So, you’ve noticed that your valves or piping are shaking, and you don’t know why. If you clicked on this article, you’re probably considering whether vortex shedding is the cause of your vibration issues. Well, you’ve come to the right place! In this article, we’re diving deep into the topic of vortex shedding to help you understand what it is, why it occurs, and how you can fix your vibration problems before they become a real catastrophe for your piping system.
Before we dive in, let’s first unpack a general definition of vortex shedding:
Vortex shedding is a fluid dynamics phenomenon in which fluid flows around a bluff body. This interference causes low-pressure vortices to shed from either side of the body and a low-pressure zone to form in the wake behind the body, giving rise to a fluctuating force perpendicular to the direction of the flow. At high velocities, the opposing forces can cause the body to vibrate at a low frequency.
In simpler terms: Vortex shedding is when fluid flows over an object, creating an oscillating flow. In certain circumstances, this cross-flow can cause the object to vibrate.
The hypnotizing video below provides a good representation of the vortex shedding phenomenon. This video was created during a computational fluid dynamics simulation by one of our CFD specialists at Pi Engineering (more on this later!):
Vortex Shedding in Everyday Life
Although the concept of vortex shedding can seem quite technical, this phenomenon frequently occurs in everyday life. You might have even noticed it when driving down the highway on your way to work this morning.
Car antennas are a great everyday example of fluid (wind) flowing around an object (the antenna). If you’ve ever seen a car antenna bouncing around uncontrollably, vortex shedding was likely the culprit.
Another example of vortex shedding in everyday life involves tall buildings and wind. Buildings with significant height, such as the Burj Khalifa in Dubai (the tallest building in the world), are at considerable risk of flow-induced vibration due to vortex shedding. A variation in cross-section can help these buildings avoid damage caused by severe oscillation. In fact, any tall and narrow structure can be at risk of detrimental vibrations. Tall chimneys made of steel may even oscillate due to strong airflow and vortex shedding.
On a larger scale, the photo below shows atmospheric vortex shedding from an island. It’s interesting to note that the physics behind vortex shedding is universal irrespective of the magnitude of the phenomenon.
What is Vortex Shedding in Piping?
The physics of vortex shedding in piping is exactly the same as wind passing around a car antenna or atmospheric vortex shedding from an island. When you have fluid flowing over an object, it will produce oscillating flow vortices, and those oscillations can cause fluctuations in the pipe. However, it’s worth noting that there are two different ways that pipes can experience vortex shedding: inside the pipe and outside the pipe.
Vortex Shedding Outside the Pipe
Vortex shedding in piping or vessels occurs primarily when high-velocity wind passes over piping, vessels, or tall towers or stacks. The oscillating cross-flow produces forces perpendicular to the flow. If the frequency of the oscillations is similar to the natural mechanical frequency of the piping or stack, it can create resonant vibration problems.
Vortex Shedding Inside the Pipe
Vortex shedding can also occur inside pipes or vessels with internal devices, like a thermowell, injection quill, or sample quill. For example, you may need to measure the temperature of processes within process pipes. Temperature sensors are inserted into thermowells that are perpendicular to the flow within the pipe. When a certain velocity of flow crosses the thermal well, it can sometimes cause the thermowell to shake.
What Causes Vortex Shedding in Piping?
In reality, harmful vortex shedding is not incredibly common. However, there are a few variables that can make your piping more susceptible to fatigue failure and damage due to vortex shedding:
- Bends or t-junctions in piping
- Piping that spans long distances
- High wind velocity
- Internal piping devices
Why Should You Care About Vortex Shedding in Piping?
If you see vibrations occurring in your piping and don’t know the cause, it’s important to take action immediately to avoid detrimental damage to your piping system.
Resonance between the vortex shedding frequency and your pipe geometry can cause shaking through your piping that could cause the piping to break, resulting in the potential shut down of your piping system. Sometimes vortex shedding can occur, and you’ll have no issues at all. However, if the vortex shedding frequency locks into the natural piping frequency, the resonance can cause serious fatigue damage.
Don’t wait for your piping to break before you call an expert!
How Do You Determine The Vortex Shedding Frequency?
Some engineers might be familiar with old school formulas such as the one below that can be used to calculate vortex shedding frequencies (MW Kellog Circa 1975):
However, a more reliable and modern way to determine the frequency and magnitude of vortex shedding is through a computational fluid dynamics (CFD) simulation. Through CFD simulation, you can find accurate measurements of the frequency and pressure fluctuation amplitude. CFD simulations are often more accurate than traditional hand calculations.
For example, at Pi Engineering, a customer came to us with an over-spanned pipe that experienced high-velocity winds. Although the pipe had been there for almost ten years without any issues, one of their engineers performed hand calculations and noted that the vortex shedding frequency was problematic. Our team stepped in and performed a CFD analysis and discovered that there wasn’t actually an issue with the pipe, saving our customer a significant amount of money.
CFD simulations have a range of benefits for piping operators. For example, suppose vortex shedding is upstream from a pump or other piping equipment. In that case, a CFD simulation could tell you how much distance you need to add between the pipe experiencing vortex shedding and the pump/equipment to eliminate resonant vibration.
How Do You Reduce Vortex Shedding in Piping?
Typically, flow-induced vibration is not considered during the design stage because pipe sizing is based on expected operating conditions. However, if you are experiencing resonant vibration problems down the line, there are a few ways that piping engineers can help you reduce vortex shedding in your piping.
Reducing Vortex Shedding Outside the Pipe
One of the best ways to reduce resonant vibration is to add more supports to the piping to increase the stiffness. For example, if you have a pipe with a natural frequency of 1 Hz that spans 20 meters, you could add support in the middle of the piping to increase the natural frequency above 4 Hz so that the piping doesn’t shake in the wind.
Reducing Vortex Shedding Inside the Pipe
We discussed how a thermal well could lead to resonant vibration in the “Vortex Shedding Inside the Pipe” section above. One of the most effective ways to prevent this vibration is to add a helix to the thermowell shape. Instead of a straight, smooth tube, a helix spiraling around the tube can prevent flow-induced vibrations. In fact, the same goes for car antennas! You’ll often see a helix around car antennas that prevents the antenna from vibrating in the wind.
How Pi Engineering Can Help Eliminate Your Resonant Vibration Issues Due to Vortex Shedding
Although CFD is a transformative way to analyze vortex shedding, it is a relatively new type of analysis, and very few piping engineering firms offer this service. At Pi Engineering, we’re proud to offer CFD to our customers to help them address their unexpected resonance vibration issues.
Contact us today if you notice vibrations in your piping. We’ll help you identify the cause of the vibration and the best approach to eliminate your vibration issues before they become detrimental to your piping system. The specialized team at Pi Engineering is here to help you.