FLUID MECHANICS : CLASSIFICATION OF FLUIDS ( General )

Fluid Mechanics

INTRODUCTION

Fluid mechanics may be defined as that branch of Engineering-science which deals with the behaviour of fluid under the conditions of rest and motion. The fluid mechanics may be divided into three parts: Statics, kinematics and dynamics. 

Statics.-The study of incompressible fluids under static conditions is called hydrostatics and that dealing with the compressible static gases is termed as aerostatics.

 

Kinematics- It deals with the velocities, accelerations and the patterns of flow only. Forces or energy causing velocity and acceleration are not dealt under this heading.

 

Dynamics- It deals with the relations between velocities, accelerations of fluid with the forces or energy causing them

 

 

FLUID- A fluid may be defined as follows: “A fluid is a substance which is capable of flowing.” or “A fluid is a substance which deforms continuously when subjected to external shearing force.”

A fluid has the following characteristics:

 

1. It has no definite shape of its own, but conforms to the shape of the containing vessel.

 

2. Even a small amount of shear force exerted on a liquid/fluid will cause it to undergo a deformation which continues as long as the force continues to be applied.

 

A fluid may be classified as follows:

 A. (i) Liquid, (ii) Gas, (iii) Vapour.

 

B. (i) Ideal fluids (ii) Real fluids.

 

Liquid 

•  A liquid is a fluid which possesses a definite volume (which varies only slightly with temperature and pressure). 
• Liquids have bulk elastic modulus when under compression and will store up energy in the same manner as a solid. As the contraction of volume of a liquid under compression is extremely small, it is usually ignored and the liquid is assumed to be incompressible. A liquid will withstand a slight amount of tension due to molecular attraction between the particles which will cause an apparent shear resistance, between two adjacent layers. This phenomenon is known as viscosity. 
• All known liquids vaporise at narrow pressures above zero, depending on the temperature.

 

Gas -It possesses no definite volume and is compressible.

 

Vapour -It is a gas whose temperature and pressure are such that it is very near the liquid state 

Ideal fluids- An ideal fluid is one which has no viscosity and surface tension and is incompressible. In true sense no such fluid exists in nature. However fluids which have low viscosities such as water and air can be treated as ideal fluids under certain conditions. The assumption of ideal fluids helps in simplifying the mathematical analysis.

Real fluids- A real practical fluid is one which has viscosity, surface tension and compressibility in addition to the density. The real fluids are actually available in nature.

 

Continuum- A continuous and homogeneous medium is called continuum. From the continuum view point, the overall properties and behaviour of fluids can be studied without regard for its atomic and molecular structure