Week 4.2: Project : Transient simulation of flow over a throttle body
TRANSIENT SIMULATION OF FLOW OVER THROTTLE BODY:
Aim: To carry out a simulation of air flow inside an Elbow consisting of a Throttle using Converge 3.0 CFD program and then post-process the results on Paraview.
Introduction: We need to observe the parameters of Air as a medium flowing inside an Elbow of specific size consisting of a throttle inside. The parameters to be observed are change in the pressure, velocity and mass flow rate.
Theory: In this process of simulation, the throttle angles are varied through the flow, so it creates obstruction for medium flowing. The variation of pressure and velocity can be observed at the throttle body.
The throttle body simulation is a depiction of a component of IC engine in which a throttle controls the amount of air or fuel entering the engine. This can also be a depiction of a water pipeline consisting of butterfly valve to control the flow of water.
Simulation End Time Calculation:
Approximate Length of the Flow Path = 0.2 m
Maximum Velocity observed from Steady State Simulation = 192 m/s
Flow Through Time: [0.2 / 192 = 0.00104 seconds]
End Time considering 10 time steps = 0.001 x 10 = 0.01 seconds.
Geometry:
- In the Converge Studio first we import the ‘STL’ file of the geometry. The geometry has a thickness which was cleaned and only inner walls were kept to perform the simulation.
- Then we carry out a ‘Diagnosis’ to check for any errors. If there is no error then the geometry is correct.
- Then check for the vector ‘Normal’, initially the normal point out of the shape which are then transformed to point inward. Since the flow is inside the channel, so the correct position for the normal is pointing inward.
- Now the correct geometry is created, so we move to case set up.
Case Set up Procedure:
- Application Type: Time-based
- Material: Predefined Mixture = Air
- Select Gas Simulation & Reaction Mechanism.
- Reaction Mechanism: O2 & N2 are available elements.
- Simulation Parameters:
- Run Parameter: Transient Solver used. Steady State Monitor is Turned Off.
- Simulation Time Parameter:
- End Time = 0.01 second.
- Maximum Time Step = 1 second
- Maximum Convection CFL limit = 1
- Solver Parameters:
- Navier-Stokes Solver Type = ‘Density based’ selected.
- Misc. Setting:
- Boundary Conditions:
- Inflow: Inlet Pressure = 150000 Pa
- Outflow: Outlet Pressure = 100000 Pa
- Elbow Walls = Law of the Walls condition.
- Throttle Walls = Law of the Walls condition With Rotating Motion.
- Initial Conditions & Events:
- Regions: Initial Pressure = set to 101325 Pa; Temperature = 300 K
- Species Mass Fraction: O2 = 0.23 ; N2 = 0.77
- Physical Models:
Turbulence Modelling= set to RANS [RNG K-Ɛ]
- Grid Control:
Base Grid:
Base Grid (in m): dx = dy = dz = 0.002.
- Fixed Embedding:
- Post Processing:
- Post variables Selection:
- Typical Variables/Geometry: Select desired variables such as Density, Velocity, pressure, temperature.
- Output Files:
- Time Interval for writing 3D output data files = 0.001 seconds
- Time Interval for Writing Text Output = 1e-6
- Time interval for writing restarting output = 0.001 seconds
- Maximum number of restart files saved = 3.
Case setup is complete with this.
- Export case setup: The case setup files are then ‘Exported’ to the destination folders to save the details.
- The destination folders should consist of following files in order to run the simulation and then post process the result on paraview. The files are:
(i) converge.exe (name changed for the ease of run command on cygwin)
(ii) mpiexec.exe (name changed for the ease of run command on cygwin)
(iii) post_convert.exe (under output folder)
- Open Cygwin: Now the simulation is run using Cygwin. We navigate to the desired folder where the setup files were exported.
- Command to Run Simulation: The command to run Cygwin for Converge Studio 3.0 version is as below:
“ mpiexec.exe –n 2 converge.exe restricted “
- Now the simulation starts running due to which output files are generated under a separate output folder.
- These output files are then post processed to convert the data in a format suitable for Paraview.
- Post Convert in Converge Studio: To post process, we use file type as ‘Paraview VTK in-line binary format’. Now the files are post processed with a case name ‘test’.
- This generates ‘test.vtm’ group files which are generally used in Paraview to observe the results.
- Inline Plot: In the Converge Studio we use Inline Plotting option to the curves during simulation run. We can observe the change in the plots and see the simulation reach the steady state condition in the end of the plot.
- Paraview: Open Paraview to check for the results.
Observations from Simulation:
Mesh Formation on Elbow:
Mesh Formation on Throttle due to Fixed Embedding:
Velocity Contour:
Pressure Contour:
Throttle Contour:
Mass Flow Rate Plot:
Velocity Plot:
Pressure Plot:
Total Cell Count:
Velocity Animation:
Pressure Animation:
Throttle Animation:
Results:
- The simulation was run for throttle by varying the angle 0°C for the time step 0 to 0.002 ms, 25°C from 0.002 to 0.004 ms, 25°C from 0.004 to 0.008 ms, 0°C from 0.008 ms till the end of time.
- The Velocity reaches at 172 m/s as shown in the velocity plot. The velocity is minimum as the flow hits the throttle.
- The Pressure rises as the flow hits throttle at the inlet upto 126000 Pa and gradually varies during the movement of the throttle.
- The Mesh refinement with fixed embedding was given at the throttle to capture the various conditions accurately. This is clearly visible in the throttle animation.
Conclusion:
We can conclude from simulation with transient condition was performed using Converge CFD program with varied throttle angles. The finer mesh sizes are able to capture the results precisely. The change in the parameters for a flow can be observed from the various animations, plots, contours and profiles and understand how the results becomes smooth towards the end of the cycle.