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is there any difference between phi1t and pd(phi1,t).?I have to specify partial differential of phi1 w.r.t time(t)

Posted Jan 14, 2011, 4:36 a.m. EST Parameters, Variables, & Functions Version 3.5a 8 Replies

GaneshM Battery Modelling
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Hai..

i have to solve a model including PDE general form equations. In source term i have to specify partial derivative of (phi1-phi2) wrt.time. first, i tried to simulate by specifying phi1t-phi2t. It didn't work.

if i specify the model with operator pd(phi1,t), my simulation is running in place of phi1t.


Could you please tell me why is this happening?. I am in a confusion weather my simulation is giving correct results or not.


thanks
ganesh
8 Replies Last Post Jan 16, 2011, 11:38 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 14, 2011, 6:46 a.m. EST
Hi

I suspect this is depending if you have selected or not the "time derivatives on" in the solver settings.

COMSOL operates sometimes differently in 3.5 and v4 (and you have not mentionned in which version you are).

In anycase check all GUI of the solver settings (an sequencing) somewhere there is a "time derivative" check box

It could be (without really knowing for sure) that there might be some differences when the "phi1t" is evaluated and the "d(phi1,t)" is done, but I do not beleive it is very different

--
Good luck
Ivar
Hi I suspect this is depending if you have selected or not the "time derivatives on" in the solver settings. COMSOL operates sometimes differently in 3.5 and v4 (and you have not mentionned in which version you are). In anycase check all GUI of the solver settings (an sequencing) somewhere there is a "time derivative" check box It could be (without really knowing for sure) that there might be some differences when the "phi1t" is evaluated and the "d(phi1,t)" is done, but I do not beleive it is very different -- Good luck Ivar
GaneshM Battery Modelling
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Posted: 1 decade ago Jan 14, 2011, 9:14 a.m. EST
hi Ivar,

thanks for reply.

My version is 3.5a.

I have turned "include time derivative" on in out put of solve manager. Still I am facing the same problem. problem is solvable when i give pd(phi1,t) pd(phi2,t) in place of phi1t, phi2t.

is phi1t equivalent to ordinary derivative d(phi1,t) or partial derivative pd(phi1,t)?

regards
ganesh
hi Ivar, thanks for reply. My version is 3.5a. I have turned "include time derivative" on in out put of solve manager. Still I am facing the same problem. problem is solvable when i give pd(phi1,t) pd(phi2,t) in place of phi1t, phi2t. is phi1t equivalent to ordinary derivative d(phi1,t) or partial derivative pd(phi1,t)? regards ganesh
Magnus Ringh COMSOL Employee

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Posted: 1 decade ago Jan 14, 2011, 10:11 a.m. EST
Hi,

If phi1 is a dependent variable, phi1t and p(phi1,t) are equivalent but pd(phi1,t) is 0.
With the pd operator (contrary to the d operator), phi1 is considered independent of anything but itself. For example,
d(u+x,x) equals ux+1 whereas pd(u+x,x) equals 1.

The "Include time derivatives" setting in version 3.5a affects the output from the COMSOL simulation only. By storing the time derivatives you allow for more accurate interpolation of time derivatives; without including the time derivatives, the evaluation of second-order time derivatives like utt returns 0.

Best regards,
Magnus Ringh, COMSOL
Hi, If phi1 is a dependent variable, phi1t and p(phi1,t) are equivalent but pd(phi1,t) is 0. With the pd operator (contrary to the d operator), phi1 is considered independent of anything but itself. For example, d(u+x,x) equals ux+1 whereas pd(u+x,x) equals 1. The "Include time derivatives" setting in version 3.5a affects the output from the COMSOL simulation only. By storing the time derivatives you allow for more accurate interpolation of time derivatives; without including the time derivatives, the evaluation of second-order time derivatives like utt returns 0. Best regards, Magnus Ringh, COMSOL
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 14, 2011, 3:50 p.m. EST
Hi Magnus,

thanks for the info, I see I need to refresh my d(), pd() operator use again, an check too that the v4 way is understood. I do not expect this to have canged, apart that I have noticed that the "save time derivatives" is "on" by default in v4.1

