[Users] Synchronization and MoL

[Erik Schnetter]

On Sep 21, 2006, at 13:20:13, Ian Hinder wrote:

> On Tue, Sep 19, 2006 at 04:40:35PM -0500, Erik Schnetter wrote:
>> On Sep 19, 2006, at 14:24:35, Ian Hinder wrote:
>
>>>> You have two choices; you can either sync the state vector or its
>>>> RHS.  This is somewhat independent of where you apply the physical
>>>> (outer) boundary condition.  In our multi-block code, we apply the
>>>> boundary conditions to our RHS via penalties, and we also sync the
>>>> RHS.
>>>>
>>>> Using the boundary interface is another issue; the boundary  
>>>> interface
>>>> also applies symmetry and inter-patch boundary conditions.  We
>>>> therefore also use the boundary interface to apply boundary
>>>> conditions onto the RHS.
>>>>
>>>> In principle, you can mix and match applying the triple
>>>> (synchronisation, boundary interface, physical boundaries) to  
>>>> either
>>>> the state vector or the RHS.  We chose to apply all to the RHS for
>>>> consistency reasons, but you can do otherwise.
>>>
>>> Do you mean the whole triple can either be applied to the RHS or to
>>> the state vector, or that I can choose either for each element of  
>>> the
>>> triple?
>>
>> I think you can mix and match, that is, you can choose either for
>> each element of the triple independently.  Of course, this may not
>> work for specific boundary conditions, e.g. when a boundary condition
>> takes tangential derivatives on the boundary.
>>
>>> For example, can I set my interior RHSs in MoL_CalcRHS (with no SYNC
>>> statement), apply my physical boundary condition to the RHS in
>>> MoL_RHSBoundaries and synchronize the RHS variables with a SYNC line
>>> there.  At this point all the RHSs are correct except in the  
>>> symmetry
>>> zones.  The MoL update then happens and updates the state vector.  I
>>> then call the Boundary interface in MoL_PostStep on the state  
>>> vector,
>>> and all the symmetry boundary conditions are applied.  This seems
>>> consistent; it is correct?
>>
>> Yes, this seems correct.  However, there does not seem to be a reason
>> for doing so -- you may as well apply the boundary conditions to the
>> RHS, simplifying the logic and ensuring that  all RHS grid points are
>> correct.  There is a group MoL_rhs_boundary or so for that effect.
>
> And SYNC in that group, and rely on MoL updating the ghost zones?  But
> if we only SYNC the RHS variables, and they only have one time level,
> how can prolongation occur?  Am I misunderstanding?  It seems that
> prolongating the RHSs is quite different to prolongating the state
> vector.

This is correct; you need to sync the state vector.

-erik

-- 
Erik Schnetter <schnetter at cct.lsu.edu>

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