The Donaldson trailing edge, no more fin hum!
At last, I have found a good video explaining how to suppress the vibration of fins and foils, and giving the name of this technique : the Donaldson trailing edge, described in 1956 by ... Donaldson
Up to now, I had only seen the diagrams for symmetrical foils (just sand only one side, not both) but this video goes into the details and show that you must sand the inner face of asymmetrical foil (flat one), not the curved one.
Thanks to Mario Legenstein from Levitaz for the video:
I thus also learned that the effect causing the humming is "Vortex Shedding", aka the "Von Kármán effect". On thick trailing edges, vortexes are created alternatively on the two sides of the trailing edge, and they push and pull on it at a frequency depending of the speed and the edge geometry. These alternating forces make the foil or fin vibrate and hum.
You can google "Donaldson trailing edge" to find many detailed references. For instance:
https://iopscience.iop.org/article/10.1088/1755-1315/22/3/032045 https://pdfs.semanticscholar.org/6c26/a77b5f59145758ad1fa3ab72f179bb2601cd.pdf
And these wind tinnel images show the alternating vortexes creating the vibrations on a blunt edge on the left, and how the Donaldson edge locks the vortex in place on the right:
Here is a comparison of the amount of vibrations for different shapes of the trailing edge:
This von Kármán effect is what makes ropes whistle in the wind, can destroy underwater pipelines, and can destroy bridges when they vibrate in sync with the wind, such as the famous Tacoma bridge: https://aerospaceengineeringblog.com/the-von-karman-vortex-street-and-tacoma-narrows-disaster/
Spiral ridges are added to chimneys to prevent the effect: https://www.spartaengineering.com/vortex-shedding-and-tall-structures/
And it happens also at wide scale, and can be seen from space:
- https://earthsky.org/earth/these-are-von-karman-vortices
- https://earthobservatory.nasa.gov/images/90734/two-views-of-von-karman-vortices
- https://oceancolor.gsfc.nasa.gov/outreach/ocsciencefocus/VariousViewsofvonKarmanVortices_o.pdf
Note: I first posted this article on: https://www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/The-definitive-solution-to-fins-and-foils-whistling-humming--The--Donaldson-trailing-edge-_
Page history:
- 2020-12-14 The page has now the English version only, the French has been moved to its own page: TheDonaldsonTrailingEdgeFR
- 2020-07-26 First version of the page, that mixed the English and French texts around the video and images