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High-order derivatives
Posted Sep 29, 2009, 4:35 p.m. EDT 7 Replies
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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
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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
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Niklas
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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..
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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
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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)
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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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