How it works: laminar flow

What is the difference between turbulent and laminar flow?

Flow is described as laminar when all the fluid (gas or liquid) flows in layers. It is turbulent when the fluid swirls and mixes across the diameter of the pipe.

Ensuring that flow stays laminar changes the relationship between flow and pressure, which Alicat uses to make our flow instruments highly accurate across a very wide turndown range.

The types of flow

Gas and liquid flows (fluid flows) can be described as being in one of three states; turbulent, transitional, or laminar.

  • Turbulent flow through a pipe

    Turbulent flow through a pipe

    Turbulent flow is by nature chaotic. The fluid mixes irregularly during turbulent flow. Constant changes in the flow’s behavior (wakes, vortexes, eddies) make flow rates difficult, if not impossible, to accurately measure. Turbulent flow usually occurs at high flow rates and/or in larger diameter pipes. Turbulent flow is usually desirable when solids must remain suspended in the fluid to prevent settling or blockages.

  • transitional flow through a pipe

    Transitional flow through a pipe

    Transitional flow exhibits characteristics of both laminar and turbulent flow, depending where the fluid is within the cross-section of the pipe. The edges of the fluid against the walls of the pipe flow in a laminar state, but the center of the fluid flow remains turbulent. More or less of the flow will be turbulent or laminar, depending on how near the conditions are to fully laminar or fully turbulent. Like turbulent flows, transitional flows are difficult, if not impossible, to accurately measure.

  • Laminar flow through a pipe

    Laminar flow through a pipe

    Laminar flow (smooth flow) tends to occur at lower flow rates through smaller pipes. In essence, the fluid particles flow in cylinders. The outermost cylinder, touching the pipe wall, does not move due to viscosity. The next cylinder flows against the unmoving fluid cylinder, which exhibits less frictional “pull” than the pipe wall. This cylinder will move the slowest. This continues, with the centermost cylinder having the greatest velocity

Reynolds number

How do you know if a flow is turbulent, transitional or laminar? In the late 1800’s, Osbourne Reynolds discovered that the type of fluid flow is related to the fluid’s density, mean velocity and absolute viscosity, plus the diameter of the pipe through which the fluid is flowing. The Reynolds Number (Re) is a dimensionless number (having no units) that helps us predict the flow type under a certain set of conditions.

In simple terms, the Reynolds Number (Re) can be written as:

Re = ρVD/η


ρ = Fluid density
V = Mean velocity
D = Pipe diameter
η = Absolute viscosity of the fluid

It is generally accepted that flow is laminar if the Reynolds Number is less than 2000. Transitional flows have a Reynolds Number between 2000 and 4000. Flows are considered turbulent when the Reynolds Number is greater than 4000.

Using the Reynolds equation, we can see that reducing the density, mean velocity and/or pipe diameter for a turbulent fluid flow will move it towards laminar flow. This can also be accomplished by increasing the fluid viscosity.

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