[Nek5000-users] Building mass and stiffness matrices

[nek5000-users at lists.mcs.anl.gov]

Hi Paul,
thank you for your answer, now I understand why I couldn't find the matrices in the code.
For the mass matrix I think it is clear how to proceed, but how could I do the same for the stiffness matrix and the convective matrix?
My problem consists in about 3000 elements, with lx1=10, but I plan to move to 6000 or 9000 elements.
I will now work on a subroutine for this task, then I will come back if I find other issues.
Giuseppe




Il giorno 13/feb/2014, alle ore 13:34, <nek5000-users at lists.mcs.anl.gov>
 <nek5000-users at lists.mcs.anl.gov> ha scritto:

> 
> Hi Giuseppe,
> 
> Nek uses a completely matrix-free approach.  The standard way to generate
> a matrix in this case is to start passing in unit column vectors, e_j
> (the jth column of the Identity matrix) and saving the result a_j, the
> jth column of A.   For sparse matrices there is a fair amount of bookeeping required so that you store only the nonzeros.  (I normally
> use compressed sparse row format to do so.)
> 
> The mass matrix is diagonal and therefore readily retrieved.
> 
> How large is your problem (i.e., how many elements and what is the
> order, lxl) ?
> 
> Note that the number of nonzeros scales like E*(lx1^6).
> 
> Paul
> 
> On Thu, 13 Feb 2014, nek5000-users at lists.mcs.anl.gov wrote:
> 
>> Dear users and developers,
>> I am using NEK5000 for a 3d unsteady simulation with mixed Dirichlet and
>> periodic bc. For the post-processing (a POD-based dynamics) I need the global
>> mass and stiffness matrices as built by NEK5000 on my mesh. I would also need
>> the matrix having as entries (phi_i, grad(phi_j)) where phi are the basis
>> polynomials.  Is it possible to have NEK build these matrices and then save
>> them on file? The ultimate goal is to import them on a PETSc program to
>> perform some algebraic manipulations. I already found a way to import the
>> simulations' results.  Surfing the code, I have found some 1-d routines, but
>> I don't know how to extend them to my needs.  Thank you in advance for any
>> help or hint.
>> Best regards,
>> Giuseppe
>> 
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