Viscosity

Viscosity is the resistance of a fluid to flow. Virtually all fluids have viscosity which generally changes as a function of temperature; although different types of fluids exhibit different types of fluid–shear velocity dependencies.

“When a fluid or semisolid is subjected to a constant shearing force it flows, i.e., it deforms continuously at a velocity that increases as the applied shearing force increases.” Viscosity quantifies the resistance of the fluid to flow

Introduction

Viscosity is a quantitative measure of fluid’s resistance to flow (shear stress) at a given temperature. This resistance arises from the attractive forces between the molecules of the fluid. A fluid will only flow if enough energy is supplied to overcome these forces.

Dynamic Viscosity / Viscosity

The dynamic viscosity (η) of a fluid is a quantitative measure of the resistance it offers to relative shearing motion.
Dynamic viscosity, which is also referred to as absolute viscosity, or just viscosity, is the quantitative expression of a fluid’s resistance to flow (shear). Fluid dynamicists, chemical engineers and mechanical engineers commonly consider the use of the Greek letter mu (µ) as the symbol to denote dynamic viscosity.

Units

The SI unit is pascal-second [Pa.s] or millipascal-second [mPa.s]:

    1 Pa.s = 1000 mPa.s
    The SI unit is named after Blaise Pascal.

Other commonly used units are poise [P] or centipoise [cP]:

    1 P = 100 cP
    This unit is named after Jean Poiseuille

    1 cP = 1 mPa.s = 0.001 Pa.s = 0.01 P

However, the most common expression is centipoise (cP), which is mainly used in ASTM standards.

Kinematic Viscosity

It is defined as the ratio of absolute viscosity to the density of fluid. Kinematic viscosity describes a substance's flow behavior under the influence of Earth's gravity. It is dynamic viscosity divided by density ρ, rho, which is defined as mass per volume. The quantity mass carries the gravitational influence. Kinematic viscosity is sometimes called the diffusivity of momentum.

        ν= η/ρ

Units

The SI unit is square-meters per second  [m2/s] or square-millimeters per second [mm2/s]:

    1 m2/s = 1 000 000 mm2/s

Other commonly used units are stokes [St] or centistokes [cSt]:

    1 St = 100 cSt

This unit is named after George G. Stokes.

    1 cSt = 1 mm2/s

 It should be noted that water (H2O) at 20 degrees centigrade is about 1 cSt.

Relation of Kinematic Viscosity with Dynamic Viscosity

Limitations

The above equation holds only when

1-Fluid is Newtonian

2-Specific Gravity Remains the Same

Application

Liquids are generally considered viscous if viscosity is more then 40 centipoise (cp). Centrifugal pumps are not recommended for fluid having viscosity more then 300 centipoise (cp).

The viscosity of liquids decreases with increase the  temperature. Typically 2% per degree C. For some materials (fruit juices) the Temperature effect follows an Arrhenius relationship. 

Viscosity of gases increases with the increase the temperature.