If you subject a fluid to a sudden change
in pressure, some interesting things can happen. You can cause tremendous damage to moving
parts, or you can harness this destructive power in many beneficial ways. From mantis shrimp killing their prey to ultrasonic
cleaning, so many things rely on this fluid phenomenon. Hey I’m Grady and this is Practical Engineering. Today we’re talking about fluid cavitation. You might even call this video a treat especial,
because this is the story what may be one of the most inept YouTube collaborations of
all time, thanks to me. It all started with a sketch of a venturi. A venturi is a device that constricts the
flow of a fluid to take advantage of Bernoulli’s principle. You may have heard of this principle, which
basically says that all the energy in a fluid can take one of three forms: kinetic, potential,
or internal energy. And the total amount of energy is the same
along a streamline. So if you change one - for example you increase
the kinetic energy of the fluid by speeding it up - the others have to accomodate - in
this example the fluid’s pressure goes down. Being able to lower the pressure of a fluid
(also known as a vacuum) just by constricting the flow area makes a venturi a very useful
tool that can be found in all kinds of devices from engines to trombones to scuba diving
regulators. So, I thought, I’d like to have one of these
venturis, and I knew just the guy to make it for me. You may have heard of his YouTube channel:
Arduino versus Evil, now cryptically shortened to AvE. We’ve never seen his face but we’re pretty
sure he’s handsome. Him and I had been emailing ideas across the
U.S. - Canadian border, and this seemed perfect. I have a channel centered around practical
demonstrations of engineering principles - he has a clapped out bridgeport milling machine. It was a match made in YouTube heaven. So I sent that sketch over to AvE and said,
“Could you make something like this.” And he said, “The drawings are never right. There’s details left off. The guy doesn’t know his a** from his elbows.” But, he tried to make it anyway, providing
us with many excellent lessons about manual machining. “There’s three ways to do this…” And in a second video, the prototype was finished,
and we were left with these parting words: “If I was a betting man - and I am - I’d
bet that this ain’t going to work.” And it didn’t. Or at least I have to assume it didn’t,
because 10 months later I got this in the mail. Instead of giving me the hard truth - that
my sketch was poorly considered and I wasted his weekend - he gave me something even better:
a care package including a clear acrylic liquid flow meter that was designed by someone who
knew what they were doing. And, if you look closely at this flow meter,
you might recognize the shape as a venturi, which is perfect, because I need a venturi
to show you this fluid phenomenon. Here’s my setup: I have my garden hose running
into the garage and a pressure boost pump feeding a manifold that connects to a pressure
tank, a pressure gage, and this flow meter. I modified the meter so it acts like a venturi
by gluing the weight to the center post so it can’t slide up and down. And I have a differential pressure gauge to
measure the pressure drop across the venturi. The drop in pressure is the whole purpose
of this demonstration. To understand why, we need to look at the
phase diagram of water. We know that water changes state based on
temperature. It’s a solid (ice) when it’s cold, a liquid
at room temperature, and a gas (steam) when it’s hot. But, the phase of any substance also depends
on the ambient pressure. You can see that, even at room temperature,
water can turn to steam at very low pressures. This is true for a lot of liquids. If I force this water through a small enough
opening in the venturi, according to Bernoulli, I’m decreasing the internal energy (aka
the pressure) and converting it to kinetic energy (aka the flow velocity). And if I get the flow going extremely fast,
I can decrease the pressure below the vapor pressure of the water, converting to steam. Steam by itself isn’t a problem, but the
issue comes when the pressure goes back up and the steam collapses back into a liquid. On a larger scale, this collapse can lead
to thermal shock. Check out my video on steam hammer to learn
more. But, on a smaller scale, collapsing steam
bubbles are called cavitation. And even though the scale is smaller, the
damage cavitation can cause can be just as destructive. This is because collapsing steam causes water
to accelerate and decelerate violently. Water isn’t compressible, so it slams into
itself creating a shockwave. It’s like a thousand tiny water hammers. In many cases where cavitation is occuring,
you even can hear these shockwaves, which often sound like gravel moving through a pipe. If I build up enough pressure in this tank
and open the valve to the venturi, you can clearly see (and hear) the cavitation occurring. I can’t measure the pressure at the constriction
of the venturi, which will be a very strong vacuum, but this gauge measures the total
loss in pressure caused by the turbulence and cavitation, just for reference and because
it looks cool. Needless to say, in most cases, cavitation
is bad news. It can erode pipes, impellers, and other moving
parts, leading to accelerated wear or catastrophic failure. It can even cause damage to the spillways
of very tall dams. So engineers generally avoid designs that
might subject liquids to sudden changes in pressure. Pipes get smooth bends rather than abrupt
changes in size or direction. Boat propellers and pump impellers are carefully
designed to match with the speed and power of the motor to which they are attached. And dam spillways are designed to avoid any
protrusions into the high velocity flow. However, although it is generally avoided
in all kinds of industries, cavitation can also be a force for good. Ultrasonic cleaners use cavitation to agitate
a solvent and break the strong bonds between a contaminants and parts. Some industries use cavitation to mix compounds
that are difficult to combine (like paints). Finally, some shrimp can move so quickly,
they create a cavitation bubble to kill their prey. As for this flow meter, it seems to be holding
up fairly well so far. The acrylic seems to be able to absorb the
shockwaves better than metal would. So, it’s probably best that our collaboration
worked out the way it did. Thanks to AvE for supplying the demonstration
for this video. If you like seeing the insides of tools and
industrial machinery and don’t mind a little bit of language, check out his channel and
tell him I sent you. Also, thank you for watching, and let me know
what you think. Thanks to Blue Apron for sponsoring this video. Blue Apron delivers all the fresh ingredients
you need, right to your doorstep, in exactly the right proportions to create delicious
recipes at home. I cook about as well as can be expected for
civil engineer, but even I can follow these instructions. These meals are some of the best I’ve ever
had, and we really enjoy cooking them together. If that sounds like something you’d like
to try, the first 100 people to click the link in the description will get $50 off your
first two weeks of Blue Apron! Again, thank you for watching, and let me
know what you think!
ave and PE? wow this should be great.
AMA hydraulics engineer, I can’t even tell you how many times I’ve seen pump/ valve failure because of this.
Excellent video bravo human!
That's a nice video and all, but the important thing is that now that I know tiny scales exist I have to have one!
this is why we supply multi-ro's for high pressure drop applications!
Can someone explain to me why the original design wouldn't have worked? Wasn't explained in the AvE video either.
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That venturi looked far from skookum... And the host seems kinda chintzy :/