Meaning of Dynamic and Kinematic Viscosity

What are Dynamic and Kinematic Viscosity:

The dynamic and kinematic viscosity are values ​​that determine the movement that a certain liquid or fluid has under specific conditions.

In hydraulics or fluid mechanics, dynamic viscosity and kinematic viscosity are necessary concepts to relate the forces that generate movement and velocity in a liquid. Thus, it is important to know how liquids move in order to understand how mechanisms actuated by liquid fluids work.

To understand both concepts, it is necessary to take into account that the Viscosity of a fluid is determined by the level of cohesion of the molecules. Liquids have a cohesion between their molecules that is weaker than a solid and stronger than a gas, which gives them the fluidity that characterizes them. The lower the interaction between the molecules of a liquid, the lower its viscosity, therefore there is less friction.

dynamic viscosity

Dynamic viscosity, also called absolute viscosity, is the internal resistance between the molecules of a moving fluid and determines the forces that move and deform it.

Isaac Newton (1643-1727) observes this behavior of liquids by placing it between two parallel plates. The static base plate and the upper one with a constant movement of one centimeter per second. In this way, he reaches the Newton’s law of viscosity represented in the following formula:

tangential force

Liquids slide in layers or sheets, which means that the velocity of the fluid is zero at the contact surface and increases as it becomes more distant creating a tangent which is called a tangential force.

For the calculation of the dynamic viscosity, the specific unit in the Cegesimal System of Units (CGS) Poise (P) is used.

You may be interested:  Law of Conservation of Matter

Kinematic viscosity

Kinematic viscosity relates dynamic viscosity to the density of the liquid. Taking the value of the dynamic viscosity, it is possible to calculate the Kinematic viscosity of a fluid with the following formula:

Kinematic viscosity

To this extent, viscosity is the resistance of a fluid to slipping, and density is the specific weight (mass/volume) divided by gravity. For example, a viscous motor oil slowly slides down a tube, but it will still be less dense than water when floating on top of it. In this case, the water is less viscous, but more dense than the oil.

To calculate the kinematic viscosity, the specific unit in the Cegesimal System of Units (CGS) Stoke (St) is used.

It is important to note that both dynamic and kinematic viscosity depend on the nature of the liquid and the temperature, for example, the higher the temperature of a liquid, the less viscous it is, since the cohesion of the molecules becomes more weak.

See also Viscosity.