This is a paper describing the CactusEinstein hydro interface.
Erik Schnetter. Multi-physics coupling of Einstein and hydrodynamics evolution: A case study of the Einstein Toolkit. CBHPC 2008 (Component-Based High Performance Computing), 2008. (PDF <>, 9 pages, 546.3 kbyte)
http://www.cct.lsu.edu/~eschnett/doc/Karlsruhe-2008-CBHPC-EinsteinToolkit.pd...
-erik