We were discussing the basic definition and significance of fluid mechanics and various properties of fluid in our previous
post.

Now we will understand here the term “Viscosity” of
fluid with the help of this post.

###
**Viscosity
or Dynamic viscosity**

Viscosity, also termed as dynamic viscosity, is
basically defined as the resistance provided to a layer of fluid when it will
move over another layer of fluid.

Let us consider two parallel plates of area A of
each plate and separated by a distance y with each other as displayed in
following figure. Let us consider that bottom plate is fixed and liquid is
filled between both parallel plates.

Let us consider that a force F is required to move
the top plate in a direction parallel with fluid flow.

When Reynold’s Number Re will be less than 2000, the
flow will be considered as laminar flow and we will have one linear velocity
distribution and such a parallel flow of uniform velocity gradient will be
termed as Couette flow.

Force per unit area, required to move the upper plate in a direction parallel with fluid flow, will be termed as shear stress and this shear stress will be directionally proportional to fluid flow velocity and inversely proportional to gap between both plates.

Shear stress α (Fluid flow velocity/Gap between
plates)

Shear stress, τ α (U/y)

Shear stress, τ = μ (U/y)

Where, μ is the constant of proportionality and this
constant of proportionality will be termed as dynamic viscosity of fluid or
simply viscosity of fluid.

###
*Understanding
the viscosity in simple way*

*Understanding the viscosity in simple way*

If we consider one layer of fluid of thickness y
flowing over a fixed surface as displayed in following figure. Fluid which will
be in contact with fixed surface will have zero velocity. As we will go away
from fixed surface, fluid flow velocity will be increasing. At top most surface
i.e. at a distance y from the fixed surface, velocity of fluid will be maximum.

Considering these two values of fluid velocity i.e. velocity
at fixed surface and velocity at top most surface, we will draw the velocity
profile as displayed in following figure.

Let us consider a small layer of fluid of thickness
dy as displayed in above figure. Velocity of fluid will be u at bottom of small
layer of thickness dy. Velocity of fluid will be u + du at top of small layer
of thickness dy.

In simple, we can say that fluid layers will have different velocity according to its distance from the fixed surface. Hence, there will be possibilities of shearing action when fluid layers flowing over each other with different velocity.

Shear stress will be directionally proportional to
the velocity gradient and therefore we will have following equation as
mentioned here.

Shear stress α Velocity gradient

Shear stress α (du/dy)

Shear stress, τ = μ (du/dy)

Where, μ is the constant of proportionality and this
constant of proportionality will be termed as dynamic viscosity of fluid or
simply viscosity of fluid.

Above expression was given by Newton through
experiment and therefore above expression is also termed as Newton’s law of
viscosity.

###
**Unit
of viscosity**

Unit of viscosity in S.I system will be N.s/m

^{2}or Pa.s
Viscosity is also provided with unit Poise or Centipoise.

1 Poise = 100 Centipoise = 0.1 N-s/m

^{2}
Viscosity is also given in terms of centistokes in
various engineering applications such as hydraulic oil viscosity will be
provided in terms of centistokes or cSt.

###
**
Viscosity
Vs temperature**

Viscosity will be increased with increase in
temperature in case of gases. While in case of liquid, viscosity will be
decreased with increase in temperature. Relation between viscosity and
temperature is reversed for liquid and gas.

Let us see here the curve indicating the change in
viscosity with temperature for liquid and gas.

We will now discuss the basic concept of kinematic viscosity in the
category of fluid mechanics in our next post.

Do you have suggestions? Please write in comment
box.

###
**Reference:**

Fluid mechanics by Y. Nakayama and R F Boucher

Image Courtesy: Google