Week 4.2: Project - Transient simulation of flow over a throttle body

AIM - Transient simulation of flow over a throttle body.

OBJECTIVE - 

Setup and run transient state simulation for flow over a throttle body.

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.

Also show the calculations on how you calculated an end time for the simulation.

CALCULATIONS - 

From the last project of steady state simulation of flow over a throttle body, the velocity flow rate plot shows that the inlet flow velocity at steady state is about 192 m/s and from the geometry of the throttle body we can see that the total length of the pipe is almost close to 0.2 m so if we calculate the flow time that will be

`sf"flow time " = sf"length of the throttle body"/sf"velocity of flow through the body"`

`sf"flow`time " = 0.2/192 = 0.00104 sec`

so from here if we want to perform a transient simulation we will be running the simualtion for a time period of 4 or 5 times of the flow time so almost 5 milli seconds.

PRE-PROCESSING -

In the pre processing the case is setup up so that it can be input files for the converge processing can be generated.

The application type is set to time based.

For the Materials Air is selected as the predefined mixture and the box for gas simulation and reaction mechanism is checked. 

The setting for the gas simulation, global transport equations and the reaction mechanisms are kept as the default and no edits are made.

For the simulation parameters is we have run parameters, Simulation time parameter and solver parameters options turned on.

To set up the run parameters set the solver type as transient, temporal type as time-base simulaiton and the simulation mode as fully hydrodynamic.

to set up the simulaiton time parameters set the end time for the simulation as 5 ms that is 0.005 s and initial time step and the minimum time step is set as 1e-9 and the maximum time step is set as 0.005 s. Also make sure that the CFL limits are 1, 2, 50 for convection, diffusion and mach respectively. 

In the solver parameters section the Navier-Stokes solver scheme is set as PISO and the solver type is set as Density-based.

Boundary Conditions:-

Elbow wall-  the elbow wall has boundary type as WALL and the wall motion is stationary with fixed surface movements. The temperature at the elbow wall is 300K. 

The Inlet has INFLOW boundary type where the Pressure is 150000Pa and the temperature is 300K and the species at the inlet inflow are standard air mixture of oxygen and nitrogen having mass fractions 0.23 and 0.77 respectively.  

The outlet boundary has OUTFLOW boundary type and with pressure as 100000Pa and the temperature and the species being same as the inlet boundary condition.

The throttle has a boundary type set to WALL and the wall motion is rotating and the surface movement as MOVING.

The rotate center and rotate about are measured using the following steps,

select any three points on the surface of the circular arc of the throttle axis as shown below and then select apply the button at the bottom of the measure tab.

Now to set up the initial conditions again go to the case setup tab and select the regions and initialization option

Here in the above window we have our region name as Region 0 . Also we have to select the +Air button next to species and it will load the oxygen and nitrogen in the standard air mixture mass fraction ratio as shown in table below that.  

To set up the physical model we will use turbulence modelling for this simulation and to do that check the box in front of turbulence modelling.

We will use k-epsilon turbulence modelling here and keep all the settings as default for the case.

MESHING - 

To setup the grid control we will turn on the fixed embedding option this will help in creating finer grid at the region of inportance which in this case is the throttle body.

The base grid size is kept as 2 mm in all the three directions

to setup the fixed embedding we have selected the entity type as BOUNDARY and the boundary is the THROTTLE because we have to inspect the flow close to the throttle baoundary also the mode for embedding is set as PERMENANT mode.

Here we have a scale of 3 and the number of embedded layer is 2.

 To set up the output files we have set the time interval to write 3D output data files and 5e-5 and to write the text output is 1e-6 and also the time interval to write the restarting output files is 5e-5. 

 Now the case setup is ready so the input file are exported at a location where the simulation is needed to run.

to export the input files go to Files>>Export>>Export Input Files and then a window will open there select appropiate location of the folder to save the input file to run the simulation.

PROCESSING -

Go to the cygwin terminal with run in admin option and locate the folder where the input files are exported. Now to run the simualtion there use the command as shown below

mpiexec.exe -n 4 converge.exe restricted </dev/null> logfile &

the mpiexec.exe -n 4 helps to run the simualtion in parallel using 4 number of processors.

the converge.exe is the main command that is used to run any converge simualtion 

and lastly all the output that is showed in terminal is moved to a logfile and can be saved for further use and the & sign run the process in background so that the terminal can be used for any other purpose.

Also to check the status of the simulation while running in background by using the following command

tail -n 20 logfile

once the simaltion is completed we have to now convert the output files into a form that can be used by paraview to do so go to the output folder that is generated within the same location and use the following command 

mpiexec.exe -n 4 post_convert.exe

while running the command it will as to name the file and we can give any name after that it will ask for the format in which the files are need to be converted so select the vtk inline binary format and then select all for rest of the options it will ask to convert all the files.

POST-PROCESSING - 

Now open simualtion is paraview and we can see the results.

MESH-Generated :

At time t = 0 s

at time t = 0.005 s

Fixed embedding in the mesh close to the throttle can be seen below and it is good enough to be used for further analysis.  

Cell count for the simulation is given in the plot below. we can see that the fluctuation in the cell count is due to the motion of throttle body and mesh froming again at each timestep.

Now we will see various results for the contours, vectors and plots.

Contours:

Pressure contour-

Density Contour-

Velocity Contour-

Velocity Streamline -

Plots :

Mass Flow rate at inlet & outlet-

Video Animation for the flow over a throttle body:

CONCLUSION -

1. Transient flow over a throttle body is simulated using converge. 

2. The rotating wall motion type and the moving surface boundary conditions set for the throttle body help to for the movement during the simulation.

3. There is a low pressure zone just after the throttle.

4. From the velocity contours and streamline we can see that the velocity is increased after the throttle and there is a swirling motion in the flow.