Document units

git-svn-id: http://svn.einsteintoolkit.org/cactus/EinsteinBase/HydroBase/trunk@19 57fe0bb3-ccba-405f-9b23-de0201f165b7

2 files changed, 53 insertions, 1 deletions

diff --git a/doc/documentation.tex b/doc/documentation.tex
index cdf33f7..601a123 100644
--- a/doc/documentation.tex
+++ b/doc/documentation.tex
@@ -170,7 +170,42 @@ In this way the initiation of the primitive variables, methods to recover the co
 variables, and basic atmosphere handling can be implemented in different thorns while allowing
 a central access point for analysis thorns.
 
-\subsection{Acknowledgments}
+\section{Units}
+
+HydroBase uses a specific set of units, so that there are no
+misunderstandings between thorns.  These units are derived from the
+conventions
+\begin{eqnarray}
+  M = M_\mathrm{sun} &;& c = G = 1
+\end{eqnarray}
+which are commonly used in astrophysics and in relativity.
+
+We assume the following definitions and constants of nature:
+\begin{eqnarray}
+  c             & = & 299792458\, \mathrm{m/s}
+  \\
+  G             & = & 6.67428\cdot 10^{-11}\, \mathrm{m^3/kg/s^2}
+  \\
+  \mu_0         & = & 4 \pi 10^{-7}\, \mathrm{N/A^2}
+  \\
+  \epsilon_0    & = & \frac{1}{\mu_0 c^2}
+  \\
+  M_\mathrm{sun} & = & 1.98892\cdot10^{30}\, \mathrm{kg}
+\end{eqnarray}
+
+This corresponds to the following units for mass, length, time, and
+magnetic field:
+\begin{eqnarray}
+  [M] & = & M_\mathrm{sun}
+  \\{}
+  [L] & = & [M]\; G/c^2
+  \\{}
+  [T] & = & [L]\; / c
+  \\{}
+  [B] & = & 1/[L]\; / \sqrt{\epsilon_0 G / c^2}
+\end{eqnarray}
+
+\section{Acknowledgments}
 
 This thorn was produced by Tanja Bode, Roland Haas, Frank Löffler, and Erik Schnetter.
 
diff --git a/interface.ccl b/interface.ccl
index 2806eac..7b892a0 100755
--- a/interface.ccl
+++ b/interface.ccl
@@ -8,6 +8,23 @@ public:
 
 # These variables correspond to the Valencia formulation
 
+# HydroBase uses the following conventions:
+#    M = M_sun
+#    c = G = 1
+
+# We also assume the following constants of nature:
+#    c    = 299792458.0 m/s
+#    G    = 6.67428e-11 m^3/kg/s^2
+#    mu0  = 4 pi 1.0e-7 N/A^2
+#    eps0 = 1 / (mu0 c^2)
+#    M_sun = 1.98892e+30 kg
+
+# This corresponds to the following units:
+#    [M] = M_sun
+#    [L] = [M] (G / c^2)
+#    [T] = [L] / c
+#    [B] = 1 / [L] / sqrt(eps0 G / c^2))
+
 CCTK_REAL rho     type = GF Timelevels = 3 tags='ProlongationParameter="HydroBase::prolongation_type" tensortypealias="Scalar" interpolator="matter"' "rest mass density"
 CCTK_REAL press   type = GF Timelevels = 3 tags='ProlongationParameter="HydroBase::prolongation_type" tensortypealias="Scalar" interpolator="matter"' "gas pressure"
 CCTK_REAL eps     type = GF Timelevels = 3 tags='ProlongationParameter="HydroBase::prolongation_type" tensortypealias="Scalar" interpolator="matter"' "internal energy density"