In fluid dynamics, the movement of a fluid (which may be water or air) over a solid object may become incapable of following the object’s surface. As the flow shifts away from the object a “separation bubble” is created, in which circulating regions of air or water move divorced from the rest of the flow. Eventually, these vortices break away and move downstream in the flow. This phenomenon is called “vortex shedding.”
Then things get exciting.
These vortices, by their nature, exert less pressure on the solid than the rest of the flow. As the vortices move down the object they introduce a repeating pattern of higher and lower pressure at a constant frequency, also known as a Kármán vortex street. The force from this oscillating pressure is perpendicular to the direction of the flow, while the drag force of the object is parallel to the flow. Assuming the solid object doesn’t deform, the force due to vortex shedding will vary at a constant frequency which can cause the object to vibrate [1].
This vortex-induced vibration can result in potentially large movements of the object and loud noise, particularly if the frequency of vibration is close to the object’s natural frequency causing harmonic oscillations.
Vibrating to Destruction
Vortex shedding was the cause for the demise of the VertiGo ride, initially installed at Cedar Point in Ohio and Knott’s Berry Farm in California. The ride, which consisted of three 265-foot (81 meters) towers in a triangular arrangement was designed to use pneumatic air pressure to launch riders to heights of up to 300 feet.
The ride opened in Ohio in August of 2021 and operated for a few months before closing for the winter. In January of 2002, one of the towers broke off approximately 65 feet up, causing a 200 ft section of the tower to crash to the ground. As the park was closed for the winter at the time, no one witnessed the crash, however, investigators were later able to attribute the cause of the collapse to vortex shedding. This was confirmed several weeks later when one of the remaining towers began oscillating 15 feet (4.5 meters). Both parks removed the ride later that year.
Designing for Vortex Shedding
Any fluid flow around an object can potentially lead to vortex shedding, which in turn can cause excessive vibration and potentially damaging forces. Understanding these forces is important when designing products that will be exposed to flowing fluids or that will move relative to a stationary fluid (air, water, etc.).
The first step in designing a product against applied fluid forces is to develop an understanding of the flow pattern. Here, Computational Fluid Dynamics (CFD) software can help by modeling these forces. For example, Porticos used CFD software to evaluate the design of a panel array mounted on a vertical support pole. The effects of wind on the array needed to be understood before beginning a detailed design of the supporting structure.
The CFD analysis considered wind loads from five different directions to assess the potential impacts of vortex shedding.
Of the wind directions analyzed, the 45° flow direction was observed to generate the largest forces. Using the data to determine the maximum resultant force, a structural Finite Element Analysis (FEA) could be run to identify the maximum stresses on the pole. Finally, this model could be used in a vibration FEA to check that its natural frequency was sufficiently far away from the oscillation frequency observed in the CFD results.
If vortex-induced vibration is determined to be a potential issue, there are several ways to mitigate the problem. One is to specify a different (usually lower) fluid velocity. This is an option when the structure is moving through a fluid, for example, aircraft or watercraft, but not viable when the object is static.
If this option is not practical, one alternative is to change the design so that the product is sheltered from the fluid flow. While this option has merit, the testing must now examine the forces on the new barrier, meaning it may simply move the problem.
A third solution might be to add helical “strakes” to the external geometry in order to prevent the formation of a vortex street [2]. Similarly, a final option may be to design adjustments to make the product stiffer using different materials or geometry. This should adjust the natural frequency of the solid so that it’s sufficiently far away from the frequency of vortex shedding.
Closing Thoughts
Vortex shedding is just one of the multitude of issues to be considered and tested in the product design process. Porticos has years of experience with the product development and testing process. Our engineers have the tools and insights to make your product sing – without destruction.
Sources
[1] Munson, Bruce, et al. Fundamentals of Fluid Mechanics. 7th Ed., John Wiley & Sons Inc., 2013, pages 363 and 520.
[2] DeLancey, Adam. “How to save thermowells from vortex-induced vibrations and mechanical fatigue.” https://www.piprocessinstrumentation.com/instrumentation/temperature-measurement/article/15564048/how-to-save-thermowells-from-vortexinduced-vibrations-and-mechanical-fatigue
