[Nek5000-users] opdiv and multd

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

Thank you Paul,
that solved the problem.

The correct result is obtained by :
       call multd (RES(1),vx,rym2,sym2,tym2,2,iflg)
       call dssum(RES(1))
       call col2(RES(1),binvm1,lt)

francesco



On 12/08/2010 02:17 PM, nek5000-users at lists.mcs.anl.gov wrote:
>
> Hi Francesco,
>
> Several of the routines internal to nek are the so-called weak
> derivatives, implying that they somehow involve the mass matrix
> or the transpose of the operator, so it takes a bit of investigation
> to understand precisely what each operator is doing.  (Note that the
> functionality also differs slightly between PN-PN and PN-PN-2 
> formulations.)
>
> As an example, axhelm does not multiply by the (negative) Laplacian.
> Instead, it creates (for h1==1, h2==0):
>
>    w = A * u
>
>      = grad^T B grad u
>
> where B is the mass matrix.
>
> Similarly, opdiv is most likely returning   d = B*grad u (or perhaps
> the negative of this).     Moreover, d is returned on the pressure
> mesh (mesh 2), which is the same as the velocity mesh (mesh 1) for the 
> PN-PN formulation that you use.
>
> In Nek, the mass matrix is diagonal and the mesh 1 inverse is stored
> in binvm1.  It seems probable that you'll get the correct result
> after
>
>       call col2(d,binvm1,n)
>
> where d holds the output of opdiv and n is the number of points on
> mesh 1, assuming you're using PN-Pn.
>
> Paul
>
>
>
> On Wed, 8 Dec 2010, nek5000-users at lists.mcs.anl.gov wrote:
>
>> Hi all,
>>
>> I am having some problems using the subroutines opdiv and multd.
>> For some reasons they do not give me correct results.
>> Do they need any special treatment or precautions?
>>
>> I was trying to compute the divergence of a 3D field, and i realized 
>> that in my case the multd subroutine used
>> by opdiv does not gives good results, while the same derivative 
>> computed from gram1 gives no problems.
>>
>> For example, I overwrite an analytical field :
>>
>>         ...
>>         parameter(lt=lx1*ly1*lz1*lelt)
>>         real       Res(lt)
>>         ...
>>                    vx(i,j,k,e)=   1.0*sin(x)*cos(y)*cos(z)
>>         ...
>>             call print_line(vx)
>>         ...
>>         iflg = 1
>>         call multd (RES(1),vx,rym2,sym2,tym2,2,iflg)
>>            call print_line(Res(1) )
>>
>>        call gradm1(work1,Res(1),work2,vx)
>>            call print_line(Res(1) )
>>         ...
>>         if (lx2.ne.lx1) then
>>         ...
>>         STOP
>>         endif
>>
>>
>> and  print output a (xr,:,zr) the result  (performed by print_line) .
>> In vxi.eps i compare the result (lines) with the analytical solution( 
>> symbols). The result of multd is not correct while all
>> the others match perfectly the analytical solution.
>> In vxy.eps i plot only the result of multd. Note that on y i have 12 
>> elements the same number of peaks of the solution.
>>
>> Does anybody have an idea on how to fix this?
>>
>> Thank you very much for any help or suggestion
>>
>> francesco
>>
>>
>>
>>
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