Bulb drag calculation

How does one calculate induced drag from a bulb?

 

Is it a bulb by itself, or a bulb as in a bulbous bow at the fore end of a hull?

 

Sorry for the lack of detail,

its a 4' long lead bulb at the base of a fin keel. I'm specifically intersted in how to deal with the induced drag and the lift it produces but any tips are greatly appreciated

 

It is basically form drag (shape presented to flow on the front 'face') and frictional, wetted surface area.

You get get the basic drag of the 'form' from tables in books such as Fluid Drag by Hoerner ( http://www.abebooks.co.uk/book-search/title/fluid-dynamic-drag/author/hoerner/sortby/3/ ) then use the ITTC 1957 friction calculation based upon WSA.

This would give you a good approximation.

 

Actually, it doesn't "produce" any lift, what it does is act as a fence and changes the apparent aspect ratio of the keel, see Hoerner, but this time use Fluid Dynamic Lift, the companion volume to Fluid Dynamic Drag. Be very careful with bulbs though, as intersection losses can double the drag of the keel. The better the bulb acts as a fence, the more interference drag it has.

 

I thought a long slender, symmetric body generally produces a small normal force, ie lift, when at an angle of attack. However the normal force is small compared to the axial force, ie drag. An example is an aircraft fuselage. Most aircraft are designed to cruise with the nose up.

 

Small is the the operative word here. The lift of the bulb is trivial, and washed out by roll, pitch, and yaw AoA effects, compared to the keel and hull. Aircraft fuselages are compareable in size to the wings, like a hull is large compared to a keel. A bulb is neither, it is just thick compared to the foil, preventing tip roll-over. The only reasons to add a bulb is if you have a draft restriction or motion/stability problem. In a restriction free case, it is better to extend the span of the keel.

 

Agree that a bulb which is essentially a thickening of the keel probably has negligable effect on lift.

But I've seen racing sailboats with high aspect keels and a bulb. The length of the bulb is a number of keel chords, and the side-view area of the bulb is a considerable fraction of the keel area. Proportions are getting close to an aircraft wing and fuselage. I suspect that for a design such as that the lift from the bulb is not entirely negligable.

 

Bulb works as end plate and increases effective aspect ratio of keel fin thus effecting lift generated by fin. Bulb itself produces negligible lift.

Viscous resistance of bulb can be calculated using formula for revolved bodies:

CV=[1+0.5*D/L+3*(D/L)^3]*CF0

where CV - viscous resistance coefficient; D, L - diameter and length of body; CF0 - friction coefficient of plate.

Hope this helps.

 

You pull it through the water and measure the drag.

Why do you ask?

-Tom

 

And if you don't have a tow tank available???

 

You don't need a tow tank.

-Tom

 

You could use a panel code such as CMARC. And of course, even more sophisticated CFD methods.

 

There are several different definitions for induced drag in use.

The strictest one is the drag due to trailing vorticity which arises from the generation of a normal force.

But the more common one seems to be the change in drag associated with an angle of attack / leeway which would also include drag due to changes in the boundary layer and local separation.

 

Induced drag only? Or did you mean to say "total drag" or just "drag"?
Anyways, you could take a look at this discussion: http://www.boatdesign.net/forums/sailboats/theories-estimating-bulb-drag-14022.html
Cheers