dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Sun, Dec 07, 2008 at 02:57:01PM +0100, Martin Sandve Alnæs wrote:
> 2008/12/6 Johan Hake <hake@xxxxxxxxx>:
> > On Saturday 06 December 2008 20:54:34 Anders Logg wrote:
> >> On Sat, Dec 06, 2008 at 04:03:25PM +0100, Johan Hake wrote:
> >> > On Saturday 06 December 2008 15:43:13 Anders Logg wrote:
> >> > > On Sat, Dec 06, 2008 at 03:34:40PM +0100, Johan Hake wrote:
> >> > > > On Saturday 06 December 2008 14:56:59 Anders Logg wrote:
> >> > > > > On Sat, Dec 06, 2008 at 12:05:41PM +0100, DOLFIN wrote:
> >> > > > > > One or more new changesets pushed to the primary dolfin
> >> > > > > > repository. A short summary of the last three changesets is
> >> > > > > > included below.
> >> > > > > >
> >> > > > > > changeset:   5262:0e349fbe09ce4179252652fe5c1d58725951bc3f
> >> > > > > > tag:         tip
> >> > > > > > user:        "Johan Hake <hake@xxxxxxxxx>"
> >> > > > > > date:        Sat Dec 06 12:05:42 2008 +0100
> >> > > > > > files:       demo/pde/stokes/taylor-hood/python/demo.py
> >> > > > > > dolfin/function/SpecialFunctions.h
> >> > > > > > dolfin/swig/dolfin_function_pre.i
> >> > > > > > site-packages/dolfin/function.py description:
> >> > > > > > Added support for f.split()
> >> > > > > >  - Renamed operator[] to f._sub instead of f.sub
> >> > > > > >  - f.sub(i) now returns an instantiated sub function
> >> > > > > >  - f.split() uses f.sub() to return a tuple of all sub functions
> >> > > > > >  - stoke/taylor-hood demo now runs.
> >> > > > >
> >> > > > > Excellent!
> >> > > > >
> >> > > > > It runs now but the solution looks completely crazy. The problem is
> >> > > > > that the boundary conditions are not set correctly since we use V
> >> > > > > and Q to set the boundary conditions for the sub systems and they
> >> > > > > don't know the offsets (DofMap::offset()).
> >> > > >
> >> > > > The problem here is that these spaces need to be SubSpaces?
> >> > >
> >> > > Yes.
> >> > >
> >> > > > If that is the case, we could extract the subspaces after a
> >> > > > MixedFunctionSpace is created and then store these in the spaces
> >> > > > attribute, either as pure cpp.SubSpaces or add another python class,
> >> > > > SubSpace, which is a cpp.SubSpace and stores the original ffc.element
> >> > > > too, or something?
> >> > >
> >> > > I was thinking something like this:
> >> > >
> >> > > W = VectorFunctionSpace(mesh, "triangle", 2) + FunctionSpace(mesh,
> >> > > "triangle", 1) V, Q = W.split()
> >> >
> >> > Doesn't look too nice, and it will create a non intuitive work flow.
> >> > Combining two interfaces aren't easy!
> >> >
> >> > > The split function needs to both create SubSpaces (which will lead to
> >> > > DofMaps with correct offsets) and set the element correctly, which can
> >> > > be done by looking at the spaces attribute in MixedFunctionSpace.
> >> > >
> >> > > It's a bit weird since what we really do is
> >> > >
> >> > > V = VectorFunctionSpace(mesh, "triangle", 2)
> >> > > Q = FunctionSpace(mesh, "triangle", 1)
> >> > > W = V + Q
> >> > > V, Q = W.split()
> >> > >
> >> > > First, we put V and Q together to create a mixed function space. Then
> >> > > we split W again into V and Q. After the split V and Q know that they
> >> > > are part of the bigger space (at least they know the offset into the
> >> > > bigger space).
> >> >
> >> > Yes I see this. Instead, or in addition, of using split, we could
> >> > implement W.sub(i) that returns a subspace, which is then used when
> >> > setting the bc, and other relevant places.
> >> >
> >> > > The other option would be to let + have a side effect on V and Q but
> >> > > that does not seem to be a good solution.
> >> >
> >> > I thought of this too. But I cannot se how we could do it with the
> >> > present implementation of FunctionSpace/Subspace.
> >>
> >> If we can get make_subspace working as you suggest below, then it
> >> would be easy to modify __add__ for FunctionSpaceBase:
> >>
> >>   W = MixedFunctionSpace([self, other])
> >>   self.make_subspace(W, 0)
> >>   other.make_subspace(W, 1)
> >>   return W
> >
> > Yes something like that.
> >
> >> This would make the interface look nice but someone (you know who you
> >> are) might think we are insane... :-)
> >
> > *laugh*
> >
> > I am not very happy with it either, as the __add__ operator should not do such
> > a thing. What we are saying is that V and Q are individual FunctionSpaces
> > before we make make the add. After we have made it they are suddenly
> > subspaces of the result of an addition. Not very intuitive.
> >
> > One slution could be to remove the __add__ operator and instead force the user
> > to instantiate the Mixed space with a constructor, e.g.
> >
> >   W = MixedFunctionSpace(V,Q)
> >
> > With this it is easier to justify that we are doing somthing with V and Q.
> 
> No it isn't.
> 
> This isn't a matter of taste, it just doesn't scale outside a minimal example.
> 
> What will happen here?
>   W0 = MixedFunctionSpace(V0,V1) # or V0 + V1, same thing
>   W1 = MixedFunctionSpace(V1,V2)
>   W2 = MixedFunctionSpace(V2,V3)
>
> >> > If we could just call a member function in a FunctionSpace, let say
> >> > V.make_subspace(W,i), and this would then create the needed stuff in V,
> >> > to make it SubSpaceable. But now SubSpace is a subclass which effectively
> >> > prevents this approach.
> >> >
> >> > This approach is abit intrucive too. But what difference would it make
> >> > for the user, i.e., if V in addition to be a FunctionSpace now also is a> >> > subspace of W?
> >>
> >> The only reason we need this is to be able to set boundary conditions
> >> for sub systems. For that we need two things: the offset into the
> >> global system vector and the DofMap for the subsystem in question.
> >> There might be other ways to define the interface for DirichletBC,
> >> for example:
> >>
> >>   bc = DirichletBC(W, 0, ...)
> >
> >
> > This is maybee the best one? The DirichletBC operates at the global vector
> > from the mixed space, and then it is more intuitive to actually use the mixed
> > space to set the BC.
> 
> Agree.

I'm still not convinced. The thing is that the C++ implementation of
DirichletBC assumes that it gets a FunctionSpace. Sometimes, this may
be a SubSpace (which is a subclass of FunctionSpace) but DirichletBC
doesn't care, it just sets the bcs for the FunctionSpace. Before, in
0.8.1, there were 6 different constructors and an extra SubSystem
argument was required, but the introduction of SubSpace simplified the
implementation.

This all works very fine in C++, but only because we don't see the
initial FunctionSpaces V and Q that are used to create the
MixedFunctionSpace W. We just get the MixedFunctionSpace W directly
and then it's natural to define V and Q from W. The problem in Python
is that we see the original V and Q, so it becomes awkward to put them
together and then split them:

  W = V + Q
  (V, Q) = W.split()

How about the following (just renaming the variables):

  W = V + Q
  (W0, W1) = W.split()

  bc0 = DirichletBC(W0, domain, u0)
  bc1 = DirichletBC(W1, domain, p0)

?

-- 
Anders

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