# I Venturi effect and pressure?

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1. Aug 8, 2016

### Metals

So I recently looked into the venturi effect, only to be confused. I had always been aware that a lower available volume would result in a higher pressure, yet I had read that fluid passing through a constriction is actually subjected to less pressure than the diverging sides of a pipe. I had thought that the increase in fluid velocity was due to an increase in pressure (from a smaller volume).

Why is there less fluid pressure in a constriction as opposed to a section of a tube with a larger volume?

Is the decrease in pressure from the constriction the reason why fluid flows faster through it? I thought fluid went faster because it had to maintain the same volume/time ratio through a smaller volume, which should result in a higher pressure (force/volume=pressure).

2. Aug 8, 2016

### rcgldr

In the simplified case, no compression occurs, mass flow is constant throughout the venturi, and no external forces act on the fluid, so the total energy per unit volume of the gas or fluid remains constant. If the kinetic energy per unit volume increases due to an increase in speed, then the static pressure (energy per unit volume) decreases.

http://en.wikipedia.org/wiki/Venturi_effect

http://en.wikipedia.org/wiki/Bernoulli's_principle

There is a frame invariant form of Bernoulli, which is described in this pdf file:

http://www.loreto.unican.es/Carpeta2012/EJP%28Mungan%29Bernoulli%282011%29.pdf [Broken]

Last edited by a moderator: May 8, 2017