Dam Solution?

Unfortunately not. Have a read of the Wikipedia article on Siphons. While it agrees Bernoulli equations provide a 'fair approximation' of the flow, the section on 'Modern Research doesn't come to a solid conclusion as to how Siphons actually work. At least not the way I read it.

It's been shown that siphons do not depend on atmospheric pressure because they work in a vacuum. Operation looks to be more to do with gravity and molecular cohesion. I wonder if anyone has tried to eliminate gravity by siphoning liquids in outer space? 

The latest research - Hughes and Gurung, Queensland University of Technology, 2014 - concluded "It follows from the above analysis that there must be a direct cohesive connection between water molecules flowing in and out of a siphon. This is true at all atmospheric pressures in which the pressure in the apex of the siphon is above the vapour pressure of water, an exception being ionic liquids." Why are ionic liquids exceptional?  Of course being Australian they were upside down when they tried the experiment...

Dave

Edited By SillyOldDuffer on 03/08/2019 18:17:16

Also... coming back from the supermarket earlier, a Chinook went over heading NW at about 4-500 feet.

Heading for Whaley?

Neil

Edited By Neil Wyatt on 03/08/2019 18:15:00

On a similar thought can anyone explain how a hydraulic ram works ? I've seen drawings but have never seen one in the flesh so to speak

Roy

.

The one that's been working is, apparently, based at RAF Odiham : **LINK**

https://www.forces.net/news/raf-chinooks-called-help-stop-derbyshire-dam-collapse

... So, I don't know

I was told this morning that the 1-ton bags are being delivered to part of Long Hill [which should be recognisable on the map that I posted], and then the Chinook is shuttling them to the dam.

MichaelG.

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https://m.youtube.com/watch?v=r7LMuLmSun8

Edited By Michael Gilligan on 03/08/2019 18:59:05

Dave

I have just had a look at Wikipedia. I treat Wikipedia as a thoughly untrustworthy source of information. It has a vast amount about what is generally a very simple hydraulic device, a lot of it seems to be grounded in mumbo-jumbo. Away from problems due to pressure approaching a vacuum at the top of the siphon Bernoulli’s equation applies. When the height of the siphon is around 30ft the water starts to boil and the simple Bernoulli analysis fails. The modern studies referred to in Wikipedia concern these low pressure conditions.

JA

This is just trying to make life difficult. Syphons work mainly by gravity, but you need air pressure on the top reservoir to start them off by sucking on the bottom end of the tube. Yes you can have a broken column of water, but then you again need air pressure on the free surface in the top reservoir. Even with a continuous stream of water, if you try to work in too much of a vacuum the 'suction' height will be reduced, because the water boiling point is reduced.

To take a slight detour, all the weirs on the Mayenne River in France have hydro electric turbines perched on top which work by syphoning water over the weir. Neat, the weirs were already there to make the river navigable

Mayenne generators

Re Hydraulic rams there was a good article in ME a few years ago, on making a ram using plastic pipe and fittings. I think it is a large volume of water moving a small volume, John

Rather than decrying Wikipedia as 'inaccurate why not take a look at some of the plentiful references in the article?

Past research has shown it to be no less accurate than the Encyclopaedia Britannica.

Neil

P.S. Looks to me that the pumps are in the floating yellow bits (sorry Original thought was wrong...)

Edited By Neil Wyatt on 03/08/2019 20:26:32

They are

Come from the High volume pump sets they have been sending.

The are 50 + sets distributed around the fire brigades that do about 420 m3/hr

High Volume pump pods

Saw somewhere they have 16 sets in place now

Steve

|On a similar thought can anyone explain how a hydraulic ram works ?

It is based on using the surge pressure or transient caused by the kinetic energy release delivered by the instantaneous stop of a moving body of water. Also known as water hammer to anyone with old plumbing who has fitted an instant cut off tap. The 'clunk' that you may hear or feel through the house on slamming the tap shut is a pressure transient that can be many times the ambient pressure in the system. If you are experiencing this frequently, I suggest you get it sorted before you find your compression fittings starting to leak. (or not find until too late as in my case)

T hydraulic pump functions as water under some sort of pressure flows out of the waste valve of the pump, the fluid drag then shuts the 'waste' valve triggering a positive transient that opens a non return valve (nrv) on the delivery pipe and moves a small quantity of water up the delivery pipe.

Milliseconds later, the reflected negative transient causes the delivery nrv to shut and the waste valve to open allowing flow through the 'power' section of the pump again. The process repeats ad infinitum.

At each pulse, only a small volume is moved, but the process can continue endlessly as long as the flow and pump internals are maintained so over time domestically useful volumes can be moved.

'Stonishingly low tech, but very effective, however in the complex and demanding world we live in they are of limited application these days.

If you have a stream at the bottom of your garden, and wanted to move water up hill to storage at the top of your garden at practically zero cost, this is the device for you.

For the most up-to-date local weather reporting, traffic news, etc. : **LINK**

https://www.buxtonweather.co.uk/

MichaelG.

I can't see why anyone should have the slightest difficulty in understanding how a siphon works. Atmospheric pressure can support a column of water about thirty feet high, which is why a pump can only lift water from that far below itself. Really the pump just removes the air from the pipe, and the atmospheric pressure pushes the water up the pipe.

