Thank you, for your kind replies and interesting thoughts!

Though I am relatively new to the JRA, it appeared to me pretty soon that the one thing missing (or at least very rare) among us is the possibility to evaluate and put numbers to those ideas that a lot of clever people develop in the JRA. If those wonderful ideas can’t be evaluated and tested, they stay theories and speculation. And that would be an absolute pity! Wind tunnel tests are naturally very expensive, and for a reason: huge machinery involved, very sensitive sensors and data acquisition, lots of people involved. Though CFD is still a huge amount of work, frustration and debugging, it is so much less compared to wind tunnel tests.

One thing: I would like to give advice from my professional experience: we all tend to like the idea of looking at the whole thing. The whole airplane, the whole car, the whole junk rig in 3D. From what I experienced, that is a very human wish. However, there is a lot to be learned from only partial views, like a 2D case. You do not need to simulate the entire car if you want to install a new side mirror :-) Also, the budget is always limited. Always. It would also be limited in case crowdfunding would produce reasonable results. In my opinion, it is best to simulate as simple cases as possible – sticking with the KISS principle. For example: mast balance can be easily investigated in 2D, without relevant compromises. On the other hand: the different sail plan forms, yard angle, batten angle  or the tip vortex would have to be simulated in 3D. Looking at the complete rig in 3D in a CFD simulation is what I, personally, would really love to do. But let’s stick to what we have available right now, 2D, which is already huge and can be used for a vast amount of information.

I’m not so sure if crowdfunding from the JRA members would be sufficient to pay the machine rent necessary. I can only estimate right now, but I think a 3D case simulation would be about 500-2000€ for each run. A run is one data point in my graphs. To produce the graphs of my first post below, I already had to do 70 runs. If done so in 3D, that would be 35000 – 140000 €. Of course, a lot of those data points can be omitted (more to that later), but still. Now imagine doing different velocities on top, different mast balances, etc…

However, I could imagine doing a phd about this topic (in which case I would also publish in peer reviewed journals, Mauro ;-) ). I am actively looking for universities/insitutes to host such a topic – and also pay a little salary to feed me. Most of the technical universities are equipped with quite some computational power, so the costs of machine rent wouldn’t be such an issue. Let’s see if the topic is relevant enough to be of interest. If you have contacts to anyone related, please let me know!

Graeme, Paul: about a JRA CFD/aerodynamic seminar: why not? That would be an interesting group!

Back to the technical part:

Good/bad tack

Graeme, I agree with you. Good questions from your side! Other than with the flat cut sail, I also think that the difference is hardly sensible out on the water with a cambered sail, but maybe measurable. The L/D ratio is not directly translatable to windward performance, other factors like the hull shape might “reduce” (but not turn) the difference between good and bad tack.

Alpha tolerance

Again, Graeme, your explanation is spot on! The sharpness/bluntness of those lines, with respect to the tangent, is the alpha-tolerance. To have it clearer, I made a graph of L/D ratio over angle of attack. Same data, just a different way of plotting it:

The blunter the curves, the more alpha-tolerant the profile. The sharper the hill (the area of max L/D), the more you need to focus while at the helm. The flat cut profile on stb tack definitely has the highest alpha-tolerance compared to the three other curves – however, at a price: it is least efficient. Actually, it’s alpha-tolerance is so good because it stalls almost immediately when sheeting in…

Further calculations

Yes, the mast balance is one of the things I will have a look at. Not sure which results to expect in advance, quite adventurous :-) Also, the SJR, wingsail and aerojunk are on top of my list.

Process Speedup

Those first simulations I did from 0 to +-15°/20° AoA, to get an impression how everything works out. However, to evaluate the performance of the junk rig, we do not need that much data points. Of main interest will be the L/Dmax point, which is around 4-5°, as well as the alpha-tolerance, which is described be the curvature of the L/D over AoA graph. The points of interest of the already finished simulations I did mark in the following graph:

Also, the empty data in between the data points can be easily interpolated by a spline curve. So, instead of calculating every degree from -15° to +15° (30 simulation runs), I would only do every second degree from -8° to -2° and +2° to +8° (8 simulation runs). Quite a speedup! If in the data evaluation phase, after all runs are finished, it appears that it would be good to go i.e. up to 10° or have one further data point necessary between 2° and 4°, it is no problem to execute another run afterwards.

