[FLASH-USERS] energy inaccuracy in Sedov simulation

[Robert Fisher]

Hi Jonathan :

  I've recently been looking into the Sedov problem as a test case for some
remnant work I am doing. The Sedov problem is easy to set up as a point
blast and get "sort of" correct, but because of its self-similar nature, it
is singular at [image: t = 0], and consequently it is extremely challenging
to get precisely correct in all its details.

  By default, FLASH initializes a point-like energy deposition in the
central zones, in an approximation to a delta function. Firstly, if you
carefully read through the code, you will find that the explosion energy
parameter is not actually used in this case at all -- the code is simply
initializing density and pressure. Additionally, adding more resolution is
a somewhat better approximation to the delta function, but is still not
quite what you may need if you are looking to do a very accurate
initialization. A better procedure is to set the initial time (tinitial in
flash.par) to a small but non-zero value. This will kick in a second piece
of the initialization code in Simulation_initBlock.F90, which initializes
the exact solution at the specified time, and actually *does use the
explosion energy parameter sim_expEnergy*, though this code is currently
hard-coded to work only for [image: \gamma = 7/5] and for 3D. This
initialization procedure has the advantage of avoiding the initial
singularity of the solution. In this case I believe you will find better
convergence behavior with added resolution, but there are limitations in
the hardcoded solution.

  Lastly, depending on what sort of accuracy you may be demanding, there
are also some errors in the well-known texts on this problem which will
reveal themselves as you test against the exact solutions. There is much
more on these issues on Frank Timmes website :
http://cococubed.asu.edu/research_pages/sedov.shtml, where you can also
find a highly-accurate initialization method which can be used as an
initialization.

  Best wishes,

  Bob

On Tue, Jul 26, 2016 at 10:48 AM, Marissa Adams <madams at pas.rochester.edu>
wrote:

> Hi Jonathan,
>
> I have also noticed this problem.
>
> The energy one sees in the .dat file is typically less than the input
> energy for sim_expEnergy. Also noticed that this is geometry dependent.
>
> I input on the order of 1e10 ergs, and in cartesian I get out 9.2e9; for
> cylindrical I get even less! 3.2e8... which is huge. Note I am going based
> off of memory here. I cannot get into my HPC system now.
>
> If I am remembering things correctly: seems that in the source code's
> volume function in Simulation_init.f90 doesn't always choose the volume
> you're choosing to distribute the energy over.
>
> I think it is the section where you'll find a comment that notes the code
> will calculate the initial volume and interior pressure. It is assuming pi
> r^2 for to get the energy density... where as you may want something
> different for other chosen geometries.
>
> If I remember correctly the output energy also changes depending where you
> put the hotspot.
>
> Sorry for the haphazard e-mail, but I am experiencing similar issues!
>
> Best,
> Marissa
>
>
>
>
> On Tue, Jul 26, 2016 at 10:30 AM, Slavin, Jonathan <
> jslavin at cfa.harvard.edu> wrote:
>
>> Hi all,
>>
>> I've been running supernova remnant evolution simulations.  The
>> initialization is done with code taken from the supplied Sedov simulation
>> with certain small changes.  One issue that I've run into is that the total
>> energy is not very accurate.  That is, though the energy is conserved, the
>> total energy, initially input as thermal energy, is below that which is
>> given in the sim_expEnergy parameter by ~5%.  Has anyone else run into
>> this?  I don't think that it has to do with any of the changes that I've
>> made to Simulation_initBlock or Simulation_init.  I'm wondering if
>> increasing the sim_nsubzones parameter might help.  Any advice would be
>> appreciated.
>>
>> Thanks,
>> Jon
>>
>> --
>> ________________________________________________________
>> Jonathan D. Slavin                 Harvard-Smithsonian CfA
>> jslavin at cfa.harvard.edu       60 Garden Street, MS 83
>> phone: (617) 496-7981       Cambridge, MA 02138-1516
>> cell: (781) 363-0035             USA
>> ________________________________________________________
>>
>>
>
>
> --
> Marissa Adams                         E-mail: madams at pas.rochester.edu
> Graduate Student                      Ph: (585) 402-5779
> Department of Physics & Astronomy     Website:
> http://www.pas.rochester.edu/~madams/
> University of Rochester
> 478 Bausch & Lomb Hall
> P.O. Box 270171
>
> --
> Dr. Robert Fisher
> Associate Professor / Graduate Program Director
> University of Massachusetts/Dartmouth
> Department of Physics
> 285 Old Westport Road
> North Dartmouth, Massachusetts 02747
> robert.fisher at umassd.edu
> http://www.novastella.org
>
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