The answer to every part of this video is the back pressure that's being generated. The air has to go somewhere, so as you get closer, the pressure goes up as there is more resistance for the air to flow out of the way out of the back of the fan. Eventually it become too great, so the flow rate drops down and you stall the blades (literally like an air plane wing, which the air stops being laminar over the whole blade and it starts to detach). For the end of the video, the hand is more wavy than the plywood, causing less efficiently flow around it, causing a slightly higher back pressure. It's really cool that you could so decisively measure this.

Basically, in the distant past, fans and hands had a pact of peace spanning thousands of years. A few centuries ago, a rebellious hand broke this pact, causing a fan and hand turf war. This is why the fan refuses to occupy the same space as your hand, and chooses to fall over, rather than touch it. I hope that helps.

@AlexLTDLX

Жыл бұрын

Lol. Yes, it helps a lot. Great comment.

It's simple your cupped hand let's the air have a place to go... the flat piece of wood does not. So pressure builds higher with your hand. My 2 cents.

What you’re not noticing, is the affect the air pressure is having on your skin! Everyone mentioned the cupping between your fingers and also your palm. But at the highest pressure points, the air is going to indent your skin, creating a bigger parachute pocket. Something else to ponder, a Harrier jet. I’d imagine that it requires more force to lift off, with the flap down, than if the engine were directly facing the ground. The simplified answer, is back pressure! New air is trying to occupy space before old air can leave it, which causes a chain reaction of air. The FAA hasn’t hired enough Air Traffic Controllers to prevent delays in the flow of things.🤣

Alex it’s the same concept in aeronautics that makes “Ground Effect” with low wing airplanes vs high wing ones, and hovercrafts that don’t create a lot of pressure just high volume of moving air and it makes a cushion of pressure at the collision point of the plank or your hand. Since your hand is cup shaped it magnifies the collision pocket to be more effective at catching the air velocity, similar to a spinnaker on a sailboat vs a flat sail or panel to catch wind. And not only does your hand, catch the velocity and make pressure in the pocket, the pressure also pushes towards the fan, increasing its thrust force back on the fan.

@vercingetorige400

8 ай бұрын

seems far much easier to understand it in terms of pressure: by blocking you're creating a higher pressure zone than in the back, so you create a force and you can flip it

We need more of this thank you 👍🏾👍🏾😎

@AlexLTDLX

Жыл бұрын

Thanks. I'll probably do a follow up video.

Fingers have pockets in between and your palm acts as a big pocket. Think of pushing a sail with some give vs pushing a flat sheet. More pressure behind the sail than the sheet. That's my basic theory. All the micro cracks on your hand may also create an air friction or surface effect as well vs the relatively smooth board surface. One may generate a better boundary layer or essentially directional flow surface than the other. You can also intuitively move your hand to catch the most air, but holding the board perfectly level is difficult and even a slight tilt gives the air an easier path which also creates a suction effect as the air speeds up off the board. I'd suspect some combo of all of the above. Aerodynamics are so complex, but I think it's mostly intuitive answers in this case.

eddy currents around the base of the fan and table

And it will generate less pressure because your hand has more resistance

Whenever you create a restriction, you create a pressure and the velocity can increases. That’s why. You literally funnel the air and create more thrust against the back of the fan.

You're creating a zone of high pressure where the air had previously dispersed quickly enough. The denser air requires more energy to displace, ergo, a force is exerted on the fan blades themselves (and the housing as well, a little), causing it to tip over Dense air is seeking equilibrium and your hand's presence alters where that occurs Your concave hand helps create a pocket of high pressure & high density, ergo, there's more for the fan to push against and create force. The lines and creases in your flesh and other structures of your hand help to trap air and make it more difficult to displace I'm thinking of this whole system as a liquid, and maybe that's helping me? I may be wrong too

@AlexLTDLX

Жыл бұрын

I should've included this in the video - but when I was playing around with this, I tried a dish, concave side facing the fan - it didn't help; the results were more similar to the flat board. I agree with your pressure idea - it's basically what I'm thinking too, just phrased differently. I'd love to think of a way to prove or disprove this empirically...

@krakhedd

Жыл бұрын

@@AlexLTDLX you could try to wire some pressure probes. Or mount a single one up on a stick or something so you can move it around and sample different zones. Might need a spring dyno, constructed for lateral force (or it could be a fun rabbit hole purely for shits & giggles) I'm positive it's happening due to lack of equilibrium pressures!

Build a hopper that feeds flour into the airflow so you can visualize what the paths the wind takes.... Haha just kidding, wanted to mess your desk up

@AlexLTDLX

10 ай бұрын

lol.

