Anonymous wrote:

Failure and Progress

 David, I did not let simulations run with your provided wingsail profile yet. I want to ensure solid results for the trivial test cases before commencing to more interesting geometries.

Bonjour

I would also be interested on a perticular wingsail profile.

I've already tried a demonstrator of a junkwing. The next step, under slow motion progress is to develop a prototype for Mingming.

I used a very simple profile : I started with a  classical Clarck Y profile. I only kept the extrado. I made a symetrical profile with two extrado. Then I rotated the two extrado until thry crossed at 70% of the profile.It produced a wishbone forward with two tails.

The wishbone part is the forward part of the profile. The tails are the positions, on each tack, of the flexible part of the profile in order to have an assymetrical profile.

To hold the flexible part in position, I use a double sheating system with two points of sheating on each side of the battens.

Eric

Failure and Progress

Short update from the simulation area: Unfortunately, I discovered that some of those simulation results I posted here in the last weeks were taken from not yet converged runs. My fault! It is mainly an issue for AoA’s > 8°. Reason for this: with higher AoA comes more flow separation/ detachment, which creates huge eddies behind the profile. Those simply take longer to resolve numerically than attached flow. It is a pure simulation issue and has nothing to do with real world physics.

Anyways, I let the simulations rerun up to 30 s (instead of 12-15 s as before). See the results below:

Sorry for the graph being a bit cramped. Basically, the data with “30s” in its name is from the rerun simulations.

Good thing is, the big picture doesn’t change. However, there are still some significant deviations. I have to bite the apple and let future simulations run long enough – and double check their convergence.

It also appears that slightly higher AoA’s would be of interest, too. Let’s say up to 20-30° – specially for the flat cut profile, the maximum of the Cl/Cd-line is not yet reached.

Also, after some good nights sleep and some head-scratching, I came to realize what some of you already suggested: For comparison, the Cl/Cd-plot might be not the right choice. What is most interesting for close hauled performance is mainly (or only?) Cl. I’ll go and make some Cl/AoA-plots, next time.

One last thing: A recent conversation with Prof. Graf from the Kiel “Yacht Research Unit” suggested that it might be sufficient to use a steady-state solver. Until now, I used a transient solver (that’s why I talk about computed seconds, i.e. have my simulation run to 30 s – which is not the time it takes to calculate that simulation). If that would speedup things while still being accurate, I’ll go for that! I'll do some comparison runs, then lets see! More hard work for my laptop to come…

Cheers,

Paul

PS: David, I did not let simulations run with your provided wingsail profile yet. I want to ensure solid results for the trivial test cases before commencing to more interesting geometries.

".. the line was always there, it is just so thin that you could not see it in that view ...I just highlighted it for you..."

"I can imagine that this sounds wierd...  "   

No, perfectly clear explanation - thanks.

Thanks Paul

I know which side is which (leeward/windward  low pressure/high pressure)

What I could not see on your coloured images was the boundary line between them (ie the 'sail profile' line) which is not shown on your earlier diagrams.

Now you have drawn it in with that magenta line - so it shows now -  so we can now see a clear line between the windward and the leeward side. 

But you have drawn that line in after making your your conclusion!

(Anyone could draw a line which was on one side of the blue bubble in one diagram, and on the other side of it on the other. But that would be circular reasoning).

I don't doubt your reasoning but I am still puzzled, and feel a bit stupid to admit it. For sure there is something I have missed on your earlier diagrams.

(I can follow and agree with your general reasoning and can see some real promise in where you are heading with it.  This question is a merely a minor point of interpreting a diagram, which i am lacking).

Hi,

Eric, we know that every geometry in moving air has aerodynamic forces acting on it. They can be summed up in one total force vector, having a length and an angle. It is now just a question of definition, how we decompose this total force vector. The Lift (Cl) and Drag (Cd) concept decomposes the total force vector into one part parallel to the flow direction and one part normal to the flow direction. Useful for different cases, i.e. aviation. Now, there is no objection in decomposing the total force vector into different directions: for example into the boat’s course direction and lateral direction. Useful, as well. However, just because we decide to use another convention of representing the forces on the geometrie does not change the actual total force vector. The conclusion we can draw from both conventions are the same!

Cl is the Lift, Cd is the drag, just written down in dimensionless form to get comparable coefficients instead of specific forces.

Arne, I repeat: the aim of my 2D profile simulations is not to find the perfect AoA of a real sail to the wind. This information could simply not be drawn from my 2D simulations. I chose the LDmax point to have a variable on which to compare the different profiles. I take it on my head, that this preliminary choice of mine (to use LDmax as the comparable variable) was not the best. That’s why I suggested to open up from one variable to an interval of AoA for comparison between different profiles. It would also be possible to just compare max Cl between for different profiles, but I think it won’t be good as at max Cl the profile is already stalling.

What is sure from Marchaj’s diagrams here, is that the  5° AoA, with the lowest drag angle, is not the setting which will get a boat fastest to windward. One needs some brute force as well. Don’t forget the boat. – Arne K.

Yes, exactly! I do not argue on this one.

In the end, I want to compare profiles to each other. A hull does not have an effect on this comparison. To give a real-world example: think about Ilvy, being rigged as she is now, cambered, and next year rigged as SJR. Given the same weight of the hull (plus 2-3 kg gained body weight from Christmas), the hull can be ruled out when comparing those two rigs.

To compare the profiles, my suggestion is to compare the ClCd curve, at the interval between deflating luff and fully stalling flow (which is about 5° to 15° AoA). Anyway, I’m not emotional on this: Any suggestions or better ideas on how to compare the performance of profiles in this case are very welcome!

Telltales at the leech  -  useful or not. – Arne K.

Arne, I think leech telltales are very useful, indeed! They have been in use with junk rigs as well as Bermuda rigs, airfoil flow visualizations as well as kites. I’m not debating on their usefulness, when I try to investigate what exactly they are showing, and question if they show the most efficient sheeting angle. Regardless if they do or don’t point to the most efficient angle, they depict a very effective, practical and proven way of feedback from the sail – which works!

(Btw. when watching videos on Youtube, showing stall-tests of aeroplanes, the tufted wings clearly indicate that separation starts at the trailing edge...) – Arne K.

I assume you watched stall-tests of thick profiles – at least thicker than a single-ply sailcloth can ever be. That nose radius on an airfoil has a significant effect: i.e. the luff is detaching way later when the nose is not sharp but has a radius. Slieve nicely described it in his “Some thoughts” writeup, and also David must have thought about it a lot when designing his wingsail – and for a good reason. The thickness of the nose, the nose radius, does the trick! Regarding performance, a single-ply sheet, even if cambered, can not be as good as a profile with a nose radius.

Cheers,

Paul