Playing with Venturi tubes

A Venturi tube is one of the things one hears about in hydrodynamics class all the time, but one never gets to see them for real. And even though I just said on Friday that the thing that I found most fascinating in the aerodynamics collection I got to borrow recently was to see how the flow reversed downstream of a paddle I might have to take that back, because the hands-down most exciting thing was to play with a Venturi tube!

So what is all the fuss about? This is what a Venturi tube looks like:

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Venturi tube in the no-flow state

Basically, it is a tube, open at both ends, that gets thinner in the middle and wider again. All the rest you see in the picture is props: The mouth of the fan in the top right, and then three U-tubes filled with dyed water below the Venturi tube.

The Venturi tube is so famous because it nicely demonstrates the Venturi effect, namely the reduction in pressure that occurs when a flow is accelerated. In the case of the Venturi tube, the flow is accelerated in the thin section of the tube, where – for continuity reasons – it has to go faster than in the wider sections. So what happens when we turn on the fan?

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Venturi tube. Pressure decreases in the thin section of the tube, visible by the red water being “sucked up” in the middle U-tube

Yep! The levels in the three U-tubes change. And most importantly, the pressure for the middle U-tube drops, as demonstrated by the red water being “sucked up” on the side of the U-tube that is connected to the Venturi tube.

Watch the movie below to see this in action:

[vimeo 133272156]

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