[Users] HydroBase description

[Erik Schnetter]

On Apr 27, 2010, at 10:19 , Bruno Giacomazzo wrote:

> 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).

Yes, we want to use the same convention as Whisky.  And yes, we do  
want to define the units that are used, so that there is no confusion  
about initial data or analysis methods which may provide or access  
HydroBase data, and otherwise may be tempted to use different  
conventions.

> 	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

Please do that!  Please add these also to HydroBase's README file.

> 	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
>>
>> --
>> My email is as private as my paper mail. I therefore support  
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>> signing email messages. Get my PGP key from http://keys.gnupg.net.
>>
>>
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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)
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> email : bgiacoma at astro.umd.edu
>
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-- 
Erik Schnetter <schnetter at cct.lsu.edu>   http://www.cct.lsu.edu/~eschnett/