Hi,

I was able to continue some of my CFD calculations for junk rig profiles (2D). In the meantime, I finetuned the solver and let it run long enough.

David, your wingsail was next. I took your drawing as you posted it, and made a profile out of it. The results (at 10 kn) show what is expected:

Recently, I stepped back from comparing the different junk rig types to each other with my 2D simulations. In my view, there are too many variables involved which I don't have at hand: how much camber is there in reality? How is the real profile shaped? Is it the same as designed? How much camber? Is flat really flat? If not, how much camber? The list goes on...

Instead, I focus on variables which can be determined, for instance: mast position, position of max camber, etc... Let's see how that goes.

Also, after the wingsail simulations I switched from the transient solver I used to a stationary solver. This one only gives good results until the flow is fully detached, but is way, way, way faster.

Paul

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).