Hydrodynamics of Bulbs

Hi Everyone,

Fluid Dynamics is a science, and Design is an art.

Achieve Designs that outperformed the best known references. Hardcore computations (CFD) and costfull studies are not mandatory, when one merges the intimate understanding of the Fluid Mechanics, together with the constant improvement of the know-how during years of practice.

We present here the case study of the design of a bulb for a racing sailing boat. Music Credit : cryonicPAX - 'Excessive'



Comments greatly appreciated

 

I found the text and graphics at the same time tough. I missed either all the text or all the graphics.
The type of music didn't help.
The text needs a lot of editing and proof-reading.
Hard truth.

 

Right. This should be revised. I didn't take enough time to do this video. My daughter also found that the music does not help. I'm also quite impotent in video editing...Thanks Bluebell for these kind advices.

 

I have been solicited to work on the bulb of a racing boat. From previous work, I understood that hydrodynamic drag of a bulb, or any slender body, is driven by the area of the main section. A basic understanding that gives only one direction of shape improvement. Yet, I had several other constraints, that needed to be overcome, and couldn't, if I stuck to this only principle. One of the constraint was my time, and I couldn't go mad on computing : No adjoint optimization, no gradient descent, no recurrent neural network, no programming, no GPU cooling, no waiting. I had to know the answer, in advance of its practical design.

So I've made the same search I've made each one or two years from 2008 till now : Slender bodies theory hydrodynamics & Area to volume ratio of geometric shapes. And I've come accross the same old publication from Naval Architecture and Aeronautics. Strip theory, Panel theory, Slender body theory... Every publication, a invitation to hardcore computing I was already willing to decline. So I extended the field of research to Medecine and Material Science. In Medecine, there are a lot of research on the cells and their exchange with their environnement. As a consequence, a lot of publications have been made that includes the Area to Volume ratio as parameter, or the slenderness parameters. It is also the case in Material Science, and especially in the science of Powders, as the slenderness of a grain affects the mixing properties of any powder.

And I finally found this interesting publication : https://www.researchgate.net/public...articles_with_Different_Shape_Characteristics
At last, someone has taken care of making a overview of the hydrodynamic behaviour of ellipsoids... And have used a very nice expression for the hydrodynamic drag that I didn't know, and that sums up the hydrodynamic curve of each shape into one Coefficient :

2*CDmin*CDmax / (CDmin + CDmax) .... Very usefull for further numerical usage, as it is a usefull fitness function for brutal optimization.

The overall results are gathered into on chart. Very practical to me. So as to get what we look for, at first sight. Usefull work. Great work. I hope that this work will help others, having the same design issue. Bulbs, fairing, approximation of complex shapes...

 

From my physic classes, crossection area combine with skin friction, but I also see the rate of change, how much and how fast is the fluid forced to displace, and how far, as the body moves. Bending fluid (displacing) takes effort.

 

Alan,
Are you familiar with the Carmichael bulb experiments?

 

Hi Alan,

Vacanti designed a forward swept keel with bulb for Amati when he designed our boat in 1998- I don’t know if Vacanti would share any of this or his methodology or not, so I may be extremely remiss volunteering this, since all this may be his proprietary intellectual property. Vacanti offers a design package called ‘Wings’ - I haven’t used it, but might be worth checking out? At any rate, more arrows in the quiver….

Vacanti Yacht Design LLC https://vacantisw.com/?v=7516fd43adaa

Shop – Vacanti Yacht Design LLC https://vacantisw.com/software/?v=7516fd43adaa

edit- FWIW, a demo version of wings 10, something to have fun with
Boat, Yacht and Small Ship Design and Analysis Software, Marine Radar Consultancy https://vacantisw.com/product/wings-10-demo-version/?v=7516fd43adaa

 

Many thanks for the link. Impressing to see such a complete suit.

Bulb shapes are generally not very optimised, since their contribution to the total drag of the boat is small in general. ( except for some exceptionally extreme boats, like ACC's, where bulb's weight could represent 80% of the total weight). And this contribution gets smaller and smaller with the boat speed. Meaning that hardly few architects are willling to spend time studying bulb hydrodynamics. Even less have developped a specific hydrodynamic tool. And Vacanti surely is sophisticated.

I shall precise that doesn't mean architects don't study bulbs. As an essential component of the righting moment, the bulb weight and position is always a very central question in boat design. Regarding the region of the world where the boat is sailed, or when boat's measurements are enclosed by a rule, adaptation and compromise are always to be found. For instance, I notice that more and more class40, sailing a wide variety of wind conditions, tend to lower the bulb's size, maximizing their blade area for a given draft. In that case, boat CG's are lowered by lead shots are put into the keel.

 

Unfortunately no, would you please tell us more about this ?

 

My catamaran has two bulbs. These are bow extensions, which are, of course, different from a bulb that fills with water as a ballast. A contributor here designed them. The first step to reduce drag is rather obvious, but one I learned from the designer. That is, the bulb must be sized perfectly. In our case, we added 0.15m^3 displacement to each hull. The mass of the bow extensions was blown up a bit as we had some design issues in the transistion from the geometry to the composites engineering. This was unfortunate, but there was enough reserve for the issue. So, the second important thing, before or along with, rather, the hydrodynamics is the weight of the bulb.

I can't help you with the geometry. But I wanted to point out the challenges we faced; if it helps.

by the way, our bow extensions meet the requirements and corrected the boat's trim issue, as a side bonus; we got much less hobby horsing as well, a few negatives are that my props are not getting us to wot anymore and I need a pitch change, and there were some challenges getting the noses of the extensions sharp enough to stop massive amounts of water from coming off them... We did correct this issue as well, but the corrections were all prototypes with fairing compounds that need to be modified to G10... So, the tips of a sharp entry require either metal or something sharp as well. The conventional round bulb with an impact zone below water would behave differently afa hydrodynamics.

Our shape is basically a diamond. Maybe the contributor can offer more. I'm just a builder.

Bluebell is right. Words too small and scrolling is too fast to comprehend the video without pausing. Too much work for me..

 

Off topic a bit, but with the class 40 righting test, it’s tempting to think a skinny hull wouldn’t need a bulb? Or Maybe I’ve got that backwards….

 

Have you considered the interaction between the bulb and the supporting strut, particularly when the bulb is at and angle of attack?

 

I think you had best do the research.

 

During the test, the boat is heeled @90deg with a lever, and the righting moment is deduced. At tbis moment that the boat is approaching the AVS, the angle of vanishing stability, mandatory not less than 120deg (approx). Although I notice that some new class 40 do not comply.
The feeling of things being effortless heeling at 90deg may then be due to this fact ?

 

You picked my curiosity. Well done

yes, angle of attack has been set to 5°, and the heel angle to 15° in this picture, (highlighting another source of interference at the junction between hull and keel).
The influence of the incidence can be seen by the shape of the streamlines, being at different depth from one side to another.