Steady state simulation of flow over a throttle body
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 steady-state flow through a throttle body using Converge Studio. However, the throttle body used isn\'t representative of the real-life one. The 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: Pressure-Based Steady State Solver
- Simulation Parameters-: Start Time- 0 cycles
End-Time 15000 cycles
Initial Time Step: 1e-9 seconds
Minimum-Time Step: 1e-9 seconds
Maximum Time Step: 1 second
***The default CFL Values were used for the first run. CFL values determine stability of solution.
- 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
- Turbulence Model- RNG k-`epsilon`
- Base-Grid: 2mmFixed Embedding was used under Grid-Control to refine the mesh locally near throttle.
Entity Type: Boundary
Mode: Permanent
Scale:3
Embed Layers: 2
The input files are exported and the solver is run paralelly using Cygwin.Results were then post-processed using Ensight and viewed on Paraview.
As can be seen, both the inlet and outlet mass flow rates reach a steady state value after approximately 4000 cycles.
The mesh was refined locally near the throttle(empty circle) using fixed embedding to obtain more accurate results.
The above image provides a clear view inside the throttle body with the intlet being at the top and outlet at the right.
The inlet pressure was assigned a value of 1.5e5 Pa and the outlet pressure was 1e5 Pa. In the above figure, the throttle valve experiences the maximum pressure of the flow where it faces the inlet side and towards the outlet the pressure drops to less than 1e5 Pascals. Obviously, a pressure difference is what leads to the flow of air here.
The magnitude of velocity reached a maximum of 3.1e2. Towards the inlet side of the throttle valve where there is a high pressure, the velocity is maximum.At the the outlet side of the throttle valve where there is a lower pressure, due to flow seperation the velocity reduces. The diagram on the right is a clearer representation of velocity of flow over and around the throttle valve.
The simulation was now run for a second time with same parameters as defined above but with different CFL numbers to see what effect it has on the solution. CFL numbers determine the stability of simulation and the smaller the values are the slower the simulation will be. These can be tweaked under \'Simulation Parameters\'.
Maximum convection CFL Limit- From 1 to 0.5
Maximum diffusion CFL Limit- From 2.0 to 1.5
Maximum Mach CFL Limit: From 50 to 75
The solution reached a steady state after 4000 cycles which was the same as that for the original CFL values. However, during the initial phases of the simulation, there is a lesser variation of the values of inlet and outlet mass flow rates.
The velocity magnitude remains the same for the modified CFL values. The simulation could have been run for lower CFL values too but that would just mean higher simulation time. The difference between the two sets of CFL values was suffiecient to demonstrate the changes in simulation stability.