It seems ironic that your description of a small diameter tube at the apex is described as a way to prevent air buildup, but it is also the design of a siphon break device intended to let air in.
An automatic air bleed valve is placed at the crest of a pumped water pipeline to prevent the crest being blocked by compressed air. That air bleed valve is closed by the higher density of liquid water.It seems ironic that your description of a small diameter tube at the apex is described as a way to prevent air buildup, but it is also the design of a siphon break device intended to let air in.
There are several similar possibilities that require a supply of water at the crest.What if we left this apex valve open and connected it via a small diameter pipe to the bottom of an open-top tank that had a continuous flow of water coming into it?
Would a dosing syphon work in this case? For a dosing syphon to work (look at the above link), the supply level needs to be above the high point in the inverted U at some time. Can that happen in a simple system where the upper reservoir level is always lower than the top of the U? I remember the standard system in ancient men's urinals (the urinals not the men) was a dosing syphon and the tank was constantly filled from a mains pressure supply. That doesn't seem to be the case here.It seams as if a conventional dosing siphon would maintain your levels.
Bearing in mind that the pressure at the top of a syphon can be almost zero then I can't see how, without some 'special help' that can ever fill with water.It seems ironic that your description of a small diameter tube at the apex is described as a way to prevent air buildup,
In a steady state flowing system, there is a logical problem with duplicating the outlet tube from the crest with a longer tube to a venturi. Rather than solving it, that duplicates the problem. If modification of an outlet tube could cure the problem, then one should modify the outlet of the existing tube.I did wonder whether, if the water flow is fairly fast, then a Venturi pipe, lower down the hill , could 'suck' air out of the U. There would need to be a one way valve to stop air getting back up the tube and making the problem worse.
As I suggested in post #39, the cyclic pulsing of a ram pump could be used.Or perhaps a ram-type of mechanism could act as an air pump.
The two effective columns of water, separated with the partial vacuum at the crest, could generate a cyclic positive pressure pulse at the crest, blowing the partial vacuum and a little water out of the air bleed valve at the crest each time. In effect, the siphon tube forms the drive tube of the ram pump, with the ram pump outlet valve at the crest. While generating a one bar pulse would be a trivial job for a ram pump, the system must still be primed somehow to start it operating.Given a little more fall at the outlet, it may be possible to arrange the outlet valve to cycle like a ram pump, ejecting gas from the crest of the siphon.
Is that true? The two pipes are not equivalent. One has a large volume of water, traveling fast and the other will initially have a small volume of water (initially), traveling slowly. I can't see why water will not be drawn into the Venturi section. In a static situation then obviously there will be no flow but is there there not a difference in the available power which can support the pumping action? Are you saying that the pressure difference in the two pipes will not be enough and that the level in the small tube will never reach the Venturi section? That could make sense but the Venturi section need only be a small distance below the crest. Also, the bubbles in the main pipe will reduce the water density but they could be arbitrarily few - just enough to balance the rate of formation.In a steady state flowing system, there is a logical problem with duplicating the outlet tube from the crest with a longer tube to a venturi.