Physics

Difference Between Kinematic And Dynamic Viscosity

the basic difference between kinematic and dynamic viscosity is that 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 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.

What is Viscosity?

What is viscosity? Well, the viscosity is the property that tells us how much is the resistance of a fluid to be cut when it is in motion. Only solids can withstand shear stresses without moving; in the case of liquids, necessarily when they are subjected to this type of effort, movement occurs. Something very important to understand the term is to understand that viscosity occurs because the different molecules of the fluid interact with each other.

When we are going to let a liquid come out of a container, through a hole, its speed depends in part on the viscosity. The fluid opposes the movement and therefore, this opposition, which is due to viscosity, is what regulates the speed with which the liquid comes out of the hole. Of course, there are also terms such as the dimension of the hole, the shape, the depth to which it is located, etc. that influence, but it is clear that the viscosity has an important influence.

To understand the above, let’s imagine two equal glasses, each with an equal hole in the bottom. We both fill them, one with water and the other with honey. Now, if we look at the speed at which both are emptying through the hole, I think it is clear to everyone that the glass of honey will take longer to empty than the glass of water.

Something similar happens when we circulate a fluid through a pipe. We say that there are losses and we always assume them as due to “friction.” In this case, it must be clarified that these losses occur due to the viscosity of the liquid and that, if we compare it to friction, they are due to the friction of the liquid with the same liquid (that is, to the resistance of the liquid to shear) and not to friction of the liquid with the inside of the tube. That is, to the internal friction of the liquid and not to the friction of the liquid with the tube.

What is 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.

What is 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 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.

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

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.

How does temperature influence the velocity of liquids?

In the case of liquids, the viscosity decreases as the temperature increases. This is critical in some cases, for example in vehicle engine oils where it is sought that the viscosity does not decrease much as the engine heats up, since the lubricant would not be as efficient.

Informally we do not usually talk about viscosity, but we talk about how thick a substance is. When we talk about the resistance of a fluid (see the above-mentioned example of the fluid inside a tube), we are not far from the definition of viscosity, since the force necessary to cut the fluid, and therefore so that it is move, it is due to the resistance that the molecules oppose to its movement, due to the collisions that occur between particles close to each other.

Although it does not exist, we are talking about an ideal fluid, which is one that has no viscosity. That is, the one that moves without loss; that we can mix without spending energy; etc. They approach ideal liquids, without viscosity, only some superfluids at very low temperatures.

Various methods and instruments are used to measure viscosity, such as viscometers and rheometers. Rheology is the branch of physics that is dedicated to the study of viscosity, the relationship between stress and strain in fluids.

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