types of fluid

Fluid mechanics is one of the major sciences involved in chemical engineering, so a chemical engineer is mostly interested in many aspects of the problems involved in the fluid flow. To understand the fluid mechanics, a chemical engineer must have to understand that what is fluid, and the types of the fluid.

The simplest definition of the fluid is that “Any substance which can flow under pressure is fluid”. Fluid can also be defined as “any substance that has no fixed structure, shape or size, and yields easily to the external pressure”.

There are generally two ways to classify the fluids, i.e.

> Compressible and Incompressible Fluids

> According to viscosity change

Compressible And Incompressible Fluids

The nature of the fluid is said to be compressible or incompressible according to its behavior under applied pressure, i.e.

The incompressible fluid is the one, whose volume is independent of its temperature and pressure, i.e. its volume will not be affected by the change of its temperature and pressure. There is no real fluid, which is completely incompressible, however liquids are assumed to be the incompressible fluids, as they sustain the change of temperature and pressure, more then gases.

The compressible fluids change their volume according to the change in their temperature or pressure. The gases are real example of such type of fluid. However, if the percent change is small, then for practical purposes, a gas may be treated as the compressible fluid.

Classification Of Fluids According To Viscosity Change

The fluids can also be classified according to the effects produced on the fluid by the action of the shear stress. This classification is important, as it determines the way in which the fluid will flow. This classification is based on a most important physical property “viscosity”.

Then main two types under this classification are:

> Newtonian fluid

> Non Newtonian fluid

A Newtonian fluid is the fluid, whose viscosity remains constant regardless of any applied stress. That’s why these fluids are also named as “linear viscous fluids”. The most common example of Newtonian fluid is water. The flow of water remains same, whether it flows alone, or in vigorously agitation condition. Its simplest meaning is that, the fluid will continue to flow, regardless of the forces acting on it. The Newtonian fluids behave according to following equation:

                                                            τ = µ (du/dy)

τ is the shear stress exerted by fluid

µ is fluid viscosity, constant for Newtonian fluids

(du/dy) is the velocity gradient, or the strain.

So for Newtonian fluid, according to the equation, the ratio of stress to strain is constant, there fore, the viscosity is constant.

The viscosity of the non Newtonian fluid is variable, and is dependent upon the applied stress on the fluid. These type of fluids also exhibits the rheological properties .The common examples of non Newtonian fluids are solution of corn starch and water, paints, ink, tooth paste, etc.

Posted in: fluid mechanics