# 2D Airfoil Pressure Coefficients from simpleFOAM

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January 21, 2012, 14:34
2D Airfoil Pressure Coefficients from simpleFOAM
#1
New Member

Travis
Join Date: Oct 2011
Location: seattle
Posts: 7
Rep Power: 14
After searching the forum, I have seen relatively limited discussion regarding validation of pressure distributions via simpleFOAM with experimental data.

I have a converged simpleFOAM result for 2D, incompressible, laminar, and attached flow about a thick airfoil. Using ptot as total pressure at the inlet, I have defined cp,

cp = 2(p* - ptot*)/(U0^2)

where * denotes density-normalized pressure values from simpleFOAM, p*=p/rho, ptot*=ptot/rho, and U0 is the velocity at the inlet.

Plotting these coefficients gives a puzzling non-smooth profile (Fig. attached).

Has anyone else observed this for a converged case? Any suggestions are much appreciated.
Attached Images
 cp_non_smooth.jpg (22.4 KB, 142 views)

 February 6, 2012, 16:51 Surface definition #2 New Member   Travis Join Date: Oct 2011 Location: seattle Posts: 7 Rep Power: 14 Thought I'd reply to my own post in case anyone else needs help in the future... I found my surface geometry to be way too coarse, especially near the leading edge. Once I reverted to B-Splines in my gmsh setup to refine the initial STL file, I had success with snappyHexMesh in building the 2D grid. Then with a simple RANS (k-omega) turbulence model I was able to very closely match experimental pressure profiles. The key is surface refinement. It is probably overkill as I haven't run any grid sensitivity study yet, but I used 1000 points around the leading edge. My STL file for a simple airfoil is 64MB...but I have good results s.m likes this.

February 7, 2012, 08:31
#3
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Vieri Abolaffio
Join Date: Jul 2010
Location: Always on the move.
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Quote:
 Originally Posted by seadmiral Thought I'd reply to my own post in case anyone else needs help in the future... I found my surface geometry to be way too coarse, especially near the leading edge. Once I reverted to B-Splines in my gmsh setup to refine the initial STL file, I had success with snappyHexMesh in building the 2D grid. Then with a simple RANS (k-omega) turbulence model I was able to very closely match experimental pressure profiles. The key is surface refinement. It is probably overkill as I haven't run any grid sensitivity study yet, but I used 1000 points around the leading edge. My STL file for a simple airfoil is 64MB...but I have good results
sorry, I've just seen your post after you've resolved your problem.

but for future reference, about 100-150 points in the cordwise direction should suffice, given that they are clustered near the LE.

best regards.
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February 9, 2012, 20:44
#4
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Travis
Join Date: Oct 2011
Location: seattle
Posts: 7
Rep Power: 14
Again with the hope of solving someone's problem in the future...I certainly have learned heaps from this forum.

Attached are the laminar simpleFoam & RANS results, compared to test data. There are certainly some more tweaks but the preliminary result is not bad.

@sail: Perhaps turbulence was a bigger deal here than I initially gave credit. However, the mesh is certainly much improved from the first plot.

Thanks for your tip, I'll be sure to try it out.
Attached Images
 cpLamRansTest.png (9.3 KB, 120 views)

July 8, 2013, 07:39
#5
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Join Date: Aug 2012
Posts: 229
Rep Power: 15
Quote:
 Originally Posted by seadmiral Thought I'd reply to my own post in case anyone else needs help in the future... I found my surface geometry to be way too coarse, especially near the leading edge. Once I reverted to B-Splines in my gmsh setup to refine the initial STL file, I had success with snappyHexMesh in building the 2D grid. Then with a simple RANS (k-omega) turbulence model I was able to very closely match experimental pressure profiles. The key is surface refinement. It is probably overkill as I haven't run any grid sensitivity study yet, but I used 1000 points around the leading edge. My STL file for a simple airfoil is 64MB...but I have good results
Hi,
i have a smilar problem, would you please guide me?
i am working on multi element airfoils, my pressureCoeffs figures for 3 element separately are in the attachment. As you see the pressureCoeffs has flactuation in three of them, whould you please explain more that how should i refine my STL file to have a smooth pressureCoeffs figure.
Thank you very much
your answer may help me a lot.
Attached Images
 cp-slat-3700.png (11.0 KB, 24 views) cp-flap-3700.png (9.8 KB, 20 views) cp-main-3700.png (9.6 KB, 18 views)
Attached Files
 system.tar.gz (3.3 KB, 16 views) triSurface.tar.gz (20.2 KB, 6 views)

 Tags airfoil, pressure coefficient, simplefoam

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