So OK, we want to do without the pump and start a siphon. For the siphon to work, the low end must be below the level of the water in the reservoir. The highest point must not be too high above the reservoir, eg about 30 feet for water, or 29.6 inches if you are siphoning mercury. You can start the siphon by filling the entire length of the tube with the working fluid. With a hose, this can be done by immersing the whole length, then blocking one end and transferring that end to the low point, then unblocking it.

If you were to measure the absolute pressures along the tube, you would find that at the inlet, the pressure was about 14.7 psi. At the highest point it would be less, a minimum of zero psi. So there is a pressure gradient causing the water to flow towards that point. From the high point to the outlet, it is downhill, so the well known tendency of water to flow downhill accounts for the flow there.

Overall, you can calculate the flow on the basis of the difference in height between the two ends and the resistance of the total length of tube. A siphon will not work if there is a vacuum above the fluid in the top reservoir. It will also not work if the whole system is in free fall. You can still use capillary forces in these situations, but that is a different can of worms.

Do not try to siphon petrol by sucking on the hose. You are likely to get lungful of petrol vapour, which can cause your lungs to go into spasm and can be fatal quite quickly. One guy was found lying dead beside the car with the siphon still running onto the ground. The same would apply to any other volatile liquid.

John

Now whether this would still be the case with a siphon tube a couple of feet in diameter, I have no idea.

Hi, I follow and understand the experiment in the video that peak4 has posted, however they have started with the system primed. If you stick one end of an empty tube into a container of water and then have the other end outside the container and below the end that is in the water, nothing will happen. Now assuming that the tube is large enough not it allow capillary action to take place and you evacuate the air from the low end of the tube (i.e. suck the air out) then the syphoning will proceed to take place and to my mind atmospheric pressure is the only thing that will drive the water up the tube and over the top of the container. Something that they could not achieve in the video. A vacuum has no pulling power, even in a vacuum cleaner you have to have an air flow for it to do its job.

Regards Nick.

Many thanks for the maths, I accept JA's logic entirely, but I don't think it explains how siphons actually work.

Isn't this failure to explain rather than describe generally true of all formula? Maths allow natural phenomena to be quantified and predicted, which is extremely valuable. But formula don't explain anything, they're just models. It's true formula improve understanding by eliminating explanations that don't match the numbers, but - bottom line - they don't explain physical causes. (A nice example of elimination by maths in JA's post : the sums don't feature atmospheric pressure, strongly suggesting atmospheric pressure is not what drives siphons. Gravity does feature in the formula and thus remains a player. )

Ohms Law is similar. V=IR does not explain electricity. V=IR works without the concept of electrons let alone the fact that a deeper look proves electron behave like waves (not particles) and particles (not waves), which cannot both be true in ordinary life. Just as well - it means engineers can exploit electricity even though the scientists don't fully understand it.

Are liquids in a siphon pushed from the high pressure end or pulled from the low? My feeling favours 'pull' but I wouldn't bet the farm on it!!!. Provided the pressure on the output is lower than the pressure on the input and the molecules are bound well enough to their neighbours to haul them through, I can imagine it working. Note a chain being pulled over a lip by the weight of the hanging end has no pressure on the static end - there is no push at all. Also, provided the tube is full and output is lower than input it is not necessary to suck on the end to start a siphon.

Curiosity about such things is valuable. Huge engineering benefits have come from science penetrating deeper in to the workings of the natural world. Chasing electrons has given us a much better model of the atom, resulting in laser beams, semi-conductors, advanced communications and chemical engineering, and much, much else. I doubt understanding siphons will open the door to Nirvana but who knows?

Possibly a parallel between siphons and global warming. Lack of conclusive scientific proof and a full understanding of the mechanism does not mean the results cannot be predicted...

Dave

I see it this way. A siphon runs on gravity, not air pressure. Creating a partial vacuum is a convenient way of priming the system and getting the process started, but not the only way.

.

Sounds good to me, Alan

MichaelG.

Well, if no-one really understands how syphons really work, allow me my 50 cents' worth. I've skimmed the Wikipedia entry, so am now an instant expert and world authority...

The 'flying droplet' syphon is interesting. Elsewhere in the article, it's said that a syphon will work (assuming that it has been started, presumably) in a vacuum. But would the 'flying droplet' type work in vacuo? It seems to me that the discharge tube tries to empty, by gravity, just like water running out of a simk, thus reducing the pressure in the 'air chamber'. This 'sucks' up liquid in the feed tube. But this suction should only work if air pressure pushes the liquid up the feed tube, into the 'air chamber'. So, for this type of syphon, it seems to me that air pressure is needed to 'get over the hill', and gravity takes care of the flow down the discharge tube. Of course, the cohesive property of liquids helps the bulk flow in the discharge tube from breaking up and allowing air up the tube, to break the syphon. So, a combination of gravity and air pressure are required.

Does this mechanism seem a reasonable explanation for 'conventional' syphons too? ...Well, those not in vacuo, at least.

However, on thinking about it a bit more, the laws of thermodynamics and energy conservation have to be satisfied, and I've conveniently ignored the energy accounting. I think I'll go to bed...