VISCOSITY

Viscosity is the quantity that describes a fluid's resistance to flow. It exists in both liquids and gases and is essentially a friction force between different layers of fluid as they move past one another. In liquids the viscosity is due to the cohesive forces between the molecules. in gases the viscosity is due to collisions between the molecules.

Viscosity cont’d…. represented by the symbol η "eta“ is the ratio of the shearing stress (ƒ/A) to the strain rate (v/l). SI unit of viscosity is the Pascal second [Pa s]. A common unit is the poise (P) where 1 Pa.s = 10 P

Dynamic or Simple viscosity Types of viscosity Dynamic or Simple viscosity Kinematic Viscosity

v=ŋ/ρ Kinematic Viscosity a measure of the resistive flow of a fluid under the influence of gravity. represented by the symbol ν "nu“ ratio of the viscosity of a fluid to its density. Can be measured using the Capillary Viscometer. SI unit of kinematic viscosity is the [m2/s]. Common unit used is the stroke (St) = [cm2/s] v=ŋ/ρ

Factors affecting Viscosity 1. Viscosity is very temperature dependent. Viscosity of a liquid decreases with increasing temperature. Ex. It is easier to pour skin lotion during summer than cold season. Different oils are used in hot and cold countries. • Viscosity of a gas increases with increasing temperature.

Factors affecting Viscosity 2. Viscosity is normally independent of pressure, but liquids under extreme pressure often experience an increase in viscosity

Viscous Flow

Viscosity cont’d…. Viscous fluids tend to cling to a solid surface. • Syrup and honey are more viscous than water. Grease is more viscous than engine oils. Liquids are more viscous than gases. Lava is an example of a very viscous material.

Equation of Continuity When fluids move through a full pipe, the volume of fluid that enters the pipe must equal the volume of fluid that leaves the pipe, even if the diameter of the pipe changes. This equation says that as the cross-section of the pipe gets smaller, the velocity of the fluid increases, and as the cross-section gets larger, the fluid velocity decreases.

Equation of Continuity