Coefficient of viscosity
Back to Top
When fluids flow through pipes, the outermost layer touching the sides of the pipe, have minimum velocity. As one approaches the centre of the tube, the velocity of the innermost (or centre) layer is maximum. Therefore, a velocity gradient develops within the pipe. Mathematically, it is represented by
Velocity profile
Back to Top
Layer 1 travels with maximum velocity and layer 4 travels with minimum velocity.
According to Newton, the tangential force between two layers, say A and B, separated by a distance dx, is directly proportional to the velocity gradient
and also to the area of the layer.
or F = -
hA
where '
h' represents coefficient of viscosity of the fluid. The negative sign indicates that this force opposes the relative motion of the layers.
Definition of coefficient of viscosity
Back to Top
The coefficient of viscosity of a liquid is defined as the tangential viscous force, which maintains a unit velocity gradient between two parallel layers, each of unit area.
Drag and angle of flow
Back to Top
a) The air strikes the wind at a greater angle, and the drag is increased.
Back to Top
b) The air stirred up, or "burbles," and drag is very great.
Back to Top
- The air flows smoothly past the wing and the drag is very small.
- The air strikes the wing at a greater angle and the drag is increased.
- The air is stirred up, or the air 'burbles' and the drag is very great.
Applications of viscosity
Back to Top
- The knowledge of viscosity and its variation with temperature, helps us select a suitable lubricant for a given machine.
- Viscosity of organic liquids such as proteins and cellulose, helps us calculate their molecular weight and determine their shape.
- The knowledge of viscosity helps in studying the circulation of blood through arteries and veins.
- The charge on the electron was determined by Millikan from the knowledge of viscosity.
