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On 02/02/2012 16:41, Jed Brown wrote:
<blockquote
cite="mid:CAM9tzSm40EWVREahzNL3DOiuPAvFrzisQin-g0x6G3WJCJjAOg@mail.gmail.com"
type="cite">
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<div>On Thu, Feb 2, 2012 at 17:24, gouarin <span><<a
moz-do-not-send="true"
href="mailto:loic.gouarin@math.u-psud.fr">loic.gouarin@math.u-psud.fr</a>></span>
wrote:<br>
<blockquote>
<div>No. In those examples, you have the same grid for the
velocity and the pressure. If I use 4Q1_Q1 elements, I have
not the same grid. I have one for the velocity and an other
for the pressure in the DMComposite. And for me, it is the
difficulty because as you say after, I have to do my own
preallocation step to have the good off-diagonal blocks.<br>
</div>
</blockquote>
<div><br>
Okay, I misinterpreted your notation. For your mixed elements,
you would couple with DMComposite.<br>
</div>
<blockquote>
<div> <br>
This is why I asked what is the best way to construct my
matrix. I hoped that now it is not necessary to do this
preallocation. </div>
</blockquote>
<div><br>
As long as dynamic preallocation is slow, you need to provide
it.<br>
</div>
<blockquote>
<div>This is also a difficulty to use a multigrid only on the
velocity.</div>
</blockquote>
</div>
<br>
What is hard about using multigrid only on velocity?<br>
<br>
-pc_type fieldsplit -fieldsplit_velocity_pc_type mg
-fieldsplit_velocity_pc_mg_levels 3<br>
<br>
will do geometric MG on the velocity block. Note that your
elements are slightly non-standard, so depending on how you want
to work, you might provide your own coarsening and interpolation.<br>
</blockquote>
It is not hard to use multigrid on velocity if I provide the
interpolation as I said in my first email and as you say here. But
it is for me too intrusive because I have to do a special case to
use a multigrid pcmg whereas I can use boomeramg easily. I don't
know if what I say is clear ...<br>
<br>
The idea is to test many solvers on this problem.<br>
<br>
Thanks for your replies.<br>
<br>
Loic<br>
<pre class="moz-signature" cols="72">--
Loic Gouarin
Laboratoire de Mathématiques
Université Paris-Sud
Bâtiment 425
91405 Orsay Cedex
France
Tel: (+33) 1 69 15 60 14
Fax: (+33) 1 69 15 67 18</pre>
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