Civil Engineering - What is Viscosity and its unit

Civil Engineering ⇒ Topic : Viscosity and its unit

What is Viscosity and its unit

Atharv said on : 2018-07-09 11:27:44

Viscosity

it is the property of a fluid which resists relative motion of its adjacent layers. It is primarily due to cohesion and molecular momentum exchange between fluid layers.

Shearing stress τ ∝	dv
	dy

where, dv = Relative velocity between two adjacent fluid layers
dy = Distance measured normal to the direction of flow

∴	dv	= Velocity gradient
	dy

τ ∝ μ	dv
	dy

where μ is known as the coefficient of viscosity. The unit of viscosity is poise.

1 poise =	1	kg-sec/m²
	98.1

Krishna said on : 2018-07-21 23:31:03

Viscosity is defined as the property of a fluid which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid. When two layers of a fluid, a distance 'dy' apart, move one over the other at different velocities, say u and u + du as shown below

the viscosity together with relative velocity causes a shear stress acting between the fluid lavers
The top layer causes a shear stress on the adjacent lower layer while the lower layer causes a shear stress on the adjacent top layer. This shear stress is proportional to the rate of change of velocity with respect to y. It is denoted by symbol τ (Tau).
Mathematically,

τ ∝	du
	dy

or τ ∝ μ	du
	dy

where μ (called mu) is the constant of proportionality and is known as the co-efficient of dynamic viscosity du/dy or only viscosity. represents the rate of shear strain or rate of shear deformation or velocity gradient. From above equation, we have

μ =	τ
	(du/dy)

Thus viscosity is also defined as the shear stress required to produce unit rate of shear strain.

Units of Viscosity : The units of viscosity is obtained by putting the dimensions of the quantities in above equation

Kinematic Viscosity : It is defined as the ratio between the dynamic viscosity and density of fluid. It is denoted by the Greek symbol (v) called 'nu'. Thus, mathematically,

v =	Viscosity	=	μ
	Density		ρ

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