Week 4.1: Project - Steady state simulation of flow over a throttle body

AIM - Steady State simualtion of a flow over a throttle body.

OBJECTIVE - 

• To setup and run a steady state simulation for flow over throttle body.
• To post process teh results and sow pressure and velocity contours.
• To show the mesh(i.e surface with edges)
• To show the plots for pressure, velocity, mass flow rate and total cell count.
• To create the animation for the flow generation. 

PRE-PROCESSING-

The preprocessing step includes cleaning the geometry, setting up the case then applying proper boundary conditions and finally giving the base mesh conditions. 

Geometry-

The geomety for the flow over a throttle is imported in the from of .stl file.

Now the geometry needs to be cleaned according to the requirment

First the geometry is in meters so it need to be scaled in millimeters

for that go to the geometry dock and go to the transform tab , then select scale tab now choose entire surface and give uniform scaling factor of 0.001  and then click apply 

Now the geometry will look the same but the bounding box shows that it is scale to factor of 1e-3 from earlier.

Next is to remove the outer surface of the pipe 

For that first go to the boundary tab on the geoemtry dock and then select find/clean tab after that hit find and it will create a fence at the edges as shown below 

Now the outer surface and the thickness surface can be selected easily and removed from the geometry

go to the geometry dock then select the repair tab then delete option now choose triangle selection and type of triangle selection as 'by boundary fence' then select the three surface form the geometry the outer wall the thickness wall from both ends and then hit apply to delete them.

now two open edge will be present at the ends of the pipe.

This edeges needs to be closed to from a closed geometry where flow will take place 

From the geometry dock go to the repair tab and select  patch tool, from teh patch option select Free edge loop and select 'By open edge' type of the edge selection . after that select the edge form the geometry and hit apply . it will create a patch and closed the geometry from that end after that do the same patching for the other end and the geometry is completely closed.

Now remove the boundary with Id = 1 from the boundary flag on the geometry dock and now add more boundary flag as shown below

Select and apply proper boundary using the by angle selection for triangle. also to select the throttle part hide the elbow wall and select the throttle part by usign box selection tool . 

Now the geometry will be coloured according to the colors of the boundary

Finally go to the diagnosis dock and select find to check for any errors within the geometry 

after hitting the find it will show the list of problem triangles in the above box and if the number of problem triangle is zero it means the gometry is correct and now can be used for case setup.

Case Setup-

Now move to the case setup dock and there a tree can be seen after selecting "begin case setup"

Now just following the steps according to the tree will help in setting up the case.

Select teh application type as "time based"

Now go to the material and keep the default settings and select the air as predefined mixture and click apply. 

 keep the default settings and values for the gas simulation.

 also the global transport parameter are kept the same as default.

the reaction mechanism is also the same as default.

the simulation parameters have three option selected by default that is run parameter, simulation time parameter and solver parameters.

from the run parameters go to the solver tab and choose steady state solver as the solver then simulation mode as full hydrodynamic and gas flow slover type as compressible. also ffrom the Misc. tab check the box for the steady state moniter and uncheck teh box for Use shared memory.

to stepup the steady state moniter click on add to add a new moniter then select moniter variable as mass flow rate , select target type as boundary and select target as outlet. set the number for minimum number of cycles to be executed for the steady state solver as 5000 and also set the moniter start delay as 200 and sample size as 1000.

To setup the simulation time parameters set the end time as 15000 cycles . Set the initial an minimum time step values as 1e-9 and maximum timestep as 1 also keep the maximum convection CFL limit as 1 

For teh solver parameters from the Navier stokes solver tab select the NS solver scheme as PISO and NS solver type as Density-based. now from teh steady state solver control tab set the maximum convection CFL limit (final stage) as 1. 

Now first setup teh region and initialization . select the add button to add new region and it will be names Region 0 by default. for the properties of that region add species by clicking on +Air and it will add the oxygen and nitrogen in the given mass fraction by itself.

Boundary Conditions-

To setup boundary condititions go to boundary and then select each boundary and give the region name as Region 0 for all the boundaries first by double clicking the box add it will give the list and select Region 0.

set the boundary type for the elbow wall as WALL select the wall motion as stationary and surface movement as fixed all make sure that the temperature has a law of wall option and has 300 K as the temp value.

To set the inlet boundary select the boundary type as INFLOW, set teh valure of pressure as 1.5e05 Pa, the temperature is 300 and add the species by hitting +Air button and it will add the O2 and N2 with the default mass fractions.

Base Mesh-

To setup the base grid 2mm grid size was applied as shown below.

Also fixed embedding for he throttle wall boundary was used. the embedding mode was set as permanent and with scale of 3 having embed layers as 2.

The setting for the output and postprocessing were set as shown below. The writing time intervals for the output files is set as 100 cycles. the same is for the restarting output file and the maximum number fo restart files saved is 3. 

PROCESSING-

Running the Simulation-

To run the files first the input files are exported as shown below.

save the files to the desired location where simulation is meant to run.

Now to run the simulation open cygwin using run as administrator option and then go to the same location where the above input files are stored. Now there run the simualtion use the converge.exe command.

here we will be running the simualtion in parallel using the mpiexec.exe commmand and also save the output that is generally printed on the screen into a logfile. So to do this the syntax to be run will look as shown below.

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

Now the simulation will run in the background an to check the status of simualtion we can use the "tail logfile" command.

Post Convert-

Once the simualtion is completed now we need to convert the converge output file in a paraview visible files and this can be easily done using post_convert.exe to be run in parallel using the mpiexec.exe 

Now after running this we can post process the files using paraview.

POST-PROCESSING-

Mesh Grid-

first lets take a look at the cell count which is 35518 and this is fairly good .

The grid generated 

and at the region near the throttle wih the embedding is shown below.

Contoutrs-

The post processing results show the following contours 

here we can see that the flow velocity in increasing near the small between the throttle and the elbow wall.

also we can see in the above image the pressure contour on the throttle body where as the flow hits the body there is high pressure and on the other side there are blue regions of low pressure due to dlow seperation.

The image below shows the stream lines over the throttle body.

Plots-

the inlet mass flow rate plot and the outlet mass flow rate plots are shown below and it can be seen that the solution is converged as there are no fluctuation in both the plots

Animations-

The animation for the flow simualtion is shown below.

CONCLUSIONS-

The flow simulation using fixed embedding is performed here, this helps to refine the region near the throttle body while keeping the grid size high at the region with less significance.

The region with high velocity is basically the small gap between the throttle and the elbow wall.

The pressure contour show low pressure at flow seperation .