The viscosity (of Latin the viscum ) indicates the capacity of a fluid to run out, in Mécanique of the fluids. In current language, one uses also the term of fluidity .

Property

When the viscosity increases, the capacity of the fluid to be run out decreases. Viscosity tends to decrease when the temperature increases. On the other hand, one could believe that the viscosity of a fluid increases with its density but it is not necessarily the case.

One classifies in particular the Huile S mechanics according to their viscosity, according to the needs for Lubrification of the Moteur and the temperatures to which oil will be subjected during the operation of the engine.

Modeling

There exist two types of viscosity:

• the dynamic Viscosité μ (or η ) is measured in Pascal-second (Pa.s), this unit having replaced the Poiseuille (Pl) which with the same value. One finds still sometimes the old unit: the Poise (Po); 1 Pa.s = 10 Po. The viscosity of water with 20°C is of 1cPo (centipoise).

A way of defining dynamic viscosity is to consider two noted layers of a fluid abcd and has' b' it of, the abcd layer being animated a speed relative to a' b' it of noted FD and directed according to X . Under the effect of viscosity, a force F is exerted on the layer a' b' it of. Dynamic viscosity μ is defined by the relation between the standard of this force and relative speed FD $F\; =\; \backslash \; driven\; \backslash ,\; S\; \backslash ,\; \backslash \; frac\; \{FD\}\; \{dz\}\; \backslash ;$, S being the surface of each layer, and dz the thickness of fluid separating the two layers.

• the kinematic Viscosity ν which is obtained by dividing dynamic viscosity by the density ρ . It is expressed in m ² /s. This unit is very large. In system CGS kinematic viscosity was expressed in stokes (St) or centistoke (cSt). Conversion is immediate, since 1 St = 1 cm ² /s = 10^-4 m ² /s and 1 cSt = 1 mm ² /s = 10^-6 m ² /s.

The viscosity of a fluid varies according to its temperature or from the mechanical actions to which it is subjected. See for example on this subject the phenomenon of Thixotropy. To determine the importance of the temperature on the viscosity of a fluid one uses a index of viscosity. The larger this index is, the less the temperature has of influence on the viscosity of the fluid.

Concerning a gas, it is current to use in the following way defined law of Sutherland: $\backslash \; frac\; \{\backslash \; driven\; (T)\}\{\backslash \; mu\_0\}\; =\; \backslash \; left\; (\backslash \; frac\; \{T\}\; \{T\_0\}\; \backslash \; right)\; ^\; \{3/2\}\; \backslash \; frac\; \{T\_0+S\}\; \{T+S\}$

$\backslash \; mu\_0\; =\; \backslash \; driven\; (T\_0)$ is viscosity at the temperature $T\_0$, $S$ is the temperature of Sutherland. For the air for example one takes usually the following values μ ₀ = 1,711.10^-5, T⁰=273,15 K and S=110,4, which gives a good approximation on a beach of temperature of about 170 K to approximately 1900 K.

Some values

See too

• Viscometer

External bonds

• Viscosities of various liquids according to the temperature

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