I would love to see what happend if you tried a bowl

Slap like now, lol. I haven't watched his stuff in a minute...

Only liquids can cavitate & I believe your hand not being smooth causes more of a vortex the same way vortex generators work on aircraft to prevent low speed stalls. I am not an engineer but my pimp hand is at a tier 1 level!

@AlexLTDLX

Жыл бұрын

Thanks William. I learned something from you today - always a good thing!

no the curved surfaces of your fingers gives a place for the air to be trapped and coalesced creating additional force.

Maybe your hand creates more turbulence which creates a larger dynamic pressure restriction for the air to maneuver around What happens if you Cup your hand Concave or Convex?

Put the fan over the scale blowing down I’m curious to see

@AlexLTDLX

Жыл бұрын

That would've been something to try - who knows, maybe I'll do a follow up if someone give me an explanation and a way to prove it empirically.

Pocketing or catching and throwing causes a jump pulse. Sudden increase in speed. Like racket strings on a tennis ball.

The wood have a smother surface hence the airflow find a way to flow over it, you hand is probably trapping the airflow just my guess 😊

@AlexLTDLX

Жыл бұрын

As good a guess as any, actually.

My guess is cause its pulling the air from over your hand…

it slowly starts spinning until reaching max speed. once you place your hand near it, instead of the fan just moving air, it's now comressing it. run a high speed video and compare the difference it makes falling when you slowly bring your hand in close versus bringing it in fast with some velocity. if it is to do with a compression front then the faster you bring your hand in the faster it should try to escape. the air doesn't ALL go around your hand, some bounces straight back at the fan. you can test this as well by removing the square shround from around the fan. the shroud increases static pressure because it's much harder for the air to create a vortex right back into the intake side of the blades. your hand grabs more air (air has viscosity) and is less smooth. that turbulent air creates more of a frictionous environment that can build pressure more readily. TLDR; it's pressure versus thrust.

The axial flow has to turn into radial flow. If its 360 degrees then at 25% the housing inner diameter the areas are equal. If the bottom is shrouded as it is on the desk it will be a bit more than that but all sorts of things have to happen for the direction change. Obviously the full answer is going to be complex... So with the board, especially if there is a board on the fan something else happens in conjunction. The air will go radial and quickly slow down as its stuck to the smooth surface of the board and as its slowing down it will actually create a suction where the board having a hole and a fan mounted and a flat board will be drawn to each other but will also repell each other id too close. Perhaps some of this suction is happening with your board but the hand is too caotic yet still increases acceleration on the air by the fan blades or even reflects and interacts with the blades in the reverse direction where as the radial flow over the sticky smooth surface is either creating a suction or not creating a back turbulence? Veritasium did a video on the board distance equilibrium a while back.. ive also considered this as a reason some carburetor setups just plain dont work. The prostock or otherwise high rpm types want very tall manifold plenums especially with single carbs so all ports in the tunnel ram have line of sight to the carb and fuel will reach the outer cylinders however cross ram manifolds sometimes work better than run of the mill tunnel rams, or single carb setups with low profiles often work fine as if there is a middle road that doesn't work. I feel that interaction with the manifold floor causes a radial flow and if it happens to be in this happy spot where enough air wants to be raial then fuel has no issues making it to the ports. But if its too high then there is stagnant air and the outer cylinders may pull from he slower air or the air/fuel hits the floor too slowly to stay vaporized and suspended. You see weird things like this on occasion with air filter tests that shouldnt affect fuel. Its not 100% but there are occasionally tests using filtered lids that lose power and by taping up the sides and using lid only flow the power comes back, as if the wrong mixture of radial and axial flow has a negatice effect. Thats kinda far from the thrust conversation but perhaps there is a part that's related because if it affects the amount of air flowing it also changes the energy used to accelerate the air and the trust difference may have some small factor that has to do with the conservation of momentum etc... I think the suction of radial flow over a smooth surface is a good place to investigate..m

You would have learned everything you wanted to know if you had a MAF infront of the fan and measured the voltage output.

pressure only travels so fast, your're accumulating a pressure increase behind the fan that starts pushing back against the air it's throwing out then that pressure propagates to against the blades, think of it as making it's environment more like water. Now it's acting slightly more like a scuba thruster, enough to simply tip over at least.

I teckon its a bit like why some intake manifolds have ribs below the carby, at wide open throttle and the butterfly's are 90⁰ to the floor of the manifold a small amount of turbulence is created by the reverb for the want of a better word of incoming air bouncing back . Your hand is acting like a vane of sorts directing the air flow away from the fan allowing it to be slightly .ore efficient than the flat surface.

