Colas Nahaboo personal site, with discussions about programming code, web and computing topics, surfing and SUPing, and various musings. Zola 2020-07-07T00:00:00+00:00 https://colas.nahaboo.net/tags/foil/atom.xml 2020-07-07T00:00:00+00:00 2020-07-07T00:00:00+00:00 Unknown https://colas.nahaboo.net/surf/the-donaldson-trailing-edge/ <p>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</p> <p>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.</p> <p>Thanks to Mario Legenstein from Levitaz for the video:</p> <iframe width="640" height="360" src="https://www.youtube.com/embed/6rcCk8bAF10" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe> <p>I thus also learned that the effect causing the humming is "Vortex Shedding", aka the "<a rel="noopener external" target="_blank" href="https://en.wikipedia.org/wiki/Vortex_shedding">Von Kármán effect</a>". 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.</p> <img src="2kiQcrx.png"> <p>You can google "Donaldson trailing edge" to find many detailed references. For instance:</p> <p><a href="dhttps://iopscience.iop.org/article/10.1088/1755-1315/22/3/032045">Boundary layer effects on the vortex shedding in a Donaldson-type hydrofoil</a> <br><a rel="noopener external" target="_blank" href="https://pdfs.semanticscholar.org/6c26/a77b5f59145758ad1fa3ab72f179bb2601cd.pdf">Vortex shedding from blunt and oblique trailing edge hydrofoils</a></p> <p>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:</p> <img src="jRjVij5.png"> <p>Here is a comparison of the amount of vibrations for different shapes of the trailing edge:</p> <img src="izQLDCw.jpg"> <p>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: <a rel="noopener external" target="_blank" href="https://aerospaceengineeringblog.com/the-von-karman-vortex-street-and-tacoma-narrows-disaster/">https://aerospaceengineeringblog.com/the-von-karman-vortex-street-and-tacoma-narrows-disaster/</a></p> <p>Spiral ridges are added to chimneys to prevent the effect: <a rel="noopener external" target="_blank" href="https://web.archive.org/web/20190824084241/https://spartaengineering.com/vortex-shedding-and-tall-structures/">https://web.archive.org/web/20190824084241/https://spartaengineering.com/vortex-shedding-and-tall-structures/</a></p> <p>And it happens also at wide scale, and can be seen from space:</p> <ul> <li><a rel="noopener external" target="_blank" href="https://earthsky.org/earth/these-are-von-karman-vortices">https://earthsky.org/earth/these-are-von-karman-vortices</a></li> <li><a rel="noopener external" target="_blank" href="https://earthobservatory.nasa.gov/images/90734/two-views-of-von-karman-vortices">https://earthobservatory.nasa.gov/images/90734/two-views-of-von-karman-vortices</a></li> <li><a rel="noopener external" target="_blank" href="https://oceancolor.gsfc.nasa.gov/outreach/ocsciencefocus/VariousViewsofvonKarmanVortices_o.pdf">https://oceancolor.gsfc.nasa.gov/outreach/ocsciencefocus/VariousViewsofvonKarmanVortices_o.pdf</a></li> </ul> <hr /> <blockquote> <p><strong>Note:</strong> I first posted this article on: <a rel="noopener external" target="_blank" href="https://www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/The-definitive-solution-to-fins-and-foils-whistling-humming--The--Donaldson-trailing-edge-%5C">https://www.seabreeze.com.au/forums/Stand-Up-Paddle/SUP/The-definitive-solution-to-fins-and-foils-whistling-humming--The--Donaldson-trailing-edge-\</a></p> </blockquote>