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Help me calculate drag on objects in water!

  • #1

Homework Statement



How can I calculate the drag of different missile/torpedo shapes in water? I have five different shapes, similar cylinders with different "noses" One is a long cylinder with a flat nose, another is a cylinder of the same diameter with a hemisphere nose. Third and fourth are short and long cone-shaped noses, and the fifth is a large hemisphere (twice the diameter of the cylinder, sort of a mushroom shape). They all have identical surface area (were designed that way) and are powered by identical electric motors. But they move through water at different speeds, presumably because of their different shapes or "streamlining."

We're talking about low speeds here, these are models of approx. 14-18 inches long propelled by a small 3 volt electric motor.

Is there a formula for calculating the drag or drag coefficient of these different shapes? Is there a rule of thumb (I have heard 90 %) for the amount of drag caused by the torpedos' frontal area/cross section vs. the amount of drag caused by laminar/surface friction?

Thanks, any help is appreciated!



Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
rock.freak667
Homework Helper
6,230
31
You can use the formula here, however I am unsure as to the exact values for the drag coefficients of your shapes.
 
  • #3
The link you pasted is not active, but I bet you're suggesting the formula for drag, i.e. the force of drag = one-half the density of the medium, times the velocity squared, times the cross-sectional area of the object, times the drag coefficient for that shape. If correct, I've done some searching and found drag coefficients for all my "shapes" of torpedo. Actually, it's the torpedo "noses"...flat cylinder, hemisphere, long cone, short cone.

So I can basically calculate that, and as I understand it the "frontal" drag is the main drag effect.

However each torpedo has a body too, a fuselage if you will....so how do I calculate the drag on that, the "laminar" resistance or whatever, and add it to the frontal drag for a total?

I've found lots of examples for calculating the frontal drag of different-shaped objects, but the length and shape (pure cylinder, gentle taper, rounded, etc.) of the fuselage/body must matter too...how to measure drag there?
 

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