Fluid Mechanics (lesson #01) – Basics


Fluid Mechanics is that discipline in the broad field of applied mechanics that is concerned with the behavior of liquids and gasses at rest or in motion.

It covers a vast array of phenomena that occur in nature (with or without human intervention.)

* Fluid cannot offer permanent resistance to a deforming (shearing force) 

Fluids flow How when acted upon by a shearing (deforming) force, deforming continuously for as long as the force is applied.

* A Fluid is a substance that deforms continuously under the action of shearing forces, however small they may be.

Conversely, it follows that ;

If a fluid is at rest, there can be no shearing force acting and therefore, all forces in the fluid must be perpendicular to the planes upon which they act.

Properties of Fluid

Density or Mass Density

The density or mass density of a fluid is defined as the ratio of the mass of a fluid to its volume. Thus mass per unit volume of a fluid is called density. The density of a liquid may be considered constant while that of gasses changes with the variation of pressure and temperature.

Specific Weight or Weight Density

Specific weight or weight density of a fluid is the ratio between the weight of a fluid to its Volume. Thus weight per unit volume of a fluid is called weight density.

Specific Volume 

The specific weight of a fluid is defined as the volume of a fluid occupied by a unit mass or the volume per unit mass of a fluid is called specific volume.


Viscosity is defined as the property of a fluid that 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 apart, move one over the other at different velocities see u and du as shown in the figure, the viscosity together with relative viscosity cause stress acting between the fluid layers. The top layer causes shear stress on the lower layer while the lower layer causes the shear stress on the adjacent top layer. This shear stress is proportional to the rate of change of velocity with respect to Y.

Kinematic Viscosity

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

Newtonian and Non-Newtonian Fluids

The shear stress (𝜏) on a fluid element layer is directly proportional to the rate of shear strain. The constant of proportionality is called the coefficient of viscosity.

  • Fluids that obey the above relation are known as Newtonian fluids and the fluids which do not obey the above relation are called Non-newtonian fluids.

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