@@matthiasrandomstuff2221 does a thicker pipe increase or decrease lift ? By the way, since I've got your attention, I would like to let you know that I love your videos and so does my grandpa. Even though he doesn't know English, he watches the video and we talk about it afterwards. You're awesome!
Making a pipe from thinner material lifts much less, while widening the top rim increases lift by a lot, so its gotta be coming from there. I made a short video on that, will publish it tomorrow.
In the late 70s as I was preparing to leave home for 4 years at Canoe U. and then on to a life in fighter jets. Some High School and Hockey buddies decided we should all go camping. Knowing the trouble this could be I drove myself. Just before Priest Grade outside Yosemite I stopped at a Gas Station/Mini-Mart. I noticed the old brown van ,that was the only other vehicle in the gravel lot, had a bunch of white squares all the way around the van under the window line. Each square had a Feynman diagram. A lady came out carrying bags of groceries and fumbling with her keys. I offered to take at least one of her bags and she handed me one and unlocked her door. As I handed her the bag I asked why she had Feynman diagrams all around her van? She responded, “well, I am Mrs. Feynman”. About that time, her husband came out the door carrying 2 more bags, which he too handed one to me. Then after adjusting his gear took the bag, turned around and said “Tanks”. (Not a typo). She then presented her husband with my question, to which he responded, “Hi, I’m Mrs. Feynman’s husband. Call me Richard”. I responded “Call me Richard, too”. We talked about 10 minutes, he asked Me lots of questions. I attended one of his lectures a few years later. He remembered me, told the Navy Brass I was one of the best helpers he’d ever had. I got an A in Physics that year.
Matthias, I’ve studied this effect for the past hour and I have come to the conclusion that this effect is caused by the air around the tubing being moved by the suction moving through the tube. I am contemplating making a video here on RUclips to explain, I’ll let you know if I do.
Watched your videos as a teenager. Loved your approach to building bandsaws. Now I’m 30 and just stumbling upon your videos again. I’m glad to see you still making the awesome videos you make. Inspirational.
I really appreciate your curiosity and thinking. I actually built a dish for my wifi repeater to get signal to an outbuilding. It's been a few years now and was a great solution. Much thanks for your content!
Consider if you pack the short tube with straws, would this tend to reduce or eliminate any possible recirculation effect? Rather than trying to visualize it with small threads, maybe try to remove it entirely and test? Great video as always. Thank you.
Hmm, packed full of straws = laminar flow! I think that's the right answer - when the air is rushing around the edge of the pipe on the elbow directly, you get recirculation like he showed, so little effect of the air pushing on the back of the elbow, maybe due to forces canceling, or due to the flow at the elbow still being very turbulent, but when you put a longer straight pipe in front of it, that section of pipe is dealing with the recirculation forces, and by the time it reaches the elbow, you have more laminar flow, so it pushes the elbow backwards - but that separates the elbow from the pipe, so it oscillates between the 2 conditions, and it bounces back and forth! Pretty nifty. I wonder what an elbow with a longer straight section of pipe directly attached to it would do?
I'd assume that would increase the friction between the walls and the air, producing more force in the opposite direction of the flow. Would need to test that though.
Fascinating! And Feynman was a fantastic guy indeed... More than brilliant! Anyway, thanks, Matthias! 😊 Stay safe there with your family! 🖖😊 And happy holidays!
I thought the conclusion was going to be that the fast moving air started trying to drag air in from the outside, and somehow that was going to cause the motion. I guess that could be ruled out by making a diaphragm around the moving tube, such that all of the air would be forced to go through the tube, but the tube would still be able to move up and down. But with the tiny forces involved, the diaphragm would itself be adding a variable that would probably still be significant.
The motion induced in the intake tube by placing an object into the (converging) air flow reminds me of the motion induced in the magnet in the (diverging) magnetic field of the Stern-Gerlach experiment. (but I can't identify a similarity in the physics)
Thats so cool! And it explains what I experienced just yesterday! I have 120mm dust collection tubing (ridged steel tubing) in my workshop and am currently upgrading my "main line" to 160mm. I have a little reducer piece to go from 160 down to 120mm and thats obviously a cone as well (About 100mm long). I was trying to - for fun - push the reducer piece the wrong way into the 160mm tube (So the 120mm side facing inside of the 160mm tube). And I just couldn't push it in! It kept coming out! I thought for sure it would get sucked in very hard and would be very loud with the air going through the now relatively small opening. Very fascinating indeed! thanks for sharing!
fascinating. I would think if you are sucking on the whole 160 mm diameter, that it would suck in hard enough to overcome whatever effect I was seeing with the cone moving away from the suction
The tube doesnt swing forward much because of the reaction force of the air going round the corner is cancelling out the -ve thrust. When air is flowing out the reaction force adds to the +ve thrust enhancing the effect. No idea about the straight side case at the end though.
I think that if you increase the wall thickness of the short piece of piping (and make a nice half-circle of the rim cross-section) it may become apparent: the suction on the top rim will pull it up - and as soon as it lifts and exposes the bottom rim, the same will happen there and it will be pulled down again. That would be my guess.
18:30 Off hand I'd say because the straight tube will form a vena contracta at the inlet which behaves exactly like a bellmouth would even if there is no physical material shape of the bellmouth. The flow coefficient for a plain pipe like that could be ~.5 so it will behave like a bell mouth of half that diameter. Since there is no actual surface to suck against it will suck against the air in the vena contracta which will in turn pull upwards on the tube walls as it recirculates in the vena contracta.
yes, that was my initial theory too, but I could not find the air getting sucked back up when probing with a thread. But it's something along those lines.
@@matthiasrandomstuff2221 Probably any thread will be pulled into the main stream immediately. Can you try tiny short threads on the wall or some liquid in the inside wall and see if it climbs up or not. It MUST climb up because the torridal flow in the vena contracta certainly has to be there. Failing that a thread or smoke in the top will reveal the shape of the vena contracta and you could calculate the size of the throat. This is an interesting phenomenon and I'd like to know more. Something very similar has been used in hi perf engine intake systems.
The suction causes a vacuum around the entire hose and nozzle since air is compressible/decompressible. It is the same fluid within the system as outside the system - not air pressure on non- compressible liquid. This prevents the forward motion as the overall space around the piple loses pressure. The boucing is when the pipe is in an air pocket so free-falls back to position. From the original position the rebalanced pressure starts the process again. Layman speaking with limited experience of an open-faced helmet at 140mph.
