Fluid, Characteristic of fluid, Ideal fluid, real fluid, viscosity, cause of viscosity, Newton's law of viscosity, kinematic viscosity ~ MECHTECH GURU

Fluid, Characteristic of fluid, Ideal fluid, real fluid, viscosity, cause of viscosity, Newton's law of viscosity, kinematic viscosity

Definition of fluid

A fluid is a substance which deforms continuously when subjected to external shearing forces.

Characteristics of fluid

1.  It has no definite shape of its own, but conforms to the shape of the containing vessel.

2.  Even a small amount of shear force exerted on a fluid will cause it to undergo a deformation which continues as long as the force continues to be applied.

3.  It is interesting to note that a solid suffers strain when subjected to shear forces whereas a fluid suffers Rate of Strain i.e. it flows under similar circumstances.


Ideal and Real Fluids


1.  Ideal Fluid

An ideal fluid is one which has no viscosity, no surface tension and incompressible

2.  Real Fluid

An Real fluid is one which has viscosity, surface tension and compressible

Viscosity

Viscosity is the property of a fluid which determines its resistance to shearing stresses.

 

Definition

Cause of Viscosity

It is due to cohesion and molecular momentum exchange between fluid layers.

Newton’s Law of Viscosity: 

It states that the shear stress (τ) on a fluid element layer is directly proportional to the rate of shear strain. 


                 Velocity gradient = du/dy

                  According to Newton’s lawtm du/dy




Velocity Variation near a solid boundary

Velocity Variation near a solid boundary





Units of Viscosity

S.I. Units: Pa.s or N.s/m

C.G.S Unit of viscosity is Poise= dune-sec/cm

1/100 Poise is called centi-poises.

Dynamic viscosity of water at 200 degree centigrade is approx= 1 cP

Kinematic Viscosity

It is the ratio between the dynamic viscosity and density of fluid.

 kinematic viscosity= 


Units of Kinematic Viscosity

S.I units: m

C.G.S units: stoke = cm

One stoke = 10-4 m

Thermal diffusivity and molecular diffusivity have same dimension, therefore, by analogy, the kinematic viscosity is also referred to as the

Effect of Temperature on Viscosity

With increase in temperature: 

Temperature response are neglected in case of Mercury.

The lowest viscosity is reached at the critical temperature.

Effect of Pressure on Viscosity

Pressure has very little effect on viscosity.

But if pressure increases intermolecular gap decreases then cohesion increases so viscosity would be increase.



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