Discussion Forum

Hi Gabrielle,

The simplest way is probably to use the convective boundary condition
( 3rd kind condition) witch is something like:

dc/dx= h*(cb-c).

In Comsol you may use the flux boundary condition with;

cb= your dirichlet condition,

kc >> 1 if t<t1,
kc=0 if t>t1.

In the first case, when kc is large enough, c-cb -> 0, ie c~cb,
in the second case dc/dx=0.

You may introduce the discontinuity using logical features ( ie <= or >) or Comsol smoothed
Heaviside function like flc1hs and so on or usual functions like tanh.

The value to use for kc when t<t1 may depend on various parameter of your problem. So
you have to try increasing values, for exemple 1,10,100,1000 etc.. Please notice that
using very large value may cause some instability in the solution ( jumping from 0 to infinity
in one time step is not a good idea). So you may limit the value in order to obtain a correct
approximation of c=cb)

Regards.

J.M

Hello,

I'm trying to apply a heat flux using Neumann boundary condition, but It's not working.

I'm trying to input a constant heat flux in the energy equation, for example.

-n(Gamma)=g where g=41000[W/m^2] for 0<t<ts
and
-n(Gamma)=g where g=0 for t>ts.

I tried to use a piecewise function where for 0<t<ts -n(Gamma)=g where g=41000[W/m^2]
for t>ts -n(Gamma)=g=0.
Do you know if I'm doing something wrong about this implementation?

Thanks.

Marco