Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Some questions about the way to express the differential

Please login with a confirmed email address before reporting spam

I know that in comsol ax or d(a, x) represents da/dx, a is a function of x, and x is an space or time independent variable

Are these expressions below correct in comsol? like

1. d(a, b) represents da/db, where a is a function of b, but b is not an space or time independent variable

2. d(da, x) represents d^2a/dx^2, where da = d(a, x), a is a function of x, x is an space or time independent variable

3. d(da, b) represents d^2a/dx^2, where da = d(a, b)a is a function of b, b is not an space or time independent variable

the comsol version is 4.0 or higher

3 Replies Last Post Jan 14, 2011, 4:28 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Jan 14, 2011, 6:41 a.m. EST
Hi

First of all this applies only to dependent variables w.r.t. space variables.

So if you have defined a dependent variable "u" of the space x,y,z COMSOl will generate ux, uxx, uy, uyy, uz, uzz, uxy, uxz, uyz, ut, utt (see he doc) all first or second derivative w.r.t. space variable x,y,z respectively or the time "t"

but (pls check I do not know if it i fully true) you need to use the d(,) operator to get a derivative of something else than a depedent variable, and I believe you can only derive w.r.t x,y,z, r,phi or t (to be confirmed). But it's all written in the doc, use an indexer to help you find things in there

--
Good luck
Ivar
Hi First of all this applies only to dependent variables w.r.t. space variables. So if you have defined a dependent variable "u" of the space x,y,z COMSOl will generate ux, uxx, uy, uyy, uz, uzz, uxy, uxz, uyz, ut, utt (see he doc) all first or second derivative w.r.t. space variable x,y,z respectively or the time "t" but (pls check I do not know if it i fully true) you need to use the d(,) operator to get a derivative of something else than a depedent variable, and I believe you can only derive w.r.t x,y,z, r,phi or t (to be confirmed). But it's all written in the doc, use an indexer to help you find things in there -- Good luck Ivar

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Jan 14, 2011, 11:23 a.m. EST

Hi

First of all this applies only to dependent variables w.r.t. space variables.

So if you have defined a dependent variable "u" of the space x,y,z COMSOl will generate ux, uxx, uy, uyy, uz, uzz, uxy, uxz, uyz, ut, utt (see he doc) all first or second derivative w.r.t. space variable x,y,z respectively or the time "t"

but (pls check I do not know if it i fully true) you need to use the d(,) operator to get a derivative of something else than a depedent variable, and I believe you can only derive w.r.t x,y,z, r,phi or t (to be confirmed). But it's all written in the doc, use an indexer to help you find things in there

--
Good luck
Ivar


Hi, Ivar, Thanks for your answer.
I find the content in doc about the derivative operator. there is a sentence as " The expressions d(u^2,u) and pd(u^2,u) both equal 2*u ", from this we can see that u is not a independent variable as x, y, z or t. Am I right?
[QUOTE] Hi First of all this applies only to dependent variables w.r.t. space variables. So if you have defined a dependent variable "u" of the space x,y,z COMSOl will generate ux, uxx, uy, uyy, uz, uzz, uxy, uxz, uyz, ut, utt (see he doc) all first or second derivative w.r.t. space variable x,y,z respectively or the time "t" but (pls check I do not know if it i fully true) you need to use the d(,) operator to get a derivative of something else than a depedent variable, and I believe you can only derive w.r.t x,y,z, r,phi or t (to be confirmed). But it's all written in the doc, use an indexer to help you find things in there -- Good luck Ivar [/QUOTE] Hi, Ivar, Thanks for your answer. I find the content in doc about the derivative operator. there is a sentence as " The expressions d(u^2,u) and pd(u^2,u) both equal 2*u ", from this we can see that u is not a independent variable as x, y, z or t. Am I right?

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Jan 14, 2011, 4:28 p.m. EST
Hi

take a look at Magnus (he's from COMSOL ;) reply here

www.comsol.eu/community/forums/general/thread/12911/#p34421

--
Good luck
Ivar
Hi take a look at Magnus (he's from COMSOL ;) reply here http://www.comsol.eu/community/forums/general/thread/12911/#p34421 -- Good luck Ivar

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.