I did not realise before that hammerhead sharks also have bumpy leading edges - a 'cephalofoil'.
Certainly a candidate for hydrofoil experimentation.
I made one but apparently I wasn't the first. It is unlikely though that with a hydrofoil you would need the kind of AoA at which the tubercle effect starts to kick in, all busy that you are to not pierce the surface. But to be able to take off at lower speed than a conventional foil then it can be useful, that's the reason I tried it.
I did not realise before that hammerhead sharks also have bumpy leading edges - a 'cephalofoil'.
Certainly a candidate for hydrofoil experimentation.
I made one but apparently I wasn't the first. It is unlikely though that with a hydrofoil you would need the kind of AoA at which the tubercle effect starts to kick in, all busy that you are to not pierce the surface. But to be able to take off at lower speed than a conventional foil then it can be useful, that's the reason I tried it.
Looks awesome!
But it's more of a humpback hydrofoil than a hammerhead, wrt size and number of the tubercles. Maybe it matters? I have no idea.
I decided to have another crack at learning more about fluid dynamics, especially the Reynolds number.
I found some good resources and am starting to develop an understanding of it.
This video is very good and it will help you understand why a longboard glides so much better than a short board, because of length being one factor in the Reynolds number:
Here are the Reynolds numbers for a range of chord lengths used in fins at speeds relating to surfing. Numbers are rounded for simplicity. Usefull when searching for foil profiles on the net.
Chord Length Rn
Speed: 3 m/s = 10.8 km/h = 6.7 mph
10 mm, 0.4 in. 30 600
50 mm, 2 in. 153 000
100 mm, 4 in. 306 000
150 mm, 6 in. 460 000
---------------
Speed: 5m/s = 18 km/h = 11.2 mph
10 mm, 0.4 in. 51 000
50 mm, 2 in. 255 000
100 mm, 4 in. 510 000
150 mm, 6 in. 765 000
---------------
Speed: 10m/s = 36 km/h = 22.4 mph
10 mm, 0.4 in. 100 000
50 mm, 2 in. 510 000
100 mm, 4 in. 1 000 000
150 mm, 6 in. 1 500 000
Edit: These numbers are for water at 20°C or 68°F. At 10°C/50°F you can remove 20% to the RN and 40% at 1°C/33°F just above freezing temperature. Surfers in the warm tropics benefit from more friendly Reynolds numbers ! I wonder if somebody here has surfed the same board with the same fins in the tropics and in really cold water, the difference should be noticable.
So basically the lowest Rn will be at the narrowest part of the fin at your lowest speed and the highest Rn at the widest, often the base of the fin, at your highest speed. Fins that do not taper much like the Gullwhale will have the same reynold number along the depth of the fin for a given speed.
If someone can tell me the max speed of a gun on a huge wave (70 km/h ???), I can add the numbers.
Here are the Reynolds numbers for a range of chord lengths used in fins at speeds relating to surfing. Numbers are rounded for simplicity. Usefull when searching for foil profiles on the net.
Chord Length Rn
Speed: 3 m/s = 10.8 km/h = 6.7 mph
10 mm, 0.4 in. 30 600
50 mm, 2 in. 153 000
100 mm, 4 in. 306 000
150 mm, 6 in. 460 000
---------------
Speed: 5m/s = 18 km/h = 11.2 mph
10 mm, 0.4 in. 51 000
50 mm, 2 in. 255 000
100 mm, 4 in. 510 000
150 mm, 6 in. 765 000
---------------
Speed: 10m/s = 36 km/h = 22.4 mph
10 mm, 0.4 in. 100 000
50 mm, 2 in. 510 000
100 mm, 4 in. 1 000 000
150 mm, 6 in. 1 500 000
So basically the lowest Rn will be the tip of the fin at your lowest speed and the highest Rn the base of the fin at your highest speed. Fins that do not taper much like the Gullwhale will have the same reynold number along the depth of the fin for a given speed.
If someone can tell me the max speed of a gun on a huge wave (70 km/h ???), I can add the numbers.
That sounds like the 'Extended tip' McCoy Gullwing fin is pretty much the opposite of a tapering fin, with highest Rn in the tip area.
So how do you use the Rn's to decide on a foil profile?
That sounds like the 'Extended tip' McCoy Gullwing fin is pretty much the opposite of a tapering fin, with highest Rn in the tip area.
So how do you use the Rn's to decide on a foil profile?
Yes many fins have a long chord near the tip because of the pronouced sweep. I believe that high sweep fins behave like half a delta wing, using vortex lift to delay stall at high AoA, while generating drag for control. Somewhere you had written that you thought that the Gull Wing fin was one big tubercle, which is right because tubercles are small delta wings generating vortexes to dynamise the flow and keep it attached at high AoA. So the normal Dol fin planshape can also be seen as one half of a big tubercle... There are a lot of things going on at the same time, shifting regimes, blending together during the highly changing hydrodynamic situations encountered by a fin. As I said in a previous post, fins are probably one of the most complex applications of foils.
