Transient simulation of flow over a throttle body using Converge CFD
A Throttle Body consists of a butterfly valve that opens and closes to let air inside an IC Engine. Some cars use a a single large throttle body to let in air while some have a dedicated throttle body for each cylinder. The following exercise involves simulating transient flow through a throttle body using Converge Studio. The valve will open after a certain duration. However, the throttle body used isn\'t representative of the real-life one. The actual valves are sealed with gaskets and O-rings above and below to prevent leakage of air. The image below shows the elbow and valve of the throttle body that was simulated.
After cleaning up the geometry and preparing the surface, the case setup was done. The following parameters were used in setting up the case for flow:
- Solver: Transient
- Simulation Parameters-: Start Time- 0 seconds
End-Time 0.01 seconds
Initial Time Step: 1e-9 seconds
Minimum-Time Step: 1e-9 seconds
Maximum Time Step: 1 second
- Boundary Conditions-:
Inlet: Type-INFLOW, Pressure-1.5e5 Pascals and Temperature-300K
Outlet: Type-OUTFLOW, Pressure-1e5 Pascals and Temperature:300K
Elbow: Type-Wall
Throttle/Valve: Type:Wall; Wall Motion Type: Rotating( The throttle valve rotates by certain degrees to open and let air in)
Rotation Center: -7.615316E-002 3.907758E-002 2.017420E-002 (This is the center about which the throttle rotates.)
Rotate about:-7.944360E-001 -6.073478E-001 0.000000E+000 (The axis of rotation of the throttle valve. The axis is the same as direction of normal of the valve)
Rotation Rate(degrees/seconds): At 0 seconds, Angle of rotation: 0 degrees; At 0.002 seconds, Angle of rotation:25 degrees
The figure which is a cut section of the throttle valve gives a clearer picture of the axis of rotation which is the direction cylinder is slanted towards. The center of rotation of valve is the same as the center of the ring above.
- Turbulence Model- RNG k-`epsilon`
- Base-Grid: 2mm
Fixed Embedding was used under Grid-Control to refine the mesh locally near throttle.
Entity Type: Boundary
Mode: Permanent
Scale:3
Embed Layers: 2
- Output Files: Time interval for writing 3D data output files: 0.001seconds (The simulation end time is 0.01 seconds and to generate 10 output files this should be the time interval) Time interval for writing text-output: 1e-6 seconds (to write output at every time-step)
The input files are exported and the solver is run paralelly using Cygwin.Results were then post-processed using Ensight and viewed on Paraview. The simulation is first run with \'No Hydro-Dynamic Solver\'. This mode doesn\'t compute any values but is only used as a test-setup to check the mesh.
This is when the throttle valve is closed.
The throttle valve is opened at 25 degrees. It can be observed that the mesh embedding moves along with the throttle valve.
After successfully checking the test setup, the simulation was run under \'Full-Hydro Dynamic\' condition.
When running the simulation in transient state, the cell count never reaches a steady state value, instead it keeps updating at every time-step. However in the steady-state condition the number of cells remain constant over the entire duration of simulation.
The mass flow rates also take a longer time to reach a steady state value and during the intial simulation it varies a lot.
A transparent view inside the throttle body
The magnitude of velocity reaches an upward of 3.6e2 when the throttle valve is opened. Towards the outlet side of the valve where there is a flow seperation, velocity reduces and even reaches zero.
When the flow encounters the throttle valve, the pressure is higher towards the face of the valve that is directly in line of the flow which also shows the restricted area of flow. On the opposite side of the throttle valve where the area increases due to the opening of valve, there is a pressure drop. This can be clearly visualized by plotting the velocity vector and observing the speed of flow around the throttle valve (image below)