The arsehole Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> persisting in being an Off-topic troll... -- Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:> Path: not-for-mail > From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> > Newsgroups: sci.electronics.design > Subject: Re: OT: Hydraulics query > Date: Tue, 5 Dec 2023 19:45:20 -0000 (UTC) > Organization: A noiseless patient Spider > Lines: 76 > Message-ID: <uknukg$cejd$1@dont-email.me> > References: <ukn3ce$51k5$1@dont-email.me> > <cc63c9bc-2598-44ad-8bc9-f49771b5a1cdn@googlegroups.com> > <5223fc78-187f-9134-84f6-e156a603afe9@electrooptical.net> > <uknq63$8oo0$1@dont-email.me> > MIME-Version: 1.0 > Content-Type: text/plain; charset=UTF-8 > Content-Transfer-Encoding: 8bit > Injection-Date: Tue, 5 Dec 2023 19:45:20 -0000 (UTC) > Injection-Info: dont-email.me; posting-host="b6aa74a9c2051bcd4222f3ef71e00c35"; > logging-data="408173"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/rjIH5/Yk08Kjjn9mE1gpx" > User-Agent: NewsTap/5.5 (iPhone/iPod Touch) > Cancel-Lock: sha1:zKKlVKK+y0SZM13IRluzeG9w5kQ= > sha1:+74IOaD39mNVzTooHFHI9184Mho= > X-Received-Bytes: 4323
OT: Hydraulics query
Started by ●December 5, 2023
Reply by ●December 5, 20232023-12-05
Reply by ●December 6, 20232023-12-06
On Tuesday, December 5, 2023 at 10:29:31 AM UTC-8, Clive Arthur wrote:> On 05/12/2023 17:00, Phil Hobbs wrote: > > On 2023-12-05 10:00, Fred Bloggs wrote: > >> On Tuesday, December 5, 2023 at 7:00:23 AM UTC-5, Clive Arthur wrote: > >>> If I have a U-tube, horizontal and submerged in water, with a propeller > >>> half way along inside one leg, what happens when the prop is rotated? > >>> > >>> Obviously, the same amount of water comes out of one leg at the same > >>> velocity as it goes in the other. Because the in and out are separated > >>> in distance, there will be a torque generated. But if we constrain the > >>> tube so it can't rotate - or simply attach a mirror-image tube with > >>> propeller to cancel the torque - will the tube(s) move? > >>> > >>> I think so - the water changing direction as it flows around the bend > >>> will generate a force, pushing the tube along in the direction of the U. > >>> It won't matter which way the propeller is turning. > >>> > >>> Is that right, or should I stay off the drugs? > >> > >> The tube restrains the water to turn through the radius with a > >> counter-centripetal for mv^2/r. That will push the tube forward. Then > >> the propeller by the act of moving the water induces a pressure > >> differential from front to back on its blades, pushing the tube > >> forward. Centripetal is Newton, differential pressure is Bernoulli. > >> This is my guess... > >> > >>> > >>> -- > >>> Cheers > >>> Clive > > > > If we dial back the camera a bit, and allow the angle between the tube > > axes to be anything, it gets clearer. Force is the time rate of change > > of momentum. At the intake end, the force is -m*V, where m is the mass > > of water entering per second, and V is the velocity, and at the outlet > > end it's +m*V. > > > > If the angle is pi (i.e. the tube is straight) then the answer is > > obvious--the two contributions add. If not, we have to do vector > > addition. With the inlet pointing towards positive X, and the outlet at > > some angle theta from there, the total force on the tube is > > > > F = m*V [(1 - cos theta) Xhat - sin theta Yhat). > > > > So when they're pointing the same way, we expect the force to be zero. > > > > Of course this is a zero-order approximation, because the actual motion > > of the surrounding water will be a complicated mess, and there's > > viscosity and friction and all. > > > > Cheers > > > > Phil Hobbs > > > Does the fact that in the U-tube, the water's direction is changed by > 180'. That change in direction of a moving mass must surely result in a > force? > > Isn't it a bit like blowing yourself along on a skateboard using an > umbrella held in front and a leaf-blower blowing forwards into the > umbrella? eg... > > https://www.youtube.com/shorts/1CXB7_gm8I0Yes; the force is in the U=tube, which is presumed to be a rigid item, with negligible deformation because of that force. Net force that propels the U=tube is only possible due to minor effects (like, heating of the fluid as it passes the propellor, resulting in thermal expansion, also altering the drag on the walls of the tube). Because of thermal expansion, the mass of the inlet and outlet water will be the same, but the velocity a bit different.
