Fluid mechanics is the field that studies the properties of fluids in various states. There are two areas, fluid statics and fluid dynamics. Fluid dynamics studies the forces in a fluid, either as a liquid or a gas, during motion. Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Reyonlds number. This number can indicate the different states that the fluid is moving; either in laminar flow or turbulent flow. The equation below shows the relationship between the inertial forces of the fluid as compared to the viscous forces. Reynolds number, Re, can be calculated by Equation 1:

Equation 1: Re = V * D_{h}/u

Re – Reynolds Number (no dimensions)

V – Velocity (feet/sec or meters/sec)

D_{h} – hydraulic diameter (feet or meters)

u – Kinematic Viscosity (feet^2/sec or meter^2/sec)

The value of Re will mark the region in which the fluid (liquid or gas) is moving. If the Reynolds number, Re, is below 2300, then it is considered to be laminar (streamline and predictable). If Re is greater than 4000, then the fluid is considered to be turbulent (chaotic and violent). The area between these two numbers is called the transitional area where you can have small eddy currents and some non-linear velocities. To better show the differences between each state, I have a picture below that shows water flowing from a drain pipe into a channel. The water in the channel is loud and disorderly; traveling in different directions, even upstream. With the high speed coming from the drain pipe, the inertial forces are greater than the viscous forces of the water. The Reynolds number is larger than 4000 which indicates turbulent flow. As the water travels into the mouth of the river after the channel, the waves transform from a disorderly mess into a more uniform stream. This is the transitional region. A bit further downstream, the stream becomes calm and quiet, flowing in the same direction. This is the laminar flow region where Re is less than 2300. Air, like the water in the picture, is also a fluid, and it will behave exactly in the same way depending on the Reynolds number.

Why is this important to know? In certain applications, one state may be better suited than the other. For mixing, particle suspension and heat transfer; turbulent flows are needed. But, when it comes to effective blowing, lower pressure drops and lower noise levels; laminar flows are required. In many compressed air applications, the laminar flow region is the best area to use compressed air. EXAIR offers a large line of products, including the Super Air Knives and Super Air Nozzles that uses that laminar flow to generate a strong force efficiently and quietly. If you would like to discuss further how laminar flows could benefit your process, an EXAIR Application Engineer will be happy to assist you.

John Ball

Application Engineer

Email: johnball@exair.com

Twitter: @EXAIR_jb