Boundary Layer: Laminar and Turbulent flow

Fluid mechanics is the field that studies the properties of fluids in various states.  Fluid dynamics studies the forces on a fluid, either as a liquid or a gas, during motion.  Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Re. This number determines the state in which the fluid is moving; either laminar flow, transitional flow, or turbulent flow.  For compressed air, Re < 2300 will have laminar flow while Re > 4000 will have turbulent flow.  Equation 1 below shows the relationship between the inertial forces of the fluid as compared to the viscous forces. 

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)

To dive deeper into this, we will need to examine the boundary layer.  The boundary layer is the area that is near the surface of the object.  This could refer to a wing on an airplane or a blade from a turbine.  In this blog, I will target pipes, tubes, and hoses that are used for transporting fluids.  The profile across the area (reference diagram below) is a velocity gradient.  The boundary layer is the distance from the wall or surface to 99% of the maximum velocity of the fluid stream.  At the surface, the velocity of the fluid is zero because the fluid is in a “no slip” condition.  As we move away from the wall, the velocity starts to increase.  The boundary layer distance measures that area where the velocity is not uniform.  If you reach 99% of the maximum velocity very close to the wall of the pipe, the air flow is turbulent.  If the boundary layer reaches the radius of the pipe, then the velocity is fully developed, or laminar. 

Boundary Layer Concept

The calculation is shown in Equation 2.

Equation 2:

d = 5 * X / (Re1/2)

d – Boundary layer thickness (feet or meter)

X – distance in pipe or on surface (feet or meter)

Re – Reynolds Number (no dimensions) at distance X

This equation can be very beneficial for determining the thickness where the velocity is not uniform along the cross-section.  As an analogy, imagine an expressway as the velocity profile, and the on-ramp as the boundary layer.  If the on-ramp is long and smooth, a car can reach the speed of traffic and merge without disrupting the flow.  This would be considered Laminar Flow.  If the on-ramp is curved but short, the car has to merge into traffic at a much slower speed.  This will disrupt the flow of some of the traffic.  I would consider this as the transitional range.  Now imagine an on-ramp to be very short and perpendicular to the expressway. As the car goes to merge into traffic, it will cause chaos and accidents.  This is what I would consider to be turbulent flow.      

EXAIR Digital Flowmeter

In a compressed air system, similar things happen within the piping scheme.  Valves, tees, elbows, pipe reducers, filters, etc. are common items that will affect the flow.  Let’s look at a scenario with the EXAIR Digital Flowmeters.  In the instruction manual, we require the meter to be placed 30 pipe diameters from any disruptions.  The reason is to get a laminar air flow for accurate flow measurements.  In order to get laminar flow, we need the boundary layer thickness to reach the radius of the pipe.  So, let’s see how that number was calculated.  

Within the piping system, high Reynold’s numbers generate high pressure drops which makes the system inefficient.  For this reason, we should keep Re < 90,000.  As an example, let’s look at the 2” EXAIR Digital Flowmeter.  The maximum flow range is 400 SCFM (standard cubic feet per min).  In looking at Equation 2, the 2” Digital Flowmeter is mounted to a 2” Sch40 pipe with an inner diameter of 2.067” (52.5mm).  The radius of this pipe is 1.0335” (26.2 mm) or 0.086 ft (0.026m).  If we make the Boundary Layer Thickness equal to the radius of the pipe, then we will have laminar flow.  To solve for X which is the distance in the pipe, we can rearrange the terms to:

X = d * (Re)1/2 / 5 = 0.086ft * (90,000)1/2 / 5 = 5.16 ft or 62”

If we look at this number, we will need 62” of pipe to get a laminar air flow for the worse-case condition.  If you know the Re value, then you can change that length of pipe to match it and still get valid flow readings.  From the note above, the Digital Flowmeter will need to be mounted 30 pipe diameters.  So, the pipe diameter is 2.067” and at 30 pipe diameters, we will need to be at 30 * 2.067 = 62”.  So, with any type of common disruptions in the air stream, you will always get good flow data at that distance. 

Why is this important to know?  In many compressed air applications, the laminar region is the best method to generate a strong force efficiently and quietly.  Allowing the compressed air to have a more uniform boundary layer will optimize your compressed air system.  And for the Digital Flowmeter, it helps to measure the flow correctly and consistently.  If you would like to discuss further how to reduce “traffic jams” in your process, an EXAIR Application Engineer will be happy to help you.

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

EXAIR’s Super Air Amplifier Amplification Ratio’s, Explained

Much like the popular song from decades ago that was about “money for nothing”,  EXAIR can provide you with “air for free”.  What we mean by this is that when you choose to use our Super Air Amplifiers, you will produce a large volume of air while only requiring a small amount of compressed air. This is because Air Amplifiers amplify total output flow up to 25 times by entraining (pulling in) ambient air.