Reply by ●December 6, 20232023-12-06
On 05/12/2023 19:45, Phil Hobbs wrote:> Clive Arthur <clive@nowaytoday.co.uk> wrote: >> On 05/12/2023 17:00, Phil Hobbs wrote: >>> On 2023-12-05 10:00, Fred Bloggs wrote: >>>> On Tuesday, December 5, 2023 at 7:00:23 AM UTC-5, Clive Arthur wrote: >>>>> If I have a U-tube, horizontal and submerged in water, with a propeller >>>>> half way along inside one leg, what happens when the prop is rotated? >>>>> >>>>> Obviously, the same amount of water comes out of one leg at the same >>>>> velocity as it goes in the other. Because the in and out are separated >>>>> in distance, there will be a torque generated. But if we constrain the >>>>> tube so it can't rotate - or simply attach a mirror-image tube with >>>>> propeller to cancel the torque - will the tube(s) move? >>>>> >>>>> I think so - the water changing direction as it flows around the bend >>>>> will generate a force, pushing the tube along in the direction of the U. >>>>> It won't matter which way the propeller is turning. >>>>> >>>>> Is that right, or should I stay off the drugs? >>>> >>>> The tube restrains the water to turn through the radius with a >>>> counter-centripetal for mv^2/r. That will push the tube forward. Then >>>> the propeller by the act of moving the water induces a pressure >>>> differential from front to back on its blades, pushing the tube >>>> forward. Centripetal is Newton, differential pressure is Bernoulli. >>>> This is my guess... >>>> >>>>> >>>>> -- >>>>> Cheers >>>>> Clive >>> >>> If we dial back the camera a bit, and allow the angle between the tube >>> axes to be anything, it gets clearer. Force is the time rate of change >>> of momentum. At the intake end, the force is -m*V, where m is the mass >>> of water entering per second, and V is the velocity, and at the outlet >>> end it's +m*V. >>> >>> If the angle is pi (i.e. the tube is straight) then the answer is >>> obvious--the two contributions add. If not, we have to do vector >>> addition. With the inlet pointing towards positive X, and the outlet at >>> some angle theta from there, the total force on the tube is >>> >>> F = m*V [(1 - cos theta) Xhat - sin theta Yhat). >>> >>> So when they're pointing the same way, we expect the force to be zero. >>> >>> Of course this is a zero-order approximation, because the actual motion >>> of the surrounding water will be a complicated mess, and there's >>> viscosity and friction and all. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> Does the fact that in the U-tube, the water's direction is changed by >> 180'. That change in direction of a moving mass must surely result in a >> force? >> >> Isn't it a bit like blowing yourself along on a skateboard using an >> umbrella held in front and a leaf-blower blowing forwards into the >> umbrella? eg... >> >> https://www.youtube.com/shorts/1CXB7_gm8I0 >> >> > > Sure. That force is internal to the device, though—the net change in > momentum equals minus that in the fluid outside.Actually, I think Fred (and my initial guess) was right. The tube will move. I've read a few things, and my reasoning, such as it is, follows... The Feynman Sprinkler. This is a conventional lawn sprinkler having a rotatable S shaped tube with a nozzle at each end and a central water pipe. Of course, the direction of rotation is obvious. If one of these is placed in a tank of water, then the direction of rotation is equally obvious, albeit slower. If you instead suck water through the supply tube (or pressurise the tank to force water out of the 'supply' tube) then you might expect the direction to reverse, but it doesn't. Essentially, the sprinkler does not rotate, and this has been demonstrated. So now modify the sprinkler - have one leg sucking and the other blowing. This could be done simply by removing the supply pipe and fitting a pump (propeller) at the S mid point. Then, twist one of the arms so instead of an S shape, you have a 3 or C shape - pretty close to the U-tube originally mooted. Now, whichever direction the pump operates, you have one leg having no effect and one rotating the sprinkler. Clearly, you can constrain this torque to produce a linear force, or as suggested earlier, just affix a mirror image device to cancel it. As to why the Feynman Reverse Sprinkler doesn't turn, I'd favour Fred's counter-centripetal force cancelling the suction force (technical term). -- Cheers Clive
Reply by ●December 6, 20232023-12-06
Am 05.12.2023 um 13:00 schrieb Clive Arthur:> If I have a U-tube, horizontal and submerged in water, with a propeller > half way along inside one leg, what happens when the prop is rotated? > > Obviously, the same amount of water comes out of one leg at the same > velocity as it goes in the other. Because the in and out are separated > in distance, there will be a torque generated. But if we constrain the > tube so it can't rotate - or simply attach a mirror-image tube with > propeller to cancel the torque - will the tube(s) move? > > I think so - the water changing direction as it flows around the bend > will generate a force, pushing the tube along in the direction of the U. > It won't matter which way the propeller is turning. >I agree, that the water changing direction will generate a force. But doesn't the propeller, which is somehow fixed to the tube, create a force in the opposite direction?
