- 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 = F̅/A Δvx/Δz η = F/A dvx/dz
- often expressed using Newton's equation for fluids (which is similar to Newton's second law of motion).
F̅ = η Δvx A Δz F = η dvx A dz
- 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
- 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.