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What is Dynamic Viscosity ?

What is Dynamic Viscosity ?


Viscosity is defined as the internal friction of a flowing fluid. Different fluids have different viscosities and the viscosities can be changed through temperature and pressure. For further information regarding the viscosity, click on the link below –

Viscosity Definition 

Now talking about Dynamic Viscosity, it is the resistance to the shearing flows where the adjacent fluids layers flow at different speeds parallel to each other. It is simply defined as Couette flow. Couette Flow is the laminar flow in the space between two parallel plates, out of which one must be moving relative to each other and also at different speeds. When a layer of fluid gets trapped, the other layer moves relatively parallel to the other layer at a constant speed. When there are three layers, the first layer on the bottom moves slower than the second layer and the second layer in the middle moves slower than the third layer present at the top of the two layers. The speed varies from zero to a certain limit from the bottom layer to the top layer. We have the equation as follows –

F = Magnitude,
A = Area of each plate,
y = distance of each plate,
u = speed.

It is clear from the equation that the Magnitude of Force is directly proportional to Area and Speed but inversely proportional to the distance of the plates ( Separation between the plates ). The proportionality factor mentioned in the equation is known as Dynamic Viscosity of the fluid.

The ratio of speed and distance is known as Shear Velocity and also known as Shear Deformation and the relation between Force and Area with local shear will be expressed as follows –

The equation mentioned above can be used where velocity does not vary linearly.

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