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Old   June 29, 2009, 07:27
Default Bug in CEL?
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Lance
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Hi,
I have a CEL expression that looks like:

Code:
80[Pa]+30[Pa]*(1+sin(5[Hz]*pi*t-pi/2))
and if I plot the expression during 20 s it looks like:


which isnt really what it should look like...rather, the waves should be equal in size.

Changing the time to 10 s gives the following plot:


It seems like there is something strange with CFX when it plot the expression. Anyone else found the same thing?
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Old   June 29, 2009, 10:47
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Increase the number of points in the plot
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Old   June 29, 2009, 15:09
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Quote:
Originally Posted by Rui View Post
Increase the number of points in the plot
good one mate, didn't think of that one
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Old   June 30, 2009, 04:25
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Quote:
Originally Posted by Rui View Post
Increase the number of points in the plot
d'oh, didnt think of that
Thanks
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Old   June 30, 2009, 06:01
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It was evident from your 2nd plot (the vertices in the curve are well visible).

The frequency of your expression is 2.5 Hz, or T = 0.4s. CFX uses, by default, 100 points to make the plot.
In the 1st plot (20 s), the points will be spaced by 20 s/ 99 intervals = 0.202 s. So, the plot shows approximately only 2 points for each period of the sinusoidal expression.
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Old   June 30, 2009, 07:52
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Hi,

Yes, this is classic frequency aliasing. Remember the Nyquist critereon (http://en.wikipedia.org/wiki/Nyquist...ling_theorem)? In short to resolve a given frequency you need to sample it at least at twice the frequency you wish to resolve. Frequencies you do not have enough speed to resolve will result in aliased frequencies at lower frequencies which are spurious and not real. It usually manifests itself as beats in ampiltude of a high frequncy wave - see Lance's plots for a classic example.

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