In this article we will discuss about:- 1. Meaning of Fluid 2. Perfect and Actual Fluid 3. Units of Fluid Measurement 4. Properties.

**Meaning of Fluid:**

A fluid is a substance which offers no resistance to shear deformation and will continue to deform when subjected to shear stresses. A fluid has no definite shape, and it takes the shape of the container in which it is contained. A shearing force on a fluid will change its shape. Hence when shearing forces act on a fluid, it will flow.

Conversely if a fluid is at rest there can be no shearing forces on it and thus all forces are normal to the planes on which they act. What is important to realize is any shear stress, however small it may be, will distort a fluid and the shape of the fluid will continue to change. But, when the fluid is at rest, since its shape does not change, we conclude that shear stresses are totally absent.

Fluids may be classified into liquids and gases. When subjected to compression all fluids diminish in their volume, but reduction in volume is so small in the case of liquids that for all practical purposes, liquids may be considered to be incompressible. Gases are, no doubt, readily compressible.

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A liquid is generally considered to have a constant density. Yet it is worth realizing the fact that a liquid, say water has a bulk modulus much less than that of steel. This means a liquid is also compressible though to a very low degree, at a very high pressure.

**The main points of difference between liquids and gases are given below: **

**Perfect and Actual Fluid****: **

A perfect or ideal fluid is a fluid in which only pressure forces exist whether the fluid is at rest or in motion, i.e. in an ideal fluid the internal forces on any internal section are entirely normal to the section even when the fluid is in motion. Since no tangential forces exist, an ideal fluid is absolutely frictionless. In reality such a fluid does not exist.

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In a practical fluid or actual fluid or real fluid, in addition to pressure forces, shearing or tangential stresses occur whenever the fluid is in motion. In other words, fluid friction exists when an actual fluid is in motion. Tangential stresses in an actual fluid in motion are possible due to a property called the viscosity of the fluid.

**Units of Fluid Measurement****: **

Measurements of quantities are made in terms of units. Certain quantities are selected as basic quantities which have well defined units. The units of all other quantities can be determined in terms of the units of the basic quantities. In this book, the SI units (Systeme International d’ unites) have been adopted.

The SI units were recommended by the General Conference on Weights and Measures in 1960 and are now being adopted in all engineering subjects.

This system adopts six fundamental quantities namely Length, Mass, Time, Electric Current, Thermodynamic Temperature and Luminous intensity. The system has also two supplementary units, namely the Plane angle and the Solid angle. The table below shows the units of the fundamental and supplementary quantities.

Any derived quantity is related to the fundamental quantities according to some known physical laws. By such laws we can obtain the units of derived quantities. Units of some of the derived quantities are given some names. Where such names are not given, the unit for the derived quantity must be expressed in terms of the units of the fundamental quantities.

**The SI units of physical quantities commonly met with in this subject are given in the table below:**

**Subscripts for Multiples and Sub Multiples: **

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**The following subscripts are used to express multiples and sub multiples of quantities: **

Kilo – 10^{3}

Mega – 10^{6}

Mega – 10^{9}

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Terra – 10^{12}

Milli – 10^{-3 }

Micro – 10^{-6 }

Nano – 10^{-9 }

Pico – 10^{-12}

**Properties of Fluids****: **

Density, also called mass density or specific mass (ρ). This is the mass of the fluid per unit volume. Density of water at 4°C at mean sea level (760 mm of mercury) = 1000 kg/m^{3}

Specific weight (w) – This is the weight of the fluid per unit volume.

Specific weight of water at 4° C at mean sea level = 9810 N/m^{3}.

Specific weight and specific mass are related by the relation w = ρg, where g is the acceleration due to gravity.

Specific volume – This is the volume of the fluid per unit weight. This is the reciprocal of specific weight.

Specific gravity (S) – This is the ratio of the specific mass of the fluid to the specific mass of water at 4°C. This is also equal to the ratio of specific weight of the fluid to the specific weight of water.