Fun with Physics
 

https://www.youtube.com/watch?v=cp5gdUHFGIQ

In fluid dynamics, Bernoulli's principle states that for an inviscid flow, an increase in the speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738.

Fluid dynamics in a backyard pool at 27,500 frames per second.

VERY cool, and I learned something today...

His you tube channel is full of good stuff.

more science talk around this subject.

Outrageous Acts of Science: Firing A Gun Underwater

Here's another Russian weapon that uses supercavitation and goes way farther than 5 feet (at 200 knots!). VA-111 Shkval - Wikipedia, the free encyclopedia

Great find on the YouTube channel, anyone teaching physics using guns is OK by me!

Some more physics discussion seems relevant.

• Water obeys similar flow behavior as slow moving air, and they share being incompressible and both having a low Reynold's number. The latter is of course the ratio of inertial to viscous forces in a fluid, and has no unit dimension, it is just a number.

• The term inviscid or perfectly thin, is used to denote the separation of inertial and viscous properties in fluid modeling. This is done for simplification purposes mainly, it is easier to start with simple stuff than to tackle the whole thing at once. Full flow modeling can easily use up a Cray supercomputer for a few seconds of computation on a complex flow; you need to simplify the inputs and dropping the viscosity is a valid way to do it.

• Viscosity is the measure of resistance of a fluid to shear and tension forces placed on it. We can ignore the viscosity if it is trivial, and we will still get accurate flow modeling as mentioned above. However when it is not trivial, viscosity and inertia are related by the Reynolds number. They shot this video on a cold day (40F), but the results would have been interesting on a hot day to compare. You would have a higher Reynold’s number and a more turbulent flow, but the cavitation would probably be less.

• Cavitation takes place because the fluid cannot react fast enough to flow smoothly around the object it "sees" (or flows) around. The pressure drops behind the bullet, or the column of water being pushed ahead of the bullet, and the slower surrounding water tries to fill in but has trouble with energy required for such a movement and instead reverts to a boiling state as gaseous vapor. It is the same thing seen in boiling water on a stove, just for different reasons. The water boils behind the bullet and produces vapor for a short while. It is an adiabatic event involving low pressure, as opposed to the mostly thermal event we see in water boiling in a saucepot. He explains this well in the video.

• The rest of the cavitation story is explained pretty well in the video too, especially the part about oscillations due to a dynamic system set up by cavitation pockets and the liquid water around it.

Slow moving air is compressible. Our internal combustion engines, yea verily, even our jets engines rely on that characteristic.


But, we assume it is incompressible "for simplification purposes mainly." ;)






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Couldn't have said it better.

Not the same apple cake. Aerodynamics considers air going less than about 225mph (or M= 0.3) to be incompressible. It's a fluid dynamics concept not meant to be applied to flows where the pressure is raised using direct mechanical means. Turbines definitely are affected by aerodynamic compressibility since the flow rates they use are at a speed more than Mach 0.3.

Compressible Aerodynamics wiki

Apple cake?


Considers. That's another term for assumes.


You said slow moving air is incompressible.

I simply pointed out the mistake.






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