ffc team mailing list archive

[Mehdi <m.nikbakht@xxxxxxxxxx>]

On Wed, 2010-06-16 at 16:20 +0200, Marie Rognes wrote:
> On 16. juni 2010 15:51, Mehdi wrote:
> > On Wed, 2010-06-16 at 14:03 +0200, Marie Rognes wrote:
> >   
> >> On 16. juni 2010 13:41, Mehdi wrote:
> >>     
> >>> On Tue, 2010-06-15 at 15:35 +0200, Marie Rognes wrote:
> >>>   
> >>>       
> >>>> -------- Original Message -------- 
> >>>>                           Subject: 
> >>>> Re: function on EnrichedElement
> >>>>                              Date: 
> >>>> Tue, 15 Jun 2010 15:27:44 +0200
> >>>>                              From: 
> >>>> Marie Rognes <meg@xxxxxxxxx>
> >>>>                                To: 
> >>>> Mehdi <m.nikbakht@xxxxxxxxxx>
> >>>>                                CC: 
> >>>> gnw20@xxxxxxxxx, Anders Logg
> >>>> <logg@xxxxxxxxx>
> >>>>
> >>>>
> >>>> On 15. juni 2010 15:12, Mehdi wrote:
> >>>>     
> >>>>         
> >>>>> On Tue, 2010-06-15 at 14:26 +0200, Marie Rognes wrote:
> >>>>>   
> >>>>>       
> >>>>>           
> >>>>>> On 14. juni 2010 19:57, Marie Rognes wrote:
> >>>>>>     
> >>>>>>         
> >>>>>>             
> >>>>>>> On 14. juni 2010 19:37, Mehdi wrote:
> >>>>>>>   
> >>>>>>>       
> >>>>>>>           
> >>>>>>>               
> >>>>>>>> On Mon, 2010-06-14 at 19:18 +0200, Marie Rognes wrote:
> >>>>>>>>   
> >>>>>>>>     
> >>>>>>>>         
> >>>>>>>>             
> >>>>>>>>                 
> >>>>>>>>> On 14. juni 2010 18:10, Mehdi wrote:
> >>>>>>>>>     
> >>>>>>>>>       
> >>>>>>>>>           
> >>>>>>>>>               
> >>>>>>>>>                   
> >>>>>>>>>> Hi Marie,
> >>>>>>>>>>
> >>>>>>>>>> I have a function defined on Enriched Element. I want to have access to
> >>>>>>>>>> the subfunctions defined on this space. How ffc/ufl can be extended to
> >>>>>>>>>> handle this issue? 
> >>>>>>>>>>
> >>>>>>>>>>   
> >>>>>>>>>>       
> >>>>>>>>>>         
> >>>>>>>>>>             
> >>>>>>>>>>                 
> >>>>>>>>>>                     
> >>>>>>>>> Could you give me a piece of sample code?
> >>>>>>>>>     
> >>>>>>>>>       
> >>>>>>>>>           
> >>>>>>>>>               
> >>>>>>>>>                   
> >>>>>>>> The input is something like this,
> >>>>>>>>
> >>>>>>>> Elem1 = VectorElement("Lagrange", triangle, 2)
> >>>>>>>> Elem2 = VectorElement("Lagrange", triangle, 1)
> >>>>>>>>
> >>>>>>>> Element = Elem1 + Elem2
> >>>>>>>>
> >>>>>>>> u = Coefficient(Element)
> >>>>>>>>
> >>>>>>>> # I want to have this functionality
> >>>>>>>> u1, u2 = split(u)
> >>>>>>>>
> >>>>>>>> Thank you in advance.
> >>>>>>>>
> >>>>>>>>   
> >>>>>>>>     
> >>>>>>>>         
> >>>>>>>>             
> >>>>>>>>                 
> >>>>>>> Will see what I can do tomorrow.
> >>>>>>>
> >>>>>>> Note that functions on enriched spaces can be a bit treacherous since
> >>>>>>> the basis is not a nodal basis.
> >>>>>>>
> >>>>>>>       
> >>>>>>>           
> >>>>>>>               
> >>>>>> I've been thinking some about this. Let me try to explain why this is
> >>>>>> nontrivial.
> >>>>>>
> >>>>>> Say, we have two element spaces
> >>>>>>
> >>>>>>     V = span ( {v_i} )
> >>>>>>     Q = span ( {q_j})
> >>>>>>
> >>>>>> and create an "enriched space"
> >>>>>>
> >>>>>>     W = V + Q
> >>>>>>
> >>>>>> so that
> >>>>>>
> >>>>>>     W = span ( {v_i, q_j})
> >>>>>>
> >>>>>> We also define the degrees of freedom on W by taking the set of
> >>>>>> all degrees of freedom on V (L_i) and all degrees of freedom on Q (K_j)
> >>>>>>
> >>>>>>     ( {L_i}, {K_j} )
> >>>>>>
> >>>>>> At this point, we are starting to tweak the ffc element framework a bit,
> >>>>>> because the basis for W is _not_ a nodal basis with respect to the
> >>>>>> degrees of freedom: it might be (actually, quite often is) that
> >>>>>>
> >>>>>>     L_i (q_j) \not = 0
> >>>>>>
> >>>>>> for all i, j and vice versa for K_j (v_i).)
> >>>>>>
> >>>>>> A function f in W can be represented as
> >>>>>>
> >>>>>>     f = \alpha_i v_i + beta_j q_j
> >>>>>>
> >>>>>> However, note that for some degree of freedom L_k,
> >>>>>>
> >>>>>>     L_k(f) = \alpha_k + \beta_j L_k(q_j)
> >>>>>>
> >>>>>> Hence, the coefficients in the vector do _not_ directly correspond to
> >>>>>> the degrees of freedom, unless L_k(q_j) = 0 for all k, j. This means
> >>>>>> that most kinds of function manipulation on enriched elements give you
> >>>>>> something different than what you might think you get.
> >>>>>>    
> >>>>>> There are however exceptions where things are good: Take for instance
> >>>>>> continuous linears plus bubble:
> >>>>>>
> >>>>>>     V = FiniteElement("CG", "triangle", 1)
> >>>>>>     Q = FiniteElement("B", "triangle", 3)
> >>>>>>     W = V + Q
> >>>>>>
> >>>>>> Since the bubble basis functions are zero on the boundary of each
> >>>>>> element, with the above notation
> >>>>>>
> >>>>>>     L_k (q_j) = 0
> >>>>>>         
> >>>>>>             
> >>> This is not the case for the enrichment space(Q) in the partition of
> >>> unity method. They are often defined on the same dofs that we have
> >>> defined standard space V.
> >>>
> >>>   
> >>>       
> >>>>>> for all k, j. This means that you can do
> >>>>>>
> >>>>>>     f = Function(W)
> >>>>>>     g = interpolate(f, V)
> >>>>>>
> >>>>>> Then g will be what I think you want from
> >>>>>>
> >>>>>>     (g, h) = split(f)
> >>>>>>    
> >>>>>> In order to get h, you could do
> >>>>>>
> >>>>>>     h = f - g
> >>>>>>
> >>>>>>
> >>>>>> It would be possible to extract the vectors {\alpha_i} and {\beta_j}
> >>>>>> from a Function on an enriched elements, but this requires quite a bit
> >>>>>> of work, and is mainly DOLFIN related (rather than FFC/UFL).
> >>>>>>
> >>>>>>         
> >>>>>>             
> >>> To handle this inside DOLFIN, we extract required data form vectors. Why
> >>> performing the approach inisde FFC/UFL is difficult? Isn't it just
> >>> enough to pick up the components corresponding to a specific space from
> >>> enriched space?
> >>>
> >>>   
> >>>       
> >>
> >> I don't see how to do this. If you or anyone else know how, go ahead :)
> >>     
> > OK. let me give it a try. Maybe this is a hack, but I think we may need
> > to attach offsets for the sub functions derived from enriched space. Is
> > there any reason why not this should work?
> >
> >   
> 
> 
> I probably misunderstood your original request, thinking that you wanted
> to extract
> (v, w) from {v + w}...

