| Author | Message | |||
     Penia Kresnowati (penia_kdh) New member Username: penia_kdh Post Number: 1 Registered: 05-2009 |
Hi all... I am a new user of flexPDE. Now I am using it to model diffusion and convection of a subtance inside a reactor. Is there any way to stop the simulation when the value of the variable reach zero? Negative concentration is not realistic and it the negative value may lead to strange/unrealistic overall result. | |||
| Robert G. Nelson (rgnelson) Moderator Username: rgnelson Post Number: 1258 Registered: 06-2003 |
1. FlexPDE (and the finite element method in general) uses polynomial patches to approximate the solution. In cases of sharp fronts, if the mesh is not sufficiently dense, the interpolation can produce ringing and over/undershoots. This is not in itself a disaster, as energy is still conserved, and when the front smooths out the energy content will still be correct and the over/undershoots will disappear. 2. If your solution has become negative over a significant area of the domain, then you should consider the possibility that you have written your equations wrong. 3. You can use the HALT clause to stop the simulation when any logical condition is met. See HALT in the Help Index. | |||
| Penia Kresnowati (penia_kdh) New member Username: penia_kdh Post Number: 2 Registered: 05-2009 |
Thanks for the reply Robert. I could not use halt in my simulation as it is not a time dependent simulation. I rewrite the boundary to make reaction rate zero for the value of dependent variable equal to zero or less. But it doesnot always work well if I change the value of one parameter (for example vx in the attached file) and it give out a strange result. Do you have any other suggestion?
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| Robert G. Nelson (rgnelson) Moderator Username: rgnelson Post Number: 1261 Registered: 06-2003 |
For reasons I don't yet understand, the "Mesh_Density" command you have used is confusing the Student version of FlexPDE 5. Take this command out, and everything seems to work correctly. | |||
| Penia Kresnowati (penia_kdh) Junior Member Username: penia_kdh Post Number: 3 Registered: 05-2009 |
It works better now... Thanks Robert |
diffusion, convection and reaction