Sucking down on this cone makes it rise, and Feynman sprinkler

if you have an oldfashioned s-shaped sprinkler or water squirting out the ends like this obviously that will make it turn like this but what if he were to suck water through it which way would it go and I read about this experiment in Richard Fan's book 35 years ago but he never mentioned a result so finally I want to try it myself now fman mentioned making a huge mess trying this experiment with water so I'm going to try a different fluid air and here's essentially half of my sprinkler so even if I blow into this just gently you can see it moves backwards now let's try sucking on it well I don't have enough suction so I made this hose adapter for my thing here which can pivot very nicely in here hose goes on here and that hose goes to a blower in a closet that's far away so I don't hear the noise so much of it all right let's see what happens I think it's did move a little bit but it's hard to say but it is pulling air in right now so if I put my hand near it you can see it's sucking towards my head and now with a pointer to see how much it actually moves so it definitely moved a little bit initially but now it's maybe a few millimet forward from where it was before so just about nothing and now it seems to be going backwards I just tried it again and now forwards H this is very sensitive to the environment if I just Puff the air at it just a little bit that moves it way more so whatever Force it's getting from suck in there is extremely small but I noticed it's quite sensitive to changes in air flow so if I start the air flow suddenly or stop it suddenly that moves me quite a lot and I think that response to Transit is just reaction forces to the air in the horizontal part of the pipe accelerating and decelerating I was curious if the shape of this nozzle thing matters so I put a disc on there maybe that's moving a little bit more forward but again the force is MCU now with a cone maybe a tiny bit more now if I make it suck through another piece of pipe uh that'll provide some resistance so presumably it's just going to get sucked against that pipe whoa that's unexpected so if it's very close to that pipe it's actually repelled by it I'll push it against there and it gets pushed away let's move that pipe a bit closer so now it's still getting repelled by the pipe I just know a lot of you won't believe me so I'm just going to move that pipe against the other one so it's pushing against there to begin with and then we'll turn it on and see what happens so obviously it can't be bouncing right from the start and it got pushed away when at the right distance it gets pushed away now this effect is is much stronger if I use a cone instead of a pipe so let's push that way up against here so it's already pushed quite a ways back so I have to actually push quite a bit to get that up against the cone it just wants to get away from that cone and this relates to an exam problem from way back when I was taking fluid dynamics and it was a simple problem you have a tank full of water that has a spout that's squirting out water against a plate on another tank which has got an opening of the same size and the water is just deflected up and down not bouncing off it or anything like that so the question is how high a column of water can we put in here before it pushes that plate off of this opening and to me it was pretty obvious that the other column would be the same height because whatever force that the water from the first tank isn't exerting across the opening is imparted on that stream and that's going to push against the plate so same height in the other tank and then it has the same Force right but just for practice I did the math anyways and it came out wrong it came out twice the height for the other tank and clearly that's wrong so I checked my math and checked it and checked it and checked it and checked it and it just came out the same way and I just knew it couldn't be right because whatever pressure is missing on here has to to be the pressure that gets exerted on the plate if the pressure on here is twice as much uh basically this whole thing if you put it on a carriage would want to move this way because we've got twice as much pressure on here as we're missing here it just doesn't work out so I was stuck thinking about this problem for a months luckily this was a practice final so it didn't affect my grades and I finally figured out the key is the shape of this nozzle here so as the water goes into this hole of course it has to to come like this around here like that and as the water accelerates around the corner here basically centrifugal force pushes it this way a it which means there's less pressure on here and on here so whatever momentum the stream gets isn't just a force that's missing across the opening but also a lower pressure here and here but what if we change the nodle to this shape where we can't have less pressure on here because there's no area here well as it turns out this sort of way of getting water out of here is very inefficient and you have a pressure trop across here which means less flow and so less momentum so if you wanted to take the water out of the middle of a container you need something like this this is called a bellmouth and then you have the water going across here like this again the pressure here would be less than the pressure on here so essentially this bellmouth actually gets pulled into the Container but being attached to a pipe it doesn't move so when I had my cone attached to the Elbow here that was essentially like a bellmouth and no doubt that was being pulled this way but the increased flow just caused more Force pushing that way in the pipe so the whole thing was in equilibrium but if that cone or bellmouth is separate from the pipe then presumably this gets pushed this way and this gets pushed that way and so it gets pushed away so if all this is true sucking air down through this cone should make it want to rise and look it does rise and some of you no doubt are saying oh he's blowing instead of sucking but uh let me show you definitely sucking but there's still some unanswered questions I explained why the bellmouth would get pulled this way because of the centrifugal force but if my Inlet is a long cone I don't really have much of that but Bern's equation says that the pressure drops as the speed increases and as I'm in this cone the speed does increase which means I have less pressure pushing on here than I would on the outside here and the same thing here and the net sum of this is that I have more pressure going this way than this way and so that cone also gets pulled this way but why did I get that same effect with a straight piece of pipe to a lesser degree it is much weaker and here I've got a shorter piece of pipe and if I turn on the suction [Applause] it also looks up and the surfaces on this pipe are all vertical so where does it even apply the force so I made this little ble to rule out that it's a flow on the outside of the pipe that's causing it and I don't think it's the end so it has to be something about flow inside the pipe itself I thought this might be due to Inlet flow recirculation so if you've got uh air coming into an open end like that it actually tends to constrict itself a little bit because it's got all this inward momentum from coming in this way and you actually get a little eddy current recirculation here and that could pull up on the pipe but I've been sticking pieces of thread in there through holes I made on the side I even shortened the pipe to try to catch this recirculation and I see no strong evidence of it so this is still a mystery to me so is there some flu expert out there that hasn't answer to that one I'd like to hear it

Channel: Matthias random stuff

Views: 113,447

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Id: ued2cEcfAio

Length: 9min 32sec (572 seconds)

Published: Thu Nov 30 2023