[Users] GRHydro - magnetic field divergence

Fri Feb 10 17:34:09 CST 2017

Hi,

thank you Roland & Francesco, that really helped!

Everything seems to run smoothly, except that after the merger/collapse
the magnetic field diverges, especially near the refinement boundaries.
(using constrained transport)

Is this problem fixed in the current version of GRHydro?
Or is this more related to Carpet and the possible
interpolation/restriction of B?

In some parameter file (2013mhd @ ETK) I saw
"CarpetLib::use_averaged_restriction_for_bvec", but it doesn't seem to
be part of the recent release anymore?
I also saw a comment in the Illinois code, which said something about
refinement boundaries and the magnetic field.
(it seems to be fixed there?)

What is the currently recommended way of doing a magnetic field binary?
I guess constrained transport + some special AMR setting?
Or divergence cleaning, with carefully tuned parameters?
It's obvious that I can't run the whole thing on unigrid.

My issues are similar to
http://lists.einsteintoolkit.org/pipermail/users/2016-March/004746.html
and
http://users.einsteintoolkit.narkive.com/ws0R9Zfm/question-about-mhd-tests-for-grhydro
.
The GRHydro paper also mentions problems with AMR, but it is rather old.

Thanks in advance!

Best regards,
Daniel

On 01/14/2017 01:18 AM, Roland Haas wrote:
> Hello all,
>
> there's a version of adding a magnetic field that was used for the
> original GRHydroMHD paper (implemented most likely by Bruno Mundim,
> Josh Faber or Philipp Moesta) that is in GRHydro_InitData (the
> parameters for "For Poloidal Magnetic field test" ie poloidal_XXX) and
> you can find a description of them in section 5.8 (page 38) of
> https://arxiv.org/pdf/1304.5544.
>
> Note though that I have not used them myself so your mileage may vary.
>
> Yours,
> Roland
>
>> Hi Daniel,
>>
>> first of all, please be aware that the option for setting an initial
>> magnetic field for stars in a binary system is not yet part of the
>> GRHydro_InitData thorn in the official toolkit release.
>> It was implemented in the companion material for the paper you cited, but
>> it hasn't been tested in production runs yet.
>>
>> The initial magnetic field is imposed setting its potential. It is set as
>> a poloidal field inside each star, with an expression similar to the one
>> used also in the official thorn for a single star.
>>
>> From the parameter files, the initial position of the star centers is taken:
>>
>>>> GRHydro_InitData::Xc_1 = -13.5
>>>> GRHydro_InitData::Xc_2 = 13.5
>>>> GRHydro_InitData::Yc_1 = 0.0
>>>> GRHydro_InitData::Yc_2 = 0.0
>>>> GRHydro_InitData::Zc_1 = 0.0
>>>> GRHydro_InitData::Zc_2 = 0.0  
>>
>> To set the magnetic potential in a grid point, the following rule is
>> employed:
>>
>> If the distance of the point from a star center is less than the star
>> distance from the origin, the vector potential is set as:
>>
>> Ax = -(y -
>> Yc_{1/2})*poloidal_A_b_{1/2}*rhofac**poloidal_n_p*maxP_Pcut_1**poloidal_P_p
>>
>> Ay = (x - Xc_{1/2})*
>> poloidal_A_b_{1/2}*rhofac**poloidal_n_p*maxP_Pcut_{1/2}**poloidal_P_p
>>
>> Az = 0
>>
>> where rhofac = 1-rho/poloidal_rho_max and maxP_Pcut = max(P -
>> Poloidal_P_cut_{1/2}, 0)
>>
>> Else it is set to zero.
>>
>> Therefore, GRHydro_InitData::Poloidal_P_cut_{1/2} sets a threshold, based
>> on the pressure, to confine the B field inside the star. A common value is
>> to set it to 4% of the maximum pressure, following, for example, Franci et
>> al., Phys.Rev.D 88, 104028 (2013).
>>
>> Instead, GRHydro_InitData::poloidal_n_p is the exponent of the rho term in
>> the vector potential, which can be used to "move" the maximum of the
>> magnetic field to star regions with higher or lower density and
