# Question related to fill in boundary conditions in comsol

1. Oct 1, 2008

Firstly I really feel so lucky to find this forum. Since I don't have a strong physics background but now dealing with many problems directly related to physics.

I'm now doing some simulation in comsol and need to solve some PDEs. I'm using this PDE coefficient form in comsol. The equations need to be solved are:

-$$\rho$$$$\omega$$2$$\vec{u}$$- $$\nabla$$T=0
$$\nabla \vec{D}$$ = 0
$$\vec{E}$$ = -$$\nabla V$$
T=cES-eEi
Di=$$\epsilon$$S

The equations specify the behavior of a piezoelectric substrate when subjected to an electric field. In these equations the variable need to be solved is $$\vec{u}$$=$$\vec{u}$$(u,v,w,V).

Then PDE coefficient form in comsol is: -$$\nabla$$$$\cdot$$(C$$\nabla$$$$\vec{u}$$+$$\alpha$$$$\vec{u}$$-$$\gamma$$)+a$$\vec{u}$$+$$\beta$$$$\cdot$$$$\nabla$$$$\vec{u}$$=$$\vec{f}$$

in this step I need to transfer my equations to this PDE coefficient form.

Then I need to specify the boundary condition:

The neumann boundary condition in comsol coefficient form specifies:
n$$\cdot$$(C$$\nabla$$$$\vec{u}$$+$$\alpha$$$$\vec{u}$$-$$\gamma$$)+q$$\vec{u}$$=$$\vec{g}$$

The mixed boundary condition in comsol coefficient form specifies:
n$$\cdot$$(C$$\nabla$$$$\vec{u}$$+$$\alpha$$$$\vec{u}$$-$$\gamma$$)+q$$\vec{u}$$=$$\vec{g}$$-hT$$\mu$$
h$$\vec{u}$$=$$\vec{r}$$

q, g,h,r are coefficients I need to fill in according to my specicial case.

one of the boundary conditions writes V=Vp, n$$\cdot$$=0. And I really not sure how to define this in the mixed boundary condition and decide the value of q,g,h,r. Could anyone with comsol experience give me some hint?

2. Oct 3, 2008