I hope to start a lot of discussions with this topic, some ruffling of feathers but also some calming of feathers. I am very open to all of your input! For example, I would be very interested if one of the SJR/wingsail/aerojunk experts might draw a typical profile curve, which I could then use for simulation. Also, I would be very interested in feedback to the cambered profile I used (and simply made up).

Cheers,

Paul

Edit:

David, you have been faster typing than me :) The flat cut sail develops a detached area almost immediately when sheeting in. I'll post some more material to illustrate this.

I fully agree about your 3D panel shape comment. That's why this CFD data still has to be interpreted wisely, and not taken granted as it is. Do you have any actual shape measurements of the cambered panel at hand?

Yes, I intend to do a magazine article about this. But I would like to discuss things first here, to soften the edges, rule out errors from my side and put together a more round story.

The CFD modeler Paul S said that to go 3D, the costs would have 3 trailing zeros.  Somone else mentioned creating a crowdfunding site.  Well - do it - I would send some money - maybe amongst us all we have to necessary 3 trailng zeros.  And the idea of a Zoom-based seminar on CFD modeling and real world tests of hypotheses generated by the CFD modeling - wow, I would be an enthusiastic participant.

Another Paul S, currently steaming in the late fall Mexican coastal climate of Puerto Vallarta

WOW!!!

It would be great to investigate entire junk rigs in its different forms, this would be a project worth to be financially sustained by a crowdfunding among the community…

Thank you for sharing Paul…

Ps1:….4 degrees to the apparent wind=max L/D: gespeichert!

Ps2: in which peer reviewed journal are you going to publish the full report? ;)

Hi,

I’ve been busy the last weeks with a project I wanted to tackle since tufting woolen strings into Ilvy’s sail in the Swedish archipelago: CFD calculations of junk rigs.

A little bit of background

CFD (computational fluid dynamics) is basically the possibility to have a virtual wind tunnel, simulated and calculated purely by numerical means with a computer. While this may sound high-tec, it absolutely is: Lots of math (solving several differential equations like the famous Navier-Stokes-equations), programming (scripting and coding for multi-core machines/high performance clusters) and data evaluation. I do not want to go into detail here. However, setting up and running those fluiddynamic calculations is one of the things I do in my profession as a shipbuilding engineer. So, I thought I might as well set up a simulation of the junk rig(s). So far, it took me about the same time as sewing a new sail. But it is interesting, and I hope we could learn a lot from the outcome!

How is it done? I define a geometry (junk rig) that I want to simulate, and a “room” around it – wide enough to not interfere with the flow around the geometry. Then, the space inside the room is filled with cells. The cells are arranged in a way to snap exactly to the junk rig and as such resemble its geometry. They are very small close to the surface, and bigger far away. All those cells are called a mesh, and it is a science for itself to create a good mesh. Have a look at a slice of the mesh in the middle of a panel:

For each of those cells about ten equations (some differential) have to be solved, for each time step. The time step has to be very low, about 1e-5 sec, to give accurate results. The simulation is running for 15 sec simulated time. The amount of cells only in that 2D figure above is about 150000. If you do the math, there are quite some numbers to be juggled to solve that simulation. Now, as we live in the fantastic modern times of 2024, my Laptop – I admit, it is a rather strong one with 6 cores – is able to calculate such a 2D slice in about 1-2 hours (running full speed and drawing quite some power).

The output of such a simulation would be all forces acting on the geometry, as well as the full image of flow – for one state of flow: one air velocity and one angle of attack of the junk to the wind. From that, I can then (rather easily) calculate Lift and Drag, Cl, Cd, L/D, Cl/Cd, and the like – for that one state of flow. However, only that one point doesn’t help that much. Therefore, I do need to run several simulations to simulate several angle of attacks or velocities. And suddenly, my laptop keeps hard working for days nonstop before all results are calculated.

Now imagine a 3D simulation: all those thin 2D slices would have to be stacked upon each other and calculated. I estimate the amount of cells to about 30-50 millions. I did simulations like those regularly for industrie applications in the past - but not on my laptop. For such things, you need high performance clusters (also called supercomputers) which are nothing to put into your living room or cellar. It is possible to rent such high computational capabilities, and run it “in the cloud”. AWS from Amazon would be an example. But - you may have guessed it - at a price: the bill to rent their computational power for each simulation would have 3 trailing zeros…  

Conclusion: Privately, I am only able to do 2D simulations. Thus, I will only be able to simulate “profiles” rather than “rigs”. Drawback: Those 2D simulations cannot include 3D effects like i.e. the tip vortex. Thus, the results I am going to present further down are not to be simply extrapolated to actual rigs. However, there are still a lot of information to be derived from the 2D calculations. More later.

