Constraints Log Out | Topics | Search
Moderators | Register | Edit Profile

FlexPDE User's Forum » User Postings » Constraints « Previous Next »

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Olexiy Orel (scorpion)
Member
Username: scorpion

Post Number: 11
Registered: 03-2007
Posted on Tuesday, February 24, 2009 - 11:28 am:   

Hello friends.
Please explain me a few moments about constraints.
"CONSTRAINTS should not be used with steady state systems which are unambiguously defined by their boundary conditions, or in time-dependent systems."
Is a simple diffusion equation system "unambiguously defined by its boundary conditions"? For example:
del2(C)=0.
C equals 1 on one peace of the boundary, 0 on another, natural(C)=0 on the rest. Is this system defined unambiguously? What do I do when enabling constraints (like charge conservation)? Does it mean that the solution will not satisfy the equation and BC's strictly any more? Thanks.
Top of pagePrevious messageNext messageBottom of page Link to this message

Jerry Brown (jerrybrown11743)
Member
Username: jerrybrown11743

Post Number: 53
Registered: 03-2004
Posted on Tuesday, February 24, 2009 - 05:19 pm:   

If you look in the folder of sample problems that come with FlexPDE you will find a folder named "constraints". It's a sub-folder of "misc".
Also Prestube in the "Samples\steady_state\stress" folder shows how constraints are used to eliminate rigid body motion in an elasticity problem.
Top of pagePrevious messageNext messageBottom of page Link to this message

Archie Campbell (amc1)
Member
Username: amc1

Post Number: 8
Registered: 03-2008
Posted on Wednesday, March 04, 2009 - 03:40 pm:   

THe question of whether a solution is unambiguously defined is far from straightforward and leads to problems with constraints, as I have found, (see previous postings). The easisest way of trying to determine this is to think of the physical situation, heat flow is particularly useful. In this case value(C) defines a uniform temperature boundary and natural(C)=0 an insulator. What your problem describes is a box with one section at one temperature and a second at another temperature with the remaining surfaces insulators. This is clearly a well defined system with a single steady state solution. Methematical uniqueness theorems which define unique solutions are more general, but much more difficult to understand by anyone except mathematicians.

Add Your Message Here
Post:
Username: Posting Information:
This is a private posting area. Only registered users and moderators may post messages here.
Password:
Options: Enable HTML code in message
Automatically activate URLs in message
Action:

Topics | Last Day | Last Week | Tree View | Search | Help/Instructions | Program Credits Administration