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Yes, you are right, it's an issue with the null space due to
boundary conditions.<br>
<br>
Thomas<br>
<br>
Am 03.02.2012 10:57, schrieb Jed Brown:
<blockquote
cite="mid:CAM9tzSkcnz0vEYyey5DudxN_KKk41d0jnR7DhN_anEL_ee2n+g@mail.gmail.com"
type="cite">
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<div class="gmail_quote">On Fri, Feb 3, 2012 at 10:48, Thomas
Witkowski <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:thomas.witkowski@tu-dresden.de">thomas.witkowski@tu-dresden.de</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
Shouldn't be, but it seems that is is close to singular in
computer arithmetic. I would like to understand we it's so.
The matrix is a 2x2 block matrix with no coupling between
the main blocks. I know that this does not make much sense
but its for tests only and I would like to add some
couplings later. Both blocks are nonsingular and easy
solvable with direct solvers. But when adding both together,
the condition number rise to something around 10^23. Is it
only a question of scaling both matrices to the same order?</blockquote>
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<br>
<div>If it's *very* poorly scaled, then yes, it could be. You
can try to correct it with -ksp_diagonal_scale
-ksp_diagonal_scale_fix.</div>
<div><br>
</div>
<div>It seems more likely to me that it's a null space issue.
How many near-zero eigenvalues are there? Perhaps you
effectively have an all-Neumann boundary condition (e.g.
incompressible flow with all Dirichlet velocity boundary
conditions leaves the pressure undetermined up to a constant).</div>
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