Ahhh, thanks for the input ChemE...the viscocity and reynolds number were exactly what I needed to hear. I went to a hydrodynamics site and found exactly what I needed. It turns out to be a bit of 2 things. Simply put, turbulence only matters if it gets into the most restricted point where the air gets sucked in. Being that the water up until the point of intake doesnt have any pressure behind it beyond static, it has no turbulence. So the water entering the venturi has only the turbulence from the suction of the venturi...which is present no matter what on any venturi...but this is still much less than the turbulence coming out of a pump and then going into a venturi...so its not as big a deal. On an asperating venturi, turbulence after the air inlet doesnt matter...its in the pump at this point in the middle of turbulence central! But on a regular venturi, the slope is there on the outlet to try and restore as much of the pressure to keep the bubbles flowing out and continue in their path. With an asperating...not an issue, so you often see no slope or only a slight one after the air inlet goint into the skimmer.
Ok, this is the other reason why asperating intakes dont need as gradual of a slope (little to none) compared to a venturi on the output. It has to do with the delta in the pressure.
On most regular or mazzei type venturis, the methos is > add pressure > pressure causes the water to gain velocity through the oriface > air is educted > mixture must keep its pressure so it continues its path. When you stand back from what our experience is with reefing and pumps...one might ask...why would you hook up an intake to the outlet of a pump...and this is a valid point. The venturi must not only overcome the natural static pressure of trying to draw air down a tube that would otherwise be under a few feet of water, but the pressure of the pump itself. If you hooked up a T to the output of a pump (a venturi w/o a restriction), the water level in the pipe would be higher than the water level in the skimmer or body of water you are pumping into. With an asperating intake...the process starts at the pump and runs in reverse. The mixture doesnt need to continue on its way after the intake because it has the pump there to mechanically push it. On the intake side, its only the relative vacuum that matters...sure...it turns out some slope it good...but not as much is needed...a 45degree cone is plenty compared to the 15degree slope you see in a mazzei.... This is because if you hook up a 'T' to the intake side of a pump like before, and ran the T like a venturi again, the relative water level in the T would be lower than the body of water you are pumping into. The pressure from the pump is working with you.
This is because a venturi on a pump outlet is trying to make a vacuum out of what is really the higher pressure end of a pump. When you look at the slope on the inside of a mazzei, it is very gradual, and that is because the restriction at its narrowest point is much greater. For example, a 3/4" pipe might need a 1/4" oriface to accelerate the water enough to overcome not only the depth pressure, but also the pressure of the output of the pump.
In the pressure equasions, on an asperating intake, the pump's pressure works with you in that the relative pressure difference between the pump and intake is more in the direction you desire (lower).
Look at a regular asperating intake... a 3/4" intake might only need to reduce its oriface to what...1/2", 3/8" to function properly?
The degree of the slope on the inside of a venturi is inversely proportional to the amount of acceleration that the water goes through...or the relative pressure drop that is desired. When its on the output of a pump, the pump's pressure works against the drop in pressure, and so the water must be accelerated that much more to overcome...hence a smaller oriface and more slope. When on the intake of a pump, the acceleration is much less because you have the pump's intake side, which aids you, and rather than the pump's output pressure working against you, it works with you because its on the vacuum side.
Its late, and I hope maybe someone can follow. The bottom line is that the degree of acceleration needed to make an asperating venturi work is less, so the slope can be much less since according to bernoulli, as pressure increases by 1.42x, flow increases by 2x...so less pressure difference (larger oriface on the venturi) means that much less pressure change...meaning that much less turbulence. So the slope you need to have on an asperating venturi can be 45 degrees or less (some asperating venturis have no slope on the pump side) and work just as well as a mazzei venturi with a 15 degree slope on the intake.
The thing that got me thinking was how the same pump with a needlewheel can make 2x as many bubbles as the same pump on a venturi. Of course there must be a difference or this commonly accepted fact wouldnt hold true. Your typical intake on a asperating is much larger than that on a mazzei type...so the slope doesnt have to be as gradual.
