## 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 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 or turbulent flow.  Equation 1 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 viscosity of the water.  This indicates turbulent flow with a Reynolds number larger than 4000.  As the water flows 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 laminar flow.  Air is also a fluid, and it will behave in a similar 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, 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 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

## EXAIR Super Air Amplifiers Compared to Fans

EXAIR Super Air Amplifiers and fans are designed to move air.  Fans use motors and blades to push the air toward the target.  There are two types, centrifugal fans and axial fans.  Centrifugal fans are also called blowers or “squirrel” cages.  The air enters into the side of the fan and is redirected 90 degrees to the outlet.  The axial fans are box fans, ceiling fans, and industrial fans.  The motor and spindle are attached to blades.  The air enters from directly behind the fan, and the blades “slap” the air forward to the target. The EXAIR Super Air Amplifiers does not have any blades or motors to push the air.  They use a Coanda profile with a patented shim to create a low pressure to draw in the air.   (You can read more about it here: Intelligent Compressed Air: Utilization of the Coanda Effect.)  I will expand a bit more in this blog about how each one performs in moving ambient air.

The reason to move air can vary by application from cooling, drying, cleaning, and conveying.  The more air that can be moved, the better the performance for each of these functions.  With the Super Air Amplifiers and fans, these products can move the air, but what affects air flow?  Velocity, turbulence, and static or back pressure.  As we look at each one, we can start to see the effectiveness within each application.

Velocity is air flow per unit area.  This is the speed at which the air is traveling.  Some fan designs can affect the velocity, like the motor and spindle in the center of the axial fan.  Some of the area is removed from the middle of the flow region.  So, the velocity is very weak in the center.  (Reference diagram below).  With the centrifugal fan, the air velocity has to be redirected and pushed out the exhaust.  The velocity profile is very disoriented and will work against itself within the flow region.  If we look at the EXAIR Super Air Amplifier, the center is open as shown above.  There are no obstructions.  Since we are drawing in the ambient air, the velocity profile is laminar meaning that the flow is even across the entire flow region.  Laminar flow is optimum for a uniform force and effective blowing.

Turbulence is the “action” of the air flow.  If the turbulence is high, the air flow pattern is interrupted and chaotic.  It causes the velocity of the air to decrease quickly.  By the time the air reaches the target, it has low energy and force.  As a result of turbulence, noise levels can become very loud.  With a centrifugal fan or blower, the air is forced to move at a right angle and pushed out through an exhaust port.  This creates a very turbulent air flow.  The axial fan has less turbulence than its counterpart, but the blades still “slap” the air to push it forward.  This disruption in the flow pattern for both fans create turbulence and disarray.  The EXAIR Super Air Amplifier draws the air into the device to generate very little turbulence on the exhaust end.  The flow pattern is consistent, working together in the same direction.  This will allow for more air to reach the target.

Static pressure is important as it relates to the amount of resistance or blockage.  When blowing air through or around products, this resistance will determine the effectiveness and distance for efficient blowing.  To find the maximum resistance, this would be considered at the dead-end pressure.  When the exhaust is totally blocked, the maximum pressure is created.  In an application, the higher the resistance, the less air that can flow through and around to be utilized.  With fans, it is dependent on the blade types, motor size, and RPM.  Since the EXAIR Super Air Amplifiers do not have motors or blades, it is determined by the inlet air pressure.  So, the higher amount of static pressure, the more resistance that the blowing device can handle.

In comparison, I created a table below to show a model 120024 4” Super Air Amplifier against two different types of fans.  The first thing that you notice is the small package area of the model 120024 as compared to the fans that create similar air flows.  The centrifugal fan requires an addition electrical motor which increases the cost and generates a larger footprint.  The reason for the smaller flow area is the laminar air flow that the Super Air Amplifiers generate.  As stated above, the velocity pattern works together in the same direction.  So, a smaller profile can produce a lot more air movement.  In addition, this helps to create a larger static pressure.  Also referenced above, it will move the air much further to do more work.  With high turbulence, the air movement works against itself causing inefficiencies and louder noise levels.

In physics, it is much easier to pull than it is to push.  The same goes for moving air.  Fans are designed to “push” the air and the Super Air Amplifiers are designed to “pull” the air.  This method of pulling makes it simple to create a laminar flow in a small package which is more efficient, effective, and quiet.  Being powered by compressed air, there is no need for electric motors or blades to “push” the air ineffectively.  With the patented shims inside the Super Air Amplifiers, they maximize the amplification by “pulling” in large amounts of ambient air while using less compressed air.  If you want to move away from blower systems or axial fan systems to get better cooling, drying, cleaning, and conveying; you can contact an Application Engineer for more details.

John Ball
Application Engineer
Email: johnball@exair.com