Sail CFD - Help interperate my results

[m_liddell]

The boom was left out of the cad model to simplify it. It is weird that there isn't any turbulance around that area, it might be to do with the post processing, I released particles from the mast in the images.

[Skippy]

The boom isn't really the most important thing, the edge of the sail is, with the boom or without. Maybe you're right about particles that start from the mast. I'm just surprised the particles near the foot seem to be unaffected, they're traveling almost horizontally.

[jbettencourt]

Hi there,

I think that you might have some problems with your results. I see that there are no edge vortices released from the foot of the sail and also from the top edge. They should be there since these are the zones were the sail unloads. I'd sugest that you calculate the global forces (lift and drag - especially the induced drag) with empirical models and compare them with your CFD results. If you find a great difference between the results, then probably you might have overlooked some detail in your flow modelling. Especially with respect to the induced drag, since if there are no trailing vortices from the edges then this component should be very low.

Best Regards

Joao

[brian eiland]

Just thought I would reference a few other forum discussions that spoke to the CFD subject:
1) Sail Wakes and other Forum Discussions http://www.boatdesign.net/forums/sho...1&postcount=39
....a comment by Tom Speer...
There's been discussion in the past here about the flow around sails and the wakes they shed. I just ran across this picture,

<http://www.ship.saic.com/pictures/acup-flow.jpg>
http://www.ship.saic.com/overview_fans.html

of the flow around an IACC yacht. Although it doesn't say exactly, I believe what you're looking at is the velocity on the sail surfaces and in a vertical plane behind the boat. The three lines at the bottom show traces of air particles approaching from near deck level.

What I found striking was the dark blue regions in the wake indicating the vortices shed by the rig. The upper one is more in line with the hounds than it is with the head of the main. There's also a concentrated, powerful vortex shed off of the foot of the rig, too, with a substantial wake extending some distance up from the boom.

2) Sail Aerodynamics, Sail Wake http://www.boatdesign.net/forums/sho...3&postcount=44

....a comment by Brian Eiland...
What I am finding in my review of some of this computational methods of
investigating sail forces and flow analysis (CFA, CFD, vortex lattice models,
etc) is that generally there are so many assumptions made upfront in order to
simplify the equations so the computer can solve them, that the results get
skewed quite a bit from reality, ie, a quote from one of the annalist;

"CFD = Computer Fluid Dynamics. CFD is a great tool for visualizing and
explaining flow phenomena. While the latest flow software is very powerful and capable of calculating amazing things at astonishing accuracy, the old saying "garbage in, garbage out" is more true than ever. Besides of presenting the problem in a meaningful way, one needs lots of knowledge and experience to interpret the results correctly. Simulation through CFD is especially useful at giving qualitative information - when it comes to quantitative results or hard numbers, you have to be even more cautious when drawing conclusions about the merits of one design over another. Wind tunnel tests are needed to calibrate and validate the CFD code before reliable results are obtained."

"With the power of modern CFD at the desktop, it is too easy to produce
beautiful pictures with little connection to reality. Often these pictures are
produced by flow experts with little sail-specific knowledge, and then
interpreted by sail designers without sufficient understanding of the CFD
tool, and as a result you get just that - pretty pictures."