Transient flow simulation through throttle

                            Transient Flow Simulation through a Throttle

SITUATION

        The flow of air through a throttle body in Converge studio and post-processes the results in ParaView and CYGWIN was used to run the commands for simulation and converting the files for the post-processor. The airflow in the channel is considered to be turbulent.

TASK

        The simulation is to be run for three different cases steady-state and transient state conditions

         Steady-state - The fluid property at a point does not vary with time (Pressure based steady solver).

         Transient state - The fluid property at a point varies with time (Transient solver). 

Throttle valve

The throttle valve is used to obstruct the fluid flow in the IC engine to increase or decrease the power of the engine by restricting fuel flow. In this mechanism, a disc is fitted inside the pipe through which fuel is flowing. The valve rotates about its center to control the flow by restricting the space required for the fuel flow. In this problem, the throttle rotates about its axis by varying its angle.

The calculation to find the end time of simulation

The throttle valve rotates about its axis and the value of the fluid property vary with respect to the time. To total time period is calculated from the value of the velocity & dimensions of the elbow. Assuming the velocity to be 100 m/s and dimension of an elbow to be 0.2m.

Flowtime = dimension / velocity

              = 0.2/100 = 0.002 sec

The flowtime is the time required for the single fluid to move from inlet to outlet. If we want to cover at least 5 cycles per period.

Total time - 0.002*5 = 0.01 sec.        

ACTION

i) Workflow for a CONVERGE CFD Simulation

    1.Pre-processing(preparing the surface geometry and configuring the input and data files).

    2. Running the simulation.

    3. Post-processing(analyzing the *.out ASCII files in the Case Directory and using a visualization program to view the information in the post*.out).

Pre-processing

File --> Import --> Import STL file, this option is used to import the elbow.stl file.

Boundary dialog box

Model

Select on the Normal toggle option and direction on the normal must be in the direction of the flow of air. The CONVERGE is mainly developed for running the IC engine simulation & other types of flow simulation are considered to be General flow. So General flow is selected.

1. Materials – Select Air as a predefined mixture & validate all the remaining files under it

    2. Simulation Parameter – i) Run Parameters – Pressure based steady solver is chosen.                                                ii) Simulation time parameter – start time – 0.

                                                                                        end-time  - 0.01s.

                                                                                        Initial time step – 1e-9.

                                                                                        Min time step – 1e-9.

                                                                                        Max time step – 1.

1. Regions – Add a region & select air under the species area.

    4. Physical models – Select RNG k-e model is chosen under the turbulence model.

    5. Grid control – i) Base Grid – the value of the grid depending on the case (0.002mm).                                ii) Final embedding - Entity type - Boundary.

                                                           Boundary ID - Throttle.

                                                           Mode - Permanent.

                                                           Scale - 2.

                                                           Embed layer - 2.

    6. Boundary – i) Inlet – Type – inflow.

                                      Total pressure – 1e5.

                                      Species B.C’s – Air.

                         ii) Outlet – Type – outflow.

                                         Total pressure – 1e5.

                                         Species B.C’s – Air.

                         iii) Wall – Type – wall.

                         iv) Throttle - Type - wall.

                                            Surface movement - moving.

                                            Rotation center - Copy the arc center value and paste it.

                                            Rotation about - Copy the arc normal and paste it. 

To calculate the arc center and arc normal use Measure --> Direction --> Arc normal & select any three points in the throttle it provides the value of the arc center, arc dia, arc normal.

     7. Output/post-processing – i) Output files

                                                 Time interval for writing 3D output data - 0.001.

                                                 Time interval for writing text output files - 1e-6 & validate other files under it. 

Running the Simulation:

File --> export – To export the case set up for the running the simulation using CYGWIN.

Mpiexec.exe -n 4 converge-2.3.26-msmpi-win-64.exe </dev/null>logfile & - The command is used to run the simulation using 4 processors & the following command is used to store the file in the name of the log file and & symbol is used to store the files in the background.

Output data 

Results of various plot

1. Total cell count

The variation in the cell count is mainly due to the rotation of the throttle valve which can be easily depicted from the plot since the plot is constant where the rotation of the valve is zero.

2. Total pressure

3. Static pressure 

4. Mass flow rate 

The plot helps us to infer the flow rate for the angle of the throttle valve. 

5. Velocity

6. Density

Post-processing

Provide a suitable case name & select the Paraview VTK inline binary format as the file type and enter the directory which contains the output files which are to be post-processed & then click all in the files & cell variables and click convert.

Open the Paraview application and open the file using File -- > Open -- > test..vtm.

Click apply & select the slice option & choose the z normal to divide the geometry along the z-axis.

Geometry

Mesh element

i) Wall                                                          ii) Inlet

iii) Outlet                                                   iv) Throttle

Profile contour

Wall profile

i) Pressure

ii) Velocity

Throttle profile

i) Pressure                                                  ii) Velocity

Stream Tracer :

A streamline is a path traced by a massless particle as it moves with the flow.

Youtube links :

i) Velocity Simulation 

ii) Pressure Simulation

iii) Stream Tracer

RESULTS      

Transient simulation of an elbow with a throttle was performed converging to 0.01 seconds, obtaining velocity and pressure contours, as well as the mass average flow characteristic variables such as average velocity, pressure, mass flow rate and total cells in the geometry mesh. 

The total cells vary with the rotation of the throttle valve which is clearly depicted from the plots. The plot of mass flow rate & velocity is similar for each angle, the value of the mass flow rate decreases from 0 to 90 degrees and increases for angle value from 90 to 180 degrees. The decrease of pressure in the throttle which causes cavity which increases the velocity of flow.