AC75 : How to get rid of the additionnal parasitic drag when foiling ?

Yes. quite a wide range of conditions have been covered, and a lot of results are presented, as well as the method of analysis used. A few observations can be made about the numerical approach, but nevertheless, a very interesting background has been used as reference, and by itself, the whole study do represent quite well how does look like a simulation loop in CFD. It lacks, however, a few definitions : Forces & Moment computations, as well as the Wall Shear Stress computation.

I had a very similar approach, in order to make the small study that I present in this thread. 1. Find polars / 2. Extract Navigations conditions / 3. Set a numerical model up / 4. Extract Aerodynamic coefficients and Efforts / 5. Analyse & Comment.
With regards with my own work, I find interesting this flow visualization, which shows the slot effect between the wings, the "cockpit" and the sail foot, as well as the development of the vortex that forms on the bottom of the hull.


My main remarks would be :
- The boundaries of the domain of computations are not very well defined. As a consequence, in 4.5, the authors couldn't perform a steady calculations and had to realize unsteady simulations, which are time consumming. Being students working with laboratories computers gave them, indeed, this alternative.
- At this scale, it would have been interessant to also put Nestor onboard, since his wetted surface is quite important, relative to the other wetted surfaces of the model.

 

Gosh...It takes me back some years in the past... Thierry Bielak (Windsurf speed record 1991-2004) was a local rider, and I had the chance to talk several times with him, although he was not the kind of man to talk very much. Every bit of his advices are to be found in the article you gave - except the part about the flexibility, as Thierry was a granit block-, so it seems that there is quite a good concensus on how to go fast with a sail. It makes me remember the time when it was like that, with the sail's foot pressed on the deck :

and now, the foot cut and the boom positioning is more like what you showed with the picture of Björn : Foot parallel to the deck.


I understand the endplate effect, but still find no reason why we should put a maximum cord lenght closing the gap, as in the first picture. Intuitively, the second picture makes more sense to me. Flow disturbance are high in the lowest region of the sail, and the lift generated near the plate is very small. Would it be more advantageous use of this endplate effect on the top of the sail, like it is shown for rotary sails ? Do someone know if it has been done on a regular sail ?

I've done several years ago some genetic optimization of a daggerboard for the speed record accross the Atlantic of a racing catamaran -which we broke-. Given a specific surface, the software returned this kind of shape :


And here comes again the gap...

 

a few decades ago I tried a fence at the top of a Squarehead, but it really seemed to need to rotate vertically, and I ran out of $$ before I could get the balance point right- kind of like a balanced rudder - that and the shape of the thing was unfathomable. But that was before computer modeling outside of Boeing, and it was shaking the head of the sail. The things you can discover when you’re actually holding a wing in your hands….

 

Very interessant point here, I think. Did you make this test on a sailing boat or a windsurf ? In the case of a windsurf, the sail and the mast have the same rotation, right ?

 

Yes, the flow goes around the whole boat. Here is a visualization on the longitudinal component of the velocity 1m below the mast step.

 

Windsurfer CRIT 650. If I understand correctly mast and sail must have the same rotation, so the fence will adapt, ideally, to the direction of flow to limit vortical drag and with any luck create forward thrust ?

 

Like a rudder with fences, rotating together around the rudder's stock, It then maybe more straightforward to adapt a fence (winglet), on the top of a windsurf sail than on the fixed-mast of a sailing craft. The idea that I have isn't to create a forward thrust, but to try to minimize the strenght of the tip vortex, and so, the drag induced by the sail. Was it also your idea with the fence on the top of a squarehead ?

For now, sails are flatten and twisted, so that no lift is generated at the tip. It's a very simple and elegant solution, but with the flaw of limiting the lift that can be generated out of a given sail. To put it simple, a sail is tensioned to the limit where you go fetch a smaller sail in your van. Now, my question is, if, instead of using a smaller sail, you put some lead in your pocket, and run with a bigger sail, would you go faster with a sail with an idealized winglet, or without it ?

Also, I understand that there is a balance issue with this concept of fence. Can you sketch your concept and explain a little more how the issue manifests itself ?

 

The main problem was flow moving up (or sometimes down) the sail relative to the angle of the top edge of the fence on top of my of my squarehead sail, rather than across the sail parallel to the squarehead fence, depending on the inclination of the sail tip- rake or heel to lee or windward, or moving the tip longitudinally. (I was also influenced by a sailplane designer named Schumacher (IIRR?) who was messing with angling the wing tip leading edge to create more coherent span wise flow.) Anyway, when the flow was moving up towards the tip (Neil Pryde square head surf RAF, not camber induced) the tip tufts were getting slammed all over the place over the initial fence, and at times the head was shaking. The tufts at the top edge rarely curved around the fence like a tip vortex, or for that matter around the top of the sail without the fence. Since the 650 didn’t have a mast track, there were times when I could not optimize mast rake to get the flow horizontal parallel to the top of the squarehead, which led me to the idea of a fence that would have to rotate vertically to adapt to non parallel flow, otherwise it would be a tiny air brake. No matter what I tried to get it to balance on its own, using various single attachment points for it to swivel on, kind of like a vertical weather vane, it was usually pointing all over the place, so I wound up trying to use bungee chords to control it, but the attachment points and the ends of the wooden fence were tearing up the sailcloth, I tried a soft fence but the foam deformed and let air underneath the fence (among other things), & different thickness sail battens messed with camber, and like the stiff wooden fence tore up the sail cloth. Sewing a flexible fence under sailcloth was beyond me. After a few vi$it’s for sail repair, I gave up. My conclusion was that a fence needs to be rigid (in the wrong dimension ) to work, but that rigidity messes with sail shape- unless the sail is designed with a straight non cambered head, and while that was possible back then, my sailmaker just started laughing at the cost to develop it- and that was before computer designed sails. And then there was weight. I suppose an upside down park avenue boom shaped gaff might work, or a small wishbone with a fence of soft sailcloth on both sides of the top of a square top attached to each arm of the wishbone would work least on the weather side (while the leeward side folds against the wishbone) or a wishbone shaped laterally like a fence (that would be a leeward only fence?) But windage and weight up high, and the aforementioned tiny air brake….

