Converting units

Example 1:

>> from pNbody import *
>>
>> # define a new system of units : kpc,Msol,Myr
>> system_of_units = units.UnitSystem('local',[units.Unit_kpc,  10**10*units.Unit_Ms, units.Unit_Myr])
>>
>> # trivial conversion factor into kpc,10**10Msol,Myr
>> print system_of_units.convertionFactorTo(units.Unit_kpc)
>> print system_of_units.convertionFactorTo(10**10*units.Unit_Ms)
>> print system_of_units.convertionFactorTo(units.Unit_Myr)
>>
>> # length
>> #
>> print system_of_units.convertionFactorTo(units.Unit_cm)
>> print system_of_units.convertionFactorTo(units.Unit_m)
>> print system_of_units.convertionFactorTo(units.Unit_km)
>>
>> # velocity
>> print system_of_units.convertionFactorTo(units.Unit_km/units.Unit_s)
>> print system_of_units.convertionFactorTo(units.Unit_kms)
>>
>>
>> # energy
>> print system_of_units.convertionFactorTo(units.Unit_kg*units.Unit_ms**2)
>> print system_of_units.convertionFactorTo(units.Unit_J)
>>
>> # density
>> print system_of_units.convertionFactorTo(units.Unit_g/units.Unit_cm**3)

Example 2:

>> from pNbody import *
>>
>> nb = Nbody("dsph.dat",ftype='gadget',unitsfile='params.dsph')
>>
>> nb.localsystem_of_units.info()
>>
>>
>> # trivial conversion factor into kpc,10**10Msol,Myr
>> print "to km/s    : *",nb.localsystem_of_units.convertionFactorTo(units.Unit_kms)
>> print "to g/cm**3 : *",nb.localsystem_of_units.convertionFactorTo(units.Unit_g/units.Unit_cm**3)
>> print "to Msol    : *",nb.localsystem_of_units.convertionFactorTo(units.Unit_Msol)
>> print "to Msol/yr : *",nb.localsystem_of_units.convertionFactorTo(units.Unit_Msol/units.Unit_yr)

Example 3:

>> from pNbody import *
>>
>> ##############################
>> # define some system of units
>> ##############################
>>
>> # output system of units (cgs)
>> cgs_units = units.UnitSystem('cgs',[units.Unit_cm,units.Unit_g,units.Unit_s,units.Unit_K])
>>
>> # gear system of units
>> params = {}
>> params['UnitVelocity_in_cm_per_s']      = 20725573.785998672
>> params['UnitMass_in_g']                 = 1.989e+43
>> params['UnitLength_in_cm']              = 3.085678e+21
>> gear_units = units.Set_SystemUnits_From_Params(params)
>>
>>
>> ##############################
>> # compute constants
>> ##############################
>>
>> # compute mu
>> Xi=0.76
>> mu = 4./(8.-5.*(1.-Xi))
>>
>>
>> # get k (boltzmann) in cgs_units
>> k = ctes.BOLTZMANN.into(cgs_units)
>>
>> # get mh (hydrogen mass) in cgs_units
>> mh = ctes.PROTONMASS.into(cgs_units)
>>
>>
>> ##############################
>> # specific thermal energy
>> ##############################
>> T=1e4
>> ui = 3/2.*k/(mu*mh) *T
>>
>>
>> ##############################
>> # mass
>> ##############################
>>
>> mi = 5e-08 # gear_units (500Msol)
>> mi = mi*gear_units.convertionFactorTo(cgs_units.UnitMass)
>>
>>
>> ##############################
>> # energy
>> ##############################
>>
>> Nsn = 5
>> Nngb= 10.
>>
>> Esn  = Nsn * 1e51 * 0.03   # already in cgs
>> Esni = Esn/Nngb                    # already in cgs
>>
>>
>>
>> ##############################
>> # now compute, all in cgs
>> ##############################
>>
>> ei = mi*ui
>> eip= ei + Esni
>>
>> print "ratio",eip/ei
>>
>>
>>
>> ##############################
>> # now compute, all in gear
>> ##############################
>>
>> ui  = ui*cgs_units.convertionFactorTo(gear_units.UnitSpecEnergy)
>> mi  = mi*cgs_units.convertionFactorTo(gear_units.UnitMass)
>> Esni= Esni*cgs_units.convertionFactorTo(gear_units.UnitEnergy)
>>
>> print "------------"
>> print ui
>> print mi
>> print Esni

Example 3:

Convert gear density units to atom/cm 3:

>> from pNbody import *
>> ##############################
>> # define some system of units
>> ##############################
>>
>> # cgs units (exits in pNbody)
>> units.cgs
>>
>> # output system of units (the mass units is the hydrogen mass)
>> Unit_atom = ctes.PROTONMASS.into(units.cgs)*units.Unit_g
>> Unit_atom.set_symbol('atom')
>> out_units = units.UnitSystem('local',[units.Unit_cm,Unit_atom,units.Unit_s,units.Unit_K])
>>
>>
>> # gear system of units
>> params = {}
>> params['UnitVelocity_in_cm_per_s']      = 20725573.785998672
>> params['UnitMass_in_g']                 = 1.989e+43
>> params['UnitLength_in_cm']              = 3.085678e+21
>> gear_units = units.Set_SystemUnits_From_Params(params)
>>
>>
>> ##############################
>> # now, use to convert
>> ##############################
>>
>> print gear_units.convertionFactorTo(out_units.UnitDensity)         # gear to atom/cm^3
>> print gear_units.convertionFactorTo(units.cgs.UnitDensity)         # gear to g/cm^3
>> print 0.1*out_units.convertionFactorTo(units.cgs.UnitDensity)              # atom/cm^3 to g/cm^3
>> print 1.67e-25*units.cgs.convertionFactorTo(out_units.UnitDensity)

Example 4:

>> from pNbody import units,io,ctes
>> from numpy import *
>>
>> gadgetparameterfile="params.dSphCZ"
>> params = io.read_params(gadgetparameterfile)
>>
>> #system_of_units = units.Set_SystemUnits_From_Params(params)
>> system_of_units = units.Set_SystemUnits_From_File(gadgetparameterfile)
>>
>> G=ctes.GRAVITY.into(system_of_units)
>> H = ctes.HUBBLE.into(system_of_units)
>> HubbleParam = params['HubbleParam']
>>
>> rhoc = pow(HubbleParam*H,2)*3/(8*pi*G)
>> print rhoc,rhoc*80,"free of h"
>>
>> rhoc = pow(H,2)*3/(8*pi*G)
>> print rhoc,rhoc*80,"in code units, including h"
>>
>> H=0.1
>> G=43000.1
>>
>> print pow(H,2)*3/(8*pi*G)