Reply by ●December 6, 20232023-12-06
On 06/12/2023 12:35, Matthias Czech wrote:> Am 05.12.2023 um 13:00 schrieb Clive Arthur: >> If I have a U-tube, horizontal and submerged in water, with a >> propeller half way along inside one leg, what happens when the prop is >> rotated? >> >> Obviously, the same amount of water comes out of one leg at the same >> velocity as it goes in the other. Because the in and out are >> separated in distance, there will be a torque generated. But if we >> constrain the tube so it can't rotate - or simply attach a >> mirror-image tube with propeller to cancel the torque - will the >> tube(s) move? >> >> I think so - the water changing direction as it flows around the bend >> will generate a force, pushing the tube along in the direction of the >> U. It won't matter which way the propeller is turning. >> > I agree, that the water changing direction will generate a force. > But doesn't the propeller, which is somehow fixed to the tube, create a > force in the opposite direction?I think there are three approximately equal magnitude forces. The 'pushing' force generated by the change in direction (change in velocity) as the water rounds the corner, the 'pushing' force from the leg with water flowing out, and a 'pulling' force from the leg with water being sucked in. So two out of three, a net push. Reading about the 'Feynman Sprinkler' has pretty much convinced me. -- Cheers Clive
Reply by ●December 6, 20232023-12-06
The idiot Clive Arthur <clive@nowaytoday.co.uk> persisting in being an Off-topic troll... -- Clive Arthur <clive@nowaytoday.co.uk> wrote:> Path: not-for-mail > From: Clive Arthur <clive@nowaytoday.co.uk> > Newsgroups: sci.electronics.design > Subject: Re: OT: Hydraulics query > Date: Wed, 6 Dec 2023 13:08:05 +0000 > Organization: A noiseless patient Spider > Lines: 32 > Message-ID: <ukprnm$p50t$1@dont-email.me> > References: <ukn3ce$51k5$1@dont-email.me> <ktb84dFqoheU1@mid.individual.net> > Reply-To: clive@nowaytoday.co.uk > MIME-Version: 1.0 > Content-Type: text/plain; charset=UTF-8; format=flowed > Content-Transfer-Encoding: 8bit > Injection-Date: Wed, 6 Dec 2023 13:08:07 -0000 (UTC) > Injection-Info: dont-email.me; posting-host="485733799119872ab2474f10bd12fbc1"; > logging-data="824349"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18sV3igcvFBOeUCg3ccWCKqsc6uUx5MycU=" > User-Agent: Mozilla Thunderbird > Cancel-Lock: sha1:KQKVKXHHWVbqtf1kr1vd+nwUCoY= > Content-Language: en-GB > In-Reply-To: <ktb84dFqoheU1@mid.individual.net> > X-Received-Bytes: 2681
Reply by ●December 6, 20232023-12-06
The arsehole Clive Arthur <clive@nowaytoday.co.uk> persisting in being an Off-topic troll... -- Clive Arthur <clive@nowaytoday.co.uk> wrote:> Path: not-for-mail > From: Clive Arthur <clive@nowaytoday.co.uk> > Newsgroups: sci.electronics.design > Subject: Re: OT: Hydraulics query > Date: Wed, 6 Dec 2023 10:57:57 +0000 > Organization: A noiseless patient Spider > Lines: 103 > Message-ID: <ukpk3m$o0rj$1@dont-email.me> > References: <ukn3ce$51k5$1@dont-email.me> > <cc63c9bc-2598-44ad-8bc9-f49771b5a1cdn@googlegroups.com> > <5223fc78-187f-9134-84f6-e156a603afe9@electrooptical.net> > <uknq63$8oo0$1@dont-email.me> <uknukg$cejd$1@dont-email.me> > Reply-To: clive@nowaytoday.co.uk > MIME-Version: 1.0 > Content-Type: text/plain; charset=UTF-8; format=flowed > Content-Transfer-Encoding: 8bit > Injection-Date: Wed, 6 Dec 2023 10:57:59 -0000 (UTC) > Injection-Info: dont-email.me; posting-host="485733799119872ab2474f10bd12fbc1"; > logging-data="787315"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18mcY0ie3MvcN6oPBZbQ2uh2HqEF8HgcAg=" > User-Agent: Mozilla Thunderbird > Cancel-Lock: sha1:hP2WqQJnMoS+Sgl+0u7iEQYmb+U= > Content-Language: en-GB > In-Reply-To: <uknukg$cejd$1@dont-email.me> > X-Received-Bytes: 5752
Reply by ●December 6, 20232023-12-06
The idiot Matthias Czech <matthias.czech@t-online.de> persisting in being an Off-topic troll... -- Matthias Czech <matthias.czech@t-online.de> wrote:> Path: not-for-mail > From: Matthias Czech <matthias.czech@t-online.de> > Newsgroups: sci.electronics.design > Subject: Re: OT: Hydraulics query > Date: Wed, 6 Dec 2023 13:35:25 +0100 > Lines: 18 > Message-ID: <ktb84dFqoheU1@mid.individual.net> > References: <ukn3ce$51k5$1@dont-email.me> > Mime-Version: 1.0 > Content-Type: text/plain; charset=UTF-8; format=flowed > Content-Transfer-Encoding: 8bit > X-Trace: individual.net daSJypzGbWOoZD/MXpzT3wRZjHi81B0IJK8K2bIj/4i+2g5YDj > Cancel-Lock: sha1:k2RDQuAELLxY0vI2cjvXNBb/yWA= sha256:gkqVDxLBmgZAtOBINyaR3p2hHk/H0rj4LahKXbaIrdU= > User-Agent: Mozilla Thunderbird > In-Reply-To: <ukn3ce$51k5$1@dont-email.me> > X-Received-Bytes: 1707
