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Evaluating pressure coefficient  at the top wall of the channel

•  pressure coefficient is in general4.1:

 (4.1)

• We are going to evaluate pressure coefficient at the boundary, specifically at the UpperWall patch.
• It means we need to know value of pressure  at the boundary.
• We use OpenFOAM post-processing functionality for sampling value of pressure.
• The postProcess utility employs function object framework.
• We use function object surfaceFieldValue.
• An example of using this function object can be seen in its header file

# cat $FOAM_SRC/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.H • We create a new file called sampleUpperPatch in system directory. • (It can be done for example by command # touch system/sampleUpperPatch . The ‘touch’ command creates an empty text file. Use mcedit, or other text editor, to add following text to the file.) type surfaceFieldValue; libs ("libfieldFunctionObjects.so"); writeControl writeTime; writeFields true; surfaceFormat raw; regionType patch; name upperWall; operation none; fields ( p );  • We would like to sample field ‘p’ at the regionType ‘patch’ with name ‘upperWall’. The data will be stored in raw format. • The function object can be launched from case directory: # cd$FOAM_RUN/pitzDaily

# postProcess -func sampleUpperPatch

• Output is following:
/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  dev                                   |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : dev-e2ccbbbb
Exec   : postProcess -func sampleUpperPatch
Date   : Jun 16 2017
Time   : 12:00:00
Host   : $HOSTNAME PID :$$Case :$FOAM_RUN/pitzDaily
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster (fileModificationSkew 10)
allowSystemOperations : Allowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create mesh for time = 0

surfaceFieldValue sampleUpperPatch:
total faces  = 223
total area   = 0.00031106

Time = 0

volScalarFields: p

Executing functionObjects

Time = 100

volScalarFields: p

Executing functionObjects

Time = 200

volScalarFields: p

Executing functionObjects

Time = 287

volScalarFields: p

Executing functionObjects

End


• In test case directory is directory postProcessing containing new folder sampleUpperPatch, look inside:
# cd ./postProcessing
# ls

sampleUpperPatch  sets

# cd ./sampleUpperPatch
# ls

surface


# cd ./surface
# ls

0  100  200  287


# cd ./287
# ls

patch_upperWall.raw  p_patch_upperWall.raw

• Look at file data:
# more ./p_patch_upperWall.raw
# p  FACE_DATA 223
#  x  y  z  p
-0.0198086 0.0254 0 -1.90829
-0.0182574 0.0254 0 -3.67105
-0.0167681 0.0254 0 -4.67569
-0.0153384 0.0254 0 -4.97477
-0.0139658 0.0254 0 -5.12385
-0.012648 0.0254 0 -5.26736
-0.0113829 0.0254 0 -5.38125
-0.0101683 0.0254 0 -5.46206


• For plotting  at top wall channel we use e.g. an open-source program gnuplot
•  Run gnuplot:
# gnuplot
•  Run gnuplot outside of terminal. Use File > Change Directory and navigate to the location of file p_upperWallData.raw and confirm with OK.
• In gnuplot environment type command:
gnuplot> plot './p_patch_upperWall.raw' using 1:(\$4/(0.5*100)) with lines

Figure:  at the top wall, Backward-Facing-Step OpenFOAM tutorial