Bumps in rudder

i was just looking at that nice render... what about bumps in rudder ? i've heard something about and I wanted to make some analysis... someone can tell me more ?

 

It's a leading-edge modification which has been introduced by someone as a revolutionary way of increasing the rudder lift and delaying the stall: http://www.gizmag.com/bumpy-whale-fins-set-to-spark-a-revolution-in-aerodynamics/9020/ , inspired by humpback whale's fin tubercles: http://www.asknature.org/product/49269092db17515de04ac92a75f68bb1



Apparently, the articles which present the solution as revolutionary ignore the existance of the wing-mounted vortex-generators, which have been in use in aeronautics for decades - to assure that the airflow remains attached over a wing surface at high angles - thus increasing the maximum Cl and delaying stall. So, that's imo what you essentially see there - yet another of possible shapes of leading-edge-mounted vortex generators.

It has been discussed to some extent in this thread too: http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-29.html#post451704 - from the post #429 on.

Cheers

 

Thank you very much I think i'll made some analysis for next project ;-)

 

Presumably a pain to build and to keep clean, however, it is possible that you get some benefit. However, I am very skeptical until I see some research by a respected organisation which actually shows a benefit at the Reynolds numbers we deal with in yacht design. This is an interesting article, and it highlights the reynolds number dependence of this addition. http://www.google.com/url?sa=t&rct=...sg=AFQjCNFTVOdgPal-KX7Ye6TYIMt7ohPzBg&cad=rja

Cheers,

Tim B.

 

I know of 2 cases where pitted rather than a flat surface helps I don't know anything about advantages for a bumped surface?

1) The golf ball.

2) A new truck paneling system that like a golf ball has semi-circular pits all along the panels. I think they claim a fuel economy increase of 20 % or more?

A well polished sailboat will have 2/3 or more turbulent flow over the hull.
I guess (but I'm too lazy to look it up) that these pits absorb the turbulent flow allowing linear flow from the back edge of each pit?
I assume with good geometry you could limit the turbulent flow to the pit areas and have linear flow over the rest of the surface?
However I'm not sure you get the same results with water rather than air.
It would be an interesting experiment e.g.:
Does it only work with a compressible gas?
How should the size of the pits be scaled given the difference in densities of the 2 mediums?

Colin
http://www.cg.xbug.ca

 

The bumped leading edge concept is currently being tested by a few firms, but the biggest concern, is the limited range in which they operate efficiently. If sailboats operated at one speed, then the shapes could be optimized for that speed, but since this isn't the case, they wander in and out of their efficiency range, with a net lose except on narrow performance envelop craft (racers).

Dimples aren't as effective as bumps, but again both have a limited range of usefulness. I was friends with the guy that invented the bumped golf ball. He was a DuPont engineer, who in the 60's developed the bumped ball, which promptly made every PGA course in the world too short. The ball was banned for that reason, though it was a better approach to laminar flow manipulation.

Vortice generators are something that's been played with too, but again and unfortunately, sailboats tend to operate in a widely variable set of conditions, which can limit usefulness of these too. There will come a time, likely not too far off where automatically adjustable vortice control will exist. Computer controlled surfaces dominate high preformance aircraft, so it's logical for this to also happen in sail, but the big difference is on aircraft, the flow is relatively constant, so computer manipulation is relatively easy, with some attitude sensors. Sailboats have continuously changing conditions, much more so than aircraft, so "keeping up" with control surfaces will be much more difficult.