bulb design & optimisation

Hello everybody!
I am starting my dissertation aiming to optimise a bulb for a consistent keel in terms of minimising induced drag (not particularly interested in viscous drag). The free surface effect is going to be ignored whatsoever.

To this end I am going to consider 3 (or more) different shape versions of the 3d bulb of the same required volume (which can be assumed if the ballast weight gets a value..) and investigate the flow characteristics around it with cfd tools (Fluent solver and Gambit for geometry modelling) finally obtaining values for induced drag.

What I am looking for (and can't find in the web) is
- a full design of a bulb with or without a set of experimental data for this bulb if it has been tested (I know I ask a lot but any hints will be appreciated),
- (or) the full design or at least dimensions as precise as possible for the bulb of the keel of a Volvo Ocean Racing yacht or any other well known sailing class (beneteau od 25??) !!

Even without these data I can still design from scratch a pretty simple bulb but it then ends up to be a huge practice in cfd tools but the naval architecture context becomes somewhat ambigious or at least with no possible practical application... :-( what do u think??

Thanks for any help!!

 

Is this a free bulb, something like a torpedo, or is it attached to a sailboat keel? The flow of water in a sailboat keel is not linear fore and aft.

 

hey thanks for reply, sorry i forgot to mention that my initial plans it is going to be just the bulb!! looking for a design of the bulb itself and ideally I would like to know just to which boat this bulb belongs! cheers!

 

Strather,

The first thing you'll find is that a bulb on it's own acts very differently than a bulb on a keel. I think what you are looking for is actually the performance effect on the keel, as the performance of the bulb is largely as a heavy drag-producing device.

Unless I'm much mistaken, your department has a towing tank and Glasgow Uni has a wind-tunnel. Does the department have any keel models that you can find data for? It might be sensible to test one of these yourself (as long as you don't have to build it). These sort of tests have been carried out in the past, but they have almost all been for commercial purposes, so they are covered by non disclosure agreements. You might find some existing data, and I would suggest looking in the UNI Library.

The other thing you can do is to go to a marina and take a camera and some blue (and yellow) masking tape and a ruler. This allows you to take a set of photos from which you can reproduce a bulb. You may find taking cardboard and scissors is also useful to produce templates that you can then digitise.

Hope some of this helps,

Tim B.

 

Seems like a rotated NACA profile is what I've always seen

 

Strather,

I would be happy to let you have the 3-D surface models for the keels that I designed for Bagatelle and Saint Barbara if you:

a. Protect the data files against release to third parties,
b. Include them in your testing and analysis (you may size appropriately to be equivalent volume)
c. Provide me with a copy of your results.

Please contact me privately through my website:
http://www.sponbergyachtdesign.com/Bagatelle.htm
http://www.sponbergyachtdesign.com/SaintBarbara.htm

As you will see, these are both vanilla-brand keel designs with straight blades and flattened bulbs. Bagatelle is in an L-configuration, and Saint Barbara is in a T-configuration.

Eric

 

Velocity?

strather,

What is your target velocity or range?

What is your surface finish, incredibly smooth or dimpled?

What is the trajectory, linear or multi-directional?

Completely submerged operation or surface and near surface?

Have you heard of the Carmichael Bulb?

Search this forum under the Search option in the header and you will find

quite a lot of dialogue on this topic.

I will be very interested in your final conclusion...

Tom

 

guys thank you all for responses!

@Tim B: you are not mistaken about the towing tank and the wind tunnel, but it turns out to be really complicated to include experiments in the project (there are many reasons for that I 'd rather not mention them right here). As for the real 3d measurement of a bulb, it would be a decent option if I get too desperate....

@gonzo: an appropriate symmetrical NACA profile revolved round the x-axis produces indeed realistic shapes. I have already tried some of them. It is convenient because at the end of the day you've got something and that's important.
While changing the shape though (to get alternative versions), the fact that I demand the volume to stay the same causes some headache but I guess this is all included in the procedure..
But still with these improvisations the project lacks practical application

@Eric Sponberg: What you suggest is extremely interesting, I checked the boats. I am willing to go along with your terms, I think I will contact you very soon!!!

@Submarine Tom:
- the velocities considered will probably be for 3 cases (4knots, 6knots & 8knots). But this is something to be determined later on depending on which the yacht under consideration will be (if any particular)
- the trajectory will be linear
- I will try to introduce typical smoothness values (gelcoat mould finish with no fowling) even though the frictional resistance is not of much interest in this case.
- the case will be deeply submerged (as metioned above), no free surface effects.
Indeed there's some material within the forum so I keep you all posted of my decision or any further queries. Cheers!

one more minor thing if you are gambit users: do you think there'll be a problem later on if I have designed the mesh at Rhino and import it to gambit?? Because so far it seems to work but I haven't reached the Fluent part of course...

 

Have you tried these guys?

http://www.marskeel.com/