= Fluid dynamics
= Hydrodynamics
{synonym}
<Fluid dynamics> studies the <motion> of <liquids> and <gases>, including the <forces> and transport processes that determine their evolution. Its central variables include <velocity>, <pressure>, <mass density> and <temperature>, all varying with position and time.
<Conservation of mass> gives the <continuity equation>. Combining <Newton's second law> with a constitutive model for <viscous stress> gives the <Navier-Stokes equations>. The relative importance of <inertia> and <viscosity>, measured by the <Reynolds number>, explains why slow flow around a microscopic particle differs so sharply from a turbulent river. Boundary conditions and material properties are essential parts of a fluid model.
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