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# Viscosity

## Summary

• Viscosity (also known as dynamic viscosity, absolute viscosity, or simple viscosity) is…
• represented by the Greek letter η (eta).
• defined informally as the quantity that describes a fluid's resistance to flow.
• defined mathematically as the ratio of the shearing stress to the velocity gradient in a fluid.  η = F/A Δvx/Δz
or  η = F/A dvx/dz
• often expressed using Newton's equation for fluids (which is similar to Newton's second law of motion).  F = η Δvx A Δz
or F = η dvx A dz
• Units
• The SI unit of viscosity is the pascal second [Pa s].
• The Gaussian unit of viscosity is the poise [P = dyne s/cm2].
• Ten poise equal one pascal second [10 P = 1 Pa s].
• Kinematic viscosity is…
• represented by the Greek letter ν (nu).
• defined informally as a measure of the resistive flow of a fluid under the influence of gravity.
• defined mathematically as the ratio of the viscosity of a fluid to its density.  ν = η ρ
• Units
• The SI unit of kinematic viscosity is the square meter per second [m2/s].
• The Gaussian unit of kinematic viscosity is the stokes [St = cm2/s].
• Ten thousand stokes equal one square meter per second [10,000 stokes = 1 m2/s].
• Factors Affecting Viscosity
• Viscosity varies with material. (Viscosity is a property of materials.)
• The viscosity of simple liquids…
• decreases with increasing temperature
• increases under very high pressures.
• The viscosity of gases…
• increases with increasing temperature
• is independent of pressure and density.