Bulb drag calculation

Discussion in 'Hydrodynamics and Aerodynamics' started by Cpalm, Oct 6, 2011.

  1. Cpalm
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    Cpalm Junior Member

    How does one calculate induced drag from a bulb?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Is it a bulb by itself, or a bulb as in a bulbous bow at the fore end of a hull?
     
  3. Cpalm
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    Cpalm Junior Member

    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
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

  5. jehardiman
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    jehardiman Senior Member

    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.
     
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  6. DCockey
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    DCockey Senior Member

    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.
     
  7. jehardiman
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    jehardiman Senior Member

    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.
     
  8. DCockey
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    DCockey Senior Member

    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.
     
  9. Alik
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    Alik Senior Member

    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.
     
  10. Submarine Tom

    Submarine Tom Previous Member

    You pull it through the water and measure the drag.

    Why do you ask?

    -Tom
     
  11. DCockey
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    DCockey Senior Member

    And if you don't have a tow tank available???
     
  12. Submarine Tom

    Submarine Tom Previous Member

    You don't need a tow tank.

    -Tom
     
  13. tspeer
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    tspeer Senior Member

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

    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.
     

  15. daiquiri
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    daiquiri Engineering and Design

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