I think the behavior you see makes more sense if you change your perspective. The fan is lowering the pressure inside and at the end of the tube. The surrounding air in the room is at a higher pressure and so that surrounding air is pushing the air near the end into the tube. It explains why the "reverse sprinkler" doesn't turn backwards and also why you see that separation. The air from the room is trying to squeeze through the crack into the low pressure area inside the tube/pipe. The air in the high pressure zone is trying to find the easiest path to the low pressure zone. Thanks for making these kinds of videos. I'm trained in mechanical engineering but this kind of stuff is non-intuitive, even after studying. The simple "grade-school" or common sense explanations are often wrong.
The movement of the suction pipe with the knee in front of the fixed straight pipe and the straight pipe above the suction use different effects for the movement. 0. The Bend pipe don't move much because it will suck in air from all directions around the opening of the pipe. Only a small section in the middle will make the pipe move forward. 1. The "loose bend pipe" experiment oscillated because of the weight of moving air that was changing direction and with that impulse. The air gets accelerated in the straight section of pipe and the enters the bend section, it has to change direction. It has to loose the horizontal momentum. This pushes the pipe away from the straight section. The movement in the fixed pipe stops and the bend pipe falls back into the default position. It starts to oscillate. 2. You're sucking air through the short loose end of a pipe that is not exactly the same diameter or orientation than the pipe sucking the air, these misalignment can result in wired forces pulling or pushing on the loose pipe. My guess here is that some of air is hitting the suction pipe not exactly straight causing sight changes of air movement and with that turbulence in the suction pipe that results in an repelling effect as long as the loose pipe section is close enough. As the pipe moves away the air flow changes and the loose pipe gets sucked towards the fixed pipe. EDIT: changed some wired wordings and typos. It's hard to write in a non native language after midnight.
When the straight tube is closed to the end of the vacuum hose, the entire column of air is below ambient pressure because you are pulling a vacuum. This means that there is a force pulling the entire column forward towards the atmospheric pressure. Since the only articulating part of the machine is the end of the tube, it is pulled forward by the low pressure.
My first thought was entrainment. The high pressure atmosphere wants to push into that gap where the low pressure stream is, and if it's not able to form an airtight seal, there will always be a crack where it can start expanding and blow open. When the gap opens enough for the atmosphere to rush in, the velocity into the gap increases, and the pressure equalizes with the cone or tube. then the air current is able to pull the two pieces together again. I'm curious what would happen if you use a straight tube that's smaller than the inlet? (Mounted to something so it doesn't get sucked through.) Maybe there's an optimum size where the inlet would exert the most force on the tube, while still allowing atmospheric entrainment.
I'll preface this with: I'm not an expert, but I'm at least an engineer. I wonder if there could be some resonance going on in the long extractor pipe - maybe adding and removing the tube causes pressure waves that cause the air to accelerate and decelerate in the tube, and so cause the 'bouncing' we see. The fast-moving air in the short tube would suddenly come up against the slow-moving air in the extractor pipe and cause it to be pushed away. It'd probably be possible to disprove this by shortening the extractor fan pipe or adding a weight to the short tube. Fascinating video as always! Looking forward to the update🤞
This was an amazing video. I really enjoyed it, though I understand very little of the math / physics / fluid dynamics behind it. For some reason, the way you presented this one or the problem itself perhaps, I found incredibly enjoyable. Thanks!
I have not read all the comments so this may have been covered. I believe the lift is being generated by eddy currents as air gets sucked in from outside the tube. This air initially has less resistance and the tube is pushed away as it rushes in, then gravity overcomes the tube’s momentum and it falls so the process repeats. Adjusting the variables should act to control what can be viewed as a machine.
Before watching the end, I'm guessing the major force is the inertia of the air exerting a force on the outside bend. It doesn't matter which way the air flows, as the pipe is always moved to the direction of the outside bend as the air exerts force when having to change direction.
I have 8.5 years of experience designing turbo machinery, is just Bernoulli. The gap in between pulls air inward which separates both ends. Try inverting the suction cone you will see is not the slope of the first section is the gap that you have between both sections. The inlet also is pulled upward, you are right! Bernoulli again.
This is so amazingly interesting. I’m watching this and try to get my spouse to come and watch and she goes. That’s nice dear. Clearly we have different interests :). Thanks Mattias
I think the reason the vibe repels the vacuum is that the vacuum is sucking in air from the outside of the cone and the tube. Same thing with the cone rising, is sucking the air from around the cone and the shape of the cone is what causes it to rise.
3:00 is neat. it makes sense that the air in the tube provides resistance...but in unintuitive ways: when pressed up close, it seals well and the straight horizontal tube air begins to speed up. This lowers the pressure in the tube, and creates the pull force...but then as it reaches steady state the pressure at the elbow and the pressure in the tube are equal-ish. ...but then the air has a high inertia as it hits the elbow and provides a large leftward force. Once the seal breaks there is a big glut of air available to the low pressure elbow, so the elbow is drawn forward once again starting the oscillation.
As the air is pulled through the gap between the hose and the tube by the venturi effect it is accelerated and constricted by the small gap. To constrict flow you need to confine it, this requires a force. With no force holding the gap constant the opening is forced open until the air rushing through the gap is greater than the air coming through the tube at which point the tube falls back to the hose. It may be possible to set up a stable arraignment where the pressure through the gap matches the weight of the tube, but this seems unstable. I think because once the gap between the tube and the hose gets too large the flow through the tube slows the venturi effect drops and the gap closes. As the gap closes it exceeds the stable point due to momentum and so the tube rises again.
This is quite interesting from a horn loud speaker. We use horns to better couple the acoustic energy to air and to make the speaker more efficient. Basically the reverse of what the bell opening does.
The air rushing in is pushing left as it hits the 90 elbow so it's canceling out all the pulling force, when you lengthen the tube after the bend the air is accelerating faster and why it pushes the whole arm left away from the longer tube
Fun experiment. I love playing around with things like this in the shop. There is also a phenomenon called the Coanda effect with respect to flow and curved surfaces.
Matthias the momentum change from restriction with tube or cone into vacuum is the greatest. As the vac tube moves away from the straight or cone the vena cava of the air flow reduces the momentum change and pendulum effect of weight moves vac tube back in proximity to the restriction and the system oscillates back an forth.
8:18 From the center of the holes perspective, the tube is cone shaped, and so it still makes low pressure on the outside. Air get sucked towards the center of the hole, at an angle towards the outer tube walls, rather than parallel along the entire flow-path. An analogy would be a 3D drawing or render, with perspective vs. no perspective; since the hole sees perspective, it effectively sees the tube as a cone.
Welcome to the new "Mr. Wizard's World". I love these videos. I would love to hear what The Physics Girl would have to say about this phenomenon...hopefully she can recover much sooner than later.
3:55 the lack of an air-tight fitting allows slightly more air to enter around the tube than through it. If the added tube was slightly larger the oscillation would not occur.