>> GRHydro_InitData::poloidal_P_p the exponent of the pressure term. Setting
>> poloidal_P_p = 2 tries to accomplish a vector potential which has
>> continuous first derivative at the star surface, but it is by no means a
>> mandatory choice.
>>
>> This initial field definition is taken from Liu et al., Phys. Rev. D 78,
>> 024012 (2008).
>>
>> Given this definition for the vector potential, the maximum value of the
>> resulting magnetic field is given also by the pressure and density terms
>> (and their spacial derivatives, when computing the curl of A), and not
>> only by setting poloidal_A_b.
>>
>> Hope this can be helpful,
>>
>> Francesco
>>
>>
>>
>>
>>
>>> Hi!
>>>
>>> I have a question regarding the initial data thorn of GRHydro.
>>> The documentation and ccl files didn't really help me.
>>>
>>> The following was taken from paper [1] and now I am trying to add
>>> magnetic fields to the stars.
>>>
>>> My questions are:
>>>
>>> 1) What are the units of (A) and (B) below? Is the following conversion
>>> factor correct?
>>> The star should have a field strength of 10^16 Gauss.
>>> Using the following conversion would give "0.00011973228161339154" for
>>> (A) and (B).
>>>
>>> 2) What are reasonable values for (C..F)? Can I just use the default
>>> ones given in param.ccl?
>>> Is there a reason not to use the default ones?
>>> My plan is to analyse the structure of the field after the collapse and
>>> the influence on the wave signal.
>>>  
>>>> # constants, in SI
>>>> G     = 6.673e-11       # m^3/(kg s^2)
>>>> c     = 299792458       # m/s
>>>> M_sun = 1.98892e30      # kg
>>>> mu0   = 1.2566370614e-6 # Newton/Ampere^2
>>>> Kb    = 1.3806488e-23   # Joule/K
>>>> Mparsec = 3.08567758e19 # km
>>>> CU_to_Tesla = c**4 / M_sun / G**(1.5)* mu0**(0.5)
>>>> CU_to_Gauss = c**4 / M_sun / G**(1.5)* mu0**(0.5) * 10000
>>>> #-------------------------------------------------
>>>> # Magentic Fields:
>>>> #-------------------------------------------------
>>>> (A) GRHydro_InitData::poloidal_A_b_1      = 0.00011973228161339154
>>>> (B) GRHydro_InitData::poloidal_A_b_2      = 0.00011973228161339154
>>>> (C) GRHydro_InitData::poloidal_P_cut_1    = 3.72e-6 #Non è in
>>>> percentuale!!
>>>> (D) GRHydro_InitData::poloidal_P_cut_2    = 3.72e-6 #Non è in
>>>> percentuale!!
>>>> (E) GRHydro_InitData::poloidal_n_p = 1 #Esponente di
>>>> max((rho-rho_cut),0)
>>>> (F) GRHydro_InitData::poloidal_P_p = 2 #Esponente di max((P-P_cut),0)
>>>> GRHydro_InitData::Xc_1 = -13.5
>>>> GRHydro_InitData::Xc_2 = 13.5
>>>> GRHydro_InitData::Yc_1 = 0.0
>>>> GRHydro_InitData::Yc_2 = 0.0
>>>> GRHydro_InitData::Zc_1 = 0.0
>>>> GRHydro_InitData::Zc_2 = 0.0  
>>> 3) These should be fine, right?
>>>  
>>>> GRHydro::transport_constraints     = "yes"
>>>> GRHydro::track_divB                = "yes"
>>>> GRHydro::calculate_bcom            = "yes"
>>>> #GRHydro::clean_divergence            = "no"
>>>> GRHydro::Grhydro_MaxNumConstrainedVars = 33
>>>> GRHydro::GRHydro_MaxNumEvolvedVars     = 10
>>>>
>>>> HydroBase::initial_Bvec       = "bin_ns_poloidalmagfield"  
>>> The field is evolved and everything seems okay, I am just not sure about
>>> the units and (C..F).
>>>
>>> 4) Is there a good method to visualize the B-field?
>>> Visit + Carpet hdf5 reader + Streamlines? (seems to be "buggy")
>>>
>>> Thank you!
>>>
>>> Best regards,
>>> Daniel
>>>
>>> References:
>>> [1] "R. De Pietri, A. Feo, F. Maione, and F. Löffler, “Modeling Equal
>>> and Unequal Mass Binary Neutron Star Mergers Using Public Codes,” Phys.
>>> Rev., vol. D93, no. 6, p. 064047, 2016."
>>>
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>>>  
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