Dear Benoît.
First, it seems that in FreeFem++, you are solving a potential flow. Did you
succeed with ff3d doing the same?
Second, since ff3d's discretization is based on a Q1-Q1 element
(velocity-pressure), it needs to be stabilized with a 'penalty' term (eps in
this case). The pressure solution depends a lot on the choice of this term.
ff3d does not support Q2-Q1 coupled discretization yet. But, you can already
use a projection algorithm to do so. I can send you an example of such a
scheme which provides much better solution than the direct resolution.
I am quite busy now but I can send you the example by the end of the week.
Remind me if I forget.
Concerning the second way of approximating the force, it requires that the
null velocity is imposed weakly using a volume penalty (in your case, you
used a strong formulation --- elimination --- for Dirichlet on the cylinder).
If I remember, this is described in this paper:
http://www.math.u-psud.fr/~maury/Files/OP60DelpinoMaury.pdf
If not, you can check B. Maury's web page, you should find an answer.
I hope this gives you enlightenment...
Best regards,
Stéphane.
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