COMSOL Forums: how can I impose a boundary condition which contain derivative of other dependent variable

Re: how can I impose a boundary condition which contain derivative of other dependent variable

Wed, 25 Aug 2010 09:58:10 +0000

Hi,

I am experiencing a similar problem: I want to use the time derivative of an Integration Coupling Variable (on a boundary) as BC (normal inflow velocity of a fluid dynamics simulation).

The integration variable is V. Typing d(V,t) in the BC for normal inflow velocity does not work. The velocity remains zero throughout the simulation.

Any clue what to do (V. 3.5a)?

Thanks. Alois

Re: how can I impose a boundary condition which contain derivative of other dependent variable

Mon, 21 Sep 2009 20:38:12 +0000

Hello, Kamil. You can implement derivatives very easily in COMSOL.

The notation is very simple: you can write d(A,B), where A is the variable you wish to derive with respect to B. You can also use Ab, though I think this only works up to 2nd order, and I'm not sure if it works of every variable.

So, for example, dT/dx = d(T,x) = Tx

You can nest these to get higher orders, such as:

d2T/dx2 = d(d(T,x),x) = Txx

d3T/dx3 = d(d(d(T,x),x),x)

You can also derivate with respect to different variables while nesting.

Hope this helps.

how can I impose a boundary condition which contain derivative of other dependent variable

Fri, 18 Sep 2009 14:41:31 +0000

I am using pde general mode for solving a natural convection problem and boundary condition for omega equals to second derivative of the dependent variable psi with respect to x. That is,

omega=d2psi/dx2