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Some questions about the way to express the differential

Posted 14 janv. 2011, 00:21 UTC−5 Version 4.0 3 Replies

Xiaoyi Yu
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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 14 janv. 2011, 16:28 UTC−5
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 14 janv. 2011, 06:41 UTC−5
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
Xiaoyi Yu
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Posted: 1 decade ago 14 janv. 2011, 11:23 UTC−5

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)
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Posted: 1 decade ago 14 janv. 2011, 16:28 UTC−5
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

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