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Definition of fluid, uniform, non uniform, steady and non steady flow

Fluid flow: When a fluid flows past a point or through a path different parameters associated with the flow of the fluid, certain parameters vary and others may remain constant

When a fluid flows past a point or through a path different parameters associated with the flow of the fluid vary in different patterns. In this article we will study classification of fluid flow in different types according to the conditional variations of the flow parameters with space and time.

The two basic parameters of any fluid flow are velocity of the fluid particle or element and the pressure of the fluid at the point under consideration. The flow of fluids can be classified in different patterns based on the variation of the flow parameters with time and distance. The benefit of characterizing the fluid flow as certain patterns helps in analyzing it under the appropriate solution paradigm.

    Classification Based on Variation with Time

The classification of the fluid flow based on the variation of the fluid flow parameters with time characterizes the flow in two categories,

  1. Steady flow and
  2. Unsteady flow.

Steady flow: If the flow parameters, such as velocity, pressure, density and discharge do not vary with time or are independent of time then the flow is steady.

Unsteady flow: If the flow parameters , such as velocity, pressure, density and discharge vary with time then the flow is categorized as unsteady.

 

In real conditions it is very rare to have such flows with parameters exactly constant with time. The parameters usually vary with time but variation is within a small range such as the average of particular parameter is constant for certain duration of time.

Classification Based on Variation with Space

 

The other classification criterion for the fluid flow is based on the variation of the flow parameters with distance or space. It characterizes the flow as,

  1. Uniform flow and
  2. Non-uniform flow.

Uniform flow: The fluid flow is a uniform flow if the flow parameters remain constant with distance along the flow path.

Or, flow of a fluid in which each particle moves along its line of flow with constant speed and in which the cross section of each stream tube remains unchanged.

Non-uniform flow : The fluid flow is non-uniform if the flow parameters vary and are different at different points on the flow path.

Or, Fluid flow which does not have the same velocity at all points in a medium, at a given instant.

Example:  For a uniform flow, by its definition, the area of the cross section of the flow should remain constant. So a fitting example of the uniform flow is the flow of a liquid thorough a pipeline of constant diameter. And contrary to this the flow through a pipeline of variable diameter would be necessarily non-uniform.

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