Week 4.2: Project - Transient simulation of flow over a throttle body
Aim:
Set up and run the transient state simulation for flow over the throttle body.
Objectives
- Post-process the results and show pressure and velocity contours.
- Show the mesh (i.e. surface with edges)
- Show the plots for pressure, velocity, mass flow rate, and total cell count.
Introduction
A throttle valve is used in a fluid-flowing pipe to regulate the amount of fluid passing through the pipe. The throttles can be operated manually and automatically based on the need. There are different kinds of throttle valves used in different applications. Those are,
- Ball valves
- Butterfly valves
- Gate valve
- Diverter valve etc.
Here in this simulation, we will briefly look into the operation of the throttle valve.
Methodology
Here flow properties when air is flowing through a throttle body are studied using CONVERGE CFD with the help of CYGWIN and ParaView. The geometry is imported from a 3d file. The dimensions are in meters.
To create the geometry and to set up the problem, Converge studio is used.
To simulate the problem, CYGWIN is used.
To post-process the results ParaView and converge are used.
Throughout the simulation, the flow is considered to be steady and turbulent.
- Pre-processing
Model Creation
After opening converge studio import the 3D part file.
Remove all the unwanted surfaces in the geometry and close all the open edges.
The next step is to create boundaries by grouping the triangles. Go to boundaries, click on the flag and create the boundaries as required.
Here 4 boundaries are created, inlet, outlet, wall, and throttle. The throttle is inside the wall which is not visible in the following image.
Enable the normal vectors to see whether the normals are pointing towards the fluid region. To run the simulation, the normals should point towards the fluid region. If it is not, click on transform, go to normal, select the triangle, and click on apply. Now the normals are pointed towards the fluid region. To check for any errors in the geometry, go to diagnosis and click on find. If all the items are green then the geometry is clean and we are ready to proceed with case setup.
Case setup
Click on begin case setup under case setup. Since we are not dealing with the IC engine problems, select Time based and click Apply.
Now converge will give access to edit all the simulation setups. Now click on materials to assign material. Select Air as material. Since we are not interested in species uncheck species and click on apply.
Validate gas simulation and global transport parameters and move on to run parameters. Change the solver to the transient solver. Under the misc tab, check the steady-state monitor and uncheck use shared memory. Click on ok and for load recommended defaults click cancel.
Next, we will define the fluid region and initialization parameters by clicking on regions and initialization. Click on add and create a region. Select air as species in that region and click ok.
After defining the regions go to boundary to define the boundary conditions. The following boundary conditions are used, and select the above defined region as region for all BC.
Inlet – INFLOW
Specified value = 150000 Pa
Temperature = 300 k
Species = Air
Outlet – OUTFLOW
Specified value = 100000 Pa
Temperature = 300 K
Backflow species = Air
Wall – WALL
Law of wall
Temperature = 300 K
Zero normal gradients (NE)
As we are dealing with transient case, the throttle is set to rotate about an axis up to 25 degrees, the following changes were made to the throttle boundary conditions,
Wall motion is set to rotating and surface movement is set to moving
A rotation rate file is created to define the rotation of throttle as follows and it is saved as rotation.in,
After accepting all the changes the rotation axis and center are defined. These values are obtained by going into measure-direction and clicking on arc normal. Now select three points on the throttle axis geometry and click apply.
Since we are dealing with turbulent flow, select RNG k-epsilon from turbulence.
Grid sizes are changed as per the case setup defined in the methodology. We go for fixed embedding in grid size options. Click on add, select BOUNDARY as entity type, select wall, and select permanent as mode.
The following changes were made to the steady-state monitor. The solver will run at least for 1000 cycles even if the solution gets converged before.
The simulation is set to run for 15000 cycles with the following time parameters.
From the steady-state simulation, it was observed that the average velocity is 195m/s.
The characteristic length of the geometry is found to be 0.2m.
Hence the time = 0.2/195 = 0.001s.
Considering 10 times the above time as flow time, we have 0.01s as time.
Under solver parameters, change solver type to pressure-based solver
Select all the required variables required from the Post variable selection and click on ok.
Another important thing to note is the time interval for writing the 3D output data files.
The following changes were made to get at least 20 3d output files. (0.01/20=0.0005)
Click on final validate to validate all the setup. If all are fine, converge will say that no issues were found.
Now go to file then click on export and click on export input files to export the setup files. Save these files to Cygwin run folder to make the simulation run through CYGWIN.
The simulation is run using the CYGWIN interface and the results were then converted to binary form using CONVERGE so that ParaView can understand the result data and post-process it.
Results and Discussion
- Mesh
Refined adaptive meshes near the throttle.
- Velocity contour open position
When the throttle is opened, the velocity distribution is almost same everywhere.
- Velocity contour 25 degree position
After 2milli seconds, the throttle is at a 25-degree closed position. During that position, the flow is interrupted by the throttle. A sudden change of velocity change can be observed after the throttle region. Also, a small wake region is generated just behind the throttles.
- Pressure contour open position
The pressure distribution shows that there are no significant pressure changes inside the pipe except at the throttle where the flow directly hits the throttle.
- Pressure contour 25-degree position
After 2 milliseconds, the throttle is at a 25-degree position. We can observe a sudden pressure build-up before throttle. Also, a low-pressure region is created just after the throttle resulting in a wake region. - Mass flow rate plot
From the above plot, it can be observed that the mass flow rate is varying between 2 and 4 milliseconds where the throttle is at 25 degrees, after 8 milliseconds the flow rate is constant.
The flow is more when the throttle is open.
- Average Velocity
The same trend as in the mass flow rate can be observed here also. The velocity is increased when the throttle is opened.
- Total Pressure
The inlet pressure is trying to reach the outlet pressure.
- Total cell count
- Streamlines animation
https://www.youtube.com/watch?v=MKa_Wf5jq_8ab_channel=AlwinThomas - Velocity animation
https://youtu.be/s2fks7TiQfg - Pressure animation
https://youtu.be/CrPpQ5t55PQ
Conclusion
- The velocity contour shows the velocity distribution throughout the flow region. Just after the throttle, we can observe the formation of wake regions.
- When the throttle is at the maximum open position, the maximum amount of fluid passes through the throttle. At 25 degree position, the flow is restricted, and correspondingly the velocity, as well as the flow, got reduced.
- We can observe a pressure build-up on the fluid strike face of the throttle in the pressure contour. As it passes over the throttle, the pressure decreases and the velocity increases and the fluid passes over the throttle region by creating a wake region just behind the throttle body.
- It is clear from the mass flow rate plot that the mass flow rate at the inlet is the same as at the outlet.
- The total pressure plot indicates that the outlet pressure is increasing to reach up to the inlet pressure.
- The attached animations give a clear visualization of the flow field throughout the simulation. It clearly shows Separation also.