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High-order derivatives

Posted Sep 29, 2009, 4:35 p.m. EDT 7 Replies

Yun Jing
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Hello,

I am trying to solve a wave equation by the PDE coefficient module, the variable is u. I want to look at the second time derivative of u, i.e., utt. But when I plot it, all I got is zero, which is weird....Is there anything I might be doing wrong? The some thing happened when I used the acoustic module and tried plot ptt.

Thanks in advance
Lee
7 Replies Last Post Sep 14, 2010, 2:51 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 2 decades ago Sep 30, 2009, 3:55 a.m. EDT
Hi

for higher order derivatives have you checed out

www.comsol.com/support/knowledgebase/816/

And do not forget that there is a few settings as "click" "objects" on the bottom left of the
"solver manager output tab" concerning "time derivatives"

good luck
Ivar
Hi for higher order derivatives have you checed out http://www.comsol.com/support/knowledgebase/816/ And do not forget that there is a few settings as "click" "objects" on the bottom left of the "solver manager output tab" concerning "time derivatives" good luck Ivar
Certified Consultant
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Posted: 2 decades ago Sep 30, 2009, 9:10 a.m. EDT
You need to open the Solver Manager and go to the Output page. There, check "Include time derivatives". Solve the problem again. Then you should be able to plot utt.

Niklas
You need to open the Solver Manager and go to the Output page. There, check "Include time derivatives". Solve the problem again. Then you should be able to plot utt. Niklas
Yun Jing
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Posted: 2 decades ago Sep 30, 2009, 11:32 a.m. EDT
thanks for your reply

Is there a way to also look at the third order derivative?
I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either
But diff(diff(diff(u^2,t),t),t) did work, why is that?

Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything..
thanks for your reply Is there a way to also look at the third order derivative? I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either But diff(diff(diff(u^2,t),t),t) did work, why is that? Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything..
Linlin Yu
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Posted: 2 decades ago Sep 30, 2009, 9:23 p.m. EDT

thanks for your reply

Is there a way to also look at the third order derivative?
I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either
But diff(diff(diff(u^2,t),t),t) did work, why is that?

Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything..



I saw your post in simwe.forum, sorry for late responding.

1) If you want to get thirder or higher derivatives of sloving variable u(x,t) with respect to time, a intermediate variable v(x,t)=diff(u(x,t),t) should be introduced. In other words, just keep the highest dirivative order of time that appears in any equation less than 3, otherwise you need to break it into sevral systems.

2) Acctually, diff(diff(diff(u(t)^2,t),t),t) =6*ut*utt+2*u*uttt.

Good Luck~

TBE
[QUOTE] thanks for your reply Is there a way to also look at the third order derivative? I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either But diff(diff(diff(u^2,t),t),t) did work, why is that? Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything.. [/QUOTE] I saw your post in simwe.forum, sorry for late responding. 1) If you want to get thirder or higher derivatives of sloving variable u(x,t) with respect to time, a intermediate variable v(x,t)=diff(u(x,t),t) should be introduced. In other words, just keep the highest dirivative order of time that appears in any equation less than 3, otherwise you need to break it into sevral systems. 2) Acctually, diff(diff(diff(u(t)^2,t),t),t) =6*ut*utt+2*u*uttt. Good Luck~ TBE
Yun Jing
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Posted: 2 decades ago Oct 1, 2009, 9:51 a.m. EDT

thanks for your reply

Is there a way to also look at the third order derivative?
I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either
But diff(diff(diff(u^2,t),t),t) did work, why is that?

Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything..


sorry for the confusion, it should be diff(diff(u^2,t),t) not diff(diff(diff(u^2,t),t),t)
[QUOTE] thanks for your reply Is there a way to also look at the third order derivative? I tried diff(utt,t), and got zero, diff(diff(diff(u,t),t),t) did not work either But diff(diff(diff(u^2,t),t),t) did work, why is that? Eventually I want to put a third order derivative into the equation, and I wrote a term in the weak form to represent this as utt*test(ut), but it turned out it's not doing anything.. [/QUOTE] sorry for the confusion, it should be diff(diff(u^2,t),t) not diff(diff(diff(u^2,t),t),t)
Roozbeh Foroozan
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Posted: 1 decade ago Sep 13, 2010, 12:42 p.m. EDT
diff is not included in V.4 any idea how to get around it?
diff is not included in V.4 any idea how to get around it?
Magnus Ringh COMSOL Employee

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Posted: 1 decade ago Sep 14, 2010, 2:51 a.m. EDT
Hi,

The "diff" and "pdiff" operators changed name to "d" and "pd", respectively, starting from version 3.5a, so, for example,

d(u,x)

is now the correct syntax for du/dx.

Magnus Ringh
COMSOL
Hi, The "diff" and "pdiff" operators changed name to "d" and "pd", respectively, starting from version 3.5a, so, for example, d(u,x) is now the correct syntax for du/dx. Magnus Ringh COMSOL

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