dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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.

-- 
Anders