losed loop' of circulation this was discounted; 'In contrast, in "closed" systems, like the circulation, gravity does not hinder uphill flow nor does it cause downhill flow, because gravity acts equally on the ascending and descending limbs of the circuit', but for 'historical reasons', the term is used. One hypothesis (in 1989) was that a siphon existed in the circulation of the giraffe. But further research, in 2004 found that, 'There is no hydrostatic gradient and since the "fall" of fluid does not assist the ascending arm, there is no siphon. The giraffe’s high arterial pressure, which is sufficient to raise the blood 2 m from heart to head with sufficient remaining pressure to perfuse the brain, supports this concept.' However, a paper written in 2005 urged more research on the hypothesis;
'The principle of the siphon is not species specific and should be a fundamental principle of closed circulatory systems. Therefore, the controversy surrounding the role of the siphon principle may best be resolved by a comparative approach. Analyses of blood pressure on a variety of long-necked and long-bodied animals, which take into account phylogenetic relatedness, will be important. In addition experimental studies that combined measurements of arterial and venous blood pressures, with cerebral blood flow, under a variety of gravitational stresses (different head positions), will ultimately resolve this controversy.'
Some species are named after siphons because they resemble siphons in whole or in part. Geosiphons are fungi. There are species of alga belonging to the phylum Chlorophyta in the genus Siphonocladaceae which have tube-like structures. Ruellia villosa is a tropical plant in the Acanthaceae family that is also known by the botanical synonym, 'Siphonacanthus villosus Nees'.
In speleology, a siphon or a sump is that part of a cave passage that lies under water and through which cavers have to dive to progress further into the cave system, it is not an actual siphon.
Explanation using Bernoulli's equation
Bernoulli's equation may be applied to a siphon to derive the flow rate and maximum height of the siphon.
- Let the surface of the upper reservoir be the reference elevation.
- Let point A be the start point of siphon, immersed within the higher reservoir and at a depth −d below the surface of the upper reservoir.
- Let point B be the intermediate high point on the siphon tube at height +hB above the surface of the upper reservoir.
- Let point C be the drain point of the siphon at height −hC below the surface of the upper reservoir.
- = fluid velocity along the streamline
- = gravitational acceleration downwards
- = elevation in gravity field
- = pressure along the streamline
- = fluid density
Apply Bernoulli's equation to the surface of the upper reservoir. The surface is technically falling as the upper reservoir is being drained. However, for this example we will assume the reservoir to be infinite and the velocity of the surface may be set to zero. Furthermore, the pressure at both the surface and the exit point C is atmospheric pressure. Thus:
- (Equation 1.)
Apply Bernoulli's equation to point A at the start of the siphon tube in the upper reservoir where P = PA, v = vA and y = −d
- (Equation 2.)
Apply Bernoulli's equation to point B at the intermediate high point of the siphon tube where P = PB, v = vB and y = hB
- (Equation 3.)
Apply Bernoulli's equation to point C where the siphon empties. Where v = vC and y = −hC. Furthermore, the pressure at the exit point is atmospheric pressure. Thus:
- (Equation 4.)
As the siphon is a single system, the constant in all four equations is the same. Setting equations 1 and 4 equal to each other gives:
Solving for vC:
- Velocity of siphon:
The velocity of the siphon is thus driven solely by the height difference between the surface of the upper reservoir and the drain point. The height of the intermediate high point, hB, does not affect the velocity of the siphon. However, as the siphon is a single system, vB = vC and the intermediate high point does limit the maximum veloc