Week 1: Channel flow simulation using CONVERGE CFD

AIM - To run a channel flow simualtion using Converge CFD

 

OBJECTIVES - 

• To set up a channel flow simulation using Converge CFD.
• To run the case for 3 different mesh sizes and compare the results of the 3 simulations.
• To show the postporcessing results for all the 3 cases using velocity and contour plots; mesh(i.e surface with edges); plots of velocity, pressure, mass flow rate and total cell count. 
• To provide the animation for the developement of flow.

 

Theory:

Channel flow is an internal flow in which the confining walls change the hydrodynamic structure of the flow from an arbitrary state at the channel inlet to a certain state at the outlet.

 

Geometry:

First step for the simulation is to prepare a geometry model. The channel geometry is prepared in Converge itself as shown below.

Go to the geometry dock 

select the create option, then go to the shape tab and select box , after giving the co ordinates of center and the length of box in each direction, hit create.

 

Now we can see the box generated in the GUI

Now the geometry is ready.

Now moving to the boundary option on the geometry dock itself, the boundary are defined as shown below 

This will give the number of boundaries to be defined.

no it can be seen in the table as shown below

The first boundary will be inlet and will be defined by selecting the triangle on the inlet face and applying to the boundary1, also rename the boundary 1 as inlet. similary the remaining boundaries will be set as shown below

Finally check the normal to the face and it should be facing inside the domain, if not then reverse the direction by going to the transform option on the geometry dock.

Now the geometry is ready to setup the simulation.

 

CASE SETUP:

Now move to the case setup dock on the right hand side and follow through the steps shown there.

first is setting the application type as time based and hit apply.

after that select the materials and check the gas simulation and species boxes.

Now keep the gas simualtion and global transform parameters as it is 

for the species add N2 and O2 as the gas species

 

To setup the simulation parameters do not change anyting it has by defalut selected run parametes , simualtion time parameters and solver parameters

To setup the sun parameters, go to the Misc. tab and check the box for steady state moniter and uncheck the box for use shared memory option and click ok to save.

To set up the steady state moniter set the minimum munber of cycles to be executed as 5000 set the moniter variable as avg-velocity at the outlet boundary and save the moniter to teh mass_avg_flow.out file also keep the setting the moniter start delay, smaple size and min sample sise and shown below. In this way the simulation will stop when this moniter is converged and not run for very high number of cycles that are input so it will save the computation time.

To set up the simulaiton time parameters set the end time step as 30000 cycles and the initial and minimum timestep and 1e-09 s and the max time step as 1 s.

 Also set the maximum convection CFL limit as 1.

To set up the solver parameter set the NS solver type to density based, from the equations tab for pressure set the preconditioner to none, and form the steady state solver tab set the max convective CFL limit as 0.5 

 

 

 

BOUNDARY CONDITIONS: 

The BOundary Condition at each boundary is setup as shown below;

For Inlet:

The boundary type is inflow, pressure is 100001 Pa and the Temperature is set to specific value of 300 K. The Speces are Air Mixture in respective mass fraction.

For Outlet:

The boundary type for outlet is set to outflow and the temperature and pressure have specific value of 300 K and 100000 Pa.

For top and Bottom wall:

The Top and Bottom are set to wall boundary type with specific temperature value of 300 K.

For front and back : 

The front and back have boundary type 2D to convert the 3D geometry into a simple two dimensional case.

 

MESHING: 

Here there are 3 mesh sizing on which the simualiton is to be run 

CASE NUMBER	MESH SIZE (m)
case1	0.0002
case2	0.00015
case3	0.0001

 

To setup the mesh sizing in the base grid for case 1 

 

To setup the mesh sizing in the base grid for case 2

 

To setup the mesh sizing in the base grid for case 3

 

 

To setup the Post Variable selection  keep the default variable from the typical variables section and from the species tab selct mass fraction for N2 and O2.

For the output files select the write time interval as 100 cycles for writing both the 3D Output data files and restarting output.

 

RUNNING THE SIMULAITON:

To run the simulation first export the input file by going to the files menu and select export input files form the export option

 

Now select proper path of the folder to save the exported file, this folder is where the simulation will be run.

Now go to cygwin terminal and go the same folder where the input files are exported.

Run the command for the simualtion the command to run the simualiton is shown below:

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

  mpiexec.exe is used to run the simulation in parallel.

-n suggests the parallel processors to be used followed the number of processor(in this case it is 4)

 converge-intelmpi.exe  is the command to run the simulation 

</dev/null> logfile save the output in the logfile rather than printing at the terminal

& make the terminal to be used for another process by running the process in background.

 

After successfully running the simalation the file are to converted in a form that is readable by paraview so to do that go to the output folder and give the follwing command 

mpiexec.exe -n 4 post_convert_30_msmpi_64.exe 

this will help convert the converge output file into paraview readable format.

it will ask for the name by which the files are to be saved follwoed by the format where select the number before paraview vtk binary format so it will convert in that format and then give all for the variable and post files.

 

POST PROCESSING:

 

BASE MESH 1: 

 

Velocity contour:

Pressure contour:

AvgVelocity at Inlet and Outlet:

Cellcount:

Mass Flow rate:

Species Mass:

 

 

BASE MESH 2: 

Velocity contour:

Pressure contour:

AvgVelocity at Inlet and Outlet:

Cellcount:

Mass Flow rate:

Species Mass:

 

 

BASE MESH 3: 

Velocity contour:

Pressure contour:

AvgVelocity at Inlet and Outlet:

Cellcount:

Mass Flow rate:

Species Mass:

 

 

Velocity Plot at the outlet for all the three Base grid: 

 

For this plot the velocity at the outlet in the increasing oder as BaseGrid2 < BaseGrid3 < BaseGrid1

Following table gives the comparison of the three base grid sizes.

CASE	CellCount	Number of cycles	Time taken for Simulation to run	Maximum velocity
BaseGrid1	25000	11374	1370 s	1.281
BaseGrid2	45420	25721	6249 s	1.107
BaseGrid3	100000	16503	10628 s	1.229

The high number of cycles and the low maximum velocity for the BaseGrid2 is because of the cell generated at the boundary of this grid have high aspect ratio. 

 

VIDEO ANIAMITION FOR THE FORMATION OF FLOW :

The flow development video showing how the velocity of the flow through the channel develops along with the number of cycles is shown below.

BASEGRID1:

BASEGRID2

BASEGRID3

 

 

CONCLUSION:

 From the above simulaiton it is concluded that 

1. As the base grid size is reduced the cell count has increased 
2. The increase in cell count results in longer duration to run the simulation.
3. With decrease in the base grid size the number of cycles required to reach the stable solution increases, also the result tends to be more stable.