# Why is the draft behind a fan not as strong as the blast in front?

It should be equal, of course, but standing behind a fan doesn’t cool you as effectively as standing in front. Is the intake more diffuse, and if so, why?

The air is being drawn in from an area covering 180 degrees behind the fan and then being blown out the front in a fairly tight stream. So when compared to the front of the fan, the airmass behind the fan is the same but the area is bigger therefore the velocity is lower and the windchill effect is less.

The “suction” end of the fan is much less directed than the “blow” end. It’s getting the air from everywhere behind the fan, but only pushing it out an an area the diameter of the blades.

The air rushing into the fan comes from a wide area and from all directions. Air is entrained from a region that may be considerably larger in diameter than the fan face, and it sort of funnels in toward the blades. The air exhausted from the fan forms a free jet that stays more or less the same diameter as the fan for a significant distance.

Consider the cross-sectional area of the fan’s slipstream, both before and after the blades. The mass flowing through any cross-section you care to slice is the same (the fan isn’t generating or absorbing mass, after all). Since the flow area in the intake region is much larger than the area after the fan, the flow velocity will be much lower before and higher after. It may be nearly zero over much of the volume you’re testing when you stick your hand in front of the fan. (For a constant mass flow rate, area and velocity are inversely proportional…air speeds up through a contraction*)

It’s not unlike the operation of an open-circuit wind tunnel. The one were I work can flow Mach 0.5 (nearly 400 mph) in the test section, and has a giant open-air intake that’s about 40 feet tall. You can stand outside on the sidewalk in front of the inlet when the tunnel is running full blast, and there’s maybe a 5-10 mph breeze. The contraction from that huge intake area down to much narrower the test section is responsible for creating a lot of acceleration.

As for why the air flows in from all directions but out in a confined jet…it’s all about momentum. The air being sucked in starts from zero velocity and doesn’t really have a preferential direction. Air from above and behind the fan…or from below, or from the side…can be sucked in just as easily as air from directly in line with the fan’s axis. It’s rushing in from all directions, so the effective intake area is big. But after being pushed through the blades, the air has momentum in that axial direction. It doesn’t want to start moving sideways, and so it tends to stay moving in a directed jet. Eventually mixing forces take over and diffuse that jet. But at first, the axial inertia of the air keeps the slipstream well-confined.
*For subsonic conditions

Damn dude, I wish I knew as much about my SDMB name as you apparently do about yours. You truly earned the “Aero” part of your name, kudos.

So for a thought expirement, if we put a fan in the center of a long tube (tube circumference just slightly larger than the fan circumference) then compared the draft in front of and behind the fan they should be about the same, right?

Yes, they’d be about the same for a ducted fan like that. I thought about mentioning that fact in my earlier post, but it got lost somewhere between my brain and keyboard.

Note in my linked graphic of a wind tunnel, above, that the test section is ahead of the fan itself. The walls of the wind tunnel serve to guide the air and confine the slipstream, even well ahead of the fan itself. The reason you want the air to go through the test section before the fan and not after is that the air is smooth ahead of the fan, but choppy and turbulent after.

So in the case of that hypothetical ducted fan, you might notice a different quality to the flow before and after. The overall velocity would still be the same, but sticking your hand in the smooth inflow might feel a bit different than in the turbulent outflow.

Add thanks for the compliment, Mr Buttons, but all I did was elaborate. The other posters got to the important part of the answer in a much quicker fashion than I did.

Yeah, I think bouv wins the thread for the answer that uses the shortest and fewest words while still providing a perfectly correct explanation. Go thou (me, too) and do likewise!

Well, to be fair, the first couple succinct responses didn’t address the last bit of the OP: Why is the intake more diffuse.

My long-winded answer was good for something…

Indeed it was, and I speak as one who likes to fill in the corners and add interesting facts myself.

I have edited reports of testing of an Army less-than-lethal “acoustic weapon,” where an air jet is puffed out in a blast like a thickish ring of tobacco smoke, where not only is the axial momentum super high, but the air ring speeding towards the impact has itself a high rotational momentum around the axial direction. The intended result is that the bad guy goes down, and walks away mad, I guess.

1: What would the physical layout of the air gun look like to produce such a blast?
2: How does the rotational speed–which increases as the mass of air lessens during flight–affect the strength of the ultimate hit?

Please use Aerodynamics for Idiots language.