| Author | Message | ||
     Ole Tar (oletar) Junior Member Username: oletar Post Number: 3 Registered: 05-2007 |
I try to solve a time-dependent problem. It is necessary to change boundary condition from the VALUE(U)=const to NATURAL(U)=0 at a certain moment of time. Could you please explain how to do that. Thanks, oletar | ||
| Robert G. Nelson (rgnelson) Moderator Username: rgnelson Post Number: 1086 Registered: 06-2003 |
You cannot switch the basic class of a boundary condition from Value to Natural, but you can construct a Natural condition that simulates a Value, and switch it on and off. If you want to achieve a boundary value of U0, then the BC Natural(U)=Big*(U-U0) will drive U toward U0 with a driving strength Big. The sign and value of Big will depend on how your PDE is written. The Natural BC defines a flux, and the expression Big*(U-U0) should be set up so that if U>U0 the BC will draw a large flux from the system, while if U<U0 the flux will be a large source into the system. You can switch this BC on any condition you want, like Natural(U) = IF(condition) THEN Big*(U-U0) ELSE 0 | ||
| Ole Tar (oletar) Member Username: oletar Post Number: 4 Registered: 05-2007 |
Thank you Robert, oletar | ||
| teguh_kr New member Username: teguh_kr Post Number: 1 Registered: 07-2010 |
I try to use Natural(U) = IF(condition) THEN Big*(U-U0) ELSE 0 to change boundary from value(u)=0 to natural(u)=0 periodically, but the error message appear on the screen like this "timestep has fallen below 1e-009 of starting value!you may have a temporal discontinuityin parameters....." then i changed the syntax to RAMP natural(u) = ramp(v,0,1e8*(u-0),0.000001) and it give the same result as before. would you like to help solve this problem? | ||
| mgnelson Moderator Username: mgnelson Post Number: 202 Registered: 07-2007 |
If you post a copy of your script (*.pde) file, we can take a look at it. |