About inclined underwater hull form

thanks, i love those pretty pictures program determines if its lift or drag and adds lift types?

 

Sure, it predicts all forces & moments, but it makes no distinction between "vortex lift" or "other kind of lift". Those are rather just man-invented conventions associated with different flow theories. In the end it's all pressure differences and skin friction.

Here's a couple more about flow separation behind a non-optimally sheeted smaller sail.

 

Well said, Mikko!

What do you use for skin-friction? (I've seen some very expensive CFD codes that use the ITTC line).

The sail graphics are beautiful. All we need now is some hard validation on some simple cases so we can understand and believe the results

 

One concern with RANS codes and trailing vortices is grid size. If the grids are not sufficiently small the trailing vortices will be diffused, somewhat similar to a lower Reynold's number. A good test to use if sufficient computing resourses are available is to run with several grids, each one finer than the previous and see if the results become "grid independent" as the grid is refined.

 

was thinking how a program differentiates vortex lift and vortex drag, adds lift sorts and had a few more wonderings
so thanks for the answer, how simple, the dows too, and with a 3d viewer there would be even more insight

and sorry for side trackin your thread so much fredsmidt

 

Most modern RANS codes rely heavily on wall functions: close to the surface the boundary layer is solved by traditional methods, for calculation of skin friction. Further away, the turbulence is solved by averaging or "smoothing out" the different sizes of turbulent eddies in the flow. I've understood the tricky part is the interface between these two. Thus RANS yields only a sort of average snapshot of the real life flow, which is always transient and more turbulent in nature. In reality the flow is not as orderly as in RANS, but it helps us to gain understanding about the average way flow behaves.

 

It is a great tool for visually understanding the flow behaviour, (still) somewhat less good in predicting the exact numerical values. but I am confident that slowly but steadily we will arrive to a good match between the CFD predictions and the physical reality for generic flow fields.

Regarding the "vortex lift" - it is true that the flow has only two ways of acting on an immersed body, which are pressure and shear friction. But introducing the concept of vortex flow is very educative for anyone who has to deal with aero or hydrodynamics. It gives an idea about what happens in the flow field under some conditions, and why are there often sudden changes in force coefficients for a very limited change of a rigid-body configuration.

Many people are unaware of how a flow develops over various surfaces and more or less sharp edges, because flow field is invisible to a naked eye. Take, for example, the first picture in your post #137: http://www.boatdesign.net/forums/at...derwater-hull-form-sail-1-eased-sail-2_13.jpg . How many people do you think would guess that there is such intense vortex in between sails, even those who are in the business of sailmaking (apart you, of course )? But if one knows that under certain circumstances a vortex will develop over a body, if he can rationally predict when and why will it happen and know how will this vortex interact with a body and with the rest of the flow field, he can either choose to not let it happen or use it to his benefit.

It is essentially like the division of a surface-ship drag into wave and residual resistance, and all the rest of the sub-divisions. Nature knows nothing about these tricks we use but we still need them, because they help us put some order into an apparent chaos.

Cheers

 

You should read "Modelling multiple yacht sailing interactions between upwind sailing yachts" by Spenkuch T., Turnock S., Scarponi M., Shenoi R., JMST Vol 16 No.2 2011. They model these effects between yachts to gauge the interactions for better performance predictions.

 

Thanks AH, I will try to find that article.

 

Well said. With ref. to your Batana post with illustration, I doubt that for instance in case of the Star, the hull lift is achieved through vortex lift from the chine - the price paid for drag would be too large.

 

This vortex, as Daiquiri noted, is quite big, but seems to be essentially due to the (apparent) fact that the mizzen is sheeted flat while the main sheet (I suppose this distinction in the name of those sails is applicable here?) is paid out. Not that big surprise, in such circumstances and with apparently improper sheeting, that we see such a vortex.

Would certainly be very interesting, Mikko, if you have such CFD pictures of the interaction of a properly sheeted main and genoa, for example, or perhaps even more interesting, of two similar sails one behind the other, like on modern schooners between foresail and main, or on a ketch with very large mizzen.

Sorry too for the side tracking of this thread....

 

My first inclined waterline foil form:


Botton up:


Design:
http://iomdesign.files.wordpress.com/2012/06/orca.png?w=300&h=90

http://iomdesign.files.wordpress.com/2012/06/fs-65-vi-foil2-incli-30-3d.jpg?w=300&h=72
Coming soon will be in the water (30 days)

 

Very interesting and informative thread. One question is what is the effect that the amount of rocker and the shape of the chine has on all of this? The bow shape and the angle of entrance would seem to locate the longitidinal point of "lift" , but it would move around alot in real life .

 

I made some assumptions and I am now trying to prove them, but it will be a very slow process.

 

Pratice

More and more I notice that hulls with inclined water lines in foil form goes well to windward and make a good angle into the wind.
A good example is here:

http://rg-andy.blogspot.com.br/2012/06/nekst-round.html

A hull with no much form as a box, like the Star, and going good to windward. I saw in freeship the inclined waterlines in the form of foil, which could even be better, and a report of good results.

Naturally the hull has chine and in this case with a under water part, as Star.

See the comments.