[Libmesh-users] periodic boundary condition

[David K. <dav...@ba...>]

Hi guys,

I was just wondering what exactly happens in libMesh when you set a  
periodic boundary condition constraint in order to constrain, say,  
dof0 and dof1 to be the same, e.g.:

			DofConstraintRow constraint;
			constraint[dof0]=1;
			dof_map.add_constraint_row(dof1, constraint);

Does this simply reset the row in the system matrix corresponding to  
dof1 to be [0 0 0 ... 0 -1 0 ... 0 1 0 ... 0], where the dof0'th  
entry is -1 and the dof1'th entry of the row is 1? Is this all one  
needs to do to impose periodic boundary conditions in a finite  
element code? (of course the RHS here would need to have a 0 in the  
dof1'th row as well)

My understanding of periodic BCs is that, when imposing periodic BCs,  
we can think of two corresponding boundary degrees of freedom on  
opposite sides of the domain as the same DOF, and then the support of  
this DOF's basis function is split across the boundary of the domain.  
In this case, in order to take into account the split support of the  
basis function, I would think we would need to do the full matrix  
assembly, then add row dof1 to row dof0, then finally set the  
constraint row as in the above code snippet. Is this in fact what is  
required when setting periodic BCs? have a hunch this adding of  
matrix rows is unnecessary, clarification would be much appreciated.


Thanks a bunch,
David