I am not an expert in either liquid or airflow... and I definitely did not have any special training in these - but... My musing on it. The cone/tube floating over the vacuum: Are you not simply seeing the fact that a rapid flow of air creates less pressure. The outside of the container is at the normal atmospheric pressure. So basically, how a wing works. You have higher pressure on your outer surfaces and lower pressure on the inner, obtaining lift, but as it separates from the air flow that pressure drops, and you lose lift and if falls. The cone floats due to the cone shape creating more pressure within. The cone does not create as much pressure and so flutters. The hanging tube: Maybe not scientific enough for you, but... a suction itself will not move the tube forward much (partly due to weight) and the low resistance to the suction. In a liquid this would be different, and the water has higher density. When you move your hand in front you are creating higher resistance to the suction (more rapid air flow and so a vacuum) and so the tube flies forward. For the tube bouncing with the cone/tube placed in front - I suspect you are obtaining lower pressure in front of the tube due to the rapid air flow between the cone/tube and the hanging vacuum tube. This pulls the hanging tube towards the cone/tube. Once contact is made that low pressure between the two is lost (like your hand in front of the vacuum tube) and air is now flowing directly into the vacuum tube through the cone/tube. The tube then begins to settle away from the cone/tube which creates a low pressure vacuum again - and repeats, but with increased energy as it gained some from the previous actions. Just my thoughts - and again no expert at all - just thoughts.
An explication would be that Bernoulli effect. The air is faster inside the tube than on the outside opening, so there is less pressure inside the tube than on the opening. It's true there is no wall where you can picture a pressure difference but is that necessary for a force to exist? I'm not sure.
The air surrounding the pipe has no initial velocity and hence no momentum. The vacuum in the pipe starts accelerating the air just outside the pipe, and you would expect the bent tube to deflect in the direction of the outlet, but then the air hits the bend in the pipe which redirects the momentum towards the vaccum, but absorbs the imparted momentum, and practically stops deflecting.
i believe that the high velocity air being channeled though the toilet paper tube contain an amount of velocity pressure. When it hits the elbow, or any restriction, it causes a reverse pressure wave. I believe that you have rediscovered "water hammer".
For your vertical test, my thought is that it's a variation where the air is primarily moving the fastest. At the start, air is pulled through the top of the tube, which creates a low pressure region above the tube. Even though the cross sectional area of the tube is really small, the force of the atmosphere is huge with a relatively small pressure difference, and the force on the bottom edge of the tube pushes it up. As it moves up, air starts to come from the interface where they were previously joined, which makes a low pressure region there (or at least reduces the difference in pressure compared to the top), and the tube is forced back down. You tried the disc on the outlet by itself, but on your vertical test, what if you put the disc on the bottom of the tube (at the interface with the vacuum) - i would expect the effect to be larger if the above is true.
yes, it turns out that's what happens. Video on that tomorrow.
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at 2:56 (same for 8:34?), your pipes aren't the same size 100%, wondering if the fluid suction is greater at the mouth of the bend, then the extension. This would create a void difference and a path of least resistance at the first opening instead of trying to pull it through the pipe. Of course, it bounces back because of the 'inertia' of the pipe swinging back and repeating the motion. At 4:50, the pressure of the water hitting the gate, would have equal or greater pressure, but while the fluid is flowing, it's moving away from the gate so the pressure on the gate is not 2x or =
In regards to the comment about people saying you would lie about this.... I mean... What POSSIBLE motivation could this hypothetical person come up with??? I mean..... fluid dynamics.... WOWZA! That has GOT to be a top trending subject on RUclips right now 😂😂😂 Matthias is definitely cutting up paper towel tubes and hot gluing them back together solely for the money and attention Enjoyable as always, even if I never dreamed I'd be thinking so hard about how a fluid moves through an opening in my life.
my guess paused at 4:30 is that the L shaped tube pulls air through the cone, which then creates a force on the bottom of it, which pushes it back (we saw how much it was affected by small breezes just from blowing on it a minute before), and it's able to be pushed back since it's not attached.
Isn't the part being pushed away by the addition at the bottom all Bernoulli's principle? Air is being pushed though that added bottom piece and more air wants to join, thus pushing away the swinging part with force.
2:06 the difference between reaction force of prrssure to suction in this example is because there flow dynamics resulting from pused liquid leaving a tube is Very different to suction into one. Pushed out of a tube, you get a directed stream. This creates a direct, linear reaction force. Suction happens in a spere around the opening: it pulls from the shortest distance, more from the "sides" of the opening, so suction just negates itself.
Not recirculation, just the flow at the edge getting centered deeper in the tube, the flow being positive everywhere, just having a greater radial gradient deeper in the tube
The tube is affected by turbulence, spinning air at lower pressure, that draws it towards it, but the air coming from behind filling the hole is keeping the tube down, it's all about equilibrium.
That's a great demonstration for college level course. Unfortunately my fluid professor was so bad at teaching that they did away with it and changed the requirement. I had him for thermodynamics. I can attest to how bad he was. Incredibly gifted researcher though, just was not organized enough to put together a coherent lecture
Flow in the pipe, the speed increases with the length. A long enough straw and you blow into it, the exit speed at the other end would be speed of sound. Basically makes a invisible cone inside the straight pipe.
Fascinating! Try attaching little threads to the *outside* of the short straight tube; I'll bet you'll see airflow moving upward around the outside in a pulsing oscillation. This upward airflow occurs so long as the tube's bottom is resting on the vacuum inlet beneath. But the internal downward flow creates low pressure that overcomes the weight of the tube and support arm, creating a lifting force--upward flow--on the outside of the short tube (Coanda Effect). This breaks the upward flow (Coanda Effect) around the outside of the tube when it lifts off the vacuum inlet, and the tube settles back down. The motion develops into an oscillation.
Its a tolerance issue. Air is pulling in through gap and creating pressure differential. Rest the tubes together with flat collars attached to each tube. Like the disc you used in first part of video. This will restrict air access to that gap.
The suction pulls air from wherever it will come. One is through the tube, the other is from the crack at the bottom of the tube where it meets the hole. I'm wondering if the air entering at the bottom of the tube somehow causes the tube to rise. To test this the gap would have to be air-tight yet the tube still be free to move up.
Maybe the wall thickness of the tube plays a role? You should try it with a very thick wall! The air at the top end is moving and thus at lower pressure than the air at the bottom. But as soon as it moves upwards, air can flow around the bottom until the pressure is the same on both sides and the tube drops down again. This would explain why it oscillates and the cone doesnt.
I think the air flowing through the pipe extension can only move so fast so the unrestricted air from outside can force it's way in which pushes the main pipe back but only so far before gravity takes over
weight of tube could be damping the effect, possibly also some conflating airflow in the gap between the cardboard tube and the wooden plate which you can see at 0:39.
