[Users] GRHydro negative rho

Erik Schnetter schnetter at cct.lsu.edu
Fri Jul 23 11:43:57 CDT 2021


Instead of modifying an existing (and complicated, because it has many
features) thorn, I would recommend first writing a simple thorn by
yourself. I'd be happy to meet via Zoom and chat.


On Fri, Jul 23, 2021 at 8:24 AM Roland Haas <rhaas at illinois.edu> wrote:
> Hello Nicholas,
> my guess would be that the assumption that rho>0 is showing in lots of
> places in GRhydro thus not likely to be easily removed.
> Just to be sure: "rho" in GRHyro is the rest-mass density, basically
> the number density multiplied by the rest mass of the particles. For
> photons this seems to be an hard quantitiy to define.
> The energy density ("tt-component of the stress energy tensor") on the
> other hand is not actually evolved by GRhydro and is instead either
> provided part of the stress energy tensor in the TmunuBase::eTtt
> variable.
> GRHydro (and the Valencia formulation of hydrotdynamics that it uses)
> does involve (almost) this component of Tmunu in its "energy density"
> variable "E" but usually that one is not directly evolved and instead
> "tau" the difference between total energy "E" and relativistic
> rest-mass energy "D" is evolved.
> Please see:
> https://link.springer.com/article/10.12942/lrr-2008-7#Sec2.1.3
> and
> https://www.einsteintoolkit.org/thornguide/EinsteinEvolve/GRHydro/documentation.html#x1-80003
> for the definitions.
> Since you are at Waterloo you should also be able to reach out to
> people at Perimeter institute (Luis Lehner, Erik Schnetter come to
> mind) who are experts on fluid dynamics and NR.
> Yours,
> Roland
> > Hello there
> >
> > I have been learning how to use and working with the Einstein Toolkit for a
> > few months now and I have gotten to a point where I wanted to implement a
> > photon gas with (locally) negative rho as part of my research, following a
> > paper written by one of my supervisor's PhD students. While looking into it
> > I found that GRHydro automatically discards negative values of rho and
> > replaces them with rho_abs_min, and I can't set rho_ans_min to a negative
> > value because they are ignored according to the documentation.
> > I was wondering if there is a way to set rho_abs_min to be negative without
> > it being ignored, or a way to circumvent the check for negative values of
> > rho somehow to allow for negative values. I have some experience with
> > rewriting thorns for my own purposes but GRHydro isn't as simple as the
> > thorns I've been changing.
> >
> > Thanks,
> > Nicholas Olsen, University of Waterloo
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Erik Schnetter <schnetter at cct.lsu.edu>

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