# Why Cp fluctuates like this?

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September 1, 2013, 10:20
Why Cp fluctuates like this?
#1
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Hi, all.
I'm trying to validate the experimental results of 2D S809 airfoil.
You can get some information from the pictures shown below.
In this case, Maximum Yplus=0.2915.
I can't understand why the pressure fluctuates this way. The general trend matches, just the wave-like fluctuation.
Series 1 and series 2 refer to Upper Wall and Lower Wall of airfoil.
I'm using SST model, AOA=1.02degree, Re=2*10^6, intensity=1%.
Thank you so so much.
Attached Images
 1.jpg (90.8 KB, 25 views) 2.jpg (98.6 KB, 25 views) 3.jpg (92.7 KB, 28 views) 4.jpg (93.8 KB, 25 views) 5.jpg (91.0 KB, 21 views)

September 1, 2013, 10:22
#2
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Attached Images
 6.jpg (92.3 KB, 22 views) 7.jpg (39.1 KB, 20 views)

 September 1, 2013, 11:30 #3 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,528 Rep Power: 25 Screenshot number 8 might also be helpful if it is a screenshot of the mesh around the airfoil.

 September 1, 2013, 18:49 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 I have seen this before - it happens when the geometry is not resolved very well and has obvious straight line segments. Then you mesh it with a finer mesh than the geometry segments and you get funny steps int he curve like that. So the solution is to go back to your original geometry and either use a better geometry representation, or put more points on the curve describing the airfoil, or use a spline rather than straight lines to generate the curve.

September 3, 2013, 09:36
#5
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Thank you both, flotus1 and ghorrocks.
Well, I don't have the mesh file at the moment, I'll update it when I'm back.But I have the geometry. I made the mesh in ICEM.
The coordinates have two sources, one from the NREL Paper "Design and Experimental Results for the S809 Airfoil" by Dan Somers, etc. the other from the NREL website.
The noticeable difference is around the leading edge.The latter one has more points around this region. Both give more or less 32 points on each side. The two *.tin files have been attached.
Both two result in the fluctuation of Cp. And I can't find other sources to give more points. I did create Bspline from points in the ICEM.
Attached Files
 Original_Geometry.zip (5.1 KB, 5 views)

 September 3, 2013, 19:10 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 I do not have ICEM so cannot open that file. But I guarantee that if you zoom into your airfoil mesh you will find it is facetted. For a smooth Cp curve you need to stop the facetting. I have seen this many times before.

September 4, 2013, 04:36
#7
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Ok, the straight lines do exist. See attatchments.

My friend tries to create spline in Solidworks with these 60+ points, but it turns out still facetted. I suppose it's because the points are not sufficient, but I am not able to find more points. However, some scientic paper reveal that they create the geometry with exactly the same source since they put the original paper as reference, I don't know what treatment they did.(curve fitting technique maybe?)

When it comes to the mesh, the concern is Aspec Ratio, as you can see, to ensure appropriate Yplus, the ARs in the wake is too big,as well as the upper and lower vertical line. But the solver works and converges well.

What can I do now, Sirs?
Attached Images
 1.jpg (40.1 KB, 13 views) 2.jpg (35.1 KB, 12 views) 3.jpg (34.6 KB, 12 views) 4.jpg (99.0 KB, 16 views) 5.jpg (82.5 KB, 15 views)

September 4, 2013, 04:38
#8
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More on the mesh.
Attached Images
 6.jpg (92.8 KB, 10 views) 7.jpg (97.8 KB, 9 views) 8.jpg (99.1 KB, 10 views)

 September 4, 2013, 04:54 #9 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,528 Rep Power: 25 This is definitely what Glenn was talking about. The surface is facetted due to a lack of points. If you already created a spline in your CAD-tool, i guess the problem arises when you export the geometry. What file format and which settings did you use?

 September 4, 2013, 06:07 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,805 Rep Power: 107 The best solution is to regenerate the airfoil with lots more points. If that is not possible then fit a spline to the points rather than a series of straight lines. If you tried fitting a spline and that is what you got then you need to tighten the geometry tolerance or the import tolerance. Somehow you have to eliminate those facets.

 September 4, 2013, 07:17 #11 New Member   Join Date: Jul 2013 Posts: 25 Rep Power: 6 I think I have made a mistake, I will try to figure out the right way to create spline. I'll come back a.s.a.p.

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