So just how does the EXAIR’s Super Air Amplifier do this?   By utilizing our patented design (Patent # 5402938) that incorporates a special shim to maintain the air slots precisely.  The compressed air is released toward the center of the Super Air Amplifier  which creates a constant, high velocity outlet flow across the entire cross sectional area.  This

SAA How It Works

The amplification occurs by entraining most of the ambient air from the back of the Super Air Amplifier. Another small volume of air is added again as the air exits the Super Air Amplifier further increasing the amplification.

SAA Blog 1Super Air Amplifiers that have outlet diameter’s of 3/4″ (19mm), 1 1/4″ (32mm), 2” (51mm) and 4” (102mm) are supplied with a .003” (0.08mm) shim which is ideal for most applications, however there is the optional .006” (.15mm) and .009” (.23mm) if more air volume and force is needed. The 8” (203mm) Super Air Amplifier comes standard with a .009” (.23mm) shim and for increased performance we offer an optional .015” (.39mm).  The chart below explains how to determine the total output flow and air consumption at different operating pressures for each Super Air Amplifier model.

SAA Blog 2

When you need “air for free” or more accurately stated, to get all you can from every SCFM of compressed air you produce, put the EXAIR Super Air Amplifier to work in your facility!

If you would like to discuss the EXAIR Super Air Amplifier or any of EXAIR’s Intelligent Compressed Air® products, give us a call as we would enjoy hearing from you.

Erik Kuhnash
Application Engineer
Send me an email
Find us on the Web 
Like us on Facebook

Customizing Air Amplifiers

EXAIR’s line of Air Amplifiers can be found in a multitude of different applications across the world. They solve problems as simple as blowing debris off parts to exhausting fumes or circulating air. The Air Amplifier comes in two different styles either the Super Air Amplifier or the Adjustable Air Amplifier. Super Air Amplifiers come in a stock Aluminum Body with a diameter that ranges from ¾” to 8”. This differs from the Adjustable Air Amplifier which comes in either type 303 Stainless Steel or Aluminum and are Sized from ¾” to 4”.

The main difference between the Super Air Amplifier and the Adjustable Air Amplifier is the fact the Super Air Amplifier has a shim inside of it that sets the gap for the air flow. The standard shim thickness for the Super Air Amplifier in sizes of 3/4″ to the 4″ is 0.003” which is suitable for most applications. These shims can be exchanged for a thicker shim of thickness of either 0.006″ or 0.009″. The 8″ Super Air Amplifier is the only air amplifier that comes with a standard stock shim of 0.009″ and can be exchanged for a 0.015″ shim if needed.

Flanged Stainless Steel Adjustable Air Amplifier
Sanitary Flanged Adjustable Air Amplifier

Even though there is a wide variety of sizes and materials for the Stock Air Amplifiers they may not meet a customer’s specific application or need. Over the years EXAIR has produced many different custom Air Amplifiers for a customer’s specific need and the images throughout this blog are just a few of what we have done.

High Temp Air Amplifier

• Depending on the environment certain specific materials may be required like the food industry which requires specific Stainless Steel for various applications. One customer had a special PTFE plug made for the Adjustable Air Amplifier to help pull a sticky material through the process. The PTFE helped prevent the material form depositing on the Amplifier.
• For applications were mounting may be an issue, special attachments have been made to assist. For instances were an Amplifier may need to be mounted to a pipe a custom Stainless-Steel Adjustable Air Amplifier with a class 150 raised face flanges.
• Applications that are in a hot environment may require a special high temperature version which has be developed to operate in areas up to 700°F. The High Temperature Air Amplifier was so widely sought after that we turned it into a stock item. It is commonly used in large roto-molds and ovens to circulate air in order to maintain consistent temperatures.

Adjustable Air Amplifier with PTFE Plug Installed

No matter what your application needs are EXAIR will to work with you to create any custom Air Amplifier that fits your specific application needs.

If you have any questions about compressed air systems or want more information on any of EXAIR’s products, give us a call, we have a team of Application Engineers ready to answer your questions and recommend a solution for your applications.

Cody Biehle
Application Engineer
EXAIR Corporation
Visit us on the Web
Follow me on Twitter
Like us on Facebook

Laminar Flow Compared to Turbulent Flow

turbulent vs laminar

Fluid mechanics is the field that studies the properties of fluids in various states.  There are two main areas; fluid statics and fluid dynamics.  Fluid dynamics studies the forces on a fluid, either as a liquid or a gas, during motion.  Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Re. This number determines the state in which the fluid is moving; either laminar flow, transitional flow, or turbulent flow.  Equation 1 below shows the relationship between the inertial forces of the fluid as compared to the viscous forces.

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 it is considered to be turbulent (chaotic and violent).  The area between these two numbers is the transitional area where you can have 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 is loud and disorderly; traveling in different directions, even upstream.  With the high velocity of water coming out of the drain pipe, the inertial forces are greater than the viscous forces of the water.  This indicates turbulent flow with a Reynolds number larger than 4000.  As the water flows into the mouth of the river, 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 laminar flow.  Air is also a fluid, and it will behave in a similar way depending on the Reynolds number.

Turbulent to Laminar Water

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

John Ball
Application Engineer
Email: johnball@exair.com

Twitter: @EXAIR_jb