusion:

1. The concept of Bernoulli's effect can be seen in this simulation which states that velocity is inversely proportional to pressure.
2. Finer meshing can be provided for more accurate flow simulation.