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!