This is beginning to make sense of the information on cylinder head porting I've been wading through lately. The inlet flow back eddy explains why a smooth straight port does not flow the most amount of air fuel mixture. It also applies to intake and exhaust manifold size and shape. A smoke pencil {like in a wind tunnel} would better show where the air pressure and volume are increasing or decreasing. FYI: an old rule of thumb in HVAC duct sizing was that the pressure drop from a ninety degree bend in a 5 inch pipe was the same as an additional ten feet of straight ducting.
'FYI: an old rule of thumb in HVAC' Similarly, straight air flowing along a wall interacts with the wall, and sees it as a restriction. The wall has to angle out at ~7 degrees before it appears neutral to the airflow. So even the straight tube appears as a restricting tube. I'd bet if you reverse the cone and widen it as it goes in, the roughly 7 degree wall where it stops acting as a restriction to the airflow is where it will stop producing lift. Value may need adjustment for a cone instead of a flat wall, but the basic idea is the same..
the pipe isn't centered. Air is being pulled in past the outside of the pipe on one edge causing a net tangent force at the bottom of the pipe that is twisting it, which tilts that force vector to have a vertical component
Regarding the two tanks.....the reason that the flow from the tank on the left keeps the plate against the other tank is the velocity head acting on the left side of the plate which is greater than the hydrostatic head acting on the right side of the plate. Think of how a ram pump operates. The same principle applies here.
I think the oscillations you're seeing come from air's ability to have large pressure gradients (because it's a compressible fluid). I'm not sure water would act the same.
Air won't just come in through the top but the bottom of the cylinder. At the point of touching the wood, thereby disrupting the cylinder flow at the bottom, causing it to "bounce."
I was guessing the seal between the pipe and the cone is imperfect, so some air is sucked in via that shortcut with lower pressure drop than doing down the extra length of the cone, and this creates a dyanmic pressure which seperates the two? If you have some kind of seal around the connection which it could break away with little force, does it still seperate?
my guess would be: the pressure drop inside the pipe from friction. you can test it easily with your setup using a longer pipe. Altough it feels as unintuitive, as friction also pulls the pipe down. but the gap at the bottom of the pipe forces an ambient pressure boundary condition as does the inlet at the top (fluiddynamics can be verrry unintuitive and i do have a phd in that field)
@@Farlig69 @Farlig66 yes i could run a simulation - would be interesting, but holidays without pc ;-) maybe when i'm back... (and matthias already has an easy setup to test the hypothesis)
@@flowgeek706 Well I guess it's too late now as Metthias has found the solution but would still be interesting to see the modelling of the gap hypothesis!
The force is made by acceleration of the flow. When you suck or blow, the air is accelerated most at the bend in the pipe. If you measured the weight of the pipe somehow, you would see that its weight increases significantly. There is still nonzero acceleration of flow at the nozzle, especially when you turn it on, but the change in velocity becomes spread out in the volume so that acceleration is small. This diffusion of acceleration cannot happen confined in the pipe though.
Looks like the air from the extra tube becomes faster than the air at the intake of the pendulum and pushes the pendulum in the opposite direction. The space from the gap between the two seems to work as an air curtain that causes the speed difference. Possibly if you were able to vary the speed of the suction device you could get a broader picture.
I made videos about this for space craters and mountains...heres the experiment...level of dust in a layer ( about 1 inch)...put the vacuum above and lower it slowly ( min was a 2 inch vac tube)...it will create a mountain as it is pulling in air literally Horizontally in a vortex (not really from the vertical ,as it is in a vortex ( is this a lower pressure situation?)) the vortex goes INTO the vacuum tube so you create a mountain...so in my opinion, the paper rises and floats ON the air going IN. ( and perhaps some rise from a negative pressure in the vortex)
I'm adding to the guesses but I'm going with friction from the inside of the tube. The air drags and slows down which makes an inner area of air that's moving faster. The dragging air kind of makes an "air venturi" which does the same thing a cone does just to a lesser degree. Granted I'm not a fluid dynamics nerd so maybe I'm wrong. This was a fun thought experiment anyway.
For the vertical case I think it could be a reaction force; the air at the side of the pipe detaches from the walls (coanda) and there's an opposite force as it does that. That would be why it is such a small effect.
Coherency when something is pushed out. Doesn’t apply the same when a vacuum cause it grabs from all around the inlet. It’s like sharp point vs vague bubble.
does entering the cone fully like you would a funnel into the end say 1 cm change the effect rather the butt end will they still separate , is there a compressible counter effect due to the gap drawing air from the out side , like a air lift pump for dredging
Maybe, the low pressure caused by the air bending around the upper edges of the straight pipe is pulling it up ?
Yes, that turns out to be it! There is enouch vacuum that the wall thickness of the pipe is enough to lift it by.
@@matthiasrandomstuff2221 does a thicker pipe increase or decrease lift ?
By the way, since I've got your attention, I would like to let you know that I love your videos and so does my grandpa. Even though he doesn't know English, he watches the video and we talk about it afterwards. You're awesome!
Making a pipe from thinner material lifts much less, while widening the top rim increases lift by a lot, so its gotta be coming from there. I made a short video on that, will publish it tomorrow.
@@matthiasrandomstuff2221this is fascinating. Could/would you repeat this experiment after adding the O ring from the other experiment?
good job man!
In the late 70s as I was preparing to leave home for 4 years at Canoe U. and then on to a life in fighter jets. Some High School and Hockey buddies decided we should all go camping. Knowing the trouble this could be I drove myself. Just before Priest Grade outside Yosemite I stopped at a Gas Station/Mini-Mart. I noticed the old brown van ,that was the only other vehicle in the gravel lot, had a bunch of white squares all the way around the van under the window line. Each square had a Feynman diagram. A lady came out carrying bags of groceries and fumbling with her keys. I offered to take at least one of her bags and she handed me one and unlocked her door. As I handed her the bag I asked why she had Feynman diagrams all around her van? She responded, “well, I am Mrs. Feynman”. About that time, her husband came out the door carrying 2 more bags, which he too handed one to me. Then after adjusting his gear took the bag, turned around and said “Tanks”. (Not a typo). She then presented her husband with my question, to which he responded, “Hi, I’m Mrs. Feynman’s husband. Call me Richard”. I responded “Call me Richard, too”. We talked about 10 minutes, he asked Me lots of questions. I attended one of his lectures a few years later. He remembered me, told the Navy Brass I was one of the best helpers he’d ever had. I got an A in Physics that year.
Unfortunately, the comments are so full of random guesses, its hard to figure out which is right -- but I did figure it out in the end.
better than nothing but "this video sucks" comments
Search "quasi 1D flow" for pipes. Basically, flow in straight pipes behaves like flow in nozzles due to friction.
