Dear David,
well I do not know what exactly you mean by modelling /simulation, but a good
deal of what numerics people do (like integrating ODEs using Runge-Kutta
schemes,
matrix calculations of all sorts) are implemented in the GNU Scientific
library (GSL):
_http://www.gnu.org/software/gsl/_ (http://www.gnu.org/software/gsl/)
Once you have that installed, there are Ruby bindings available for it:
_http://rb-gsl.rubyforge.org/_ (http://rb-gsl.rubyforge.org/)
There is extensive documentation available on those two webpages.
Best regards,
Axel