Dear Gadget list,
I'm trying to understand some points in the comoving
implementation of the hydro part of Gadget2.
In "timestep.c" when ComovingIntegrationOn is true,
dt_entr = (tend - tstart) * All.Timebase_interval;
which in fact corresponds to dt_entr = da/a (where a is the scale factor).
Then, the evolution of the entropy is (as for non-comoving simulations)
SphP[i].Entropy += SphP[i].DtEntropy * dt_entr;
SphP[i].DtEntropy is computed in "hydra.c", where we find in
/* do final operations on results */ :
SphP[i].DtEntropy *= GAMMA_MINUS1 / (hubble_a2 * pow(SphP[i].Density,
GAMMA_MINUS1));
which means, that :
SphP[i].DtEntropy)comoving = DEntropy/da
= DEntropy/dt * 1/(H(a) a^2)
= SphP[i].DtEntropy)non-comoving * 1/(H(a) a^2)
Here, I expected to have :
DtEntropy/da = DtEntropy/dt * 1/(H(a))
in order to have then :
SphP[i].DtEntropy * dt_entr = DEntropy/dt * 1/(H(a)) * da/a
= DEntropy
because dt*H(a)*a = da
Probably I missed a point here.
It is also difficult to understand the fac_mu correction in "hydra.c":
1) 0.0001 * soundspeed_i / SphP[i].Hsml / fac_mu
with fac_mu = a^(3/2(GAMMA - 1))/a
Here I expected to find
fac_mu = a^(1/2(GAMMA - 1))/a
in order to write soundspeed_i / SphP[i].Hsml in non-comoving units.
2) mu_ij = fac_mu * vdotr2 / r
is also unclear to me
Another question conserns the
dt_hydrokick factor which corresponds to the integral
Int_a1^a2 da/( H(a)*a*a^3(gamma-1) )
If, following Quinn et al. 97, the origin of dt_gravkick is clear,
I fail to understand the origin of dt_hydrokick.
Thanks is advance.
--
(o o)
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Yves Revaz
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Observatoire de Paris Fax : ++ 33 (0) 1 40 51 20 02
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FRANCE
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Received on 2007-02-01 18:17:29