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@einsteintoolkit.org [mailto:users-bounces@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
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