I also have this request. As you said I want to have access to
sub-functions defined on enriched element, i.e. (v, w) from {v + w}
where v and w are scaler.

The other request, which we have discussed more, is handling enriched
mixed element, such that we can have access to the sub elements(Note
that the sub element should be itself an enriched element). 

P = V + Q

where V and Q are vector elements 

Mehdi
 
> One way of doing what I now _think_ that you want, is to add some
> functionality to FFC/UFL to convert an enriched mixed element to a mixed
> enriched element and then do split. This is feasible. But I still think
> that this should _not_ be the default behaviour for enriched mixed elements.
> 
> 
> --
> Marie
> 
> >  Mehdi
> >
> >   
> >> --
> >> Marie
> >>
> >>
> >>     
> >>> Mehdi
> >>>
> >>>   
> >>>       
> >>>>      
> >>>>     
> >>>>         
> >>>>> Actually we were handling this inside main file before, but we didn't
> >>>>> like it. It makes code dirty especially for the nonlinear problems in
> >>>>> which we need to obtain these functions in each iteration explicitly.
> >>>>>
> >>>>>  We think that handling enriched elements inside FFC/UFL is not
> >>>>> consistent with the rest. This causes problems while working with
> >>>>> enriched elements.
> >>>>>   
> >>>>>       
> >>>>>           
> >>>> So, let's take this discussion on a mailing list instead?
> >>>>
> >>>> --
> >>>> Marie
> >>>>
> >>>> _______________________________________________
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> >>>>     
> >>>>         
> >>>   
> >>>       
> >>     
> >   
>