What is Laminar Flow and Turbulent Flow?

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 * Dh/u

Re – Reynolds Number (no dimensions)

V – Velocity (feet/sec or meters/sec)

Dh – 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.

Turbulent to Laminar Flows

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

Super Air Knife Provides Tension with Fine Adjustment for a Lightweight Plastic Film

A company had a small converting machine that was winding a plastic film onto a roll. The width of the plastic film was only 3” across, and the amount of tension required for a consistent roll was small. The maximum amount of tension without damaging the plastic film was 16 ounces of force.  In converting media onto rolls, it is very important to control the tension on the web to reduce defects like wrinkles, out-of-round rolls, or stretching.

They explained the setup that they were trying. They had a 4” manifold with two 2” wide “duck-foot” nozzles attached.  They sent a hand drawing to better describe what they were using. (See below).  The issue that they were seeing was too much variation in the blowing force being applied to the film.  To get near the correct blowing force, they had to start at an air pressure of about 18 PSIG.  As they ran the process, the operator would have to adjust the pressure continuously to evenly roll the film onto the core.  The process was out of control, and they wondered if EXAIR had a better way to evenly exert this force.

Dual Flat Nozzle Manifold
Dual Flat Nozzle Manifold

In analyzing the drawing and their setup, I noticed a couple of things that could cause the variations. I modified his drawing to better explain the situation (Reference below).  As compressed air leaves the two flat nozzles, the center section will overlap.  This overlap will cause turbulence in the air flow pattern.  In order to get an even distribution of forces across the width of the product, turbulence cannot exist.  Turbulence is a mixing pattern where the velocity is not linear; thus, causing high and low pressure points on the target.  The other thing that I noticed was the low air pressure that they could not go above.  This limited the precision of the incremental forces.  Because of the fixed openings of the two nozzles, they had to have a ceiling with the air pressure at 18 PSIG for 16 ounces of force.  If they had to “bump” the force level, the change was difficult to hit exactly.  If we divided the 16 ounces of force between 0 – 18 PSIG, we would get roughly 0.9 ounce of force per PSIG.  You lose the accuracy to make fine adjustments.

Overlap of air flow pattern
Overlap of air flow pattern

I recommended our model 110003, 3” Super Air Knife and a model 110303 Shim Set. The Super Air Knife blows compressed air across the entire length.  Without any overlap, the flow is laminar, and the velocity profile is moving in the same direction.  Thus, an even force across the entire 3 inches.  The Shim Set comes with additional shim thicknesses of 0.001”, 0.003”, and 0.004” thick (the standard thickness of 0.002” is installed in the Super Air Knife). In working with such a precise force requirement, they needed additional options for more control.  They could change the shims as a coarse adjustment and adjust their pressure regulator as a fine adjustment.  This combination gave them the best results to accurately dial in the correct force and not damage the material.  With the maximum requirement of 16 ounces across 3 inches of film, they were able to change the shim to the 0.004” thickness.  For the model 110003 Super Air Knife, it put them at a maximum pressure of 86 PSIG, not 18 PSIG.  Thus the increment was now 0 – 86 PSIG for 16 ounces of force, or 0.19 ounces per PSIG.  There was much more resolution to make smaller changes to the force levels thus optimizing their adjustment range.

Super Air Knife with Shim Set
Super Air Knife with Shim Set

In replacing the competitor’s product with a Super Air Knife, our customer had all the necessary control to wrap rolls of film without issue. The setup with the nozzles on a manifold design resulted in turbulence, which was noisy and produced inconsistent results.  It also restricted their adjustment resolution in changing forces, as they do not use shims.  If you would like to exert a greater degree of precision blowing with products like the Super Air Knife, please contact us. We would be happy to discuss your application and help you meet such goals.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb