[FLASH-USERS] possible errors in viscous diffusive flux in hydro unsplit

[Fabien Thiesset]

Me again,

As a follow-up, I verified the viscous fluxes for the energy equation 
and for the split solver. I think they are fine although it could be 
better to have confirmation by someone else if ever someone is motivated.

Therefore, only the 2 errors described below need to be double-checked.

Regards

On 19/02/2024 22:16, Fabien Thiesset wrote:
> Dear all,
>
> I double checked the viscous diffusive fluxes for the momentum 
> equation in the hydro unsplit solver. I use version 4.7.1 and the file 
> under consideration is 
> source/physics/Hydro/HydroMain/unsplit/hy_uhd_addViscousFluxes.F90.
>
> In theory, the diffusive flux of velocity component i in direction j 
> should write
>
> 2 mu S_{ij} - \mu/3 \theta \delta_{ij},
>
> where S_{ij} is the strain rate tensor, \delta_{ij} the kronecker 
> delta, \theta = div(u) and \mu the dynamic viscosity.
>
> If I am not mistaken, I found 2 errors in the computations of the 
> viscous fluxes of u along x direction (it is called F02XMOM_FLUX in 
> the code)
>
> - the first appears at line 106-108, to compute du/dy that constitutes 
> the diffusive flux of u in x direction. I think a 1/2 factor is missing.
>
> - the second appears at line 122-124, to compute dw/dz that constitute 
> the diffusive flux of u in x direction. There is some mix between 
> VELY_VAR and VELZ_var to compute the gradient. It should be VELZ_VAR 
> everywhere.
>
> Can anyone have a look and confirm ? If it is indeed the case, is 
> there a git repo for flash to raise an issue and provide the fix ?
>
> I will tomorrow have a look also the diffuse flux for the energy 
> equation and the viscous heating. I need also to check the Diffusive 
> fluxes for the momentum equation for the split solver. I did it 
> quickly this morning and it looked ok. I will anyway write a follow-up.
>
> Thanks everyone !
>
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