Hydro/supersonic phenomena?

Discussion in 'Materials' started by Jesse Shepherd, May 1, 2012.

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Jesse ShepherdNew Member

According to my work in Aerodynamic supersonic research, I was told that air is incompressible at the speed of sound. Since water is incompressible, any thing that moves in water is in the same realm,as air, being a fluid it must obey fluid dynamic rules. That means that the moment a hull moves in water it starts a bow wave same as air at mock one (1). Any movement forward causes drag and it increases at a high rate if the hulls area of a cross section, and its resulting volume increases suddenly. That means that any under water hull shape should follow Whitcombs rule, ( coke-bottleing). Any object that causes a sudden change in underwater volume should require the same volume reduction in the same stations affected. Does this mean that keel leading edge should start less abruptly? Should the bulb shaped keel weight cause changes in the keel volume to compensate for the bulb shape? Could the bow wave angle be co-related to the hull's speed? In supersonics, One (1) divided by the sine of the shock wave angle is the mock number. If the bow wave is 45 deg., what would the in hull speed be ,if the supersonic mock Number would be 1.41? 'Tis a quandry that I have been curious about for 62 years !

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yipsterdesigner

Area-rule as used for supersonic jets (coke-bottle) is allready aplied by JK
keel, bulb and rudder are set in indents limiting boundry layer horse shoeing
Read Leo Lazoukas cyberiad on bow wave angle as I should be doiing again

3. Joined: Apr 2012
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Jesse ShepherdNew Member

Howdy Yip, Thanx for the acknowledgement of my supersonic stuff . By the way, What is boundary layer Horse Shoeing?? Also, I aint gonna read any more books or articles on any of this theory stuff. At 91 , I aint got time to do that ,even if it would be a nice read. I play with drawing R/C Designs on CAD and I am about to run out of time! Too many ideas, so little time ! Boats were my second love, Aircraft is my first and bread and butter as a career. I sailed when it was too still to fly Model aircraft. Jesse Shepherd DBA Aero-Crafted Model Designs I learned to loft boat hulls and used it to loft Aircraft like the B-58. JS

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DCockeySenior Member

Incorrect. Air is compressible at the speed of sound. Unfortunately that means the remainder of your comments and questions are moot.

What may be causing confusion is that at sonic speed the ratio of density change to pressure change becomes such that the flow in a converging channel goes from accelerating when the flow is subsonic to decelerating when the flow is supersonic. Fundamentally different than the flow being incompressible.

Also for the record water is compressible, though the density of water changes much more slowly with increasing pressure than for air. The speed of boats through water and the speed of the resulting flow around the boat is so much less than the speed of sound in water (even for the fastest boats) that the effects of compressibility of the water can be neglected when analyzing the flow of water around a boat.

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yipsterdesigner

DCockey your rite, its a bit relative and no boat did a a sonic boom at hullspeed yet
Whitcomb's area rule however revolutionized the design of virtually every
transonic and supersonic aircraft ever built. for boats you may say it's far fedged
but JuanK and Farr do use area rule in dents or flats around keel attachment
found this three in the snow pic that illustrates horse shoeing quit well

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DCockeySenior Member

Any connection between Whitcomb's area rule and the benefit a similar rule may provide for boat design is coincidental, particularly with regard to boundary layer effects and resulting vorticies. The Whitcomb rule has absolutely nothing to due with boundary layers and horseshoe vortices.

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baeckmoHydrodynamics

David has provided correct info regarding compressibility and velocities here. I would just add that the critical speed (speed of sound) in a mixture of fluid and gas, as in foaming water is considerably lower than the critical speed in either of the "ingredients". For example, a mixture of 90% water and 10% air (by volume) has a speed of sound of roughly 30 m/s; say 60 knots.

This will result in a dramatically changed flow over a propeller blade (tip speeds often around 60 m/s) or around shaft brackets, drive legs and rudders in high speed vessels, where there is a considerable amount of bubbles carried along the full length of planing bottom.

Just as we know from high speed aeronautics, there is an impact on profile drag in bubbly water at speeds below the speed of sound, expressed as a function of local Mach number. Already at speeds as low as 40 knots we will see a drag increase due to compressibility effects. I would say this is one of the reasons for the performance improvements we see with surface piercing propellers; the improvement goes beyond what you would expect from avoiding pure "incompressible flow drag".

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yipsterdesigner

thanks for the reply's and those specs i've been wondering about and i too said DCockey was rite on compressibility
i also said it was a bit relative just like DC mentioned, why i added super and trans sonic in reply and as i see it area rule (coke bottle) design
can be used where wings (or keels) stick out (creating horse shoes) and disturb a smooth transition of the boundery layer changing the frontal area to quick
JuanK wrote about a hollow in PB. Farr mentioned a flat, not round bottom area where keel sticks out to accomodate for horse shoeing around the keel/hull
coused by boundery layer area expansion, canting keel racers also use an indent in the hull bottom and i dont think thats only for giving it a better swing

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