reducing hull friction

Discussion in 'Hydrodynamics and Aerodynamics' started by nimblemotors, May 12, 2011.

  1. messabout
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    messabout Senior Member

    Riding downhill, as mentioned above is sometimes practiced in sailing dinghys. I have done that. It is great fun. There are instances where certain testosterone overloaded individuals have ridden their surfboard for miles behind the likes of the Staten Island ferry and other vessels. They do not operate in the propwash area but on the wave train.

    For those who are unaware: Leo has done a scholarly and comprehensive paper about ships in various formations. Some of the relative ship positions afford some economies. Useful information for ships traveling in a fleet one would think. His paper is, or was, downloadable from the internet. I cannot recall just how I happened to encounter the paper. Fascinating it was.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Kind words. Thanks.

    Remember though, my work is for ideal circumstances. There are many factors that work against the claimed improvements, some of which Daiquiri mentioned.

    The Diamond Tetrahull is one example of extreme cancellation, but it would require very precise speed and course control. A bit like precision, formation flying, but with a free-surface :eek:

    The paper on optimum families of multihulls (including some discussion of independent vessels) is at:
    http://www.cyberiad.net/library/pdf/tl98.pdf

    Leo.
     
  3. FAST FRED
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    FAST FRED Senior Member

    I have been reading in a "big ship" magazine that their tank used to measure drag is inside a building that can be pressurized!

    I wonder how tank testing above atmospheric changes hull drag or prop efficiency?

    FF
     
  4. daiquiri
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    daiquiri Engineering and Design

    About the propeller testing - it is a physical fact that an increase of the air pressure above the water surface increases the hydrostatic pressure in the water by the same amount. The result is a reduction (or elimination) of the cavitation. The prop works as if it was placed at a bigger depth.

    For the ship drag - I have no info about the relationship between the ship model drag and the atmospheric pressure (if there is any).
     
  5. DCockey
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    DCockey Senior Member

    I've heard of using a pressurized tow tank for testing hydrofoils and other high speed craft for which cavitation is significant. To match cavitation the cavitation number needs to be the same. It is:
    (Atmospheric pressure - vapor pressure) / (1/2 * water density * model velocity squared)
    Since the model velocity is set by other considerations (Froude number in a free surface test) matching cavitation number requires changing the atmospheric pressure.

    Atmospheric pressure won't affect wave drag or viscous drag unless cavitation occurs and changes the flow.
     
  6. Chickadee
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    Chickadee Junior Member

  7. tunnels

    tunnels Previous Member

    That Throws The Air And Water Increases Friction Theory Out The Window !!

    .


    Supercavitating Torpedo
    A rocket torpedo that swims in an air bubble

    By Eric AdamsPosted 06.01.2004 at 3:40 pm0 Comments



    by John Macneill Several challenges remain for the supercavitating torpedo, including how it will be steered underwater. Water-tunnel tests have already proven that speed can be achieved: In 1997, the Navy tested a supercavitating projectile that reached 5,082 feet per second, becoming the first underwater projectile to exceed Mach 1.John Macneill

    Submarines peaked in power and relevance during the Cold War; there has since been a shift in focus to aircraft-based combat, and subs have become budget-cut victims. But subs are still prized for their ability to sneak about global waters undetected and to defend surface ships from attack. Many U.S. subs are being converted from missile launchers into delivery vehicles for special operations troops.


    But the supercavitating torpedo—a rocket-propelled weapon that speeds through the water enveloped in a nearly frictionless air bubble—may render obsolete the old submarine strategy of sly maneuvering and silent running to evade the enemy. The superfast torpedo could be outfitted with conventional explosive warheads, nuclear tips or nothing at all—a 5,000-pound, 230-mph missile could do enough damage on its own. The Russians invented the concept during the Cold War, and their version of this underwater killer—dubbed the Shkval (“Squall”)—has recently been made available on the international weapons market; the United States, of course, wants a new, improved version of the original.


    The hard part about building a rocket-propelled torpedo isn’t so much the propulsion as clearing a path through the ocean. Water creates speed-sapping drag; the best way to overcome that drag is to create a bubble that envelops the torpedo—a supercavity. A gas ejected uniformly and with enough force through a cavitator in the nose of the torpedo will provide such a bubble, permitting speeds of more than 200 mph and a range of up to 5 miles (traditional torpedoes have slightly longer ranges, but lumber at only 30 to 40 mph).
    Though submerged, the torpedo remains essentially dry, with a frictionless surface. “That sounds easy, but doing it is extremely difficult, especially if you’re trying to steer,” says Kam Ng, program manager for the torpedo at the Office of Naval Research, which has been developing the weapon since 1997. “If your torpedo moves in a straight line, you just aim and shoot,” says Ng. “That capability already exists with
    Shkval. But the U.S. vehicle will be more capable—it will turn, identify objects, and home in on the target.” (Improvements to the torpedo to make it steerable likely froze when the Soviet Union collapsed, says GlobalSecurity.org’s Pike.)


    Among the greatest challenges for U.S. torpedo researchers is developing detection and homing technology that will enable the torpedo to distinguish an enemy sub from, say, a rock formation, says Ng. Also tricky is finding a way to control the gas bubble to permit those course changes. “When you turn, the bubble distorts because it is no longer symmetrical,” he says. “So you have to compensate for that by putting more bubble to one side.” This is done, Ng explains, by ejecting more gas toward the outside of the turn.