Tell everyone how much you hate them matt.
Matthias, I’ve studied this effect for the past hour and I have come to the conclusion that this effect is caused by the air around the tubing being moved by the suction moving through the tube. I am contemplating making a video here on RUclips to explain, I’ll let you know if I do.
Smoke could be helpful to see what is happening
Watched your videos as a teenager. Loved your approach to building bandsaws. Now I’m 30 and just stumbling upon your videos again. I’m glad to see you still making the awesome videos you make. Inspirational.
I've also been watching since my teenage years and am 31. Matthias has always been an insta click for me. his videos are so consistently good
Just loving the direction this channel takes. Popularisation of science. Thanks !
I always enjoy the rabbit holes!! Thank you Matthias.
Matthias, you're an absolute gem.
I really appreciate your curiosity and thinking. I actually built a dish for my wifi repeater to get signal to an outbuilding. It's been a few years now and was a great solution. Much thanks for your content!
Consider if you pack the short tube with straws, would this tend to reduce or eliminate any possible recirculation effect? Rather than trying to visualize it with small threads, maybe try to remove it entirely and test? Great video as always. Thank you.
Hmm, packed full of straws = laminar flow! I think that's the right answer - when the air is rushing around the edge of the pipe on the elbow directly, you get recirculation like he showed, so little effect of the air pushing on the back of the elbow, maybe due to forces canceling, or due to the flow at the elbow still being very turbulent, but when you put a longer straight pipe in front of it, that section of pipe is dealing with the recirculation forces, and by the time it reaches the elbow, you have more laminar flow, so it pushes the elbow backwards - but that separates the elbow from the pipe, so it oscillates between the 2 conditions, and it bounces back and forth! Pretty nifty. I wonder what an elbow with a longer straight section of pipe directly attached to it would do?
I'd assume that would increase the friction between the walls and the air, producing more force in the opposite direction of the flow. Would need to test that though.
I recently read, "Surely You're Joking, Mr. Feynman!"
A wonderful book I recommend to all.
Great video, Matthias!
This is the sort of curiosity I feel society is losing in the younger generation. Superb content, and inspiring!
I’ve got great news for you! You’re wrong.
Neil Degrasse Tyson would be proud of you 😅
Fascinating! And Feynman was a fantastic guy indeed... More than brilliant!
Anyway, thanks, Matthias! 😊
Stay safe there with your family! 🖖😊
And happy holidays!
That sir is the most interesting thing I've seen all week.
I never imagined getting a lesson on fluid dynamics on this channel. Super fascinating!
I thought the conclusion was going to be that the fast moving air started trying to drag air in from the outside, and somehow that was going to cause the motion. I guess that could be ruled out by making a diaphragm around the moving tube, such that all of the air would be forced to go through the tube, but the tube would still be able to move up and down. But with the tiny forces involved, the diaphragm would itself be adding a variable that would probably still be significant.
Yeah, maybe I'm thinking the same thing, that the air trying to get into the stream from outside the tube/cone was pushing it up.
The motion induced in the intake tube by placing an object into the (converging) air flow reminds me of the motion induced in the magnet in the (diverging) magnetic field of the Stern-Gerlach experiment. (but I can't identify a similarity in the physics)
Thats so cool! And it explains what I experienced just yesterday!
I have 120mm dust collection tubing (ridged steel tubing) in my workshop and am currently upgrading my "main line" to 160mm. I have a little reducer piece to go from 160 down to 120mm and thats obviously a cone as well (About 100mm long). I was trying to - for fun - push the reducer piece the wrong way into the 160mm tube (So the 120mm side facing inside of the 160mm tube). And I just couldn't push it in! It kept coming out! I thought for sure it would get sucked in very hard and would be very loud with the air going through the now relatively small opening.
Very fascinating indeed! thanks for sharing!
fascinating. I would think if you are sucking on the whole 160 mm diameter, that it would suck in hard enough to overcome whatever effect I was seeing with the cone moving away from the suction
The tube doesnt swing forward much because of the reaction force of the air going round the corner is cancelling out the -ve thrust. When air is flowing out the reaction force adds to the +ve thrust enhancing the effect. No idea about the straight side case at the end though.
Thank you for making me think this morning and engaging my brain.
Fascinating.
I think that if you increase the wall thickness of the short piece of piping (and make a nice half-circle of the rim cross-section) it may become apparent: the suction on the top rim will pull it up - and as soon as it lifts and exposes the bottom rim, the same will happen there and it will be pulled down again. That would be my guess.
I'm neither a scientist, engineer or mathematician but I have a fondness for experiments of this type and greatly enjoyed the video.
Try a clear tube and fine smoke source to "see" what is going on in the tube.
18:30 Off hand I'd say because the straight tube will form a vena contracta at the inlet which behaves exactly like a bellmouth would even if there is no physical material shape of the bellmouth. The flow coefficient for a plain pipe like that could be ~.5 so it will behave like a bell mouth of half that diameter.
Since there is no actual surface to suck against it will suck against the air in the vena contracta which will in turn pull upwards on the tube walls as it recirculates in the vena contracta.
yes, that was my initial theory too, but I could not find the air getting sucked back up when probing with a thread. But it's something along those lines.
@@matthiasrandomstuff2221 Probably any thread will be pulled into the main stream immediately. Can you try tiny short threads on the wall or some liquid in the inside wall and see if it climbs up or not. It MUST climb up because the torridal flow in the vena contracta certainly has to be there. Failing that a thread or smoke in the top will reveal the shape of the vena contracta and you could calculate the size of the throat.
This is an interesting phenomenon and I'd like to know more. Something very similar has been used in hi perf engine intake systems.
The suction causes a vacuum around the entire hose and nozzle since air is compressible/decompressible. It is the same fluid within the system as outside the system - not air pressure on non- compressible liquid. This prevents the forward motion as the overall space around the piple loses pressure. The boucing is when the pipe is in an air pocket so free-falls back to position. From the original position the rebalanced pressure starts the process again.
Layman speaking with limited experience of an open-faced helmet at 140mph.
I think the behavior you see makes more sense if you change your perspective. The fan is lowering the pressure inside and at the end of the tube. The surrounding air in the room is at a higher pressure and so that surrounding air is pushing the air near the end into the tube. It explains why the "reverse sprinkler" doesn't turn backwards and also why you see that separation. The air from the room is trying to squeeze through the crack into the low pressure area inside the tube/pipe. The air in the high pressure zone is trying to find the easiest path to the low pressure zone.
Thanks for making these kinds of videos. I'm trained in mechanical engineering but this kind of stuff is non-intuitive, even after studying. The simple "grade-school" or common sense explanations are often wrong.
