Types of Flow

Incompressible Fluid Flow or Compressible Fluid Flow 

• All fluids are compressible - even water - their density will change as pressure changes. 

• Under steady conditions or small changes in pressure, it is possible to simplify analysis of the flow by assuming it is incompressible and has constant density. 

• As you will appreciate, liquids are quite difficult to compress - so under most steady conditions they are treated as incompressible. 

• In some unsteady conditions very high pressure differences can occur and it is necessary to take these into account - even for liquids - .

•  Gasses, on the contrary, are very easily compressed, it is essential in most cases to treat these as compressible, taking changes in pressure into account.

Steady Flow or Unsteady Flow

It is possible to classify the type of flow which is being 
examined into small number of groups.

If we look at a fluid flowing under normal circumstances - a river for example - the conditions at one point will vary from those at another point (e.g. different velocity) we have non-uniform flow. If the conditions at one point vary as time passes then we have unsteady flow.

Under some circumstances the flow will not be as changeable as this. He following terms describe the states which are used to classify fluid flow:

• Uniform flow: If the flow velocity is the same magnitude and direction at every point in the fluid it is

said to be uniform.

• Non-uniform: If at a given instant, the velocity is not the same at every point the flow is non-uniform.

(In practice, by this definition, every fluid that flows near a solid boundary will be non-uniform - as the fluid at the boundary must take the speed of the boundary, usually zero. However if the size and shape of the of the cross-section of the stream of fluid is constant the flow is considered uniform.)

• Steady: A steady flow is one in which the conditions (velocity, pressure and cross-section) may differ from point to point but DO NOT change with time.

• Unsteady: If at any point in the fluid, the conditions change with time, the flow is described as unsteady. (In practise there is always slight variations in velocity and pressure, but if the average values are constant, the flow is considered steady.

Combining the above we can classify any flow in to one of four type:

1. Steady uniform flow. Conditions do not change with position in the stream or with time. An example is the flow of water in a pipe of constant diameter at constant velocity.

2. Steady non-uniform flow. Conditions change from point to point in the stream but do not change with time. An example is flow in a tapering pipe with constant velocity at the inlet - velocity will change as you move along the length of the pipe toward the exit.

3. Unsteady uniform flow. At a given instant in time the conditions at every point are the same, but will change with time. An example is a pipe of constant diameter connected to a pump pumping at a constant rate which is then switched off.

  

4. Unsteady non-uniform flow. Every condition of the flow may change from point to point and with time at every point. For example waves in a channel.

If you imaging the flow in each of the above classes you may imagine that one class is more complex than another. And this is the case - steady uniform flow is by far the most simple of the four. You will then be pleased to hear that this course is restricted to only this class of flow. We will not be encountering any non-uniform or unsteady effects in any of the examples (except for one or two quasi-time dependent problems which can be treated at steady).

Video for you...

Compressible Flow