Meaning of dynamic and kinematic viscosity

What are Dynamic and Kinematic Viscosity:

Dynamic viscosity and kinematics 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 motion and velocity in a liquid. In this way, it is important to know how liquids move 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 fluid in motion and determines the forces that move and deform it.

Isaac Newton (1643-1727) observes this behavior of liquids when 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 arrives at Newton's Law of viscosity represented in the following formula:

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 called a tangential force.

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

Kinematic viscosity

Kinematic viscosity relates the dynamic viscosity to the density of the liquid. Taking the value of the dynamic viscosity, the kinematic viscosity of a fluid can be calculated with the following formula:

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

For the calculation of the kinematic viscosity the specific unit is used in the Cegesimal System of Units (CGS) Stoke (St).

It is important to note that both the 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.

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