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A very simple PDE problem, but error. (mph file is attached V4.0)

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Hi, everyone

I am in trouble when testing the method that solving the PDE problem by Comsol.

A very simple problem that: du/dx=2*x is be solved with the boundary condition of u=1 when x=1. It is very strange that the result is oscillating. what wrong with it?

The mph file is in the attachment.

Thank you very much!

youf.


4 Replies Last Post Oct 11, 2010, 7:02 AM EDT

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Posted: 1 decade ago Oct 11, 2010, 2:58 AM EDT
well
the reason you have thos problem is because the way you wrote the pde is inconsistent with your choice of BC
if you set alpha ,called conserbative flux coefficient FOR A REASON. to be non zero in the pde, the asociated bc are strictlyd defined

for profund fundamental reasons AND WILL INCLUDE alpha as well [ they are writtenin the gui so you see what it is... you are not free to do what you want.

.To resolve your pde you need to use the beta coefficient in the coefficient pde and set te alpha to zero and then you can set your dirichlet condition. you need also to nmake sure that the second BC is free otherwise you will have a default BC there that will ake your problem overconstrained and again oscillating solution.
The thing to keep in mind here is that resolviong pde requires a lottle knowledhege of applied math if you want to wander beyond prepackaged prepared solution.

JF
well the reason you have thos problem is because the way you wrote the pde is inconsistent with your choice of BC if you set alpha ,called conserbative flux coefficient FOR A REASON. to be non zero in the pde, the asociated bc are strictlyd defined for profund fundamental reasons AND WILL INCLUDE alpha as well [ they are writtenin the gui so you see what it is... you are not free to do what you want. .To resolve your pde you need to use the beta coefficient in the coefficient pde and set te alpha to zero and then you can set your dirichlet condition. you need also to nmake sure that the second BC is free otherwise you will have a default BC there that will ake your problem overconstrained and again oscillating solution. The thing to keep in mind here is that resolviong pde requires a lottle knowledhege of applied math if you want to wander beyond prepackaged prepared solution. JF

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Posted: 1 decade ago Oct 11, 2010, 4:54 AM EDT
A small queation:

How to set the second BC is free in COMSOL "boundary condition"?
A small queation: How to set the second BC is free in COMSOL "boundary condition"?

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Posted: 1 decade ago Oct 11, 2010, 5:48 AM EDT
In 3.5a just put h=0 in the dirichlet BC.
I have not use pde module with 4.x so far so I dont know if it has changed or not.

JF
In 3.5a just put h=0 in the dirichlet BC. I have not use pde module with 4.x so far so I dont know if it has changed or not. JF

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Posted: 1 decade ago Oct 11, 2010, 7:02 AM EDT
Hi,JF

Thanks for your help!

The problem has been resolved after I adjusted the gui following your advices. Then I feel that I must learn the PDE profoundly.

Thank you again!

youf.
Hi,JF Thanks for your help! The problem has been resolved after I adjusted the gui following your advices. Then I feel that I must learn the PDE profoundly. Thank you again! youf.

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