Fluides

Fluides (French for fluids) are substances that can flow and assume the shape of their container. They include liquids and gases. Fluids respond to forces with deformation and do not resist shear as rigid bodies do. The study of fluids covers statics, dynamics, and transport phenomena in engineering, physics, and biology.

Key properties are density, ρ; viscosity, μ; compressibility; and surface tension. Liquids are relatively incompressible and have definite volume, while gases are compressible and fill available space. Viscosity measures internal friction; Newtonian fluids have constant viscosity, while non-Newtonian fluids change viscosity with shear.

In static fluids, pressure increases with depth according to P = ρ g h, and buoyancy arises from displaced fluid (Archimedes' principle). Boundaries transmit pressure according to Pascal's law. Surface tension causes capillary rise and droplets formation.

In motion, fluids are described by conservation laws. The Navier–Stokes equations express conservation of mass and momentum for viscous fluids. For ideal fluids, Euler equations apply. Flow regimes are characterized by Reynolds number; low Re favors laminar flow, high Re tends to turbulence. Bernoulli's principle relates pressure, speed, and height for steady, incompressible, inviscid flow along a streamline.

Examples and applications include water in pipes, air over an aircraft wing, blood flow, oil extraction, meteorology, oceanography, and microfluidics. Non-Newtonian fluids such as ketchup or cornstarch suspensions exhibit rate-dependent viscosity. Flow in porous media is often described by Darcy's law.