Increasing the Efficiency of a hydrofoil dagger board *HELP*

Discussion in 'Hydrodynamics and Aerodynamics' started by ross whitaker, Feb 16, 2018.

  1. Erwan
    Joined: Oct 2005
    Posts: 441
    Likes: 25, Points: 28, Legacy Rep: 97
    Location: France

    Erwan Senior Member

    Ross,
    Don't need a software for cavitation.
    You should find cavitation threshold anywhere and probably on wikipedia too.

    Once you get it, you have to choose a foil section which remains under this threshold for all actual AoA.

    You should visit Tom Speer homepage: Tom Speer - Home - http://www.tspeer.com/


    Cheers
     
  2. tspeer
    Joined: Feb 2002
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    Likes: 295, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    Calculating the section characteristics once cavitation has begun requires a very sophisticated CFD code that can handle multiple fluid phases. But most people are only concerned with designing to prevent cavitation, and for that you can use Xfoil.

    Before you run your polar in the OPER mode, set CINC. This will include the minimum Cp value in the polar. You can plot Cpmin vs CL or alpha to produce a cavitation bucket diagram. Or, you can calculate the incipient cavitation speed from the polar data in a spreadsheet and plot that. My favorite presentation these days has incipient cavitation speed on the X axis and CL on the Y axis. You can add lines of constant foil loading because if you know the speed and CL, you know the loading. When you plot the section's incipient cavitation curve, you have a lot of information on one graph. If the loading curve is below the section curve for your operating speed range, then cavitation will not occur. (At least for the 2D section. 3D effects can still result in cavitation.)
     

  3. tspeer
    Joined: Feb 2002
    Posts: 2,319
    Likes: 295, Points: 83, Legacy Rep: 1673
    Location: Port Gamble, Washington, USA

    tspeer Senior Member

    It's not that simple. The minimum pressure is a function of the section shape, foil loading, and operating speed. Cavitation can occur on different parts of the section at different conditions. For example, at takeoff, cavitation can occur at the leading edge because of the leading edge pressure spike at high lift coefficients. At high speed, a well designed section will experience cavitation in the middle of the foil, near the end of the "rooftop" segment of the pressure distribution. And if there is too much camber for high speed, cavitation can occur on the underside of the leading edge. Designing a subcavitating section for the widest speed range requires balancing all of these operating points, while simultaneously satisfying structural constraints like minimum thickness.
     
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