Including Operators and Expressions in a Multiphysics Simulation Is Easier Than You Think
As most skilled COMSOL users, I am sure you know that you are not limited to just selecting what is in our drop-down lists. Say that you have invented your own measure of structural stress. You want it to be equal to the quadratic mean of the Tresca and von Mises stresses. Go to Plot Parameters to find out what these predefined stresses are called (
mises_smsld if you are modeling in 3D with the Structural Mechanics Module). Now all you need to do is enter
sqrt(0.5*(tresca_smld^2+mises_smld^2)) in any of the Expression fields and click OK to see your new stress distribution.
Operators in COMSOL Multiphysics
You probably didn’t think of it, but in the expression I just mentioned,
^, and even
+ are all examples of operators. COMSOL offers a whole range of useful ones, not all equally obvious. Did you for instance know that the letter
d will differentiate any variable or expression with respect to time or space?
d(c,z) gives the derivative of a concentration
c with respect to the
d(sqrt(0.5*(tresca_smld^2+mises_smld^2)),t) is the time-derivative of your stress. If you have created your own subdomain expression
my_stress containing your stress definition,
d(my_stress,t) gives the same results.
at operator lets you access the solution at any time in postprocessing. This is handy if you want to see changes over a time interval. Plotting the expression
at(20,p)-at(10,p) overrides the Solution at time setting and shows you the pressure increase between 10 and 20 seconds. The
with operator lets you postprocess more than one parametric or eigensolution in a similar fashion.
Another handy pair of operators is
down. They live on boundaries and help you evaluate anything with discontinuities. Consider for example a temperature gradient on a boundary between two subdomains with different conductivities.
gradT_ht will silently evaluate this gradient on both sides of the boundary and give you the average. With
down(gradT_ht) however, you can decide which side you are interested in.
If you work with electromagnetics, you might have plotted the magnetic field in an eigenmode analysis only to find that it appears to be identically zero. Chances are it is nonzero but perfectly imaginary due to its 90-degree phase difference with a real-valued electric field. Use the
imag operator to show its imaginary part,
abs to plot the norm, or
arg to see the phase angle. Note that the default plot for complex fields shows the real part.
This is just the tip of the iceberg. You can find the complete list of mathematical and other operators in the COMSOL Multiphysics Quick Start and Quick Reference.
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Jing ZhouSeptember 6, 2012
Many thanks to the author for this useful information!
By the way, do you how to express an integral? Thank you!
Linus Andersson, COMSOL EmployeeSeptember 7, 2012
Hi Jing, glad you found it useful. To express an integral, just create and use an integration operator. This is done from Definitions > Model Couplings.
Jesus LucioDecember 4, 2012
Please, could you tell me how to access (for instance with the WITH() operator, or by means of the JOIN node for data) to any value of two different parameterized solutions (say in one solution the parameter goes from 0 to 1 and in the other the parameters goes form 1 to 2)? Each solution comes from one step function.
Thanks in advance.
Linus Andersson, COMSOL EmployeeDecember 5, 2012
I believe a Join data set should do the trick. Once you have created it, make sure that it has Data 1 set to Solution 1 and Data 2 set to Solution 2. Set “Solutions” to “One” in both the Data sections, and you will be able to pick any combination of parameter values. As usual, the Combination Method gives you various options for how to combine the solutions.
I hope this helps. If not, please contact firstname.lastname@example.org with your model, and we should be able to give you some more specific advice.
Randhir SinghJuly 3, 2013
Following up Jing Zhou: how to perform definite integral where the result is a field variable f(x,y) and where bounds are defined by the arbitrary space coordinates? say, integrating an field-expression within bounds of (x1,,y1) and (x,y) in a 2d model (x1<x and y1<y).
Linus Andersson, COMSOL EmployeeJuly 8, 2013
Suppose you have an integration operator
intop1and an expression
uthat you want to integrate over the rectangle
y1<y<y2. The expression to evaluate then becomes
intop1(u*(x>x1)*(xy1)*(y<y2)). The way this works is that comparisons evaluate to 1 if they are true and 0 if they are false.
Ciler ÖzenMarch 10, 2020
To my opinion above expression should be intop1(u*(x2>x>x1)*(y1<y<y2)). Isn't it more correct? Thanks!
Hossein MousaviAugust 5, 2014
I really like the new COMSOL capability to join two separate solutions into one dataset using the Join functionality. However, it seems data2 special operator returns only the real part of its argument but data1 can handle both components. Is that a bug or it this meant to be?
Mahvash AfzalSeptember 29, 2016
You’ve mentioned how the ‘d’ operator differentiates a variable wrt to time and space but how do we differentiate a variable wrt another dependent variable. I am using the d operator but it isn’t working for me.
Amir Hossein KazemiJuly 27, 2018
Hello my freinds,
(^2) can change negetive sign to positive but why we should use ”abs” before for example; abs(ewfd.s11)^2 to calculate norm of S11?
Athena SerraFebruary 22, 2019
Please: How to calculate this in Comsol 5.4:-8*Al*Al*integralof(dT/dz(r,0))*r*dr) from(0 to 1/2/Al). My Email: email@example.com
Brianne ChristopherFebruary 25, 2019
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