INTERVIEW SHORT QUESTION ANSWERS -VISCOSITY      Types of fluid flow-1       Fluid properties & forces       Cavitation        MCQ cavitation       Drag       Hydro Forces on Surfaces       Pressure measurement        Q. ANS. Pressure Drop



Viscosity of a fluid is resistance to

flow. Preferably viscosity is to less

for better and easy flow. It is

governed by Newton’s Law. High

viscosity fluids consume lot of

power for their movement. Rise of

temperature decreases viscosity.

  1. Define and differentiate dynamic and kinematic viscosity.

Sr. No.

Dynamic viscosity

Kinematic viscosity

It is resistance to fluid flow. It gives an idea about the thickness of fluid.
If the fluid is thin like water, it has less viscosity. If the thickness of the fluid
is more like honey, its viscosity is more.
It is the ratio of the fluid’s viscous force to the inertial force.
Its symbol is ‘µ’.
Its symbol is’ υ‘.=µ /ρ
Its unit is N-s/m2.
Its units is m2/s.
Dynamic viscosity is absolute viscosity or just viscosity.
Kinematic viscosity is momentum diffusivity.
  1.  What are its practical applications of dynamic viscosity?

It is ratio of dynamic viscosity to density of the fluid. Its symbol is ʋ (nu). Units of dynamic viscosity is m2/s. Mathematically, it is ʋ = μ /ρ.
It represents fast momentum transfer between different layers of the fluid. Dynamic viscosity is resistance to flow between various layers of the fluid. Then how do we say about fast momentum transfer for kinematic viscosity? This change of nature is due to density as denominator. Lower is the density, lower will be the inertia force and faster will be the momentum transfer between various layers. Higher is the density, more will be the inertia force and slow will be the momentum transfer between various layers. Hence kinematic viscosity is also momentum diffusivity. It is very much similar to thermal diffusivity in heat transfer. Kinematic viscosity is used in Naiver Stokes equations governing the fluid motion.


Effect of Temperature
Viscosity of liquids decrease with the increase of temperature and vice versa.
Viscosity of gases increase with the increase of temperature and vice versa.
Effect of Pressure 
NO effect on liquids and gases viscosity.

What is Newton’s Law of Viscosity?

τ= μ du/dy
τ is shear stress between two adjacent layers of a moving fluid
du change in velocity in x direction (direction of motion)
dy is the distance (perpendicular to du) between the two layers in y direction
NOTE du is in x direction and dy is in y direction
du/dy becomes shear strain or velocity gradient in y direction
μ is constant of proportionality & is  Dynamic viscosity.
μ = τ/ (du/dy)
τ =N/m2
du = m/s
dy = m
Therefore, Units of viscosity are Ns/m2.