You iterate to find out what new parts of the design space you have opened up and what you can do with with this new space, exactly as we have done and are continuing to do with camber. Benchmarking your new iterations against both your previous designs and whatever else out there you choose as performance references.
If subdivision and perturbation analysis, such as the try with and without tubercles approach, actually worked, we would have a complete model of foil performance and traditional optimisation tools would drop out the set of optimal designs for a given application without the need for exploring the design space empirically or numerically. Foil design is a tightly coupled problem, this type of analysis doesnt work, and so we have to keep exploring and iterating and comparing the lastest iterations from different corners of the splution space against each other.
? What is your explanation for how foil design in every arena has progressed then? It has gone exactly as I’ve outlined for everything from Moths to AC75s to foiling board sports. Are all of these engineers and designers, some on teams spending millions of dollars on this stuff just doing it wrong in your view?
@Dontsink please more carefully read what has been written, your theory is very shallow, understanding even more so. This point below is the critical one (also why foil design goes beyond non-scientist), ChatGPT can elaborate (or read this).
Back to building a theory of “why” they/he keeps using them, at a real cost to their manufacturing:
Is it for Hard carves?
I don’t have much of an idea around how high the Angle of Attack (AoA) in carves actually is. I feel like AoA is possibly not that important in carves, or for high speed stability.
I can fully appreciate the feeling for low speed pumping in aerated water, where one has to be very careful not to let the AoA get too high at any point.
When doing a hard carve, it is exactly the opposite. I know that I can push that foil as hard as I possibly want, and that inertia will accelerate me and so I can drive the foil through the water even faster.
So, I’m not that sure these tubercles have much impact at high speed, based on the idea that the foil is probably quite flat during high speed carves, and if it isn’t (in an extremely tight pocket carve link turn), then inertia will accelerate the foil.
Questions left with:
What do they do at high speed?
Is the high AoA ventilation prevention that useful for carves?
My intuition / current understanding is probably not much / not really for these questions.
I currently resonate the most strongly with the idea that they create “fences” preventing ventilation, increasing stall recovery, and helping with tip breach performance.
Here is why:
First - what they do
Key points, the following appears to be true:
Tubercles prevent stalls and ventilation from moving inward from the tips
At a higher Angle of Attack, the effect is stronger (citation?)
Second - what the designer has done
You will also notice that gen 1 Kujira (bottom right) had the tubercles on the wing tips, and were relatively smaller. All subsequent foils have them starting 1/3 from the tips, and concentrated around the primary lifting area.
This change, to me, is the best clue as to why they have persisted, and what the purpose is:
They are creating a strong series of “fences” near the primary lifting area (the middle)
This preventing as much as possible the onset of flow separation
If ventilated, reduce the persistence of it in that area (to recover from a full stall breach),
This effect is more pronounced at high AoA (or through turbulent water?)
I’ve also noticed that on the Enduro 1300, I can recover from a very flat breach more often than not. The wingtip ventilates, I drop down a bit, but if I anticipate it, I can catch it easily. Compare to 1201, where this was mostly impossible to recover. Similar on Ultra and Silk, but to less directly comparable degree.
What you want to do (as a designer) is stop or slow that stall from “spreading” down the wing (spanwise flow). In planes, stalls are designed to start at the inside as you have control surfaces at the tips, which we conveniently don’t have (yet).
Here is a series of pictures and conclusiong comparing big vs small. Clearly they need to be big enough to do anything. I wonder if this speaks to the reason why they have got bigger and are centred on the foil span.
@Matt, nice summary and additions! Trying to find a good reference for the angle of attack sensitivity. Many papers touch on the vortices created by the tubercles increasing in intensity with angle of attack, but don’t spell out the consequences of this as a form of momentum injection into the boundary layer. Will update this post if I find a nice reference.
I have learned in my search that one of the early papers on tubercles by Watts already proposed the fence effect of tubercles and associated effects on spanwise flow as a main effect, not a secondary effect. So, I have to update my assertion that the fence effect is a secondary one, as it is not even in mainstream fluid dynamics, and as with the rudder example the fence effect does seem to be a key one for board sports.
Digging a bit deeper, this paper probably has the best overview I have found of recent (2016) understanding of tubercles and it is pretty clear in pointing out we don’t know with any certainty how they do what they do beyond the vortex generation: https://doi.org/10.1061/(ASCE)AS.1943-5525.0000491
Unfortunately it isn’t open access, I reccommend SciDB for those that are interested.
Although you can science the shit out of some stuff some shapers possess a certain magic dust and a subtle touch that makes their products ride with a brilliance beyond what numbers alone could ever predict.
For me that is the only explanation why they feel so good to me.
Hi Matt! I see your point here - but I would say in my experience having tested the PURE and Enduros/Ultra - that the point of not having the greatest top end high speed is valid.
The ultra in this case is not a racing foil - it’s a downwind glider more focused on reaching longer distances and better avg times thru better avg speed handling - keeping the max glide will give you higher avg speeds more more time - but not necesarily better top end speed, here I would say the PURE 800 would go with better top end speeds but unfortunately in downwind conditions - for less sustained lapses of time -
I think they meant for racing - wing or kite racing - which can keep powered by a more controlled input of energy more consistently - no tubercles foil from my quiver will be faster or better for top speeds than the PURE 700 or 900 for example -
A carve requires a lot more lift than a forward glide at the same speed. The carve is “pulling g’s” - if we’re leaned over turning at 45deg then the foil is producing 1.7x more lift than a forward glide that only requires to produce lift equal to our weight.
At any given speed, if you want to increase lift, then the only way is to increase angle of attack.
So, yeah - angle of attack will be maximum in a carve for any given speed.
We’re going off topic here, but not sure i agree with this. Even though the theory is correct if you assume “same given speed”.
My experience is that speed (and therefore exponentially lift) increases significantly in carves, especially down steeper face/bump. A higher AoA in a 45deg banking turn would require you to ride the board nose up relative to surface and that’s a recipe for a foil breach.
If the idea is to stop ventilation progressing along the foil, maybe the mast should have tubercles and not the wings? Wings wont vent unless the breach.
Now was that for me the only mast that was freq ventilating.
With the UHM80 maybe twice a year instead of several times a sess with the Classic carbon cedrus.
