Sciencemadness Discussion Board

Breaking the Speed of Sound Underwater

Gooferking Science - 11-7-2014 at 20:01

I was sitting underwater in a pool recently listening to different muffled sounds. Suddenly I wondered what it would be like to break the speed sound underwater. Now the principle is quite simple, but actually doing this would be difficult. You would have to propel whatever device is breaking the speed of sound underwater through a medium that is much more dense than air. Also, the speed of sound underwater is substantially faster than that in air. At these speeds the water might just form some sort of a vacuum cavity around the device moving at extreme speeds. This is similar to shooting a gun underwater. Do you think a sonic boom would be created underwater? If the device was surrounded by a vacuum cavity then it would not be heard. Any ideas or thoughts in this subject?

Tdep - 12-7-2014 at 01:34

It would be possible.

You can break the light barrier in water, and this results in a sort of sonic boom but with light called Cherenkov radiation (pictured) so my guess is that once you break the sound barrier in water, you should get a sonic boom too.

250px-Advanced_Test_Reactor.jpg - 14kB

Gooferking Science - 12-7-2014 at 04:28

But if the object breaking the speed of sound was surrounded by a vacuum, you wouldn't be able to hear a sonic boom because sound can't travel in a vacuum. I also was wondering if it would form Mach cones like when a jet plane breaks the sound barrier, or is that just characteristic of breaking the speed of sound in air?

Zyklon-A - 12-7-2014 at 06:48

Sound travels at about 1497 m/s at 25 °C.
A very powerful gun can propel a bullet at over 1500 m/s.
So, it's very likely that this has already been done. (Although the bullet would slow down very fast). Maybe not with actual scientific researchers studying it though.
Are you talking about propelling a human though water at that speed? Cause that seems like suicide.

[Edited on 12-7-2014 by Zyklon-A]

Metacelsus - 12-7-2014 at 07:31

You'd probably get large-scale cavitation before you broke the sound barrier, so you wouldn't really be traveling in water anymore--just low-pressure water vapor.

IrC - 12-7-2014 at 07:48

Am I the only one that wonders if shrimpoluminescence is caused by the claws snapping exceeding the local sound speed in water?

http://stilton.tnw.utwente.nl/shrimp/shrimpoluminescence.htm

http://en.wikipedia.org/wiki/Sonoluminescence

Burner - 12-7-2014 at 08:03

^^^ Could this really just be sonoluminescence (https://en.wikipedia.org/wiki/Sonoluminescence)?

Gooferking Science - 12-7-2014 at 13:35

I wasn't necessarily talking about propelling a human underwater. That probably would mean suicide, although I can't think of a more glorious way to die :) I was just wondering about an object traveling underwater faster than the speed of sound for a prolonged period of time. Slow motion footage of this would be awesome, and so would the cavitation. I think that bullets probably decelerate before they even leave the barrel of a gun, since the barrel is filled with water. I doubt that the bullet would be traveling faster than the speed of sound after it leaves the barrel.

What kind of shape would the device be, and what system of propulsion would it use? It obviously couldn't be a conventional form of combustion propulsion, because combustion and water don't mix. Would it be some sort of a high pressure air propulsion system? Or a strong jet of water?

Sedit - 12-7-2014 at 19:48

Can pressurization prevent cavitation in anyway?

Such as with the case of the claws of the shrimp, the supersonic movement would not really have to be sustained and a short rapid burst may be possible under a pressurized environment in order to observe the effects of supersonic movement underwater.

Oscilllator - 13-7-2014 at 03:31

Breaking the sound barrier underwater with a bullet is no doubt possible, however somewhat meaningless because there is so much cavitation.
However I would have thought it would be pretty easy to do by using a piston to create an extremely high-pressure jet of water (or perhaps another liquid for that matter). Measuring the speed of this jet should be fairly easy if you use some kind of dye and view the footage with a slow-motion camera for extra awesomeness.

Since the claws of the shrimp do cause a high-pressure jet of water anyway, I feel this is definitely worth investigating. It could even be plausibly be done by the amateur, so long as they have access to some heavy-duty pipe, a welder and some gunpowder/nitrocellulose.

froot - 13-7-2014 at 09:16

I'not sure what the term is but I speculate that the result depends on how much the volume of water reduces as the pressure applied to it increases - sort of compressive elasticity. For air that property is more than noticeable easily giving way to a projectile moving through it.
While for water I presume that property is less than noticeable making it much more difficult for a projectile to propegate through it.
This means that at any given instant a projectile needs to displace far less volume of air thanks to its 'compressive elasticity' than that of water and thereby requiring exponentially higher amounts of energy to propel it through water. I would expect the force needed to propel a bullet size projectile even close to the speed of sound under water to be in the order of tons because due to it's low 'compressive elasticity' it would basically need to displace all the water ahead of it to a certain extent which would amount to a mass in the order of tons. That is if the molecular structure of the projectile itself can withstand the frictional abrasion with the water it's displacing, which is in my opinion highly improbable.

Basically the projectile would probably disintegrate before it gets near the speed of sound under water regardless of its composition.

phlogiston - 13-7-2014 at 12:27

It has been done actually:
Quote:
he Advanced High Speed Underwater Munition (AHSUM) program has already demonstrated the effectiveness of such high-speed underwater bullets. Fired from an underwater gun, <b> these projectiles have successfully broken the speed of sound in water (1,500 meters per second)</b>, bringing their future application much closer to reality.


Source: http://www.navy.mil/navydata/cno/n87/usw/issue_14/torpedoes....

Also: Warp drive underwater

Nicodem - 13-7-2014 at 23:48

There are electro-motors that can easily exceed 30,000 rpm at some torque. To beat the sound barrier underwater with an impeller of 1 m diameter you would need >28,605 rpm. The motor must have enough power to keep up with the huge torque (I suspect you would need at least 3 kW even with the most hydrodynamic impeller design). The impeller should be thin and flat enough to provide minimum resistance (perhaps a disc with an indenting at the edge), and the experiment should be done in a large enough pool to dissipate the vortex. This way, you should be able to break the sound barrier at the impeller's tip and even film what happens. I suspect that the tip of the impeller would start "flying" trough water vapors substantially before reaching the speed of sound. The cavitation might mess up things, depending on the impeller design.

www.youtube.com/watch?v=cp5gdUHFGIQ

franklyn - 28-7-2014 at 14:16

www.youtube.com/watch?v=cp5gdUHFGIQ

macckone - 29-7-2014 at 08:57

Quote: Originally posted by franklyn  
www.youtube.com/watch?v=cp5gdUHFGIQ

Excellent video of a gun being fired underwater in slow motion
with moderately good scientific explanation.

Bert - 29-7-2014 at 12:14

Ultra mega Shkval, anyone?

AJKOER - 1-8-2014 at 15:01

When an explosion occurs underwater, a powerful/deadly shock wave is formed. A commonly used tool by 'fisherman'.

Would expanding gases constitute a particle with respect to breaking the sound barrier under water?

Would an additional shock wave be discernible?

Beyond my pay grade.