The movement of the suction pipe with the knee in front of the fixed straight pipe and the straight pipe above the suction use different effects for the movement.
0. The Bend pipe don't move much because it will suck in air from all directions around the opening of the pipe. Only a small section in the middle will make the pipe move forward.
1. The "loose bend pipe" experiment oscillated because of the weight of moving air that was changing direction and with that impulse. The air gets accelerated in the straight section of pipe and the enters the bend section, it has to change direction. It has to loose the horizontal momentum. This pushes the pipe away from the straight section. The movement in the fixed pipe stops and the bend pipe falls back into the default position. It starts to oscillate.
2. You're sucking air through the short loose end of a pipe that is not exactly the same diameter or orientation than the pipe sucking the air, these misalignment can result in wired forces pulling or pushing on the loose pipe. My guess here is that some of air is hitting the suction pipe not exactly straight causing sight changes of air movement and with that turbulence in the suction pipe that results in an repelling effect as long as the loose pipe section is close enough. As the pipe moves away the air flow changes and the loose pipe gets sucked towards the fixed pipe.
EDIT: changed some wired wordings and typos. It's hard to write in a non native language after midnight.
the Wandel effect 😂. Keep it up dear sir 🙏i really enjoy the wits and creativity.
When the straight tube is closed to the end of the vacuum hose, the entire column of air is below ambient pressure because you are pulling a vacuum. This means that there is a force pulling the entire column forward towards the atmospheric pressure. Since the only articulating part of the machine is the end of the tube, it is pulled forward by the low pressure.
My first thought was entrainment. The high pressure atmosphere wants to push into that gap where the low pressure stream is, and if it's not able to form an airtight seal, there will always be a crack where it can start expanding and blow open. When the gap opens enough for the atmosphere to rush in, the velocity into the gap increases, and the pressure equalizes with the cone or tube. then the air current is able to pull the two pieces together again.
I'm curious what would happen if you use a straight tube that's smaller than the inlet? (Mounted to something so it doesn't get sucked through.) Maybe there's an optimum size where the inlet would exert the most force on the tube, while still allowing atmospheric entrainment.
As a retired electrical engineer I would never have thought I would be interested in a video about obsure effects of fluid dynamics......but I was!
And now you know something you didn't need to know :)
I'll preface this with: I'm not an expert, but I'm at least an engineer. I wonder if there could be some resonance going on in the long extractor pipe - maybe adding and removing the tube causes pressure waves that cause the air to accelerate and decelerate in the tube, and so cause the 'bouncing' we see. The fast-moving air in the short tube would suddenly come up against the slow-moving air in the extractor pipe and cause it to be pushed away.
It'd probably be possible to disprove this by shortening the extractor fan pipe or adding a weight to the short tube.
Fascinating video as always! Looking forward to the update🤞
This was an amazing video. I really enjoyed it, though I understand very little of the math / physics / fluid dynamics behind it. For some reason, the way you presented this one or the problem itself perhaps, I found incredibly enjoyable. Thanks!
I have not read all the comments so this may have been covered. I believe the lift is being generated by eddy currents as air gets sucked in from outside the tube. This air initially has less resistance and the tube is pushed away as it rushes in, then gravity overcomes the tube’s momentum and it falls so the process repeats. Adjusting the variables should act to control what can be viewed as a machine.
Before watching the end, I'm guessing the major force is the inertia of the air exerting a force on the outside bend. It doesn't matter which way the air flows, as the pipe is always moved to the direction of the outside bend as the air exerts force when having to change direction.
yeah. that's my thought too. pushing
Dude! I actually understand this now! Thank you so much. Great experiment.
That was unexpected. I should not have watched this so late in the evening. Now I'll be laying in bed, wide awake thinking about it all night long.
I have 8.5 years of experience designing turbo machinery, is just Bernoulli. The gap in between pulls air inward which separates both ends. Try inverting the suction cone you will see is not the slope of the first section is the gap that you have between both sections. The inlet also is pulled upward, you are right! Bernoulli again.
do watch the follow-up video for the answer.
really cool video, your explanations are very intuitive.
Fascinating. Fluid dynamics is (are?) amazing.
This is so amazingly interesting. I’m watching this and try to get my spouse to come and watch and she goes. That’s nice dear. Clearly we have different interests :). Thanks Mattias
My audience is overwhelmingly male for a reason.
I think the reason the vibe repels the vacuum is that the vacuum is sucking in air from the outside of the cone and the tube. Same thing with the cone rising, is sucking the air from around the cone and the shape of the cone is what causes it to rise.
You make physics fun! The TV series Mr Wizard was one of my favorites when I was a kid.
3:00 is neat. it makes sense that the air in the tube provides resistance...but in unintuitive ways:
when pressed up close, it seals well and the straight horizontal tube air begins to speed up. This lowers the pressure in the tube, and creates the pull force...but then as it reaches steady state the pressure at the elbow and the pressure in the tube are equal-ish. ...but then the air has a high inertia as it hits the elbow and provides a large leftward force. Once the seal breaks there is a big glut of air available to the low pressure elbow, so the elbow is drawn forward once again starting the oscillation.
As the air is pulled through the gap between the hose and the tube by the venturi effect it is accelerated and constricted by the small gap. To constrict flow you need to confine it, this requires a force.
With no force holding the gap constant the opening is forced open until the air rushing through the gap is greater than the air coming through the tube at which point the tube falls back to the hose.
It may be possible to set up a stable arraignment where the pressure through the gap matches the weight of the tube, but this seems unstable. I think because once the gap between the tube and the hose gets too large the flow through the tube slows the venturi effect drops and the gap closes.
As the gap closes it exceeds the stable point due to momentum and so the tube rises again.
This is quite interesting from a horn loud speaker. We use horns to better couple the acoustic energy to air and to make the speaker more efficient. Basically the reverse of what the bell opening does.
The air rushing in is pushing left as it hits the 90 elbow so it's canceling out all the pulling force, when you lengthen the tube after the bend the air is accelerating faster and why it pushes the whole arm left away from the longer tube
Awesome, what cool effects. Fluids behave in mysterious ways.
I love listening to people who know way more on a subject than I do. I have no idea what's going on, but I'm along for the ride.
Fun experiment. I love playing around with things like this in the shop. There is also a phenomenon called the Coanda effect with respect to flow and curved surfaces.
Matthias the momentum change from restriction with tube or cone into vacuum is the greatest. As the vac tube moves away from the straight or cone the vena cava of the air flow reduces the momentum change and pendulum effect of weight moves vac tube back in proximity to the restriction and the system oscillates back an forth.
Everything you do is fascinating!
