Fluid

The fluid S are deformable mediums perfectly . One gathers under this name the Gaz which are the example of the fluids compressible, and the Liquid S, which are not very compressible fluids. Under certain conditions (temperatures and/or pressures), the medium is neither Liquide, nor gas, it remains fluid (see Diagramme of phase of a pure substance).

The particles constitutive of a fluid are not bound by covalent bonds (i.e. of Chemical bond). In a gas, the interactions between particles are negligible, except when they meet (shocks). In a liquid, the molecules are so much close relations who it is difficult to compress the fluid. They interact this time strongly via forces of van der Waals, dipolar interactions (particles behaving like Dipole S electrostatic S). This type of interaction explains the properties physics and chemical of the liquids.

Assumption of the continuous medium

Various categories of fluids

Newtonian fluid

A fluid is known as Newtonian when the tensor of the viscous constraints is a linear function of the tensor of the deformations.

For this type of fluid, the movement of the fluid is governed by the Equation of Navier-Stokes which, in the case of an incompressible fluid is written:

where:

• $\backslash \; overrightarrow\; \{v\}$ is the Flight Path Vector

• $\backslash \; rho$ is the density
• $P$ is the pressure
• $f$ are forces per unit of mass (ex: action of gravity: $\backslash \; overrightarrow\; \{G\})$
• $\backslash \; nu$ is kinematic viscosity

NonNewtonian fluid

A fluid is known as not Newtonian when the tensor of the viscous constraints is not a linear function of the tensor of the deformations. In other words, when its deformation is not directly proportional to the force that one applies to him. The best example is that of the sand wet in seaside: when sand is struck, it with the high viscosity of a solid, whereas when one supports gently above, he behaves like a paste.

The nonNewtonian fluids are almost all of colloids. One can prepare some at home with water and starch of corn for example or of the plaster.

The colloids, and thus also the nonNewtonian fluids, are the subject of thorough basic research because they are very present in nature, but also because of possible military applications: for example, the realization of sufficiently flexible bullet-proof jackets to be carried without embarrassment.

To also see…

Dependant articles

• Rheology
• complex Flows

Simple: Fluid

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