You can search for foils here: In the drop down menu you can choose to order the foils by max lift/drag ratio for a given Rn. As an example I have set it up to look for symetrical foils around 10% thickness that are efficient at low Rn.
I did not realise before that hammerhead sharks also have bumpy leading edges - a 'cephalofoil'.
Certainly a candidate for hydrofoil experimentation.
Screenshot from 2020-06-29 22-31-00.png
I made one but apparently I wasn't the first. It is unlikely though that with a hydrofoil you would need the kind of AoA at which the tubercle effect starts to kick in, all busy that you are to not pierce the surface. But to be able to take off at lower speed than a conventional foil then it can be useful, that's the reason I tried it.
Tubercled foil.jpg
mg_7941.jpg
_____________
We Are One
Looks awesome!
But it's more of a humpback hydrofoil than a hammerhead, wrt size and number of the tubercles. Maybe it matters? I have no idea.
I decided to have another crack at learning more about fluid dynamics, especially the Reynolds number.
I found some good resources and am starting to develop an understanding of it.
This video is very good and it will help you understand why a longboard glides so much better than a short board, because of length being one factor in the Reynolds number:
Physics of Life - Life at Low Reynolds Number - YouTube https://www.youtube.com/watch?v=gZk2bMaqs1E
This video by the same guy is also very good: Physics of Life : The Reynolds number: https://www.youtube.com/watch?v=rtcpMK6NpLo
And I like this short article about the Boundary Layer: https://www.grc.nasa.gov/WWW/BGH/boundlay.html
Here are the Reynolds numbers for a range of chord lengths used in fins at speeds relating to surfing. Numbers are rounded for simplicity. Usefull when searching for foil profiles on the net.
Chord Length Rn
Speed: 3 m/s = 10.8 km/h = 6.7 mph
10 mm, 0.4 in. 30 600
50 mm, 2 in. 153 000
100 mm, 4 in. 306 000
150 mm, 6 in. 460 000
---------------
Speed: 5m/s = 18 km/h = 11.2 mph
10 mm, 0.4 in. 51 000
50 mm, 2 in. 255 000
100 mm, 4 in. 510 000
150 mm, 6 in. 765 000
---------------
Speed: 10m/s = 36 km/h = 22.4 mph
10 mm, 0.4 in. 100 000
50 mm, 2 in. 510 000
100 mm, 4 in. 1 000 000
150 mm, 6 in. 1 500 000
Edit: These numbers are for water at 20°C or 68°F. At 10°C/50°F you can remove 20% to the RN and 40% at 1°C/33°F just above freezing temperature. Surfers in the warm tropics benefit from more friendly Reynolds numbers ! I wonder if somebody here has surfed the same board with the same fins in the tropics and in really cold water, the difference should be noticable.
So basically the lowest Rn will be at the narrowest part of the fin at your lowest speed and the highest Rn at the widest, often the base of the fin, at your highest speed. Fins that do not taper much like the Gullwhale will have the same reynold number along the depth of the fin for a given speed.
If someone can tell me the max speed of a gun on a huge wave (70 km/h ???), I can add the numbers.
_____________
We Are One
That sounds like the 'Extended tip' McCoy Gullwing fin is pretty much the opposite of a tapering fin, with highest Rn in the tip area.
So how do you use the Rn's to decide on a foil profile?
Screenshot from 2020-06-29 22-06-20.png
Yes many fins have a long chord near the tip because of the pronouced sweep. I believe that high sweep fins behave like half a delta wing, using vortex lift to delay stall at high AoA, while generating drag for control. Somewhere you had written that you thought that the Gull Wing fin was one big tubercle, which is right because tubercles are small delta wings generating vortexes to dynamise the flow and keep it attached at high AoA. So the normal Dol fin planshape can also be seen as one half of a big tubercle... There are a lot of things going on at the same time, shifting regimes, blending together during the highly changing hydrodynamic situations encountered by a fin. As I said in a previous post, fins are probably one of the most complex applications of foils.
You can search for foils here: In the drop down menu you can choose to order the foils by max lift/drag ratio for a given Rn. As an example I have set it up to look for symetrical foils around 10% thickness that are efficient at low Rn.
http://www.airfoiltools.com/search/index?MAirfoilSearchForm%5BtextSearch...
_____________
We Are One
Here is a link showing a laminar flow chamber
Starts at 4 min
looks fairly simple to make
https://www.youtube.com/watch?v=y7Hyc3MRKno
The whole of this youtube vid is worth a watch.
Turbulent flow is more awesome than laminar flow.
https://www.youtube.com/watch?v=5zI9sG3pjVU
How cool is the process called periodic vortex shedding
Check out at least the dead fish swimming upstream at 14:37
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