[Nek5000-users] Nek5000 documentation details: Pn-Pn pressure solver

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

Dear Vlad,
the low Mach Pn-Pn approach is based on the 1997 (JSC) and 1997 (JCP)
papers you mention and it consists of 3 steps as you describe, i.e.:
a) first the velocity is updated using the extrapolated convective term,
b) then the Laplacian of pressure is calculated due to convection, after
that
c) the velocity is updated using the pressure gradient and accounts for
viscous term
The coupled Helmholtz solver is used for the velocities only when using
ifstrs=true, that
is when you want to include the full stress tensor. Otherwise, it is using
separate Helmholtz solves for each of the velocity components, similar to
Pn-Pn-2.
Hope this helps clarify things.
All the best,
Ananias


On Tue, Jun 6, 2017 at 7:29 AM, <nek5000-users at lists.mcs.anl.gov> wrote:

> Dear Neks,
>
> reading the documentation I got the impression that Pn-Pn solver (low
> Mach) first solves the pressure where the convective and viscous (!) terms
> are taken into account. After that using this p^{n+1} we solve for velocity
> field. It seems that the algorithm consists of only 2 steps (pressure +
> velocity).
>
> However, reading the paper by Tomboulides, Lee, Orszag (1996) which is
> referenced inside the code, I see the projection algorithm where first the
> velocity is updated using the extrapolated convective term, then the
> Laplacian of pressure is calculated due to convection, after that the
> velocity is updated using convection and pressure gradient. The last step
> accounts for viscous term.
>
> I am a bit confused, could you please help me out here? Which method is
> used?
>
> PS. Another thing is the coupled Helmholtz solver in Pn-Pn. I see that in
> case of Pn-Pn-2 each velocity component is treated separately (segregated
> solver). However, this coupled thing slightly confuses me, why not treating
> it separately as in Pn-Pn-2? Could you please comment there as well? Thank
> you.
>
> Best regards,
> Vlad
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