Is it possible to evaluate a variable at a given point?

[Adrien-a0]

I'm trying to evaluate the value of one variable at the minima of another, which I obtain by fitting the variable using scipy. I would like to be able to evaluate the point directly at this position using MpH, however I don't know if I can. My current work around is to export data at the point by defining the point using global variables in COMSOL. This, however, requires the solution to be computed twice, due to the use of global variables.

Alternatively, if it is possible to assign values variables that are only defined in the results interface I could also obtain this value using only one computation by using my current workaround.

[john-hen]

My current work around is to export data at the point by defining the point using global variables in COMSOL.

If I understand correctly (and that's difficult without a concrete example), you already know how to evaluate a (field) variable at a given point. I'm assuming when you say "export" you mean "evaluate", and by "global variable" you mean "global parameter". (Otherwise that's what you should look into.)

But then you don't have to compute the solution twice. Comsol let's you update the solution when parameters change. This feature arguably exist precisely for this use case, where the parameter values are only needed in post-processing.

[Adrien-a0]

Just to clarify, I export to .txt file and then load the .txt file within python as I'm interested in processing the data using python, but as far as I can tell you can't evaluate a model at a specific point using mph.

[john-hen]

You have access to the full Comsol API via MPh, so all of these things should be possible. Almost certainly are possible. It's just that MPh does not expose any "high-level" functions for this specific use case. But you can dig deeper, like in the Comsol documentation, or by inspecting the Java code of your model.

[Adrien-a0]

Hey I had a quick look over this stuff but I think it's a bit above my ability coding wise. I wonder if you know how I could update the solution of a study (named "Std1") in MPh, rather than recomputing? This method works far more quickly within the comsol GUI, so it would be hugely appreciated if you know a line of code that will do this!

[john-hen]

Looks like you'll have to call the Java method updateSolution() on the (Java) solution object.

For example, using the demo model from the tutorial (capacitor.mph), to update the "electrostatic solution", it's either

model.java.sol('sol1').updateSolution()

or, using the "node syntax" introduced by MPh:

(model/'solutions'/'electrostatic solution').java.updateSolution()

In both cases we access the Comsol API directly via the respective .java attribute.

You'll have to find out the solution name ("electrostatic solution" above) or tag ("sol1" above), which isn't the same as the study name or tag. Easiest is to hard-code it. To find out, if you don't know already, open your model in the Comsol GUI, do "update solution", then "save as" a .java file and look for the solution tag at the very end of the file. Or run mph.tree(model) in an interactive Python session to find the solution's name. Obviously, it would also be possible to have your program do the look-up, but that takes a little more effort to code.

[Adrien-a0]

Hey,

Thank you so much this works perfectly and was incredibly useful in understanding how to access the full api.