How can you determine if a fan is better at blowing or sucking?

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  • #26
russ_watters
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Please withdraw that comment before I report you to a moderator. You have no knowledge of my academic qualifications nor of my expertise. Neither is relevant here. I seem to recall Einstein was a patent clerk when he upended the scientific world.
Oy.
1. He is a moderator and is telling you, as a moderator, that you need to lose the attitude. I'll second that.
2. Einstein was not merely a "patent clerk" (as if the term implies an administrative role), he was a technical examiner, and more to the point, was a PhD physicist when he published his seminal papers. He just hadn't found a teaching job yet. That's quite an example to indicate why we should trust your credentials!

Your first post wasn't awful, but it wasn't great* and it was just unnecessarily hostile, which set this ball rolling down hill. There's no need to attack the word "suck" as if it describes something that doesn't exist. Even a basic dictionary definition properly describes how "sucking" works. I suggest you google it. There's just no need to attack the word. It doesn't matter.

The worst part, though, is this:
What a fan does is physically hit the air molecules in front of the blades from behind, just like a bat hits a ball.
This ignores most of the concept of lift; ignoring the much more interesting and important (but more difficult) things happening on the top surface of the airfoil.

I doubt if anyone who posted before you really didn't understand that a fan works by creating a pressure gradient (reduced pressure behind, increased pressure in front). And your post wasn't really all that helpful in explaining it.

*I'll note that I see one member posted and deleted a reply because he didn't care enough to try to fix what was wrong with yours, and I had the similar thoughts; I didn't care enough on first read to correct it either. I figured someone else would. But I didn't see the avalanche coming....
 
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  • #27
berkeman
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Please withdraw that comment before I report you to a moderator. You have no knowledge of my academic qualifications nor of my expertise. Neither is relevant here. I seem to recall Einstein was a patent clerk when he upended the scientific world.
No need to withdraw the comment. Your posts are already under Mentor discussion, so feel free to report away. As you can see, boneh3ad and I both keep our Profile pages up to date.
 
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  • #28
boneh3ad
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May I suggest you carefully read what I said before contradicting me. I said " How can the fan blade possibly affect air molecules to the left of it? It cannot attract them by gravity, electrostatic, electromagnetic means or by any other force. ".

It seems there are multiple reasonable ways to interpret the text you typed. The sentence "How can the fan blade possibly affect air molecules to the left of it?" can easily be interpreted as carrying a sense of incredulity. If a sentence can reasonably be interpreted multiple ways, that is on the writer, not the reader. A more clear version of the same idea might be "Consider the means by which the fan blade can affect the air molecules to the left of it." Tone matters, even in writing.

Can you perhaps tell us all how a molecule upstream of a fan blade moves to wards the fan. I state that it does so because more air molecules behind it than in front of it hit it, so it is the air molecules behind it which cause it to move forward. The fan doesn't "suck" - it pushes air molecules from behind causing a low pressure region into which the upstream air expands because the air molecules are being hit from behind.

As has been litigated extensively in this thread, "suck" is a perfectly reasonable word to use in the sense that a fan creates an area of low pressure behind it (upstream of it) which causes air upstream to rush toward it. Whether you like the word or not, it does make sense to a lot of people, but admittedly needs a careful definition to avoid the impression that air is somehow being physically pulled toward the fan, as I believe has essentially been your position.

I use air molecules because, the last time I checked, I found air was is comprised of discrete molecules and not as continuous matter. It seems sensible to describe what is actually there rather than some figment of someone's imagination.

Your car is comprised of individual molecules and not continuous matter. Do you only model it and explain its behavior on a molecular basis? The same could be said of any solid, liquid, gas, or plasma material at most common conditions here on the surface of the Earth. The fact is that continuum mechanics has been used to describe the behavior of solids, liquids, and gases for centuries with a great deal of success. Saying you are treating this problem as molecules because that is what comprises air and calling continuous media a figment of someone's imagination doesn't make your point any more clear or your science any more sound; it just means you have decided to ignore centuries of continuum mechanics for the sake of a "gotcha" moment. Enjoy your internet points.

This forum really fascinates me! When people are introduced to an idea they haven't heard of before they immediately close their minds and say "That's wrong" without spending a moment's thought as to whether it may be correct.

I am plenty familiar with the kinetic theory of gases. Air is made of molecules that buzz about randomly as we all know. That doesn't make the question in this thread any easier to understand. The fan interacts with relatively few molecules in the flow field, and to really explain how this could create a low pressure region, you'd need to start talking about mean free path and rms velocities of random motion versus bulk fluid motion of particles and it's all just a headache for the standard forum reader.

You could do it, sure, but why? A continuum approximation works just fine.

And, please don't digress by bringing aerofoils into the matter as the fan could be flat bladed.

I am not sure if you realize this, but you can fly a plane with flat plates for wings. It isn't extraordinarily efficient, but you can certainly do it. In fact, a fan blade generally acts exactly like an airfoil. It's just not usually as efficiently contoured because it doesn't need to be. (Although you can find companies who build fans that use actual airfoils for blades and holy cow do they move a lot of air.)
 
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wow checking back on this thread months after posting it. I really like where it went :D
 
  • #30
berkeman
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wow checking back on this thread months after posting it. I really like where it went :D
That's because you weren't here when it was hitting the fan... :wink:
 
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  • #31
rbelli1
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when it was hitting the fan

or at least the fan was hitting it

BoB
 

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