The real behaviour of fluids flowing is well described by an experiment carried out by Reynolds in 1883. He set up the following apparatus:

The discharge is controlled by the valve and the small ‘filament’ of dye (practically a streamline) indicates the behaviour of the flow. By changing the flow Reynolds noticed:

·         At low flows/velocities the filament remained intact and almost straight. This type of flow is known as laminar flow, and the experiment looks like this:

·       At higher flows the filament began to oscillate. This is called transitional flow and the experiment looks like:

·      Lastly, for even higher flows again, the filament is found to break up completely and gets diffused over the full cross-section. This is known as turbulent flow:

·         Re 2000 < : gives laminar flow;

·         2000 Re 4000 < < : transitional flow;

·         Re 4000 > : turbulent flow.

These values are only a rough guide however. Laminar flows have been found at Reynolds Numbers far beyond even 4000.

For example, if we consider a garden hose of 15 mm diameter then the limiting average velocity for laminar flow is:

Characteristics of Flow Types

For laminar flow:

·         Re < 2000;

·         ‘low’ velocity;

·         Dye does not mix with water;

·         Fluid particles move in straight lines;

·         Simple mathematical analysis possible;

·         Rare in practical water systems.

Transitional flow

·         2000 < Re < 4000

·         ‘medium’ velocity

·         Filament oscillates and mixes slightly.

Turbulent flow

·         Re > 4000;

·         ‘high’ velocity;

·         Dye mixes rapidly and completely;

·         Particle paths completely irregular;

·         Average motion is in the direction of the flow;

·         Mathematical analysis very difficult – experimental measures are used;

·        Most common type of flow.