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Shell diffusion solution not satisfying the governing PDE - Problem with tangential derivatives

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Hi all

In an attempt to further study tangential derivatives, I was working with the shell diffusion model available here:
www.comsol.com/model/shell-diffusion-in-a-tank-222

Since the governing PDE is just the Laplacian of the field variable (V), or precisely, div(- C*grad(V)) = 0 where C = sigma*d , one would expect that the solution to the PDE, being the Voltage, should satisfy the PDE everywhere.

Now I tried plotting the PDE, along a line in the z-direction, in the following form and I expected to get zero everywhere:
dtang(sigma*d*dtang(V,x),x)+dtang(sigma*d*dtang(V,y),y)+dtang(sigma*d*dtang(V,z),z)

or equivalently:
d(sigma*d*VTx,x)+d(sigma*d*VTy,y)+d(sigma*d*VTz,z)

to my surprise, the answer to the mentioned expression is actually very far from zero (to the order of 10^8). of course, refining the mesh won't help either. could anyone please help me with this? is there a problem with my understanding of the tangential derivatives?

0 Replies Last Post 19 gen 2015, 22:06 GMT-5
COMSOL Moderator

Hello Mohammad Miraskari

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