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Fluid mechanics is concerned with the behaviour of liquids and gases at rest and in motion. The proper understanding of mechanics of fluids is important in many branches of engineering: in biomechanics the flow of blood is of interest; ocean currents require a knowledge offluid mechanics; chemical processing of plants require a thorough knowledge offluid mechanics;
 

aeronautical engineers require knowledge of flow of air over the aircraft to reduce drag and increase lift; mechanical engineers require knowledge of fluid propel1ies to design pumps, water turbines, gas turbines and rockets;civil engineers require fluid mechanics to study river currents and erosion; and environmentalists require knowledge of fluid properties for solving pollution problems of air and water to control flood, irrigation channels, etc. There are special ised books on fluid mechanics for each of these areas and therefore this book will present only general properties of fluid flow.

Before we study fluid mechanics let us discuss the dimensions and units that will be used in this book. There are four fundamental dimensions: length, mass, time and temperature. The dimensions of all other quantities can be expressed in terms of fundamental dimensions. For example, Force can be expressed in terms of fundamental dimensions of mass,length and time. is significant. Mach number where compressibility is important in flows over aerofoils in aircraft.

The dimensionless parameters are also useful in design of prototypes from the models and can save a lot of money and effort. For example, a model can be prepared in a laboratory and tested, and predictions can be made of the prototype for large machines with the help of suitable dimensionless parameters. This is usually done in making models of large hydraulic machines used in power stations or in construction of big dams by making suitable models in the laboratory.

In fluid mechanics the pressure results from a normal compressive force acting on an area. The pressure p is defined as force per unit area. In SI units the unit of measurement of pressure is Newtons per square meter (N/m2 ) or Pascal (Pa). Since Pascal is small unit, the pressure is usually referred to in kilo Pascal (kPa) or even in Mega Pascal (M Pa). The standard atmospheric pressure at sea level is 101.3 kPa. The gauge pressure is the pressure recorded by the gauge or manometer. In engineering calculations absolute pressure is used and the conversion from gauge pressure to absolute pressure is carried out using the following equation.

Shear stresses are devel9ped when the fluid is in motion; if the particles of the fluid move relative to each other, so that they have different velocities, causing the original shape of the fluid to become distorted. A fluid at rest has no shearing forces. Usually we are concerned with the flow past a solid boundary.

The fluid in contact with the boundary sticks to it, and therefore will have the same velocity as the boundary. Considering successive layers parallel to the boundary as shown in Fig. 1.2, the velocity of the fluid varies from layer to layer in y-direction.