[petsc-users] How to add a source term for PETSCFV ?

Wed Apr 14 04:38:50 CDT 2021

Hello Matthew,

Sorry for the late answer !

Unfortunately, I couldn't find any elegant way to add a source term with a
TS+DM+PetscFV  ... The only thing I could find was to write completely my
own RHS function that i pass to DMTSSetRHSFunctionLocal. That way I have
full control over everything, but it's not as elegant as using the
DMPlexTSComputeRHSFunctionFVM that is already in PETSc.

I added you to the project on Gitlab where i do this (I have not been
continuing with this code but with another, also based on PETSc, that's why
there are no updates recently) so you can see what I coded, if it can be of
any help for a future release / for the people in the other project you
mention. What I am talking about is in timeDiscretization_PETSc.c &
fluxCalculation.c, mostly. (I did not expect to share that work so it might
be a bit poor comments-wise, sorry).
Feel free to share the code if you feel it can be useful, as long as the
headers in the source files stay as they are ;)

Thibault

Le lun. 12 avr. 2021 à 16:38, Matthew Knepley <knepley at gmail.com> a écrit :

> On Mon, Jul 20, 2020 at 11:04 AM Thibault Bridel-Bertomeu <
> thibault.bridelbertomeu at gmail.com> wrote:
>
>> Thank you Mark, Jed and Matthew for your quick answers !
>>
>> I see now where I should be more accurate in my question.
>>
>> Mark, I mentioned the hyperbolicity because I would like to keep using
>> the PetscDSSetRiemannSolver and the DMTSSetBoundaryLocal and DMTSSetRHSFunctionLocal
>> with DMPlexTSComputeRHSFunctionFVM that are quite automatic and nice and
>> efficient wrappers. Now aside from those which deal specifically with the
>> hyperbolic part of the PDE, i would like to add the diffusive terms. I
>> would rather stay in the FVM world, but if it is easier in the FEM world
>> then I am open to it.
>>
>> Jed, as for the discretization let us say indeed that the mesh can be
>> either cartesian or not, and the discretization should therefore be
>> independent of the nature of the mesh - any unstructured mesh (i handle it
>> with DMPlex in my case). I saw indeed that FV has gradient reconstruction,
>> with or without a limiter, which is great. However I have not quite
>> understood what function to use to get the gradient of any variable, be it
>> in the context (e.g. for N-S, ro, rou, rov, etc...) or an auxiliary
>> variable (e.g. the components of the strain tensor). I also agree that the
>> diffusive part is usually the one that strongly limits the time step in
>> explicit computations, but for now I would like to set up a fully explicit
>> system.
>>
>> Matthew, I'll take a look at ex 18, thanks, I missed that one.
>>
>> So basically if I wanted to summarize, I want to keep the Riemann Solver
>> capability from the DS, and use the
>> "automatic" DMPlexTSComputeRHSFunctionFVM for the hyperbolic part and add
>> on top of it a discretization of the diffusive terms. I was thinking maybe
>> one way to go would be to hack the DMTSSetForcingFunction but
>> 1/ I still am not sure what this function should return exactly, is it a
>> Vec for the flux on all faces ?
>> 2/ I still do not know how to compute all the derivatives involved in the
>> diffusive terms of the N-S using the gradient reconstruction from PetscFV
>>
>> Thank you for your help, I hope I am clear enough in where I want to go !
>>
>
> Hi Thibault,
>
> Did anything happen on this front? I have another project where people
> want to do that same thing.
>
>   Thanks,
>
>     Matt
>
>
>> Thibault
>>
>> Le lun. 20 juil. 2020 à 16:10, Matthew Knepley <knepley at gmail.com> a
>> écrit :
>>
>>> On Mon, Jul 20, 2020 at 9:36 AM Jed Brown <jed at jedbrown.org> wrote:
>>>
>>>> How would you like to discretize the diffusive terms?  The example has
>>>> a type of gradient reconstruction so you can have cellwise gradients, but
>>>> there are many techniques for discretizing diffusive terms in FV.  It's
>>>> simpler if you use an orthogonal grid, but I doubt that you are.
>>>>
>>>> As for terminology, the diffusive part is usually stiff and thus must
>>>> be treated implicitly.  In TS terminology, this would be part of the
>>>> IFunction, not the RHSFunction.
>>>>
>>>
>>> At a high level, I would say that this is doable, but complicated. You
>>> can see me trying to do something much easier (advection +
>>> visco-elasticity) in TS ex18,
>>> where I want to discretize the elliptic part with FEM and the advective
>>> part with FVM. I assume that is why Jed wants to know how you want to
>>> handle the
>>> elliptic terms, since this has a large impact on how you would implement.
>>>
>>>   Thanks,
>>>
>>>      Matt
>>>
>>>
>>>> Thibault Bridel-Bertomeu <thibault.bridelbertomeu at gmail.com> writes:
>>>>
>>>> > Dear all,
>>>> >
>>>> > I have been studying ex11.c from ts/tutorials to understand how to
>>>> solve an
>>>> > hyperbolic system of equations using PETSCFV. I first worked on the
>>>> Euler
>>>> > equations for inviscid fluids and based on what ex11.c presents, I
>>>> was able
>>>> > to add the right PETSc instructions in an already existing in-house
>>>> code
>>>> > with different gas models  to solve the problems in parallel (MPI)
>>>> and with
>>>> > the AMR capabilities offered by P4EST.
>>>> >
>>>> > Now my goal is to move to Navier-Stokes equations. Theoretically the
>>>> system
>>>> > is not completely hyperbolic and can be seen as one with an
>>>> hyperbolic part
>>>> > (identical to the Euler equations) and a parabolic part coming from
>>>> the RHS
>>>> > diffusion terms.
>>>> > I have been looking into the manual and also the sources of PETSc
>>>> around
>>>> > the DM, DMPlex, DS and FV classes but I could not find anything that
>>>> speaks
>>>> > to me as "adding a RHS to an hyperbolic system of equations" or
>>>> "adding a
>>>> > source term to an hyperbolic system of equations". What's more, that
>>>> source
>>>> > term depends on the derivatives of the context variables ...
>>>> >
>>>> > I wanted to know if anyone maybe had a suggestion regarding this
>>>> issue ?
>>>> >
>>>> > Thank you very much in advance,
>>>> >
>>>> > Thibault Bridel-Bertomeu
>>>> > —
>>>> > Eng, MSc, PhD
>>>> > Research Engineer
>>>> > CEA/CESTA
>>>> > 33114 LE BARP
>>>> > Tel.: (+33)557046924
>>>> > Mob.: (+33)611025322
>>>> > Mail: thibault.bridelbertomeu at gmail.com
>>>>
>>>
>>>
>>> --
>>> What most experimenters take for granted before they begin their
>>> experiments is infinitely more interesting than any results to which their
>>> experiments lead.
>>> -- Norbert Wiener
>>>
>>> https://www.cse.buffalo.edu/~knepley/
>>> <http://www.cse.buffalo.edu/~knepley/>
>>>
>>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
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