    Naval officials say the high-speed torpedo will enable submarines to attack enemy subs and surface ships without giving them time to respond. The U.S. military has tested a prototype, but combat-ready versions are not expected for at least 15 years.
     
  8. SURFFOILS
    Joined: Dec 2011
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    SURFFOILS Junior Member

    Theres always the superhyperphobic treatment from www.neverwet.com
    read the blurb and it souds like the perfect treatment for drag reduction.
    Retail can available mid 2012, or so they say....
     
  9. Leo Lazauskas
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    Leo Lazauskas Senior Member

    We have discussed this product and its "nano particles" before.
    I want to see independent tests on real boats and ships before I would
    pay money.

    See:
    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/superhydrophobic-product-40826.html

    The discussion "petered" out after PAR suggested that my wife looks at
    reflections of my ******** in my bald spot. People are still trying to work
    out the contact angles, I guess.
     
  10. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    How would this coating reduce drag, other than a very small reduction in wetted area due to the difference in contact angle? Would it significantly modify the no-slip condition at the water hull interface and thus change the boundary layer profile?

    I don't see any claims of drag reduction on the website.
     
  11. Leo Lazauskas
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    Leo Lazauskas Senior Member

    True. All they claim is:
    "Water beads “glide” over our surfaces like a skate gliding over ice, with almost no surface friction."
     
  12. tunnels

    tunnels Previous Member

    You are an engineer ?? wow if water dosent cling to the surface there is almost no friction ! drag !! friction !! remember DRAG!!! reduction !! sounds cool to me !! like to see it in a practical application !:):idea:
     
  13. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Boundary layers don't behave the way you are imagining they do.
    Hence, DCockey's question about the modification to the no-slip condition.
     
  14. Boston

    Boston Previous Member

    Tunnels, you might look into Tesla's turbine engines for a better understanding of boundary layer dynamics
     

  15. DCockey
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    DCockey Senior Member

    I found several papers online about hydrophobic surfaces modifying the boundary layer through a slip boundary condition. No time yet to digest them.

    http://turb.seas.ucla.edu/~jkim/papers/pof-oct-2005.pdf The effects of a hydrophobic surface on stability and transition in wall-bounded shear flows are investigated. The hydrophobic surface is represented by a slip-boundary condition on the surface. The linear stability analysis with slip-boundary conditions shows that the critical Reynolds number increases with streamwise slip. ....


    http://iopscience.iop.org/0953-8984/23/18/184104;jsessionid=7B059657DD8F954220CD6D8725424BC0.c1 Abstract only. Paper requires payment. We discuss how the wettability and roughness of a solid impacts its hydrodynamic properties. We see in particular that hydrophobic slippage can be dramatically affected by the presence of roughness. Owing to the development of refined methods for setting very well controlled micro- or nanotextures on a solid, these effects are being exploited to induce novel hydrodynamic properties, such as giant interfacial slip, superfluidity, mixing and low hydrodynamic drag, that could not be achieved without roughness.

    http://lpmcn.univ-lyon1.fr/~lbocquet/Langmuir-Sendner-2009.pdf The dynamics and structure of water at hydrophobic and hydrophilic diamond surfaces is examined via nonequilibrium Molecular Dynamics simulations. For hydrophobic surfaces under shearing conditions, the general
    hydrodynamic boundary condition involves a finite surface slip. The value of the slip length depends sensitively on the surface water interaction strength and the surface roughness; heuristic scaling relations between slip length,
    contact angle, and depletion layer thickness are proposed. Inert gas in the aqueous phase exhibits pronounced surface activity but only mildly increases the slip length. On polar hydrophilic surfaces, in contrast, slip is absent,
    but the water viscosity is found to be increased within a thin surface layer. The viscosity and the thickness of this surface layer depend on the density of polar surface groups. The dynamics of single water molecules in the surface
    layer exhibits a similar distinction: on hydrophobic surfaces the dynamics is purely diffusive, while close to a hydrophilic surface transient binding or trapping of water molecules over times of the order of hundreds of
    picoseconds occurs. We also discuss in detail the effect of the Lennard-Jones cutoff length on the interfacial properties.


    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030020475_2003024195.pdf We study the no-slip boundary conditions for water at a hydrophobic (graphite) surface using non-equilibrium molecular-dynamics simulations. For the planar Couette flow, we find a slip length of 64nm at l bar and 300K, decreasing with increasing system pressure to a value of 31 nm at 1000 bar. Changing the properties of the interface to from hydrophobic to strongly hydrophilic reduces the slip to 14 nm. Finally, we study the flow of water past an array of carbon nanotubes mounted in an inline configuration with a spacing of 16.4 x 16.4 nm. For tube diameters of 1.25 and 2.50 nm we find drag coefficients in good agreement with the macroscopic, Navier-Stokes values. For carbon nanotubes, the no-slip condition is valid to within the definition of the position of the interface.
    1. Motivation and objectives Macroscopic, Navier-Stokes modeling of problems in nano-fiuidics may prove a computationally ....
     
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