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Re: pressure spikes in Stokes demo?

 

Anders Logg wrote:
On Thu, Feb 14, 2008 at 08:39:07AM +0100, Dag Lindbo wrote:
Anders Logg wrote:
On Wed, Feb 13, 2008 at 08:52:29PM +0100, Dag Lindbo wrote:
On Tue, Feb 12, 2008 at 10:23:09AM +0100, Dag Lindbo wrote:
Hello all!

In the stokes demo (taylor-hood), I notice spurious pressure spikes
near
the corners. What is causing these?

I changed to a paraboloc inflow profile (consistent with the no-slip
BCs),
but the problem remains. Further, I ran the same problem on a
unitsquare
mesh (see plot attached). The pressure spikes are large. Is the demo
problem ill posed?

Thanks!
Dag Lindbo, KTH
I would also suspect the boundary conditions for the velocity
to be the problem, but if you still see the spikes with a parabolic
inflow profile (which is zero at the no-slip boundaries) then I have
no idea.

Is this in all corners? Could you send a plot?

/Johan
Problem solved. Thanks for the help! The error is that the facets adjacent
to the corners no get properly marked for noslip BC.

Fix to get the correct subdomain markers, in demo/mesh/subdomain/main.cpp:

class Noslip : public SubDomain
  {
    bool inside(const real* x, bool on_boundary) const
    {
      return (x[1] > 1.0 - DOLFIN_EPS || x[1] < DOLFIN_EPS) && on_boundary;
    }
  };

/Dag & Shilpa
Then you don't get no-slip on the dolphin which makes the solution
look pretty dull, don't you think?

Right. For the dolfin you can put
return ((x[1] > 1.0 - DOLFIN_EPS || x[1] < DOLFIN_EPS) ||
	(x[0] > DOLFIN_EPS && x[0] < 1.0 - DOLFIN_EPS) ) && on_boundary;

   Dag

Turns out to be even simpler:

  return on_boundary

does the trick. Inflow and outflow bcs will be applied after and then
overwrite where we should not have no-slip.

Don't you think that would be a little opaque to a new user?

-- ART

--

====================
Andy Terrel
Computer Science Dept
University of Chicago
aterrel@xxxxxxxxxxxx
---------------------

Science is what we understand well enough to explain to a computer. Art is everything else we do.
                                                     -Donald Knuth



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