Get yer minds outta the gutter.
Serious question, though. I noticed today that the airflow from my large floor fan is significantly stronger on the front, ie, when it’s blowing AT me, than it is when I stand just behind the thing.
Now, I know it’s sucking in air – a sheet of paper handily demonstrates that. But the airflow is a lot stronger on the blow side than it is on the suck side. Why is that? I thought you’d have to pull in just as much air as you’re blowing out, right?
It doesn’t. Exactly the same amount of air is sucked into the back of the fan as is blown out the front. The difference is that the air blown out the front is directed, so that most of it moves in one direction; the air coming into the back of the fan comes from all directions and is therefore less intense at any given spot.
It can’t be THAT simple! 
I guess that makes sense, but I’m having a hard time seeing how. The fan I’m looking at is cylindrical, and the fan blade is close enough to the center of the cylinder that I can’t see any difference in depth from the front to the blade vs. the back to the blade (if that makes sense); so it seems to me like the airflow is going to be forced into the cylinder at the back just as much as it’s shaped by the cylinder coming out the front.
Make sense?
So, I guess I’m not understanding how air can come from every direction, when the fan is in the middle of the cylinder like that.
Am I just being incredibly dense here?
Yes - the air entering the fan is drawn in from a hemisphere of directions, the air leaving the fan is directed mostly along the axis of rotation. it’s that simple.
It’s also how pop-pop boats manage to propel themselves by just sucking and blowing water in and out of the same tube(s)
Or to look at it another way:
the fan creates an area of low pressure near the intake - the air that surrounds this low-pressure zone (all around it) is at a higher pressure, so it rushes in to fill the gap, from all directions.
But air leaving the fan has been set moving, and so it continues to move in the direction it was pushed, until turbulence and other effects dissipate it.
Or if you want another demonstration:
Inhale
Purse your lips and blow - place your hand in front of your mouth and feel the air flowing out in a fairly narrow stream - exhale fully
Now, without changing the position of your lips, start inhaling at approximately the same speed - place your hand in front of your lips again - you can’t feel much, if anything at all
-That’s because the air entering your mouth rushes in from all directions - there’s nothing to make it do otherwise, whereas the air being blown out of your mouth does have something to make it do otherwise - it’s being expelled in a single direction from a small hole.
For these reasons, we perceive blowing more than sucking, in these sorts of contexts, even though the same volume of air is being moved in either case.
You can’t suck out a candle, no matter how hard you try (actually, you probably shouldn’t try at all), even though you’re inhaling an entire lungful of air in the attempt.
You poor, deprived person.
Go get a vacuum cleaner with a hose attachment and turn it on. The air is drawn in through the hose and blown out through a bag or collection chamber. In this case the airflow seems much stronger at the hose than at the exhaust port, because it’s entering through a narrow passage and leaving in a more diffuse manner. Clearly the actual volume of air is the same at both ends, though, or the cleaner would explode.
That… makes it very clear, thank you!
While I’m at it, can I thank you for posting the pop-pop/putt-putt boat link? I’m currently nursing a bleeding thumb due to my over-eager assault on a soda can to create one myself.
Those… are… COOOOL!!!
I’m going to try making a diaphragm model engine soon - the coiled tube version was pretty feeble, but I understand a properly built tin can engine can be loud and impressive.
I’d also like to add my thanks for posting this link - I’d never heard of pop-pop boats (and it’s clearly the perfect response to the OP worthy of SDMB). Mrs. Flex’s physics class will get a kick out of this. 
Clearly you were never a Cub Scout, because that was in one of the books. They did have interesting science lesson projects.
I think the geometrical explanations aren’t correct, even if they feel satisfying.
The answer has to do with the Reynolds number and inertial versus viscous behaviors of the air.
If you ran the fan slowly enough, the flow pattern would become symmetrical front and back. Actually, for a propeller fan, the flow would become entirely rotational, with no travel along the axis, so maybe bad example, But if you had a magic grill that sent air perpendicularly through, and ran that slowly enough, there’d be symmetrical flow patterns.
Somewhere on the web is an interesting discussion of the Reynolds number that reveals that animals with a single degree of freedom in movement, such as clams, could not locomote themselves at all in a low Reynolds number environment.
So, the flow is more directed in front of the fan because air leaving the front has all its momentum directed in the same direction. If you put a diffuser grill on the front of the fan that reproduced the directional pattern of the back, you’d create a symmetrical flow system that way too.
I’ll try to read that through later in full, but scanning it over, it looks like it’s the underlying explanation for the geometric behaviours, in some way. I could be wrong about that.
Napier, Reynolds number may have an effect on the form of the air flow geometry, but the geometric effect is definitely what you are feeling. It is the same thing that happens to air passing through the main rotor of a helicopter. The incoming air comes from a wide area, but the shaft of accelerated air actually narrows as it comes out the other end (to half the diameter of the disk if I remember my momentum theory correctly). Thus, it actually is moving faster on the blow side, just over a narrower area.
The physics basically work like this: You have the same amount of air going in and out, basic conservation of mass. The fan adds energy to the air, accelerating it as it passes through. The amount of air passing through any given plane is proportional to VA where A is the area the air goes through and V is the velocity. VA has to be the same on each side of the disk for conservation of mass to hold. Thus we see that on the blow side where we have made V bigger, A must be smaller to compensate, thus a narrow shaft of blowing air.
Note that I have ignored density, but for planes a reasonable distance from the fan the analysis holds.
But if you set a box fan on a tread mill blowing backwards, at exactly the same speed the belt is travelling, is there really any air movment?
I think I misconstrued what I referred to as the geometrical explanation.
>the air entering the fan is drawn in from a hemisphere of directions, the air leaving the fan is directed mostly along the axis of rotation. it’s that simple.
This is true, and the same airflow happening over a smaller area does feel more intense, so this seems like an explanation. But it doesn’t explain why the inlet and outlet sides behave differently. It only connects the observed issue of intensity with the next in line issue of areas.
>it seems to me like the airflow is going to be forced into the cylinder at the back just as much as it’s shaped by the cylinder coming out the front.
Here, I hear the original poster asking why the areas are different. The explanation of this requires Reynolds number or at least consideration of the momentum and inertial behavior which dominate the Nre>1 regime.
The geometry per se doesn’t create any of the effect the OP notes. It can’t - the fan can perfectly well be stipulated to have front-rear symmetry. So, something arising out of more than just geometry is needed. You folks agree?
As I see it, air molecules are forced out the front of the fan by impact with the fan blades and thus are directed. Air molecules coming in from the back are forced in by air pressure (normal atmospheric pressure filling a vacuum). This force is more uniform and randomly directed.
According to a thought experiment I just ran here in my office, the “geometry” factor is the primary one. Viz: If I place a fan blade in the center of a cylinder and position myself at the front of it, I’ll feel a pretty good force of air emerging from the cylinder. By the same token, if I stand at the back of the cylinder, I’ll feel a pretty good force pulling me in. Second, the various molecules of air moving in to the back of a floor fan are moving at fairly slow speed from all directions until they are grabbed by the blade and focused into a cylinder of air moving away from the front at a pretty high rate of speed. I believe that the source of the air, being diffuse, seems to be, and actually is, moving slowly, and that the resulting blow of air seems forceful for those reasons. I am convinced that I’m making perfect sense. Don’t know what that does for the other readers. xo, C.
