Discussion Forum

Hi
In the boundary post processing GUI you have a "Apply" and a "Plot" button, in parametric sweep mode, Apply gives you the values in the log of current location, while "Plot" gives a graph for all parameter values

--
Good luck
Ivar

Hi Ivar,

thanks for the reply.
How would I go about producing a surface plot of differences in temperature?

If I create a surface plot of temperature (via: Plot Parameters > Surface, and select Temperature in the 'Predefined uantities' drop-down) for solution 1, and then do the same for solution x (e.g. solution 5), I see a significant difference. How can I use those surface plots to create a 'difference surface' plot? So that at every point on the surface, the displayed T is T(5) - T(1)?

Ben.

Hi

A Temperature difference is simply obtained by plotting "T-T0". The trick is how to define T0.

Either you know it and define it as a Parameter/Constant

Or you calculate it as the average at a point or over a rim, or over a surface , ...

The "average" is obtained via a "coupling integration" of the variable T, divided by the integration of integrand "1" which gives the total length, area or volume respectively for 1D, 2D, 3D integration. In V4 you have the "average operator that looks after the meaning out over the respective Length, Area or Volume.

Note if you want to pick the value at a "Point" you take the "integration" over that Point of T.
Explanation: the dependent variable T is not a constant but a field it is in fact T(x,y,z) and only defined over a mesh or node of position x,y,z. Bit this dependence is not written out, it's implicit in COMSOL (something to learn). Therefore the integration over a point to get the values of the coordinates x,y,z (and potentially other values as the pressure p at that point if needed to resolve T). This is an essential concept to be understood to use COMSOL efficiently, and in fact rather subtil, and I find not at all easy to catch, before really understood ;)


--
Good luck
Ivar

I think what you're after is the with operator, see User's Guide manual, version 4.1, page 81. You can access all manuals through the Help>Helpdesk menu.

Hi

Indeed, if its the difference, point by point over an edge or area, between two "solver steps" there are also the "at()" and "with()" operators

--
Good luck
Ivar

Ho ho!
Great!

Thank you Jean-Francois, I used the following in the Plot Parameters dialogue box, 'Surface' tab, Expression field:
T-with(5,T)
and got something like what I was expecting.

I must say, i find the COMSOL help extensive, but almost inscrutable. To track down the possibility of using the with() operator inside this expression field you must wade through many cryptic explanations (cryptic to me anyway at the beginner level).

Hi Ben,

Regarding the documentation, a well-guarded secret is that 90% of all questions people ask regarding general-purpose features of the software (i.e. questions that are not specific to one application area covered by a Module) find their answer in the User's Guide manual, as opposed to all the other manuals that come with COMSOL. The other manuals do serve a purpose, but my point is that when I am looking for an answer to one of those general-purpose questions, I make a beeline to the User's Guide first.

One more thing: I recommend spending an hour or two one day that you're not busy browsing through the User's Guide, just to get familiar with how that manual is structured, what information you can expect to find in each chapter, etc. You'll probably discover a few features you didn't even know COMSOL had along the way. Doing that on a day that you are not looking for the answer to any particular question is the key. It may feel like a waste of an hour that day, but it will save you tons of time in the long run.

My two cents, based on 8 years of using COMSOL and helping people use it.
JF