0
$\begingroup$

I understand that the wings of a wind turbine are turning because the wind flows around them. Then due to their geometry a field of lower pressure is produced. And they are turning.

But a propeller is being turned by some engine. Which way then it produces lift? Why the conservation of momentum is not enough to describe the reason a ship moves? That it goes forward because it sends water behind?

$\endgroup$
0

2 Answers 2

2
$\begingroup$

Wind turbines and driven propellers are the same thing. In a wind turbine, the wind produces the relative motion between the wind and blades. In an airplane or ship, the engine spinning the propeller produces the relative motion between the air/water and blades. As far as the propeller is concerned, it doesn't care. There's relative motion between itself and the surrounding fluid. In a wind turbine the "lift" (it's not really called lift here as far as I know, but it is the component of the aerodynamic force that does what we want) is in the direction tangential to the blade rotation which causes the blades to turn, and does so more efficiently than if you relied on drag (i.e. the wind directly shoving the blades). In an driven propeller, the lift is the component of the aerodynamic force that points parallel to the axis of rotation (or perpendicular to the plane of rotation) which is forward in an airplane/ship propeller, or up in a helicopter rotor.

As for the momentum thing, I think it is enough but it is not very helpful on its own since all it is doing is taking the end product from the propeller, no longer caring about any physical characteristics of the propeller or conditions which it is operating under, and converting the result to another physical quantity (speed or movement). It's not useful in designing a propeller or predicting its performance.

It's like saying that all you need to predict a car's speed is the wheel radius and how fast the wheels are turning. Not untrue, but not very helpful. It'd be a lot better if you could describe the operation of the engine and the transmission transferring power to the wheels. Then you can design a car and predict its performance.

$\endgroup$
1
$\begingroup$

The common thing between a wind turbine and a propeller is that in both cases there is relative movement between a shape and the surrounding fluid, and there is a resulting conversion between a torque and a force. The turbine converts the axial force into a torque around that axis, and the propeller converts the torque about its axis into an axial force that thrusts the plane or ship.

Why shouldn't momentum conservation be responsible for that? There is nothing extra-mechanical to fluid dynamics. Just like a billiards ball that hits another causes both to be deflected, the propeller/turbine deflects the air/water hitting it, causing the corresponding forces/torques. The details, of course, can be complicated. Just like you need to know the position of all the balls on the table in order to predict into which pockets (if any) they will be going, you need to know the shape of the propeller/turbine and the boundary conditions of the fluid field in order to determine the efficiency of the torque/force conversion.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.