8:18 From the center of the holes perspective, the tube is cone shaped, and so it still makes low pressure on the outside. Air get sucked towards the center of the hole, at an angle towards the outer tube walls, rather than parallel along the entire flow-path. An analogy would be a 3D drawing or render, with perspective vs. no perspective; since the hole sees perspective, it effectively sees the tube as a cone.
Good ole Bernoulli. Explains so much
Actually, you haven't explained anything.
Welcome to the new "Mr. Wizard's World". I love these videos. I would love to hear what The Physics Girl would have to say about this phenomenon...hopefully she can recover much sooner than later.
3:55 the lack of an air-tight fitting allows slightly more air to enter around the tube than through it. If the added tube was slightly larger the oscillation would not occur.
Man, I love engineers!
I am not an expert in either liquid or airflow... and I definitely did not have any special training in these - but... My musing on it.
The cone/tube floating over the vacuum:
Are you not simply seeing the fact that a rapid flow of air creates less pressure. The outside of the container is at the normal atmospheric pressure. So basically, how a wing works. You have higher pressure on your outer surfaces and lower pressure on the inner, obtaining lift, but as it separates from the air flow that pressure drops, and you lose lift and if falls. The cone floats due to the cone shape creating more pressure within. The cone does not create as much pressure and so flutters.
The hanging tube:
Maybe not scientific enough for you, but... a suction itself will not move the tube forward much (partly due to weight) and the low resistance to the suction. In a liquid this would be different, and the water has higher density. When you move your hand in front you are creating higher resistance to the suction (more rapid air flow and so a vacuum) and so the tube flies forward.
For the tube bouncing with the cone/tube placed in front - I suspect you are obtaining lower pressure in front of the tube due to the rapid air flow between the cone/tube and the hanging vacuum tube. This pulls the hanging tube towards the cone/tube. Once contact is made that low pressure between the two is lost (like your hand in front of the vacuum tube) and air is now flowing directly into the vacuum tube through the cone/tube. The tube then begins to settle away from the cone/tube which creates a low pressure vacuum again - and repeats, but with increased energy as it gained some from the previous actions.
Just my thoughts - and again no expert at all - just thoughts.
An explication would be that Bernoulli effect. The air is faster inside the tube than on the outside opening, so there is less pressure inside the tube than on the opening. It's true there is no wall where you can picture a pressure difference but is that necessary for a force to exist? I'm not sure.
The air surrounding the pipe has no initial velocity and hence no momentum. The vacuum in the pipe starts accelerating the air just outside the pipe, and you would expect the bent tube to deflect in the direction of the outlet, but then the air hits the bend in the pipe which redirects the momentum towards the vaccum, but absorbs the imparted momentum, and practically stops deflecting.
i believe that the high velocity air being channeled though the toilet paper tube contain an amount of velocity pressure. When it hits the elbow, or any restriction, it causes a reverse pressure wave. I believe that you have rediscovered "water hammer".
For your vertical test, my thought is that it's a variation where the air is primarily moving the fastest. At the start, air is pulled through the top of the tube, which creates a low pressure region above the tube. Even though the cross sectional area of the tube is really small, the force of the atmosphere is huge with a relatively small pressure difference, and the force on the bottom edge of the tube pushes it up. As it moves up, air starts to come from the interface where they were previously joined, which makes a low pressure region there (or at least reduces the difference in pressure compared to the top), and the tube is forced back down.
You tried the disc on the outlet by itself, but on your vertical test, what if you put the disc on the bottom of the tube (at the interface with the vacuum) - i would expect the effect to be larger if the above is true.
yes, it turns out that's what happens. Video on that tomorrow.
at 2:56 (same for 8:34?), your pipes aren't the same size 100%, wondering if the fluid suction is greater at the mouth of the bend, then the extension. This would create a void difference and a path of least resistance at the first opening instead of trying to pull it through the pipe. Of course, it bounces back because of the 'inertia' of the pipe swinging back and repeating the motion.
At 4:50, the pressure of the water hitting the gate, would have equal or greater pressure, but while the fluid is flowing, it's moving away from the gate so the pressure on the gate is not 2x or =
In regards to the comment about people saying you would lie about this....
I mean...
What POSSIBLE motivation could this hypothetical person come up with???
I mean..... fluid dynamics.... WOWZA! That has GOT to be a top trending subject on RUclips right now 😂😂😂 Matthias is definitely cutting up paper towel tubes and hot gluing them back together solely for the money and attention
Enjoyable as always, even if I never dreamed I'd be thinking so hard about how a fluid moves through an opening in my life.
some people feel better when they accues others of things.
I certainly feel better accusing you of spelling accuse badly.
@@idomerrymake1211 He's Canadian.
@@idomerrymake1211incorrectly, not badly
@@bradley3549 He's the only Canadian that spells it like that.
my guess paused at 4:30 is that the L shaped tube pulls air through the cone, which then creates a force on the bottom of it, which pushes it back (we saw how much it was affected by small breezes just from blowing on it a minute before), and it's able to be pushed back since it's not attached.
Isn't the part being pushed away by the addition at the bottom all Bernoulli's principle? Air is being pushed though that added bottom piece and more air wants to join, thus pushing away the swinging part with force.
all pushing against the end of the tube? And do explain how that could relate to benoulli's law?
2:06 the difference between reaction force of prrssure to suction in this example is because there flow dynamics resulting from pused liquid leaving a tube is Very different to suction into one.
Pushed out of a tube, you get a directed stream. This creates a direct, linear reaction force.
Suction happens in a spere around the opening: it pulls from the shortest distance, more from the "sides" of the opening, so suction just negates itself.
Not recirculation, just the flow at the edge getting centered deeper in the tube, the flow being positive everywhere, just having a greater radial gradient deeper in the tube
The tube is affected by turbulence, spinning air at lower pressure, that draws it towards it, but the air coming from behind filling the hole is keeping the tube down, it's all about equilibrium.
I wouldn't be surprised if I saw some of the big science channels taking their turn to explain this phenomena in a couple of weeks.
That's a great demonstration for college level course. Unfortunately my fluid professor was so bad at teaching that they did away with it and changed the requirement. I had him for thermodynamics. I can attest to how bad he was.
Incredibly gifted researcher though, just was not organized enough to put together a coherent lecture
Flow in the pipe, the speed increases with the length. A long enough straw and you blow into it, the exit speed at the other end would be speed of sound. Basically makes a invisible cone inside the straight pipe.
What if you lengthen the horizontal part of the tube? Does that cause the tube to kick back similar to when you had the fixed straight pipe?
Incredible. I just learned a few things about fluid dynamics.
This explains how to repel black holes with a simple vacuum cleaner. Thanks.
Could it be a type of venturi effect as the air passing through the tube drags air in around the bottom forcing the tube to lift ?
