### Viscosity

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The viscosity (m) of a fluid measures its resistance to flow under an applied shear stress. Representative units for viscosity are kg/(m.sec), g/(cm.sec) (also known as poise designated by P). The centipoise (cP), one hundredth of a poise, is also a convenient unit, since the viscosity of water at room temperature is approximately 1 centipoise.

The kinematic viscosity (n) is the ratio of the viscosity to the density:

n = m/r,

and will be found to be important in cases in which significant viscous and gravitational forces exist.

Viscosity of liquids:

Viscosity of liquids in general, decreases with increasing temperature.

The viscosities (m) of liquids generally vary approximately with absolute temperature T according to:

ln m = a - b ln T

Viscosity of gases:

Viscosity of gases increases with increase in temperature.

The viscosity (m) of many gases is approximated by the formula:

m = mo(T/To)n

in which T is the absolute temperature, mo is the viscosity at an absolute reference temperature To, and n is an empirical exponent that best fits the experimental data.

The viscosity of an ideal gas is independent of pressure, but the viscosities of real gases and liquids usually increase with pressure.

Viscosity of liquids are generally two orders of magnitude greater than gases at atmospheric pressure. Fow example, at 25oC, mwater = 1 centipoise and mair = 1 x 10-2centipoise.