in the codes of Computational methods for fluid dynamics, density sets as 1, and viscosity is equal to 0.001. So what kind of material does it delegate? water (density=1000, viscosity=0.001), or Air (density=1.29, viscosity=0.000001). the corresponding code can be downloaded from springe's ftp

ftp://ftp.springer.de/pub/technik/peric/

The beauty of CFD, comparing to the experiment, is that you can specify any property values such as density and viscosity to the fluid. Let's say for a flow in pipe, Reynolds number determines the flow being laminar or turbulent. Re=(density*velocity*diameter/Viscosity). By adjusting density and/or viscosity, you can achieve desirable Re without changing velocity.

It solves the non-dimensional Navier-Stokes equations of Reynolds number=1000 for any fluids. If you read the Patankar's book, there is a section explaining this kind of treatment.