# Venturi Vaccum increase in Momentum

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1. Apr 30, 2015

### Timtam

I was hoping to get a more complete understanding how a Venturi vacuum increases the Momentum of the fluid.

I know Momentum is an awful way to pose this question but it really is important that it can be explained by momentum instead of conservation - Hence the post.

An initial pressure difference Sp1 -Sp2 gives rise to fluid flow with a momentum px=mx.vx As the flow enters a constriction its flow rate vx increases and a consequential fall in static pressure Sp3 , this may even fall below Atmospheric SpA and this pressure difference SpA -Sp3 will cause mass m+ to flow into the pipe in the y direction.

As it exits the constriction reverting to original radius its velocity reduces to the mean flow rate vx induced via the pressure difference Sp1 -Sp2 However this velocity is now carried by additional mass so overall px=(mx+m+).vx increases

This increase in px is not paid for by the external x direction Sp1 -Sp2 pressure difference but by the y direction pressure difference SpA -Sp3

I know this does but I want to understand how the y direction momentum py induced by the y direction pressure difference contributes to x direction momentum ?

Can someone please explain the mechanism is transferred ? as to me the y direction pressure difference (being random) has equal chance to reduce x momentum as increase it.

Bonus points for explaining the 15' requirement

2. May 2, 2015

### paisiello2

Are you sure that v2 = v1?