[Users] HydroBase description

[Bruno Giacomazzo]

Hi,
	Aaryn is right. I have indeed used polytropic units in the past (c=G=eos_kappa=1), while for our BNS simulations we usually use c=G=M_sun=1. But I have also used units such that c=G=M=1 (being M the total mass of the system). When using polytropic/ideal-fluid EOSs everything can be easily rescaled by simply changing the value of the polytropic constant (and the dynamics would just be the same). For realistic EOS the situation is different since I imagine the units are just fixed by the EOS and no rescaling is possible.

	The conversion factor for the magnetic field seems to be correct, but it's important to stress that before the conversion one should also multiply Bvec by sqrt(4 \pi) (if hydrobase wants to use the same convention used in whisky of course).

	I think it would be useful to add the references to the Valencia formulation for GRHD and GRMHD in the wiki where you define the quantities:

	GRHD: http://adsabs.harvard.edu/abs/1997ApJ...476..221B
	GRMHD: http://adsabs.harvard.edu/abs/2006ApJ...637..296A

	Any evolution code would then be free to use other definitions for B, but since hydrobase would couple evolution to analysis thorns is better to be sure that one passes the correct value of B to Bvec.

Cheers,
Bruno

On Apr 27, 2010, at 7:06 AM, Aaryn Tonita wrote:

> Actually, the units of "Whisky" is a myth I have been trying to dispel from
> some of the people at the AEI for a while now. In the hydro equations, the
> only place that a mass scale enters is through the equation of state. So
> HydroBase will work with any choice of mass unit (defined through the
> equation of state) so long as G=c=1 is used consistently. I have run quite
> successfully using polytropic units and kilometres.
> 
> As for the ideal magnetohydrodynamic equations, Whisky_Exp chooses
> div(Faraday) = 4-current without any constant of nature, so this is
> equivalent to choosing the permeability of free space (\mu_0) to be unity.
> However, this would not be defined by HydroBase unless HydroBase sets the
> stress energy tensor.
> 
> Cheers,
> 
> Aaryn
> 
> -----Original Message-----
> From: users-bounces at einsteintoolkit.org
> [mailto:users-bounces at einsteintoolkit.org] On Behalf Of Roland Haas
> Sent: Monday, April 26, 2010 10:04 PM
> To: Einstein Toolkit Users
> Subject: [Users] HydroBase description
> 
> Hello all,
> 
> I have put a short description of HydroBase, its grid-variables and the
> units used into the wiki. It is really just what can be found in the
> documentation (currently out of date) and interface.ccl.
> 
> Since I have never done MHD and don't know the conventions used in that
> field would someone who is knowledgeable mind reading through it and
> checking in particular the units I give for B and its definition in terms of
> the (dual) Faraday tensor, please?
> 
> One could also add descriptions for the parameters, but these would really
> be just copies of what is in param.ccl.
> 
> Also it would be nice to note if these choices for units of the various
> quantities exhaust the freedom to choose units (I would think that with
> temperature and specific entropy they should, even determine the mass of the
> (ideal gas) particle that makes up the fluid).
> 
> Yours,
> Roland
> 
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Dr. Bruno Giacomazzo
Department of Astronomy
University of Maryland
College Park
MD 20742-2421
USA

Tel.  : +1 301 405 0676
Fax  : +1 301 314 9067
Tel.  : +1 301 286 3809 (NASA/GSFC)
Fax  : +1 301 286 7230 (NASA/GSFC)
Cell. : +1 443 535 5846
email : bgiacoma at astro.umd.edu

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