--
Good luck
Ivar
Hi Magnus, thanks for the info, I see I need to refresh my d(), pd() operator use again, an check too that the v4 way is understood. I do not expect this to have canged, apart that I have noticed that the "save time derivatives" is "on" by default in v4.1 -- Good luck Ivar
Xiaoyi Yu
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Posted: 1 decade ago Jan 14, 2011, 10:37 p.m. EST

Hi,

If phi1 is a dependent variable, phi1t and p(phi1,t) are equivalent but pd(phi1,t) is 0.
With the pd operator (contrary to the d operator), phi1 is considered independent of anything but itself. For example,
d(u+x,x) equals ux+1 whereas pd(u+x,x) equals 1.

The "Include time derivatives" setting in version 3.5a affects the output from the COMSOL simulation only. By storing the time derivatives you allow for more accurate interpolation of time derivatives; without including the time derivatives, the evaluation of second-order time derivatives like utt returns 0.

Best regards,
Magnus Ringh, COMSOL


Hi, Magnus Ringh
can I use d(a, b) to represent da/db in comsol: (1) if "a" is not an dependent variable and "b" is not an independent variable; (2) if "a" is not an dependent variable but "b" is an independent variable; (3) if "a" is an dependent variable but "b" is not an independent variable?
thank you!
[QUOTE] Hi, If phi1 is a dependent variable, phi1t and p(phi1,t) are equivalent but pd(phi1,t) is 0. With the pd operator (contrary to the d operator), phi1 is considered independent of anything but itself. For example, d(u+x,x) equals ux+1 whereas pd(u+x,x) equals 1. The "Include time derivatives" setting in version 3.5a affects the output from the COMSOL simulation only. By storing the time derivatives you allow for more accurate interpolation of time derivatives; without including the time derivatives, the evaluation of second-order time derivatives like utt returns 0. Best regards, Magnus Ringh, COMSOL [/QUOTE] Hi, Magnus Ringh can I use d(a, b) to represent da/db in comsol: (1) if "a" is not an dependent variable and "b" is not an independent variable; (2) if "a" is not an dependent variable but "b" is an independent variable; (3) if "a" is an dependent variable but "b" is not an independent variable? thank you!
GaneshM Battery Modelling
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Posted: 1 decade ago Jan 15, 2011, 11:02 a.m. EST
Hi Magnus,

Thanks for the reply.

I have tried with p(phi1,t).But, COMSOL says its unknown function. If phi1 is a dependent variable, is it sufficient to represent phi1t as partial derivative wrt t? is d(phi1,t) represents the ordinary derivative?

In the quick reference book of multiphysics help, it was given about pd(f,x) as
"Differentiation operator. Differentiation of f with respect to x. No chain rule for dependent variables."
Could you please tell about this?


Thanks
Ganesh



Hi Magnus, Thanks for the reply. I have tried with p(phi1,t).But, COMSOL says its unknown function. If phi1 is a dependent variable, is it sufficient to represent phi1t as partial derivative wrt t? is d(phi1,t) represents the ordinary derivative? In the quick reference book of multiphysics help, it was given about pd(f,x) as "Differentiation operator. Differentiation of f with respect to x. No chain rule for dependent variables." Could you please tell about this? Thanks Ganesh
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jan 15, 2011, 3:47 p.m. EST
Hi

I believe p() is a typo and it's to be understood as d().

Check also the doc p79 of the user manuel v4.1 on "Differentiation Operators", or p163 in the 3.5a doc (user) guide.pdf

--
Good luck
Ivar
Hi I believe p() is a typo and it's to be understood as d(). Check also the doc p79 of the user manuel v4.1 on "Differentiation Operators", or p163 in the 3.5a doc (user) guide.pdf -- Good luck Ivar
GaneshM Battery Modelling
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Posted: 1 decade ago Jan 16, 2011, 11:38 a.m. EST
hi Ivar..
thanks for help..


regards
ganesh
hi Ivar.. thanks for help.. regards ganesh

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