# Coefficient of drag of an object in water

• bigdummy
In summary: However, if you're comparing two objects of different shapes (and sizes) moving at the same speed, then the Reynold's number will be different.
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 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?

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

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

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.

bigdummy said:
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.

## What is the coefficient of drag?

The coefficient of drag is a dimensionless quantity that represents the resistance an object experiences as it moves through a fluid, such as water. It is typically denoted by the symbol Cd and is calculated by dividing the drag force by the product of the fluid density, the object's velocity, and its reference area.

## How is the coefficient of drag affected by an object's shape?

The coefficient of drag is heavily influenced by an object's shape. Objects with more streamlined and aerodynamic shapes tend to have lower coefficients of drag, meaning they experience less resistance in the fluid. On the other hand, objects with irregular or bulky shapes tend to have higher coefficients of drag.

## What factors affect the coefficient of drag of an object in water?

The coefficient of drag of an object in water is affected by several factors, including the object's shape, size, and surface roughness. The speed and density of the water also play a significant role, as well as any external forces acting on the object, such as wind or currents.

## How is the coefficient of drag measured?

The coefficient of drag is typically measured experimentally in a controlled environment, such as a wind or water tunnel. The object's drag force is measured at different velocities, and the coefficient of drag is calculated using the drag equation. Computational fluid dynamics (CFD) can also be used to simulate and calculate the coefficient of drag.

## Why is the coefficient of drag important?

The coefficient of drag is an essential concept in fluid dynamics and plays a critical role in understanding and predicting the behavior of objects moving through fluids. It is crucial in the design and optimization of vehicles, structures, and other objects that interact with fluids, such as ships, airplanes, and submarines.

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