  The grand potential for open systems describes thermodynamics of fluid flows
at low Mach numbers. A new system of reduced equations for the grand potential
and the fluid momentum is derived from the compressible Navier-Stokes
equations. The incompressible Navier-Stokes equations are the quasi-stationary
solution to the new system. It is argued that the grand canonical ensemble is
the unifying concept for the derivation of models and numerical methods for
incompressible fluids, illustrated here with a simulation of a minimal
Boltzmann model in a microflow setup.
