Classification of Fluid

Classification of Fluid


a) Ideal fluid:

It is hypothetical which represents frictionless flow i.e. fluid without any viscosity. It is also called inviscid fluid. In ideal fluid the internal forces at any internal section are always normal to the section, even during motion. Hence the forces are purely pressure forces.

b) Real fluid:

In a real fluid tangential or shearing force always come into being whenever the motion takes place, thus giving the rise to fluid friction, also known as viscosity.


a) Compressible fluid:

Fluids which will change in volume with the application of force are known as compressible fluid. Gases are compressible.

b) Incompressible fluid:

It implies fluids with constant density i.e. it will not change in volume with the application of force. Though liquids are slightly compressible they are usually assumed to be incompressible.
a) Newtonian fluid: A Newtonian fluid is a fluid whose stress versus strain rate curve is linear and passes through the origin. The constant of proportionality is known as the viscosity.
A simple equation to describe Newtonian fluid behavior is
   t is the shear stress exerted by the fluid [Pa]
   ยต is the fluid viscosity - a constant of proportionality [Pa·s]
is the velocity gradient perpendicular to the direction of shear [ s-1]
For a Newtonian fluid, the viscosity, by definition, depends only on temperature and pressure (and also the chemical composition of the fluid if the fluid is not a pure substance), not on the forces acting upon it.
b) Non-Newtonian fluid: A non-Newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many polymer solutions and molten polymers are non-Newtonian fluids. In a non-Newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent.


The viscosity of a fluid is a measure of its resistance to shear or angular deformation. The friction forces in fluid flow result from the cohesion &momentum interchange between molecules in fluid.
Viscosity depends on temperature. But this property is different for liquid & gas. As temperature increases, the viscosities of all liquids decrease, while the viscosities of all gases increase.
Reason: This is because the force of cohesion, which diminishes with temperature, predominates in liquids, while with gases the predominating factor is the interchange of molecules between the layers of different velocities. Thus a rapidly moving molecule shifting into a slower-moving layer tends to speed up later. And a slow-moving molecule entering a fast-moving layer tends to slow down the faster-moving layer. This molecular interchange sets up a shear, or produces a friction force between adjacent layers. Increased molecular activity at higher temperatures causes the velocity of gases to increases with temperature.

Classification of flow

a)Laminar /streamline/ viscous flow:

It occurs when a fluid flows in parallel layers, with no disruption between the layers. The fluid appears to move by the sliding of laminations of infinitesimal thickness relative to the adjacent layer.

b)Turbulent flow:

The main characteristics of turbulent flow is its irregularity, there being no definite frequency, as in wave action, and no observable pattern.
Turbulent flow

a)Steady flow:

A steady flow is one in which all conditions at any point in a stream remain constant with respect to time, but the conditions may be different at different points. True steady flow is found only in laminar flow.

b)Unsteady flow:

An unsteady flow is one in which all conditions at any point in a stream do not remain constant with respect to time.

a)Uniform flow:

A truly uniform flow is one in which the velocity is the same in both magnitude direction at a given instant at every point in the fluid.

b) Non uniform flow:

A non-uniform flow is one in which the velocity is not same in magnitude or direction at a given instant at every point in the fluid.

a) Rotational flow:

It implies the flow where the fluid particles rotate about their own axis.
Rotational flow

b) Irrotational flow :

It implies the flow where the fluid particles do not rotate about their axis, they only retain their orientation.
Irrotational flow

Other Definitions

Path lines:

It is the trace made by a single particle over a period of time.

Stream lines:

Streamlines show the mean directions of a number of particles at the same instant of time. Streamlines are a family of curves that are instantaneously tangent to the velocity vector of the flow.

Streak lines:

Streak lines are the locus of points of all the fluid particles that have passed continuously through a particular spatial point in the past. Dye steadily injected into the fluid at a fixed point extends along a streak line. These can be thought of as a "recording" of the path a fluid element in the flow takes over a certain period. The direction the path takes will be determined by the streamlines of the fluid at each moment in time.
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