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Coefficient of drag of an object in water

  1. Apr 10, 2005 #1
    I'm working on an idea that has to do with placing wind turbines on lake beds to catch the constant undercurrents (not a wholy original idea, I know). One of the most important factors to take into consideration is the lateral force exerted on the tower by the current--which is identical to the force exerted by the wind on a wind turbine tower. The only problem is that there is a lot of information that can be found on the coefficient of drag of a spinning rotor in a gaseous fluid (air), whereas I really haven't been able to come across much information regarding the coefficent of drag of a rotor (in motion) submersed in a liquid (such as water). For example, a wind turbine has a coefficent of drag of about .9 while rotating. And the maximum value for an object (such as a parachute) in air is around 1.5. Would these values hold true for the medium of water? Is there some explanation that limits the coefficient of drag to a value of less than 2?
  2. jcsd
  3. Apr 11, 2005 #2
    Found something that may be of help to you in the design of the blades themselves. It seemed that the design you might be considering was similar to that of a boat's outdrive, so I went to one of the more popular manufacturers and found this page from http://www.mercurymarine.com/chapter_4_-_propeller_technology [Broken].

    Also found another paper http://web.mit.edu/13.012/www/handouts/propellers_reading.pdf that gives equations on figuring out the different properties and other design considerations.

    Out of curiosity, are you thinking large scale application, or a portable source for boaters or others near water?

    Last edited by a moderator: May 2, 2017
  4. Apr 11, 2005 #3


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    If the Reynold's number is similar in two applications, then the coefficients of drag will be close.
  5. Apr 11, 2005 #4
    In regards to the Reynold's number, aren't they totally different for water and air? And as for the question regarding the application of this information: I'm much more interested in small scale production. Sure, large-scale power generation is where all the money is, but who can do the R&D for something like that in their garage?

    Sidenote: There is supposedly this Dutch firm that has come up with a micro water turbine that is small enough to fit inside of the water pipes of a home and can still generate about 1.5V. Now that's not a whole lot of power, I realize, but it's something. I'd really like to play with this thing if I could get a hold of it, but it doesn't seem to be on the market anywhere. If you have any inklings, please do inform.
  6. Apr 11, 2005 #5


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    Yes, if everything were identical. If you're comparing things of different sizes (but identical shape) moving at different speeds, then the Reynold's number can be similar.
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