shrouded propeller vs nonshrouded opeller

Discussion in 'Boat Design' started by jacklynfong, Oct 29, 2008.

  1. Eric ruttan
    Joined: Jul 2018
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    Eric ruttan Senior Member

    how does the Coandă effect eliminate drag?
  2. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    There is absolutely no Coanda effect with basic ducts. The Coanda effect involves a potential jump between the fast moving fluid jet along a wall and a slower region of potential flow away from the wall. There is an abrupt boundary between the two.

    With ducted flow, you want to do the exact opposite. You want a pure potential flow of even velocity across the duct diameter in order to maximize propulsive efficiency. This is true both at the inlet and at the outlet.

    Regarding the video - the guy doesn't have a clue. Blowing over the paper strip is an example of the Coandra effect - a jet of air against a surface with slower air all around. It is not an example of Bernoulli's principle that explains the air pressure along a stream line, but rather an example of a different physical law that describes the pressure change across streamlines. We often make assumptions that let you run Bernoulli's equation way upstream into undisturbed, uniform air, and then jump across to a new stream because the conditions are uniform, then track back down stream using Bernoulli's equation. That process isn't valid here - you can't do that across machinery in a duct. And you can't connect the air in your lungs to the air outside, the conditions aren't uniform by just the amount needed to lift the paper strip up to it's new position.

    His description of the flow field around a wing is also misleading. The air going over the top of the wing is going faster, but it doesn't rejoin the same air that passed underneath. there is a jump across the wake trailing from the trailing edge, and it persists downstream. Bernoulli's equation is valid, but not terribly helpful as far as gaining an intuitive sense of why a wing generates lift. And his momentum argument as it is presented violates the conservation of energy. You have to conserve both momentum and energy at all times and everywhere. That is the fun part.

    If you want a real explanation of turbomachinery, the University of India has an excellent web series in English. If you want to know how a wing works, try the MIT videos and notes cached at EdX from Drela's Flight Vehicle Aerodynamics.

    You are worse off watching that video than not watching it.
    W9GFO likes this.
  3. BlueBell
    Joined: May 2017
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    BlueBell Ahhhhh...

    Thrust is equal to Drag. They act in opposite directions.
  4. portacruise
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    portacruise Senior Member

  5. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    The lift an airfoil generates varies linearly with the angle of attack over range of angles of attack. Beyond that range the change in lift for a incremental change in angle of attack decreases and flow separation and then stall occurs. This is true for any wing, independent of the camber (curvature) of the section.

    Camber is not required to generate lift. A symmetric section, such as used for boat keels and rudder, generates lift at non-zero angle of attack. The major effects of camber are to shift the lift (for a given speed and air density) at which minimum drag occurs, and to shift the angle of attack and amount of lift at which separation and then stall occurs. Increasing camber results in an increase in available positve lift (positive angle of attack) before separation and stall, and a decrease in the available negative lift (negative angle of attack).

    The article in the link has a flawed description of how angle of attack causes lift generation. It is not just because "This forces wind to ‘pile up’ beneath the wings." Increasing angle of attack also causes the air/water to move faster over the upper surface which lowers the pressure on the top surface. The contribution of the lowered pressure on the upper surface can be more significant than the increased pressure on the lower surface.
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