Siphon

In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, then discharging at a level lower than the surface of the reservoir from which it came.

All known published theories in modern times recognize Bernoulli's equation as a decent approximation to idealized, friction-free siphon operation.

This was initially explained by Galileo Galilei via the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resintenza del vacuo, but this was subsequently disproved by later workers, notably Evangelista Torricelli and Blaise Pascal[17] – see barometer: history.

The chain model helps to understand how a siphon can cause liquid to flow uphill, powered only by the downward force of gravity.

[19] Another difference is that under most practical circumstances, dissolved gases, vapor pressure, and (sometimes) lack of adhesion with tube walls, conspire to render the tensile strength within the liquid ineffective for siphoning.

Further, in other settings water transport does occur due to tension, most significantly in transpirational pull in the xylem of vascular plants.

In 1948, Malcolm Nokes investigated siphons working in both air pressure and in a partial vacuum; for siphons in vacuum he concluded: "The gravitational force on the column of liquid in the downtake tube less the gravitational force in the uptake tube causes the liquid to move.

[2] The research team of Boatwright, Puttick, and Licence, all at the University of Nottingham, succeeded in running a siphon in high vacuum, also in 2011.

Molecular cohesion and gravity are shown to be contributing factors in the operation of a siphon; the presence of a positive atmospheric pressure is not required".

[3] In 2014, Hughes and Gurung (at the Queensland University of Technology) ran a water siphon under varying air pressures ranging from sea level to 11.9 km (39000 ft) altitude.

In this respect, where the requirement is to match a flow into a container with a flow out of said container (to maintain a constant level in a pond fed by a stream, for example) it would be preferable to utilize two or three smaller separate parallel pipes that can be started as required rather than attempting to use a single large pipe and attempting to throttle it.

A common example of this is a public restroom with urinals regularly flushed by an automatic siphon in a small water tank overhead.

One way to do this intermittent action involves complex machinery such as floats, chains, levers, and valves, but these can corrode, wear out, or jam over time.

The most common failure is for the liquid to dribble out slowly, matching the rate that the container is filling, and the siphon enters an undesired steady-state condition.

A fourth problem involves seep holes in the mechanism, intended to slowly refill these various sealing chambers when the siphon is dry.

[citation needed] Some designs make use of an automatic system that uses the flow of water in a spiral vortex to remove the air above to prime the siphon.

While a simple siphon cannot output liquid at a level higher than the source reservoir, a more complicated device utilizing an airtight metering chamber at the crest and a system of automatic valves, may discharge liquid on an ongoing basis, at a level higher than the source reservoir, without outside pumping energy being added.

Large inverted siphons are used to convey water being carried in canals or flumes across valleys, for irrigation or gold mining.

This is especially important in sewerage systems or culverts which must be routed under rivers or other deep obstructions where the better term is "depressed sewer".

In these situations the unwanted flow is not actually the result of a siphon but suction due to reduced pressure on the water supply side.

[67] Metal baffles at the roof drain inlets reduce the injection of air which increases the efficiency of the system.

Analyses of blood pressure on a variety of long-necked and long-bodied animals, which take into account phylogenetic relatedness, will be important.

A river siphon occurs when part of the water flow passes under a submerged object like a rock or tree trunk.

Bernoulli's equation may be applied to a siphon to derive its ideal flow rate and theoretical maximum height.

Setting equations 1 and 4 equal to each other gives: Solving for vC: The velocity of the siphon is thus driven solely by the height difference between the surface of the upper reservoir and the drain point.

Setting PB = 0: Solving for hB: This means that the height of the intermediate high point is limited by pressure along the streamline being always greater than zero.

As long as this condition is satisfied (pressure greater than zero), the flow at the output of the siphon is still only governed by the height difference between the source surface and the outlet.

Experiments have shown that siphons can operate in a vacuum, via cohesion and tensile strength between molecules, provided that the liquids are pure and degassed and surfaces are very clean.

Once the liquid has been forced into the tube, typically by suction or immersion, flow continues unaided.The Encyclopædia Britannica currently describes a siphon as: Siphon, also spelled syphon, instrument, usually in the form of a tube bent to form two legs of unequal length, for conveying liquid over the edge of a vessel and delivering it at a lower level.

In civil engineering, pipelines called inverted siphons are used to carry sewage or stormwater under streams, highway cuts, or other depressions in the ground.