Discussion Forum

Hi,
I have a general question but before asking it, i am poposing the following simple model and associated headache :-)
[ file attached]
there is 2 stationary uncoupled 1d pde with obvious analytical solution
I beleive the attached model solve:
du/dx =-sin(10*pi*x) u(0)=t
dp/dx=0 p(0)=t

note that I am using the weak form.
t of course is representative of the time...
these equations have obvious analytical solutions

OK
Now if I choose the parametric solver with t as a parameter [ as in the attached file]comsol calculate the correct solution
BUT
if I want to use the time dependant solver the solution computed by comsol are GROSSLY wrong.
specifically it seems [ but this is just a feeling that it does not treat correctly the lack of constraint on the right boundary....trying to zero it at any cost....

Now I cannot find anything in the solver setting documentation that tell me how to setup the time dependant solver to calculate the solution.


I think this is an IMPORTANT example because there are a lot of time dependant multiphysic where some parts of the model can be solved in a quasistatic approximation with only the load [ represented by my constraint at 0 in my example depending upon the time and where the solution is then coupled back to another part of the model as a slowly time varying coefficient or loading term.... [ very different timescale for example]

So my questions are
1- can someone explain to me why the time dependant solver with its default setting fail to compute the correct solution providing an obvjously wrong soltion instead while the parametric solver provide the correct solution?
2- what do I need to do on the setting of the time dependant solver to get the correct solution for these equations?
3- In the light of thisexample and more generally what do I need to do in order to couple a static equation with a time varying load in a complex time dependant multiphysic modell?

I have carefully examined the doc and the knowledge base and found nothing relevant regarding this problem.

Thanks a lot in advance
JF