Pressure and Entropy Units Question

From: Ali Snedden <>
Date: Thu, 11 Sep 2014 13:51:48 -0400


I am trying to work through the units used in SphP[*].Entropy and
SphP[*].Pressure. I worked through the units of Entropy (see the
initialization of Entropy in read_ic.c and init.c) and I get units of :

(Internal Energy * (Internal Length)^2 / (Internal Mass^(5/3)) ) in
Euclidian (i.e. NOT comoving) coordinates.

However when computing the Pressure, in density.c

SphP[i].Pressure = (SphP[i].Entropy + SphP[i].DtEntropy * dt_entr) *
pow(SphP[i].Density, GAMMA).

This is confusing to me, because it appears that Density is in comoving
coordinates and Entropy is in Euclidian coordinates. I would expect that
they should be in the same coordinate system. I would think that there is
a couple of factors of the scale factor (i.e. All.Time) missing. This
would convert the Entropy into comoving coordinates so that the Pressure is
in comoving coordinates.

If Density really is in comoving coordinates and Entropy is in Euclidian
coordinates, then is the Pressure in comoving or Euclidian coordinates?

Received on 2014-09-11 19:51:58

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