Fluid Mechanics – Boundary Layer

Fluid mechanics is the field that studies the properties of fluids in various states.  Fluid dynamics studies the forces on 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; laminar, transitional, or turbulent.  For compressed air, a value of Re < 2300 will indicate a laminar air flow while the value of Re > 4000 will be in the range of 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 can 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 in a turbine.  In this blog, I will target the boundary layer inside pipes, tubes, and hoses that are used to transport 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 thickness 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.  Mathematically, laminar flow equations can be calculated, but turbulent flows require theories and experimental data to determine. 

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.     

Hot Tap DFM

In a compressed air system, similar things happen within the piping scheme.  Valves, tees, elbows, pipe reducers, filters, etc. are common items that will disrupt the flow.  Let’s look at a scenario with the EXAIR Digital Flowmeters.  In the instruction manual, we require the flow 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. 

Why is this important to know?  In many compressed air applications, the laminar region is the best flow 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 accurately 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

Photo: Smoke by SkitterphotoPixabay license

Communication & Connection Is Critical

I’ve heard it a thousand times… Communication is the key to success. No matter what avenue of industry, work, or even personal life you are in, this statement rings true. At home, communication between my wife and I, as well as our network of friends is always the easiest thing to forego and not want to spend time on. Once this easy path is chosen the work kicks in because one side of the team doesn’t know what the other side of the team is doing.  Most of the time this works for us, when it doesn’t I quickly realize it would have been a better solution to discuss everything rather than assume or just make a vague attempt at what I think we need to do.

My Rucking Community

As for the network of friends, one of the best things I have learned is, we are not alone… Chances are, if you enjoy doing something or talking about certain topics, even if you are struggling, there is someone, you probably even pass them every day and don’t know. Heck, I even found a group of people that like to get outside of their comfort zones and exhaust themselves physically and mentally through rucking. The fact is, my network of friends is like my council on tough decisions or even daily life recaps, as humans we need other people and interaction is in our being.

At work, this need for communication is just as important. During times like we are currently experiencing thanks to the pandemic, we may be seeing a worst-case scenario when it comes to communicating since we have split into shifts and moved to remote work.

We have blogged before about our response as a company, we have successfully been ahead of the curve on response and how we handled our staffing as well as social-distancing before these “rules” were put out. The largest hurdle for my team was the separation and not being able to easily discuss together due to separating into two shifts. Sharing applications, or problems customers may be experiencing with each other is one of our strong suits. To be able to collaboratively use our experiences to build the best solutions or see improvements was cut in half.

So how have we been able to keep helping customers the same as before when we are divided among two shifts? In case you can’t guess, it is an abundance of communication. We use every tool available to us every single day to effectively discuss what is going on between shifts as well as seamlessly transition notes so a customer who may need contact with both shifts isn’t re-explaining themselves in the afternoon.  Is this easy? No, in fact, we haven’t performed flawlessly this entire time yet we have always kept one focus at the forefront.

We do not want our customers to experience anything different or have any additional hurdles to getting the product they need to maximize their compressed air operation.  In fact, if you have noticed a change I would love to discuss it with you personally. You see, we can’t improve without evaluating the methods, just like the 6 Steps to Compressed Air Optimization, you have to know where you are starting, then fix the leaks.

Brian Farno
Application Engineer