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Specific plots in COMSOL
Posted 27 nov. 2022, 15:33 UTC−5 Results & Visualization Version 5.6 4 Replies
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Hello,
I calculated the acoustic modes, in preassure field 
, of a certain 3D structure and I want to make specific plots that I can't manage to make with COMSOL :
1) For each mode, I calculated 
and 
. In the table where the values are displayed, how to calculate the ratio of the 2 columns 
?
2) At each point 
of the domain, I want to know what frequency 
of the calculated modes 
gives the maximum value of the pressure field 
and plots the obtained values of the max on the domain, is it possible to do this?
Thanks in advance,
Romain L.
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1) I understand this as max and average over the domain. You can create variables for this under Definition->Variables, and then just create another variable for the ratio. However, note that p^2 is in general a complex number. Also, since the scale of an eigenmode is arbitrary, taking the ratio of something that has a linear dependence on the amplitude and something that has a quadratic dependence seems suspect.
2) Since eigenmodes have an arbitrary scaling, comparing values between modes is in general not a meaningful operation.
-------------------Henrik Sönnerlind
COMSOL
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Thank you for your quick reply.
1) I understand this as max and average over the domain. You can create variables for this under Definition->Variables, and then just create another variable for the ratio. However, note that p^2 is in general a complex number. Also, since the scale of an eigenmode is arbitrary, taking the ratio of something that has a linear dependence on the amplitude and something that has a quadratic dependence seems suspect.
Thanks for the clarification, I managed to implement it. For the problem I am considering the solution has no imaginary part. The ratio 
(I forgot the square root) is used to find out if a mode is located in a zone of the domain, if the ratio is big then the mode is mostly located. Divided by 
amounts to normalizing the mode by its L2-norm.
2) Since eigenmodes have an arbitrary scaling, comparing values between modes is in general not a meaningful operation.
In fact, it is not directly on the modes that I want to carry out this operation but on a slightly different problem where scaling has a precise meaning. I used the example of the modes to make it clearer. So, is it possible to plot such a function 
based on a frequency array in COMSOL ?
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Assuming that you do not have too many modes, you can plot an expression like (using 4 modes here)
max(with(4,pa),max(with(3,pa),max(with(2,pa),with(1,pa))))
where pa is a variable containing abs(p).
If you have hundreds of modes, it may become a bit boring to write the expression, though. Then you may want to resort to scripting, using model methods or Matlab. You can even use a script just to create the very long expression.
For time-domain analysis there is an operator called timemax() doing this type of max value search, but there is nothing similar built in for eigenmodes.
-------------------Henrik Sönnerlind
COMSOL
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Thank you very much Henrik, indeed this way of doing things works well. I used a Python script to express the forumula which can, indeed, become long.