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Totize February 11, 2013 12:28

[CFD-Post] Plotting Cp around a non horizontal airfoil
Hello all :)

I am actually studying an airflow passing a simple 2D curved airfoil.
I want now to plot the Cp distribution alongside the upper and lower 2D lines of this profile vs its chord line.

I am using CFD-Post. So I polyline the 2 lines. Then I go to Chart, use the polylines for the data series. On Y Axis tab, of course I select "Pressure Coefficient" as the variable. But as you can see on the screenshot the chord line is rotated of 58 compared to the X axis. This detail blocks me because in Chart's X Axis tab I can only use "X", "Y" or "Z" which are the axis the default coordinates system.

I tried to create a new personalized coordinates system rotated of 58 compared to the default one, but it seems to be useless for my problem as I can't select this new X axis to plot my Cp distribution.

I tried to create a new variable for this new X, but with no success.

I'm sure it's possible to easily solve this "problem" as CFD-Post is quite an advanced post processing software.

Please can you give some advice for this ?

I would be really greatfull.

Have a nice day !



flotus1 February 11, 2013 13:17


Originally Posted by Totize (Post 407179)
I'm sure it's possible to easily solve this "problem" as CFD-Post is quite an advanced post processing software.

Copy that;)

I can only guess a few issues that got you into this trouble.
First of all, make sure that you selected "XY" as the type of the chart in the "general" tab.
Next thing would be that you should not use the drop down menu for picking your x-axis variable, but the small button labeled "..." to the right of it.
Here you have all the variables (more than in the drop down list) including the "Length".

Running out of ideas now...

Totize February 11, 2013 13:43

Thanks flocus1 for your answer !

Unfortunately, I was already using "XY plot" option and i tried the "Length" X variable with no success. Because "Min" and "Max" of "Length" is "Unknown" under the Variables general tab.
When I use it for the Chart, I get the image below. Which is obviously not ok :rolleyes:

So the length variable is not ok. But in FLUENT before my calculations, I checked the reference values. The chord length is 79mm.

No more hint ?

flotus1 February 12, 2013 04:44

This is odd.
I thought I remembered from working with CFX about 3 years ago that this is how to get a plot versus chord length.
But actually I am facing the same problems now.

The only x-axis value that produces a reasonable output is "Chart count".
You could use this and simply normalize the x-axis aswell. Still this is a workaround, and I am curious why the chord length isnt available.

Totize February 12, 2013 05:23

Yes actually in FLUENT this is so simple and direct to choose the X and Y directions on which you want to plot your airfoil Cp distribution. But in CFD Post it seems not to be possible in my case. So the trick I'll use is definitely to plot the data according to the default reference frame's X axis, then export them onto Excel and dividing the abscisse coordinates by cos(58) in order to get the real Cp distribution according to Xc (along the chord).

OK I must use one more software but it's ok.

Anyway, thanks for your help ! And have a great day mate :D


flotus1 February 12, 2013 07:06

Some annotations to your procedure:

In CFX post, you can easily produce a new variable (cos[58]*X) and use it as the x variable.

But since your surface has a curvature (the angle is not 58 everywhere) this does not equal the chord length.
Just imagine a point at the leading edge which has the same x-value as a point at the top side of your airfoil. These would have the same "chord length", which obviously is not true.

As I already mentioned, only the "chart count" variable produces a reasonable output. You could still measure the chord length in a CAD program and scale the axis accordingly if you need a dimension on your axis.

Edit: The method with the chart count only works with uniform meshes of course.

Tarantino February 15, 2013 16:37

Contour plot with limit color bar
You can easily do it in FLUENT.

flotus1 February 16, 2013 05:56

And would you kindly tell us how?

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