Venturi Effect 2

[MichaelRs]

A colleague recently proposed a question to me about the Venturi effect. He stated that fluids flow from high pressure to low pressure in a pipe, but if you have a pipe restriction (like a venturi tube), then since the fluid velocity is increased, the pressure drops. However, after the venturi tube ends and the pipe returns to original diameter, then the velocity decreases and pressure goes back up. He was wondering why the fluid appears to flow from a low pressure area to a high pressure area. I wasn't sure what a good technical explanation for this would be. Any help on the subject would be appreciated.

Thanks

 

[Compositepro]

Fluid will flow from higher to lower pressure just like a rock will always roll downhill. That does not mean that you cannot push the rock and get it to roll uphill for a while. The Bernoulli equation describes how a venturi works. It is based on the principle of conservation of energy. Fluid energy can be in the form of kinetic or potential (pressure). A rock's energy is also in the form of kinetic or potential (height).

 

[zdas04]

Like Compositepro says, there is no magic here. Fluids absolutely do not flow from higher pressure to lower pressure. They flow from higher energy to lower energy.

Don't believe me? Put a pressure gauge and valve on both ends of a 50 ft hose and fill it with water then shut both valves. Climb up on the top of your roof with one end of the hose. Open the valve on the roof and look at the gauge. Zero psig. Now look at the gauge on the bottom. If you have a 30 ft tall house it reads something like 13 psig. Much higher, right? So the whole "fluid flows from high pressure to low pressure would say that the open ended pipe in your hand should be flowing towards the roof. Now have someone open the valve on the bottom. Which way does the water flow? From is from low pressure to high pressure. Dang.

Now when you factor in potential energy it all makes total sense.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[BigInch]

Total energy = Potential Energy + Kinetic Energy
Potential energy is what you can measure using a static pressure gauge.
Kinetic Energy is what you can measure using a pitot tube.
Kinetic energy v^2/2/g is 0 when velocity is 0

Neglecting friction, if we assume that Total Energy Et, over an infinitesimal distance along a flow path remains constant, then if velocity slowed down, pressure would have to increase in order that Et remains constant, or vice versa.

Just like how getting a running start going down into the valley can help you get up to the top of the next hill.

 

[MichaelRs]

Think I got it now, just had pressure stuck in my head. Thanks for the replies!