# Laminar and turbulent flow

In the case of a laminar flow A in pipes, fluid particles move in parallel in layers without mixing with each other. The velocity distribution of the fluid in the pipe is non-uniform. The fluid is decelerated in the boundary zone due to the pipe friction and moves more slowly than in the centre of the pipe. The pressure loss is proportional to the mean fluid velocity. In practice, a fully developed laminar flow is rare.

In the case of turbulent flow B the individual fluid layers swirl and exchange energy. The resulting flow field is characterised by three-dimensional, unpredictable and transient movements of the fluid particles. In some cases a laminar boundary layer remains in the boundary zone of the pipe. The velocity distribution is nearly constant over a wide range of the pipe cross-section. In contrast to laminar flow, the pressure loss is proportional to the square of the mean fluid velocity. ### Reynolds number

The distinction between laminar and turbulent flow can be determined using the Reynolds number Re. The Reynolds number is a dimensionless fi gure. A Reynolds number up to approximately 2300 refers to laminar flow.

Above a Reynolds number of 2300 the flow is known as turbulent flow. Flows with the same Reynolds number are comparable in their behaviour.

In flow through pipes the Reynolds number Re can be calculated from the inner diameter of the pipe d, the mean fluid velocity v and the kinematic viscosity ν. This tutorial explains three different types of flow:

• laminar flow (ordered),

• transitional flow, and

• turbulent flow (disordered).

It also briefly covers some of the factors that influences how a liquid will flow, including liquid velocity, viscosity, and density, and pipe diameter - and how these are combined in Reynolds number. If Reynolds number is less than 2000, the liquid is likely to be flowing in laminar flow; over 4000 it is likely flowing in turbulent flow.