Viscosity is a measure of a fluid’s resistance to flow. It describes the internal friction of a moving fluid. A fluidwith large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. Gases also have viscosity
Consider a fluid in an open channel. With the help of a plate a horizontal force F is applied in order to move the fluid. Let’s imagine that the fluid consists of horizontal layers. At the top of the fluid the layer “adheres” by cohesion forces to the plate.
This layer thus moves with the velocity of the plate. At the bottom, however, the layer does not move at all because there it is bounded by cohesion forces to the bottom of the channel. It then is clear that every layer will possess its own velocity c, evolving from the highest velocity at the top to velocity zero at the bottom.
Every layer will exercise a resistance on the adjacent layer (a shearing stress): the layer at the top will have a braking action on the second layer, the third layer will be broken by the second one, and so on…One speaks of “viscous friction” between the layers.
If one wants to maintain a velocity of a moving fluid then a force F is necessary. Newton states that in order to keep an object at constant speed no force is necessary. This force overcomes the internal friction of the fluid, i.e. the friction that the layers exercise on each other.
Absolute viscosity: It is property of any fluid which tends to resist a shearing force.
Kinematic viscosity:. The kinematic viscosity of a fluid is the absolute viscosity divided by the fluid density.
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