Ram air will increase flow. If you did this on your centrifugal it will cause it to flex and hp will decrease

fluid dynamics comes to mind. Rock this experiment with a water hose. I also think you are crazy moving to texas but then again if you moving onto all electric builds then the heat is nbd~!

@AlexLTDLX

Жыл бұрын

Thanks for the idea - I'd have to figure out a way to do it with a water hose. FWIW, where I'm moving to has 7 months of great/decent racing weather and I'm literally just a few feet above sea level - you can see the Gulf of Mexico in the background of one of the drone shots at the beginning - this is nothing like Dallas or Houston. Where I'm at now gets too cold to race in the winter, and the summer is too hot and humid to race. Basically, you have the rainier/more schizophrenic/shorter seasons to race during (i.e. fall and spring), and traffic here is unbearable. I wouldn't even dream of trailering the car anywhere from here unless I had no choice. Where I'm at in Texas virtually every road is a dragstrip - flat and straight and little traffic, except during the summer, when it's too hot. Besides, I'm planning to die down there - since almost no one will read this whole comment, google, "Rockport, TX beach" and check out the pics. Maybe my old, fat backside might even try to play beach volleyball again (though that may be a pipe dream at this point)...

@al73r

Жыл бұрын

@@AlexLTDLX get out there and let the belly fly ..no one cares :-) enjoy your self brotha!

actio = reactio: friction is needed for the air to transmit the force. By blocking the flow you create a higher Resistance, decreasing flow, but increasing pressure, which creates thrust. There is a optimal resistance, which creates pressure without stalling the fan too much. This explains, why thrust decreases as you get too close. I´m not entirely sure, why your hand is more effective than the flat surface. My theory would be that the slots between the fingers allow a more even destribution of flow compared to the "dead zone" in the middle of the wood and the curves allow more laminar flow than edges.

Try using a curved object on top instead of a board. I think your hand is enough to redirect the air and add pressure.

I believe the answer is quite simple...it's magic

Oh btw; the "stand" u mounted the fan to is WAY too close to the inlet. It is affecting flow & yes, it is affecting the results of blockage on the output. Much of aerodynamics & affects are exponential in nature

Think of an exhaust... run a straight pipe, then run a pipe with a million bends... which one will have the most pressure... theres your answer

These are restrictions.

Wish I had answers for you but sad to say I'm just as perplexed as you are. If not more so.

It's just ground effect

ground effect

@Jkauppa

Жыл бұрын

ie more thrust close to a surface

@Jkauppa

Жыл бұрын

better efficiency

@Jkauppa

Жыл бұрын

air-lev bearing

@Jkauppa

Жыл бұрын

hover craft skirt

@Jkauppa

Жыл бұрын

some bernoulli fluid flow stuff, pressure etc

Really? Push your self off s wall now push yourself off air what happens? Your hand creats a pocket the egdes of your palm restriced the the thrust from escaping

Aerodynamics. U must research it & read to understand. Way too much to explain here. A basic but mostly correct explanation would be ur hand creates more resistance than air which creates more pressure behind the fan,( & contributes to the low pressure in front of it due to velocity interactions, etc.). The reaction is the fan moves more than without your hand. Your hand also offers more pressure rise than the board due to the non-flatness compared to the board. The shape of your hand tends to act alot like an aerodynamic thrust reverser on a jet; although the thrust isn't all physically directed forward, due to boundary layer & Eddie's it is essentially pointed that way. Somewhat. That in turn takes energy,(velocity), from the air which is inversely proportional to pressure. Velocity drops, pressure rises hense the fan is moving/pushing a denser fluid. See simple. No. What will REALLY bake your noodle is using a water hose in a tub! I'll splain: turn the water on. Fill a large bucket/trough. 100 gallons or so. Once the water is at least an inch deep slowly put the end of the hose in the water. You will notice the hose will suddenly offer less resistance. ALOT less! It tends to be pulled into the water. In the direction of flow! Why?

Theres a pressure because of the small gap and huge fan trying to pull that air through that gap (between your palm and fan)

Ground effect plain and simple. it falls because of friction on the bottom, if it was on a sheet of ice it would scoot right along.

@AlexLTDLX

Жыл бұрын

Right - but the question is, why does the thrust increase with my hand there and less so with a flat board there?

@bobwinn

Жыл бұрын

Try this, use a much larger board, maybe 150x150 cm. And hold it 1cm closer jumps, starting at 30 cm. Then do the same with bubble wrap over the board.