Backyard Aquaponics
A place to discuss aquaponics

You could be right about 1 because of increased surface area but friction in pipes is linearly proportional to area and exponentially to velocity. Therefore I don't think so.

Friction would decrease as surface area increased until a point was reached when surface tension rather than flow effects came to dominate.

In a standard bell siphon the turbulence entrains bubbles and sweeps them out of the siphon bell. In a affan siphon the vortex may speed air evacuation by the point of the air needle of the free surface vortex repeatedly breaking off and sending air out of the bell in a series of large bubbles or slugs which may be faster.

Or it may not.

Water flow has to be through really small crosssections or be moving really slowly for it to be laminar rather than turbulent flow. If laminar flow occurs at siphon initiation it would only be for a fraction of a second unless the vessel that the siphon was emptying was very very large relative to inflow. Even in such a case the laminar flow would unlikely be sufficient to create either turbulence (by definition) to entrain air in the standard bell siphon or create a vortex in an Affan siphon.

Work today so I will look up one of them when I am having a beer later.

What do you do that you are working today?

I volunteer and do casual work at the local tip (refuse and recycling center)

I had a look at an Affnan Syphon.

First comments.

I thought it started with a funnel but the direction change is only apparent at the reducer.
When the water tension breaks over the lip of the edge I would guess it breaks at one point first ( not enough to bet my left you know what on it but pretty close) then runs round.
The flow would be laminar with the closest to the walls slowest and preceding out at increasing speed.
It then does another change of direction into the restriction ? the venturi and accelerates.

Couple of questions

Does the offset break of water tension induce rotation?
Does the reduced pressure pull air down into it?
Does an Affnan exhibit a vortex (not in the sense of an increased Reynolds number) but in the sense of the whirlpool effect of opposing flows?

You know this is a bit like using a search engine with a very simple question, and then getting totally sidetracked.

I think the only way it wouldn't break first at one point was if it was perfectly even. By even I mean not just level but the edge of the funnel would have to be perfectly smooth as well. Not likely. Breaking at multiple points though might happen if the change in depth in the bell were fast enough and the top of the funnel was level. More likely but still not likely.



Are you talking about a boundary effect? I would have thought that the water falling over the edge into the empty funnel would do so fast enough to create a turbulent flow regime.



It may increase it but it doesn't induce it. If you have a tank with an outlet in the center bottom and you open the valve to open the outlet you will see as the water flows out that it will form a vortex without the need to add any external force. This is called a free vortex.
If you take a tank and put it on a carousel and spin the carousel you also create a vortex but it is called a forced vortex. Alternatively if you put a paddle in the tank and stir it you also create a forced vortex.

I imagine that the flow breaking over one side may help to start a rotation in a particular direction but once flow develops and the depth of water around the circumference of the funnel increaes it will be a free vortex. Then if the velocity is great enough it will form a free surface vortex and air would be transported out of the siphon bell.



If it is the vortex that is doing the job then yes. In a vortex the centrifugal forces cause a reduction in water pressure. Since the air pressure remains constant but the water pressure has decreased you get a depression in the waters surface where the area of low pressure is.

In case I sound too clever I only know this now because of the suggested reading of Kimocal and others.

[quote="Sleepe"]Does an Affnan exhibit a vortex (not in the sense of an increased Reynolds number) but in the sense of the whirlpool effect of opposing flows?[quote="Sleepe"]

Yes an affan creates a free surface vortex (whirl pool) which is what got me thinking about this in the first place.

I found this thread quite interesting.

So would any one like an experiment Affan/normal siphon to see the effects.

I have two of these clear lid auto siphons running and can run one in both configurations.

Well I would , and it would be good if you could let us have your observations on what you think is happening.

When it cools down I will have a go. As far as my opinions we will see how I go.

My 2 cents
The problem is one of changing shape and center of of the buoyant air at the point within the stand pipe where the flow of water becomes laminar. The bell provides a smoother curve of increasing hydrodynamic pressure and leverage allowing the water to more quickly and more powerfully shape the buoyant upward force of the air in the stand pipe. The shape of this water pressure forces the the air into the elongated central tube of air rather than the more chaotic wobbling ball shape that stifles laminar flow. Practice seems to be to put some form of barrier to more air entering the tube in the form of a bend but another bell shape and a straight run at the exit would add even more leverage to the waters ability to shape the air in the stand pipe.
It would be an easy experiment to run and verify

Looking forward to your results.

@telonline: I don't understand what people in this thread are talking about when they say laminar flow. Laminar flow is a technical term and only occurs at low velocities. While it is possible I would be amazed if anyone was running their system at flows slow enough for the flow to be laminar.

Laminar flow occurs when the Reynolds number is below ~2000 although there can be transition flow between Re 2000-4000.

Re=DVρ/μ

Re=Reynolds number
D=pipe diameter (m)
V=velocity (m/s)
ρ=density of water ~1000kg/m3
μ=viscosity of water ~0.001519

Where have you been Stuart? You got the shits or something?

Yes. Literally. For a week.

But the reason I wasn't posting was because the internet went down. Had to wait for parts and then more parts. Got it working again last night.

One of the downsides of rural life.