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.