Transient simulation of flow over a throttle body

                                                                                             TRANSIENT STATE SIMULATION OF FLOW OVER A THROTTLE BODY USING CONVERGE CFD

                                                                                                                                                  (WEEK-4.2 CHALLENGE)

1. AIM

Our aim is to setup a transient state simulation of flow over a throttle body in converge, simulate it using Cygwin terminal and post process in Paraview and check the results.  

2. THEORY/EQUATIONS/FORMULAE USED

Structure of CONVERGE CFD simulations

The basic steps for a simulation are as follows,

1. Pre-processing [Converge Studio v3.0] – It is a leading CFD package for simulating 3D fluid flow. It is a user friendly interface which provides high productivity and easy-to-use workflows. Its features includes autonomous meshing, state of the art physical models, robust chemistry solver, and the ability easily accommodate complex moving geometries.
2. Solving [Cygwin] – It is a large collection of GNU and Open Source tools which provide functionality similar to a Linux distributionon Windows.
3. Post-processing [ParaView 5.9.0] – It includes analyzing the results by plotting the results as charts, contour, and exporting the data, also validating the results both qualitatively and quantitatively.

Throttle Body

Throttle Valve is a device that regulates the flow of fluid. It is mainly used in internal combustion automotive engines. It is usually located between the air intake filter and the intake manifold. It regulates amount of air that should go into the engine, based on input given through the gas pedal by the driver. As more air flows into the engine, it injects more fuel, thus allowing for more power. Throttle bodies today are mostly electronically controlled, where this throttle body is mechanical. In this steady state simulation the throttle valve is operated at 0 deg position.

Problem – Flow over a throttle body

The challenge includes transient state simulation of flow over a throttle body, and to check the results.   

                                             Mesh Size -   dx = dy = dz = 0.002 m

Total simulation time

Rough Length of the pipe = 0.2 m

Average Velocity = 190 m/s

Total Time required for single flow cycle = 0.2/190 = 0.00105 [Time = Distance/ Speed]

Total Time required for 10 flow cycle = 0.00105*10 = 0.0105.

So, End time taken = 0.01 s [10 Milliseconds]

3. PROCEDURE

• Firstly all the softwares are downloaded and installed in the system.
• Geometry is created in converge studio, boundary is flagged, and boundaries are checked for normals and orientation is done as per the requirement and finally diagnosis check is carried out to check for any errors such as intersections, open edges, non-manifold edges, overlapping triangles etc.
• Case setup is done using setup wizard, then the input files are exported to a specific directory and also converge executable application are also copied into the directory.
• The simulation is run using Cygwin commands and post converted the output files so that it is compatible for post processing in Paraview.

4. NUMERICAL ANALYSIS
   • Geometric Model

The 3D geometry file of elbow with throttle body is imported in Converge Studio as per the figure given below.   

3D Geometry – Elbow with throttle body

• Mesh

                                                                         Fig: Mesh with fixed embedding at the throttle body

• Boundaries

                                                                              Fig: Boundaries of the domain

• Case Set-up
• In Materials tab, materials – Air, species – O2, N2 and global transport parameters are selected.

• In Simulation Parameters tab, Run parameters such as Solver type, Simulation Mode and Gas flow Solver are set. In Misc. Tab uncheck the shared memory and check the steady state monitor.

• Simulation time parameters such as total number of cycles, time step, CFL limits etc are set as shown in the figure below.

• Solver parameters such as the Navier stokes solver type, equations preconditions type, solver controls are set.

• Boundary Conditions and Initial Conditions

• Throttle Boundary

                  Fig: Arc Center and Arc Normal for Rotation Axis                                      Fig: Throttle Profile Configuration

• Regions and Initialization – Fluid domain is created and the Species is added to the fluid domain.

• Turbulence Modelling – Reynolds Averaged Navier Stokes – RNG K-epsilon Model

• Base Grid – Mesh sizes are entered as per the problem and fixed embedding is done at the wall boundary.

• Post Variable Selection - Select all the necessary variables and the location that needs to be checked while post processing.

• Output Files – Output files writing time intervals, restart files are set as per the requirement.

• After the setup is done click on “Validate all” option to check for any issues with the case setup. If everything is correct then green tick will appear for all tabs as shown in the figure below. Once the Setup is done, export the input files.

• With input and executable files, navigate to the specific directory in Cygwin and run the simulation using "exe -n 4 converge.exe restricted </dev/null> logfile &"
• After the run is completed Post convert the output files using “exe -n 4 post_convert.exe” into binary inline vtk format.
• Using the vtm group files, post processing is done in ParaView in order to study the results.

5. RESULTS

                                                            Fig: Total Cell Count

                                                      Fig: Mass Flow Rate Plot

                                                       Fig: Total Pressure Plot

                                                    Fig: Average Velocity Plot

                                                                  Fig: Velocity Streamlines

                                                                                        Fig: Velocity Vector

                                                           Fig: Pressure Contour 

                                                        Fig: Velocity Contour

Animation Link

Velocity – https://youtu.be/XUQ1LOdfFMk

Pressure – https://youtu.be/NjVmQk7QwEs

Mesh - https://youtu.be/WqbgJAWLGHk

    6. CONCLUSION

• The transient state flow simulation over a throttle body is carried out for 10 milliseconds.
• The physical quantity plot shows that initially there are fluctuations; they eventually died down about 8 milliseconds.
• Cut cell meshing strategy is used in converge. The total cell count kept changing as the throttle body was rotated around its axis.
• Fixed embedding is used on the throttle valve body as the clearance between the elbow and throttle body is very fine, so fixed embedding is used in order to capture that flow domain accurately.
• Plot shows that the mass flow rate at the inlet and outlet are equal, total pressure at the outlet has increased from 1e5 to 1.2e5 Pa, still less than the inlet total pressure due to which the flow direction in maintained from inlet to outlet as shown in the velocity streamlines.
• Average Velocity at the inlet is around 190 m/s has increased to 220 m/s at the outlet due to the presence of throttle body.
• The animation shows the movement of throttle body and the corresponding change in mesh, pressure and velocity.
• Pressure contour animation shows the high pressure region at the upstream of the throttle body as the throttle body is deflected 25 deg, flow is getting injected directly on to the body and low pressure region is created at the downstream and fine clearance area.
• Velocity contour animation shows the low velocity region at the upstream of the throttle body throttle body is deflected 25 deg, flow is obstructed directly by body and it increases as it moves through the fine clearance as the area is reduced and eventually the average velocity increases at the outlet when the throttle body is deflected back to its original position.

    7. REFERENCES

• https://skill-lync.com/knowledgebase/simulation-crashed-restart-it-and-save-time
• https://www.researchgate.net/publication/278664165_Learning_Throttle_Valve_Control_Using_Policy_Search
• https://skill-lync.com/knowledgebase/how-to-change-opacity-in-paraview
• https://www.youtube.com/watch?v=wmrvnZT4aDU&ab_channel=EngineeringExplained