Oh, I forgot to mention: over the years, CFD has become so accurate that it is widely used in industrial applications. Cars, planes, ships, pipings, pumps, meteorology, sea currents, i.e. are designed by CFD results. CFD is way cheaper than wind tunnel tests or towing tests – and in general the results are so close to reality that those real world testings can often be omitted. There are exceptions, but they are far from this case I present here. What I did with the junk rig, a 2D case of a profile in air, at Reynoldsnumbers of about 1e6, is quite a standard application.

Geometries

To start with, I set up a standard case: a flat cut rig. The dimensions I borrowed from Ilvy: 4.9 m batten length, which is the same profile chord length for a flat cut rig. The mast is set 1.5 m aft of the luff, which is about 31% mast balance. Mast diameter is 180 mm, batten thickness is 40 mm.

Second case: a cambered panel. Same batten length, same mast, but cambered profile with 9% camber. Now it gets fiddly: I do not actually now the real camber shape in an inflated junk rig panel. It surely is not the sewn in curve. For a start, I assumed a curve – there is room for discussion and improvement. It gets even more fiddly: I also had to assume a curve of the distorted profile on port tack. Again, please throw in critics, hopefully ideas to improve or even best: real world measurements of the horizontal profile distribution! However, I watched out to have the length of the profile curve the same as the flat cut one: 4.9 m – which naturally lets the leech end of the profile slip a bit forward due to camber.

Have look at the following profiles. The flat cut profile is naturally the same for port and starboard tack, the cambered not. (Please be aware, these are rather ideal, academic test cases yet. Just to prove it is working. It has to be fine-tuned! For example, a flat cut rig is not really flat in reality…)

Results

I’ve been running those geometries at 10 kn, for a variety of angle of attacks (AoA). AoA here means angle of the batten line to the incoming airflow (apparent wind). It is NOT the sheeting angle on a boat. Each data point in the following graph is derived from one single simulation.

Where the tangent hits the Cl/Cd curve, the profile is working at its most efficient point. At this point, the L/D ratio is at max.

It is interesting, that although the curves are different for port and starboard tack, the most efficient AoA of the profile is about the same for both tacks (5° for flat cut, 4° for cambered). However, it now gets also very clear that there is a good and a bad tack! There is a difference observable in L/D_max as well as in the shape of the Cl/CD curves, which describes alpha-tolerance. What I find most interesting, is that the bad tack of a flat cut sail is the good tack of the cambered tack.

It gets also quite clear, that a flat cut sail is way way way behind a cambered sail regarding upwind performance. L/D_max is 2-3x better with a cambered sail than with a flat cut sail.

To give some context, this is what the flow around the cambered profile looks like, at L/D_max with +-4° AoA respectively.

One can observe, that the flow at those AoAs and profiles is fully attached – at least where the mast does not interfere. If you compare that to the flat cut profile at max L/D (+- 5° AoA), you see a huge detached bubble on the suction side, which reattaches towards the leech end.

I could go on, posting Cd/AoA curves, Cl/AoA curves, pressure distributions on the profile etc, but this post has already gotten way too long. This very first results have already put numbers to some things. The steps to do more calculations are relatively small, now that I already did put in all the upfront work to setup the simulation.

Outlook

What can this now existing 2D CFD-setup be used for next? I could use this to investigate:

• ·        Different velocities
• ·        More realistic cambered profile shapes
• ·        Performance comparison of SJR to single cambered profile to flat cut to wingsail to aero junk to …
• ·        Performance comparison of more/less camber and different camber shapes
• ·        Influence of small wrinkles or large creases on performance
• ·        Influence of mast diameter on performance
• ·        Influence of mast balance on performance
• ·        Influence of distorted luff shape
• ·        And so on and so on…

Cheers,

Paul

PS: Sorry for being so sloppy about details, format, labelling etc. If I would put this into a scientific paper, it would definitely have a more professional format and be more detailed. However, I know from experience that the amount of time to do such professional formatting is not following the 80/20 principle. Not at all.