formdrag

[yipster]

“formdrag” is sometimes mentiont. On bulb keels for example, but at high(er) speeds, say a torpedo, I suspect form drag may be getting a real factor. A body of revolution, all circles, gives the least drag but cant find how length and beam dimensions relate. And is there a easy way to tell how this adds to wave and wsa drag?

It would be nice to have some idea…

yipster

[gonzo]

It would be a simple calculation if boats traveled straight forward like a torpedo. However, there is pitching, rolling, yaw and leeway. All these introduce a lot of variables. Most of the drag is often caused by the bulb traveling in a direction other than its axis.

[yipster]

thanks gonzo,
it is a -sort off- torpedo! see my "swath" story's.
now whats that -please simple!- calculation simplest i figger is testing and using common sense...

here some links i've been reading...
http://www.maths.adelaide.edu.au/App...p/multipep.htm
http://www.maths.adelaide.edu.au/App...ep/results.htm
http://www.maths.adelaide.edu.au/App...nd/mistest.htm
http://web.nps.navy.mil/~me/papoulia...ides/index.htm
http://web.nps.navy.mil/~me/papoulia...des/sld024.htm
http://oregonstate.edu/instruct/exss...form_drag2.htm
http://home.wanadoo.nl/o.j.i.kramer/ut/bubbles.pdf

i also was reading about great grants from the usa navy for who can come up with (rocket?) propelled torpedos that can do 100 mile and more, just started thinking (not about war or money, just about that speed and what would be involved)

[Mike D]

yipster

I wish you success with your investigations even though it will take up a great deal of time and effort. You are studying theoretical aspects of marine vehicles that operate underwater, on the surface and overwater. The propulsion systems that interest you are propeller, jet and rocket. Speeds that apply seem to be from medium to very high with autonomy from normal to very far.

Do you think it might be over-ambitious? I sure as hell don't want to dampen your enthusiasm but I'd advise caution in your approach. For instance, some of your postings (and from other correspondents too) refer to model tests with the implication of accuracy. A direct comparison of model tests is meaningless.

Even if you build a series of models and run them at various speeds for comparison you must scale up the resistances to the full-size values. If all the models are rather similar then the results should be reasonably "accurate" but you have such a wide range of systems that comparisons really become a “black art” rather than “science”.

Nevertheless it is quite certain that some of your thoughts most certainly will work well but at a high cost in modern ultralight, high-strength materials, powerful lightweight propulsion systems and high operating costs. The day will come and I hope you will be able to afford to build something and have something left over to insure and operate it. Or maybe some super-power will finance it all.

Michael

[tspeer]

Form drag is the garbage can of drag accounting. Its where you put the scraps you can't account for anywhere else.

One big contribution to form drag is the pressure force due to the boundary layer changing the effective shape of the hull. Although this is a pressure force as opposed to the shear force of skin friction, it's part of the viscous boundary layer effects. The more skin friction, the thicker the boundary layer, and the bigger the change in pressure due to forcing the outer flow away from the actual hull contour. So it makes sense to estimate this pressure contribution as being proportional to the skin friction.

Another contributor to form drag is the fact that the skin friction is universally estimated based on the boundary layer on a flat plate. A flat plate has no pressure gradient and the boundary layer profiles are pretty much self-similar. This makes it easy to calculate theoretically. A reall hull will have areas with favorable pressure gradients, which produce a thinner boundary layer, and adverse pressure gradients, which thicken the boundary layer and also lead to flow separation. To get the right skin friction you'd have to calculate the pressures over the entire hull, calculate the 3D boundary layer development, and integrate the skin friction over the wetted surface. This is way beyond what most designers can do, and was not even possible until fairly recently. So the difference between the real skin friction and the skin friction calculated for a flat plate is another scrap that goes into the form factor.

You can estimate the form factor for your configuration using a drag buildup approach. Sources like Hoerner's "Fluid Dynamic Drag" have data and empirical relationships for just about any item you can name. Bruce Carmichael also used to publish a book with lots of drag info, and he did the original work on laminar flow axisymmetric bodies.

Each of the various pieces has its own reference area. You have to multiply the drag coefficient by the reference area to get the drag area (also known as the equivalent flat plate area because a flat plate broadside to the flow has a drag coefficient approximately equal to one). Airfoil section data is typically based on the plaform area, so the profile drag coefficient gets multiplied by the planform area to get the drag area of a strut, keel, or rudder. For an axisymmetric body, you'd multiply the drag coefficient times the frontal area. A hull would have its skin friction coefficient multiplied by both the wetted area and its own form factor to get its drag area. Once you have all the drag areas, you can add them up to get the total drag area.

If you then divide by the total wetted area and subtract the skin friction coefficient, what you're left with is the form drag coefficient for the whole boat. Personally, I think it's easier to just work with the drag areas. Then there's no possibility of confusion over just what the reference area is or what to do when you change the reference area.

[yipster]

http://web.nps.navy.mil/~me/papoulia...des/sld001.htm
http://web.nps.navy.mil/~me/papoulia...des/sld001.htm

new slideshow has directional stability, maneuvering predictions, rudders and much more.
seems i found some super power help (for now to make a waterbike) who knows later a real one...
must thank Tom Speer as superpower at his own, i'll get the books!

yipster

ps: the sldeshows are downloadebla (before our GPS's go bizerk!)

[yipster]

and as i understand it for form drag so far, given enough speed it sure isnt a garbage can to neglect...

[yipster]

i may be talking to much, but only now i see this site (again nicely in monteray calif.) http://www.nps.navy.mil/ has much more intersting info not only on formdrag and swath ships. Nonlinear dynamics and control, Ship/Submarine response and motion control, Global dynamics and chaotic response, Bifurcation theory, Design of Advanced Hull Forms just to name some research interests (and info) there. look for professor Fotis A. Papoulias works at: http://web.nps.navy.mil/~me/papoulia...me.html#theses

will, here some more nice informative slideshows that deal with surge and heave, pitch and roll, sway and yaw and more on swath ships see: http://web.nps.navy.mil/~me/papoulias/theses_1995.html