Understanding the Venturi Effect: A Physics Expert's Guide

[ramonegumpert]

Dear Physics experts,

I am trying to build a venturi tube.
I found some diagrams from the internet.
As we know, in the middle of the venturi tube, the pipe is narrower and this is where pressure is lowest and fluid passes through it the fastest. Imagine that this venturi tube is to pass pressured water through it and suck in some air to mix with the water as it flows out of the tube. So, there is a t-junction in the middle (low pressure region) where air can be sucked in. Let's call this the air inlet pipe. Now, when pressurised water passes through this tube, I am not able to understand why water will not escape through the air inlet pipe assuming the high and low water pressure are both higher than atmospheric presssure. So, what I mean is why doesn't the low water water flow into the air-inlet pipe IF this low pressure water is higher in pressure than the air pressure?

Hope you can clarify this doubt I have on Venturi effect.

Thanks and have a nice day.

Best regards
Ramone

 

[rcgldr]

The pressure of the water at the T intersection would have to be less than the air pressure at the intake to draw the air in. Part of this could be due to turbulence (a vortice at the T intersection), but I don't know if it would be significant.

The pressure inside the pump is higher than the pressure of the water at that depth, and the pressure of the water inside the narrow pipe is less than both and that of the air at the intake. The low pressure water has a lot of velocity and it's momentum allows it to flow out of the tube into the higher pressure water outside the tube. While in the tube, the lower pressure water slows down due to the higher pressure at the exit of the tube, and it's pressure probably increases, depending on how much reduction in pressure occurs due to wall friction and viscosity in the tube. Once out of the tube, the water slows and increases in pressure, and for a brief period during deceleration just before it effectively slows to "zero", it's pressure is higher than the outside water at that depth.

Aquarium underwater pumps optionally draw in air from a T intersection on the exit tube. A water driven pump based on venturi effect can be used to pump water out of an aquarium. Here is a link to an example. In this case a tapering cone exits into a chamber, which has a side intake and an exit pipe across from the tapering cone exit. Canadian patent link is broke. USA patent diagrams will require an downloaded add-on to view the diagram of the internals (link is provided at patent site).

http://andysworld.org.uk/aquablog/?postid=247