[FLASH-USERS] Changing the system of units

[Aaron Froese]

Hi Sean,

I have encountered the same error with PhysicalConstants_get, but have not endeavoured to fix it.  Rather than rewriting each call to PhysicalConstants_get, I think it would be more efficient to add a new unit system to pc_checkCGSMKS in pc_utilities.F90 and then add new if statements to PhysicalConstants_get.F90

if (present(unitLength)) then
          call pc_findUnit (unitLength, pc_baseUnitLength, indexUnit)
          if (indexUnit == 0) return
          scaleLength = pc_arrayUnit(indexUnit)%cgsValue
else
          scaleLength = 1.e0
          if (pc_SISystem == 2) scaleLength = 1.E2  ! meters
          if (pc_SISystem == 3) scaleLength = 3.08568e18  ! parsecs
endif

Of course, both of our methods still require the solver units to be correctly adjusted as well.

Aaron
________________________________________
From: Sean Lynch [sean.m.lynch at drexel.edu]
Sent: Sunday, August 07, 2011 10:42 AM
To: Aaron Froese
Cc: flash-users
Subject: Re: [FLASH-USERS] Changing the system of units

Aaron,

I did realize that this would require additional coding, however this doesn't seem like such an issue.

As far as I know any routine that needs some constant will have to call PhysicalConstants_get. So in order to make sure everything is in the right units I need to two things: 1) Set up my simulation in the correct units, and 2) find all the places where the modules that the simulation is using call PhysicalConstants_get. Then all I need to do is override this with my own version of the routine that is essentially the same but calls PhysicalConstants_get with units specified.

For example, one unit that would need to be modified for my simulation is Eos_init.F90 because the Eos unit needs the ideal gas constant. The original subroutine Eos_init(myPE) contains the line:

  call PhysicalConstants_get("ideal gas constant", eos_gasConstant)

So I copy the file Eos_init.F90 to my Simulation directory and change this call to

  call PhysicalConstants_get("ideal gas constant", eos_gasConstant, &
       unitLenth="pc", unitTime="yr", unitMass="Msun")

However, at compile time I get the error

 In file Eos_init.F90:25
        unitLenth="pc", unitTime="yr", unitMass="Msun")                                                     1

 Error: There is no specific subroutine for the generic 'physicalconstants_get' at (1)

Shouldn't this work? Why am I getting this error?

Thanks,
Sean

On Aug 5, 2011, at 6:33 PM, Aaron Froese wrote:

> Hi Sean,
>
> The two runtime parameters that determine what unit system the solver works in are
>
> UnitSystem = "CGS" or "SI"
> pc_unitsbase = "CGS" or "MKS"
>
> In most of the initialization routines, these parameters are read and then used to calculate constant factors.  In Hydro_init.F90, for example:
>
> call RuntimeParameters_get("UnitSystem",          hy_units)
>
> if ( hy_units == "SI" .or. hy_units == "si" ) then
>    hy_bref = hy_vref*sqrt(4.0*PI*hy_dref*1.e-7)
> else if ( hy_units == "CGS" .or. hy_units == "cgs" ) then
>    hy_bref = hy_vref*sqrt(4.0*PI*hy_dref)
> else
>    hy_bref = hy_vref*sqrt(hy_dref)
> end if
>
> As you can see, only CGS and SI are provided in the distribution copy, so changing the solver to use parsecs, solar masses, and mega years would require additional coding.  I think it would be simpler to convert the simulation results from CGS or SI in post-processing.
>
> Cheers,
> Aaron
>
>
> ________________________________________
> From: flash-users-bounces at flash.uchicago.edu [flash-users-bounces at flash.uchicago.edu] On Behalf Of flash-users-request at flash.uchicago.edu [flash-users-request at flash.uchicago.edu]
> Sent: Monday, August 01, 2011 10:00 AM
> To: flash-users at flash.uchicago.edu
> Subject: flash-users Digest, Vol 46, Issue 1
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>   1.  Changing the system of units (Sean Lynch)
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> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 1 Aug 2011 12:53:34 -0400
> From: Sean Lynch <sean.m.lynch at drexel.edu>
> Subject: [FLASH-USERS] Changing the system of units
> To: flash-users <flash-users at flash.uchicago.edu>
> Message-ID: <11522DB1-D682-4A7C-A9D8-7AB6B7D92791 at drexel.edu>
> Content-Type: text/plain; charset=us-ascii
>
> Hello,
>
> I'm trying to set up a simulation that uses parsec, solar mass, and mega year as the base units. I know that I can use PhysicalConstants_get to convert a constant to the correct units. But how would I go about getting the solvers to use the correct units. For instance, how would I make sure that the eos unit is using the correct value of the gas constant, or that the gravity solver is using the correct value of G.
>
> Thanks for your help.
>
> -Sean
>
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