basic definitions of fluid mechanics

Density (mass density): The mass per unit volume is defined as density. The unit used is kg/m³.The measurement is simple in the case of solids and liquids. In the case of gases and vaporous is rather involved. The symbol used is ρ. The characteristic equation for gases provides ameans to estimate the density from the measurement of pressure, temperature and volume.

Specific Volume: The volume occupied by unit mass is called the specific volume of the material. The symbol used is v, the unit being m³/kg. Specific volume is the reciprocal of density.In the case of solids and liquids, the change in density or specific volume with changes in pressure and temperature is rather small, whereas in the case of gases and vapours, density will change significantly due to changes in pressure and/or temperature.

Weight Density or Specific Weight: The force due to gravity on the mass in unit volume is defined as Weight Density or Specific Weight. The unit used is N/m³ .

Specific Gravity or Relative Density:  The ratio of the density of the fluid to the density of water—usually 1000 kg/m³ at a standard condition—is defined as Specific Gravity or Relative Density δ of fluids. This is a ratio and hence no dimension or unit is involved.

Compressible fluid -If the density of a fluid varies significantly due to moderate changes in pressure ortemperature, then the fluid is called compressible fluid.

Incompressible fluid-If the change in density of a fluid is small due to changes in temperature and or pressure, then the fluid is called incompressible fluid.

Buoyant force- If an object is immersed in or floated on the surface of fluid under static conditions a force acts on it due to the fluid pressure. This force is called buoyant force.


Archimedes principle -Archimedes principle can be stated as (i) a body immersed in a fluid is buoyed upby a force equal to the weight of the fluid displaced and (ii) a floating body displaces its own weight of the liquid in which it floats.

Reynolds number -Reynolds number is the ratio of inertia force to viscous force. As inertia force increases Reynolds number increases and the flow becomes turbulent.