Relation Between Viscosity And Density

In fluid dynamics, viscosity is the parameter to measure the thickness or thinness of any given fluid. Density is the measure of spaces between two particles in a given fluid. Viscosity and density are the characteristics of a fluid, but there is no direct relation between viscosity and density.

Viscosity and Density

There is no direct relation between viscosity and density. However, both Viscosity and density are affected by temperature. Which implies, for any given fluid, when the temperature is raised, the particle in it start to move apart, bringing down fluid density thereby the value of viscosity also falls down or fluid becomes less viscous.

In general, for any fluids, when the temperature is increased, its density decreases, thus the fluid becomes less viscous.

Kinematic Viscosity

In fluids, Kinematic viscosity is the ratio of dynamic viscosity to density. Mathematically given by-

$\nu =\frac{\eta }{\rho }$

Where,

• $\nu$ is the Kinematic viscosity.
• $\eta$ is the dynamic viscosity / Absolute viscosity.
• $\rho$ is the density.

Refer to the table given below for definition, units, and symbol for the above parameter.

Definition SI unit Symbol

Kinematic Viscosity

The ratio of dynamic viscosity to density m2/s $\nu$

Dynamic viscosity

The measure of internal resistance against the flow Ns/m2 $\eta$

Density

The ratio of mass per volume Kg/m3 $\rho$

Viscosity and Density of Water

The table given below gives the values of Viscosity and Density of water at 100C and 200C.

Temperature 100C 200C

1.308 1.002

0.99975 0.99804

Viscosity and Density of Air

The table given below gives the values of Kinematic Viscosity and Density of air at -100C, -150C and -200C.

 Temperature 0C Density (Kg/m3) Kinematic viscosity(m2/s) -20 1.3958 1.1622×10-5 -15 1.3687 1.2039×10-5 -10 1.3426 1.2462×10-5

Hope you understood the relation between viscosity and density for a given fluid in fluid mechanics.

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1 Comment

1. thanks for your excellent description.