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Why is the curvature of pump rotor blades "backwards"?

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IanB:

--- Quote from: RJHayward on October 15, 2021, 12:49:20 am ---Anyway, no thoughts, about a 'downward directed' air stream, is that real ?

--- End quote ---

Yes, because you cannot defy the laws of physics.

You can think about this by choosing a frame of reference. You could have a wing moving through the (still) air, or you could have a stationary wing (in a wind tunnel) and blow a stream of air over it. To understand wings, it is easier to use the wind tunnel example.

If the wing in the wind tunnel experiences an upward force, then it of necessity must be "pushing down" against something--Newton's law of action and reaction.

The only thing the wing can "push down" against is the air stream, but air will give way if you push on it. So what must happen is that the air receives a downward acceleration from the wing. To accelerate something downwards you have to push on it, which is exactly what the wing does.

This principle becomes very clear if you stand beneath a helicopter (which is an airplane with rotating wings). You will get blasted by downdraft. This is exactly the same downdraft produced by a fixed wing aircraft moving through the air. Conversely, it would be impossible for a helicopter to lift off the ground if there were no downdraft.

So what about pressure differences, you may ask? What about lower pressure above the wing and higher pressure below the wing? This is true too, but it is simply the other side of the exact same coin. Mathematically, the results are equal, but the formulas are different.

beanflying:
You also cant really include discussions or a lot of the analogies on compressible fluids and gases in a topic about incompressible ones ;)

IanB:

--- Quote from: beanflying on October 15, 2021, 02:13:19 am ---You also cant really include discussions or a lot of the analogies on compressible fluids and gases in a topic about incompressible ones ;)

--- End quote ---

Even so, the subsonic flow of air over wings, or through simple fans and blowers, behaves in almost the same way as incompressible fluids. As long as the overall change in pressure through the flow path is not that much (say +/- 10%), then the same rules can be applied with minimal error.

beanflying:
 :o err NO. Reynolds numbers and if we get into smaller impellers then low Reynolds number issues on top of that are not 'minimal'.

Berni:
Centrifugal pumps don't really use the effect of a areofoil aerodynamic lift, the curve is just there to help shape the flow, not to actually create the pumping action. It's propellers that use this effect, those are basically a wing that is actively forced trough the air. Even then you don't actually need to use wing curvature to make a propeller work, they work just fine with flat straight blades that are turned in the appropriate angle. The wing aerofoil shape just is a more efficient mechanism because it produces less drag, so your propeller can move more air for the same mechanical power input.

And yes air can be thought of as a really low viscosity fluid in a lot of cases. Most fans used to just move air around do not produce enough pressure to compress or stretch air in any meaningful amount. But once you move over to things like vacuum cleaners or turbochargers that are actually designed to produce a good deal of pressure then yes air is no longer just a simple incompressible fluid.

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