Oh, but i was talking about density-diffusion, not velocity-diffusion :)
And the reason i want to know along what lines the diffusion-rates should be is just that im curious, and that i would like to know if having it as 100 is very far from real.

The same applies for density diffusion, too: the semi-Lagrangian advection you do on it already diffuses the density field so greatly that manual diffusion really is unuseful.

>> as 100

Well, I doubt that realworld diffusion constants are scalars... they probably have some unit, too. In order to use these unit-based (metric system) diffusion constants in your simulator, you need to make sure that your fluid sim is based on real temporal/spatial paramterers (timestep in seconds, grid sizing in meters), too.

(That's not to discourage you - just an important note if you really want realworld physical constants)

Already using realworld units for everything.
I found a table of sorts that stated free-flowing water has a diffusion coefficient of 11Å, which means there's no way i'll be able to simulate that until i've found some way of decreasing the numerical errors. It also sucks that with a lower diffusion coefficient than about 100, density gets added to the system at a very noticable rate :(
Got any links to where i can find info on how to decrease the errors?