Characteristics of a fluid

Fluids are divided into liquids and gases. A liquid is hard to compress and as in the ancient saying ‘Water takes the shape of the vessel containing it’, it changes its shape according to the shape of its container with an upper free surface. Gas on the other hand is easy to compress, and fully expands to fill its container.

There is thus no free surface. Consequently, an important characteristic of a fluid from the viewpoint of fluid mechanics is its compressibility. Another characteristic is its viscosity. Whereas a solid shows its elasticity in tension, compression or shearing stress, a fluid does so only for compression. In other words, a fluid increases its pressure against compression, trying to retain its original volume.

This characteristic is called compressibility. Furthermore, a fluid shows resistance whenever two layers slide over each other. This characteristic is called viscosity. In general, liquids are called incompressible fluids and gases compressible fluids. Nevertheless, for liquids, compressibility must be taken into account whenever they are highly pressurised, and for gases compressibility may be disregarded whenever the change in pressure is small. Although a fluid is an aggregate of molecules in constant motion, the mean free path of these molecules is 0.06pm or so even for air of normal temperature and pressure, so a fluid is treated as a continuous isotropic substance.

Meanwhile, a non-existent, assumed fluid without either viscosity or compressibility is called an ideal fluid or perfect fluid. A fluid with compressibility but without viscosity is occasionally discriminated and called a perfect fluid, too. Furthermore, a gas subject to Boyle’s-Charles’ law is called a perfect or ideal gas.