Coefficient of drag of an object in water

[bigdummy]

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 coefficient of drag of a rotor (in motion) submersed in a liquid (such as water). For example, a wind turbine has a coefficient 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?

 

[akula]

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 .

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?

Akula

 

[Stingray]

If the Reynold's number is similar in two applications, then the coefficients of drag will be close.

 

[bigdummy]

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.

 

[Stingray]

In regards to the Reynold's number, aren't they totally different for water and air?

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.