Hope this makes sense. I can make a sketch, I think, if that’ll help.

 

Hi everyone,

Made two calculations in order to quantify the "slot effect", ie the aerodynamic effect of the slot between the deck and the mainsail of the AC75. I guess that one could also get a general idea of the awaited gain in closing this slot. To do that, I made CFD calculations on similar configurations, moving 1% of the mainsail surface from the foot to higher coords. From the results, I observe :

. A negligible effect on the hull drag/lift
. A Lift/Drag ratio of the sailplane increased of about 5%
. A net increase of the driving force of about 20%

These results confirm that, on an AC75, it is of a great importance for the gap, between the mainsail foot and the deck, to be closed. Giving also an order of magnitude of this advantage. Nevertheless, these results do not explain why, on a windsurf, a small gap could be beneficial. As a side note, the vortex created by the bustle can also be clearly seen in the results. Tempting invitation to work more on the flying hull shapes.

 

Without the slot, flying hulls are in more pressure, but have same hull drag as slotted main, if I’m reading things correctly. Non intuitive…. Good stuff!

 

It's freaky instructive, Paul. I wasn't aware that at least one sail designer worked on this subject. Ttests&trials that you've made are quite amazing. I like the idea of a sewn apparatus. I wonder if inflated shapes made out of heavy fabric could limit the added weight...

 

Yes, I totally agree with the non-intuitive aspect of this phenomenon. As I understand, it is a three flows story. As the upstream airflow is coming through the gap between the jib and the mainsail, the leeward flow is divided. One part separates (blue arrows) ,while the other part is sucked by the extrado of the main sail (red arrows). A un-slotted main sucks more air than an slotted one. This airstream forms a strong Görtler vortex on the main extrado, that pushes away the other part of the leeward flow. The windward airstream (green arrows) gently flows along the main intrado of a non slotted main. The gap between the deck and the main foot on a slotted main allows this windward airstream to flow under the mainsail, mixing with the leeward flow, to form an trailing vortex.

From the sail perspective, the drag induced by the trailing vortex is greater than the drag created by the Görtler vortex. But from the hull perspective, the drag generated by the Görtler vortex is equivalent to the drag created by the trailing vortex, because of the balance created by the leeward separated flow. This may partially explains why there could be a advantage in having a slotted main on a windsurf. Since there is no jib, the interference drag generated on the leeward side of the hull could not balanced. As the main clew of a windsurf is aft of the board, the trailing vortex generated by the slot could have non influence on the drag of the surfboard. The influence of the beam of the sailboard could also be investigated, to my opinion.

Figure below : Representation of the airflow (top) around an un-slotted main (bottom) arount a slotted main

 

This is delicious brain food!

Does the raised leeward side deck qualify as a concave wall (at least in Görtler land…)? Do you think the designers are trying to funnel the Görtler Vortexes along the wall? (Is there advantage to that?)

Does boundary layer thickness change on the leeward side between slotted & unslotted?

Since surface roughness might play a part in all this (?), is there some sort of relationship between deck roughness (non skid for crew safety) and Görtler Vortex drag?

I’m wondering if a fenced sail might create lift on the leeward side of the windsurfer board which would make things a bit squirrelly? My experience edging over 30 knots with the sail foot mashed on the deck was that toe pressure control became um, exaggerated…

 

Didn’t think of that! It could work, (maybe filled with hydrogen too (!?)) but what happens to the flow across/up/down the sail when you begin surfing down the back side of a wave or swell, or rake the sail back, unless the shape of the fence was amenable to large changes in aoa…and this is where a little lift in the right direction might occur?

Actually, this is where Gougeon 32 catamaran sailors might have some insight, given the friendly little blimp that lives at the top of their masts…. Calling Russell Brown!

 

From the flow perspective, the sailplane and the deck, can indeed be viewed as a concave surface which amplifies the flow instability. These vortices are shed through the gap between the main and the jib, and it may be difficult to control them. Looking at the deck configuration choosed by the different active teams, I don't see any evident attempt to do so. Just a guess, but there may be no overall advantage here. Indeed, in theory, Görtler vortices do modify the boundary thickness by increasing the level of turbulence. As you may know, turbulence is not always a bad thing (was pretty turbulent as a child...don't know if things have changed by now ...), since it also lowers the forces acting on a surface. Such as : The Drag. - see the example of a golf ball. I cannot remember exactly which brands have commercialized it, but I know it has been tested on surfboards and windsurf too -. On the case of the AC75, it is also possible that playing with the deck roughness is sufficient to obtain this desirable effect, influencing indirectly the Görtler vortex drag. I don't know if any considerations have been put in this subject. In general, this effect is windtunnel tested, not simulated. Since testing is forbidden in the actual rules, and since the overall gain could be small, compared to those obtained by mastering some more prominent and directly controlable.

Quadricepts in fire are so painfull in long down runs... A little assistance would perhaps makes us a little more at ease.