On 3/14/19 5:02 PM, [email protected] wrote:
>
> I think just a comment in the bit on how to implement the dirichlet bc in the
> weak form would be sufficient - something to say 'In the case of an
> inhomogeneous boundary condition, you would need to set local_rhs = 0 before
> adding the cell contributions for the boundary condition'.
What line of the code would that be? Do you think it would be wrong to just
*always* set local_rhs=0?
> I'm still unsure about the step-22 condition, for the normal component for
> the
> normal stress. Is this equivalent to a dirichlet condition on the pressure
> only? I'm a little confused on this one and any thoughts would be helpful.
The normal stress shows up in the weak form after integrating by parts both
the div(2*eta*eps(u)) and the grad p terms. The term is going to be something
like
(v, (2*eta*eps(u) - pI).n)_{Gamma}
So if you prescribe
* no normal flux
* no tangential stress
(i.e., "free slip"), then you will have:
* the normal component of v is zero
* the tangential component of (2*eta*eps(u) - pI).n is zero
As a consequence, the entire boundary term disappears, as you can see if you
write the product of test function and normal stress as
(v, s.n) = (v_n, (s.n).n) + (v_t, (s.n) x n)
(normal component plus tangential component of vectors), where the first of
the two terms on the right disappears because v_n=0 and the second because
(s.n) x n = 0.
Best
W.
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Wolfgang Bangerth email: [email protected]
www: http://www.math.colostate.edu/~bangerth/
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