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

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beanflying:
Berni and you if you like initially are fundamentally WRONG. Bending or worse moving the rules to suit a narrative is not Science and or Engineering.

And if disagreeing on major FLAWED technical points or claiming 'special case' raised is arguing then you might want to try some basic reading here.

https://www.mvorganizing.org/what-is-compressible-and-incompressible-flow/

https://www.chegg.com/homework-help/definitions/incompressible-and-compressible-flow-5

IanB:

--- Quote from: beanflying on October 15, 2021, 05:40:30 am --- :o err NO. Reynolds numbers and if we get into smaller impellers then low Reynolds number issues on top of that are not 'minimal'.

--- End quote ---

Here, in fact, you answered your own question, and is why the answer is "yes".

The Reynolds number is a dimensionless group, which allows us to apply the principle of similarity between systems that vary in scale and other parameters.

If the Reynolds number, and other relevant dimensionless quantities are equal, then two apparently different systems will behave similarly, and the same performance characteristics will apply.

Under the principle of similarity, air and water can be treated with the same equations when the variation in air pressure is small, such as is the case with a typical cooling fan.

beanflying:
Reynolds numbers are dimensionless but when you throw AIR at around 800-1000 less density and viscosities at circa 100 times different at the calculations compared to water it becomes clear that water and air are nothing alike. Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.

That is UNLESS you make up  :bullshit: constraints and 'special case' rubbish such as you are continuing to do.

IanB:

--- Quote from: beanflying on October 16, 2021, 02:43:38 am ---Even if we take your earlier assertion of +-10% Pressure without doing the numbers that equates to about a 25% change in Density.
--- End quote ---

If we do the numbers it is about a 7% change in density. It is always good to do the numbers.

RJHayward:
beanflying: I get your point, about compressible fluids / gases, maybe off OP topic, sorry (I'm at about 1/2 year basic physics expertise).

Please see photo, regarding 'pushing' air streams downward, VS. simple lower air pressure, up around the faster moving air over that wing 'hump' shape.
Now, easy to accept, that a downward deflected air stream causing a 'lift' force on the wing, but do course textbooks then talk about TWO lifting force types ?
My experience, they always just talked about 'pressure' is lower, where the air is moving faster, there.
Note the picture features wing totally flat, on bottom, no super curvey.

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