Mechanical Engineering

 

 

 

 

Fluid Mechanics : Viscosity

 

Speaking in plain language, Viscosity is a kind of indicator that shows how hard a fluid flow (or how hard to make a fluid flow).  With a couple of real life examples, you can get more intuitive understandings on what Viscosity is.

 

Take a spoonful of water and a spoonful of honey. Tilt each of the spoons a little and see what happens to water and honey. Water would flow and drop out of the spoon right away, but honey would slowly slide down.

Why ?

You would say honey flows slowly comparing to water since honey is thicker than water. In engieering (fluid mechnics), you would say honey flows slowly because the viscosity of honey is higher than the viscosity of water.

 

This is the end of speaking in plain language.  Speaking more like an engineer (i.e, in more dry and boring way :)), Viscosity can be defined as 'a measure of resistance to gradual deformation by the stress applied to a fluild'. The 'stress in this statement is mostly mean shear stress or tensil stress.

 

Getting back to the water / honey example.  You tilt the spoon. It means Gravity applies some stress (shear stress in this case) to water and honey, water and honey start deform (i.e, flows).

 

Viscosity is one of the most fundamental properties of a fluid and you would need to know this property in solving almost any kind of fluild mechnics problem.

 

Reference :

 

[1] Viscosity (The Physics Hypertextbook)