[Nek5000-users] Strong divergence and Laplacian operator

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

Dear Paul,

Thank you for the answer. I will use gradm1 then.

Best,
Nicolas

On Tue, Oct 3, 2017 at 11:25 PM, <nek5000-users at lists.mcs.anl.gov> wrote:

>
> Hi Nicolas,
>
>
> You can use gradm1 to get your strong divergence.
>
>
> Laplacian is a bit trickier since the 2nd derivative really is
> well-defined.
>
> Some people compute the gradient and then apply dsavg() to each component
>
> prior to taking the divergence of that field.
>
>
> Right now there are no tools to do exactly the thing you want, so using
> gradm1()
>
> is as good as any option.
>
>
> Note that gradm1() is as efficient (or more so) than some of the other
> tools
>
> to compute gradient in Nek.  In order to get (say) du/dx, you have to
> compute
>
> du/dr , du/ds , du/dt, and then get du/dx from the chain rule.   du/dy,
> du/dz
>
> also come from the chain rule, almost for free, even though you don't need
> them.
>
> So you can simplify everything just through repeated use of gradm1.
>
>
> Best, Paul
>
>
> ------------------------------
> *From:* Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> on behalf
> of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
> *Sent:* Tuesday, October 3, 2017 12:12:59 PM
> *To:* nek5000-users at lists.mcs.anl.gov
> *Subject:* [Nek5000-users] Strong divergence and Laplacian operator
>
> Hi all,
>
> I need to compute some operators in the strong form directly for a
> project, i.e. without the mass matrix included in the operator.
>
> For the convection and gradient operators, I am using the functions convop
> and gradm1 respectively. However, I would also need to compute strong
> divergence and Laplacian and I did not find the corresponding functions
> (opdiv and axhelm are computing the weak form if I understand correctly).
> Do such functions exist? Or what modifications should I bring to the
> existing routines to change them to strong form?
>
> Best regards,
> Nicolas
>
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