Concept of Fluid

There are two classes of fluids:

Liquids: are composed of relatively close‐packed molecules with strong cohesive forces. Liquids have constant volume (almost in compressible) and will form a free surface in a gravitational field if unconfined from above.

Gases: molecules are widely spaced with negligible cohesive forces. A gas is free to expand until it encounters confining walls. A gas has no definite volume, and it forms an atmosphere when it is not confined. Gravitational effects are rarely concerned. Liquids and gases can coexist in two‐phase mixtures such as steam‐water mixtures.

 We can define fluid properties and parameters, as continuous point functions, only   if the continuum approximation is made. This requires that the physical dimensions are large compared to the fluid molecules.

- Property: means any characteristic of a system and Properties of fluids determine how fluids can be used in engineering and technology. They also determine the behavior of fluids in fluid mechanics.

E.g.:  Pressure  , Temperature  , Mass  , and Volume .

* Properties considered to be:

Ø  Intensive Properties: Properties that are independent of the mass of a system such as temperature, pressure, and density.

Ø  Extensive Properties: Properties whose values depend on size-or-extend-of the system such as mass, total volume, and total momentum.

Fig 2 : Criteria to differentiate intensive and extensive properties

Ø  Specific Properties: Extensive properties per unit mass such as specific volume  and specific total energy .

Ø  State postulate: The state of a simple compressible system is completely specified by two independent, intensive properties.

2.1 CONTINUUM CONCEPT OF A FLUID:

The number of molecules involved is immense, and the separation between them is normally negligible by comparison with the distances involved in the practical situation being studied.

Although the properties of a fluid arise from its molecular structure, engineering problem are usually concerned with the bulk behavior of fluids.

            Under these conditions, it is usual to consider a fluid as a continuum - a hypothetical continuous substance – and the conditions at a point as the average of a very large number of molecules surrounding that point within a distance which is large compared with the mean inter molecular distance.