Fascinating! Try attaching little threads to the *outside* of the short straight tube; I'll bet you'll see airflow moving upward around the outside in a pulsing oscillation. This upward airflow occurs so long as the tube's bottom is resting on the vacuum inlet beneath. But the internal downward flow creates low pressure that overcomes the weight of the tube and support arm, creating a lifting force--upward flow--on the outside of the short tube (Coanda Effect). This breaks the upward flow (Coanda Effect) around the outside of the tube when it lifts off the vacuum inlet, and the tube settles back down. The motion develops into an oscillation.
To actually observe this you'd need to attach the strings to something stationary outside the tube. The tube's oscillation would spoil the results.
Its a tolerance issue. Air is pulling in through gap and creating pressure differential. Rest the tubes together with flat collars attached to each tube. Like the disc you used in first part of video. This will restrict air access to that gap.
The suction pulls air from wherever it will come. One is through the tube, the other is from the crack at the bottom of the tube where it meets the hole. I'm wondering if the air entering at the bottom of the tube somehow causes the tube to rise. To test this the gap would have to be air-tight yet the tube still be free to move up.
Maybe the wall thickness of the tube plays a role? You should try it with a very thick wall!
The air at the top end is moving and thus at lower pressure than the air at the bottom. But as soon as it moves upwards, air can flow around the bottom until the pressure is the same on both sides and the tube drops down again. This would explain why it oscillates and the cone doesnt.
I think the air flowing through the pipe extension can only move so fast so the unrestricted air from outside can force it's way in which pushes the main pipe back but only so far before gravity takes over
Now you've got me thinking about what the possible applications are for this.
Best. Video. Ever. The Feynman sprinkler. Yay.
weight of tube could be damping the effect, possibly also some conflating airflow in the gap between the cardboard tube and the wooden plate which you can see at 0:39.
This is beginning to make sense of the information on cylinder head porting I've been wading through lately. The inlet flow back eddy explains why a smooth straight port does not flow the most amount of air fuel mixture. It also applies to intake and exhaust manifold size and shape.
A smoke pencil {like in a wind tunnel} would better show where the air pressure and volume are increasing or decreasing.
FYI: an old rule of thumb in HVAC duct sizing was that the pressure drop from a ninety degree bend in a 5 inch pipe was the same as an additional ten feet of straight ducting.
'FYI: an old rule of thumb in HVAC' Similarly, straight air flowing along a wall interacts with the wall, and sees it as a restriction. The wall has to angle out at ~7 degrees before it appears neutral to the airflow.
So even the straight tube appears as a restricting tube. I'd bet if you reverse the cone and widen it as it goes in, the roughly 7 degree wall where it stops acting as a restriction to the airflow is where it will stop producing lift. Value may need adjustment for a cone instead of a flat wall, but the basic idea is the same..
the pipe isn't centered. Air is being pulled in past the outside of the pipe on one edge causing a net tangent force at the bottom of the pipe that is twisting it, which tilts that force vector to have a vertical component
4:50 No, static head is converted into kinetic head, and static head is what determines the plate staying
Regarding the two tanks.....the reason that the flow from the tank on the left keeps the plate against the other tank is the velocity head acting on the left side of the plate which is greater than the hydrostatic head acting on the right side of the plate. Think of how a ram pump operates. The same principle applies here.
I think the oscillations you're seeing come from air's ability to have large pressure gradients (because it's a compressible fluid). I'm not sure water would act the same.
Air won't just come in through the top but the bottom of the cylinder. At the point of touching the wood, thereby disrupting the cylinder flow at the bottom, causing it to "bounce."
I was guessing the seal between the pipe and the cone is imperfect, so some air is sucked in via that shortcut with lower pressure drop than doing down the extra length of the cone, and this creates a dyanmic pressure which seperates the two? If you have some kind of seal around the connection which it could break away with little force, does it still seperate?
my guess would be: the pressure drop inside the pipe from friction. you can test it easily with your setup using a longer pipe. Altough it feels as unintuitive, as friction also pulls the pipe down. but the gap at the bottom of the pipe forces an ambient pressure boundary condition as does the inlet at the top (fluiddynamics can be verrry unintuitive and i do have a phd in that field)
If you have a PhD in fluid dynamics, should you not be able to calculate & demonstrate rather than guess?
@@Farlig69 @Farlig66 yes i could run a simulation - would be interesting, but holidays without pc ;-) maybe when i'm back... (and matthias already has an easy setup to test the hypothesis)
but a simulation would be able to tell you WHY its doing that. My experiments failed to reveal that.
your gap idea -- unintuitive indeed. Please do a simulation to explain
@@flowgeek706 Well I guess it's too late now as Metthias has found the solution but would still be interesting to see the modelling of the gap hypothesis!
The force is made by acceleration of the flow. When you suck or blow, the air is accelerated most at the bend in the pipe. If you measured the weight of the pipe somehow, you would see that its weight increases significantly. There is still nonzero acceleration of flow at the nozzle, especially when you turn it on, but the change in velocity becomes spread out in the volume so that acceleration is small. This diffusion of acceleration cannot happen confined in the pipe though.
Looks like the air from the extra tube becomes faster than the air at the intake of the pendulum and pushes the pendulum in the opposite direction. The space from the gap between the two seems to work as an air curtain that causes the speed difference.
Possibly if you were able to vary the speed of the suction device you could get a broader picture.
I made videos about this for space craters and mountains...heres the experiment...level of dust in a layer ( about 1 inch)...put the vacuum above and lower it slowly ( min was a 2 inch vac tube)...it will create a mountain as it is pulling in air literally Horizontally in a vortex (not really from the vertical ,as it is in a vortex ( is this a lower pressure situation?)) the vortex goes INTO the vacuum tube so you create a mountain...so in my opinion, the paper rises and floats ON the air going IN. ( and perhaps some rise from a negative pressure in the vortex)
I'm adding to the guesses but I'm going with friction from the inside of the tube. The air drags and slows down which makes an inner area of air that's moving faster. The dragging air kind of makes an "air venturi" which does the same thing a cone does just to a lesser degree.
Granted I'm not a fluid dynamics nerd so maybe I'm wrong. This was a fun thought experiment anyway.
For the vertical case I think it could be a reaction force; the air at the side of the pipe detaches from the walls (coanda) and there's an opposite force as it does that. That would be why it is such a small effect.
9:24 can you add smoke to look for the eddies?
Coherency when something is pushed out.
Doesn’t apply the same when a vacuum cause it grabs from all around the inlet.
It’s like sharp point vs vague bubble.
does entering the cone fully like you would a funnel into the end say 1 cm change the effect rather the butt end will they still separate , is there a compressible counter effect due to the gap drawing air from the out side , like a air lift pump for dredging
I love the curiosity you have combined with your determination to measure and explore truth. Wonderful!