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Averaging only positive values of a parameter
Posted 28 lug 2017, 09:34 GMT-4 2 Replies
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Hello everyone,
I have a question about post processing:
Is it possible to only average the positive (or negative) values of a parameter over a surface?
For example, in my special case, I have a 3D velocity field on a cut plane which in some areas of this plane goes into positive and in some areas goes into negative x, y, and z-direction.
Now I want to determine the average velocity only in positive x-direction and neglect the negative values.
I know how to integrate or average a parameter with "Global Evaluation" -> "Surface Average" etc., but do not know how to express, that only the positive values (like u>0) shall be evaluated.
Mayby it is quite simple, but I don't see it.
Thank you in advance for any suggestions.
Best Regards
Markus
I have a question about post processing:
Is it possible to only average the positive (or negative) values of a parameter over a surface?
For example, in my special case, I have a 3D velocity field on a cut plane which in some areas of this plane goes into positive and in some areas goes into negative x, y, and z-direction.
Now I want to determine the average velocity only in positive x-direction and neglect the negative values.
I know how to integrate or average a parameter with "Global Evaluation" -> "Surface Average" etc., but do not know how to express, that only the positive values (like u>0) shall be evaluated.
Mayby it is quite simple, but I don't see it.
Thank you in advance for any suggestions.
Best Regards
Markus
2 Replies Last Post 3 ago 2017, 04:32 GMT-4
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Posted:
7 years ago
Updated:
7 years ago
Hi Markus,
In COMSOL, you can include boolean expressions in the expressions to integrate, so you can specify that you want to integrate
u*(u>0)
by typing that in the Expression field in your Surface Integral.
For the average, you can't simply type the expression above in the integration tool because that would return the integral of the positive part of u divided by the entire surface area (as opposed to divided by only the surface area of the part of the boundary where u is positive). A workaround is to integrate u*(u>0) and to divide the result by the integral of (u>0).
Best,
Jeff
In COMSOL, you can include boolean expressions in the expressions to integrate, so you can specify that you want to integrate
u*(u>0)
by typing that in the Expression field in your Surface Integral.
For the average, you can't simply type the expression above in the integration tool because that would return the integral of the positive part of u divided by the entire surface area (as opposed to divided by only the surface area of the part of the boundary where u is positive). A workaround is to integrate u*(u>0) and to divide the result by the integral of (u>0).
Best,
Jeff
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Posted:
7 years ago
Thank You very much, Jeff! This is exactly what I needed.
What would be the right expression in the surface integration tool, if I wanted to calculate the mass flow in positive x-direction? spf.rho*(u*(u>0)*nx) or spf.rho*(u*(u>0)) do not work.
Is it maybe not possible to do this in a single calulation step directly in the Integration tool and I have to calculate the integral of u*(u>0) and then multiply this by spf.rho afterwards?
Thanks in advance
Markus
What would be the right expression in the surface integration tool, if I wanted to calculate the mass flow in positive x-direction? spf.rho*(u*(u>0)*nx) or spf.rho*(u*(u>0)) do not work.
Is it maybe not possible to do this in a single calulation step directly in the Integration tool and I have to calculate the integral of u*(u>0) and then multiply this by spf.rho afterwards?
Thanks in advance
Markus