[FLASH-USERS] confused about st_energy parameter in Stir unit

[Christoph Federrath]

Hi Jon,

st_energy is meant to be the specific (per unit mass) energy injection rate, i.e., also per time (usually referred to as epsilon in the literature; see e.g., Frisch 1995). However, in practise, one normally wants to set a target turbulent Mach number, as this is usually what determines the physical behaviour of the system and can be compared to previous studies. Setting the Mach number requires some experimenting with st_energy and noting that it is proportional to Mach^3 (see notes in source/Simulation/SimulationMain/StirFromFile/forcing_generator/forcing_generator.F90). Since the resulting Mach number depends on many factors, most importantly st_energy, but also the mixture of solenoidal and compressive modes as well as the dissipation properties of the numerical hydro/MHD solver, some experimenting and iterating (setting st_energy close to what you think is required for your proposes and then measuring the Mach number; source/Simulation/SimulationMain/StirFromFile/IO_writeIntegralQuantities.F90 does that in column 9 of the resulting .dat file and adjusting st_energy accordingly in a follow-up run) is required to get to the target Mach number. Hope this helps; please let me know if you have any questions on the StirMain/FromFile unit.

Best regards,
Christoph

> On 4 Jan 2019, at 06:51, Slavin, Jonathan <jslavin at cfa.harvard.edu <mailto:jslavin at cfa.harvard.edu>> wrote:
> 
> Hi,
> 
> I'm looking to use the Stir unit in a simulation using the FromFile implementation. I've been struggling a bit with setting the input parameters.  I think I understand most of them, but the st_energy one is confusing me.  When I follow through the code it seems like the only dimensional parameters used in setting the acceleration are st_aka and st_akb which in turn are proportional to st_OUphases.  Those are proportional to the variance used to create them which is proportional to st_OUvar. Finally st_OUvar = sqrt(st_energy/st_decay).  So, if I've followed that correctly the acceleration is proportional to sqrt(st_energy/st_decay).  It seems that st_decay has units of time (s), and acceleration should of course have units of distance/time**2 (cm/s**2), which leads to st_energy having units cm^2/s^3, which seems odd to me. I can see dividing out the mass, which leaves cm^2/s^2, but the extra s dimension is strange. Should st_energy be interpreted as power per unit mass added by stirring?
> 
> Regards,
> Jon
> -- 
> 
> Jonathan D. Slavin
> Astrophysicist - High Energy Astrophysics Division
> Center for Astrophysics | Harvard & Smithsonian
> Office: (617) 496-7981 | Cell: (781) 363-0035
> 60 Garden Street | MS 83 | Cambridge, MA 02138
> 

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