1. ## Underwater sound barrier

I understand that as an aircraft flies breaks the sound barrier, the air cannot move aside fast enough for the aircraft to pass though. When this happens, the aircraft pushes though the air, compressing it and producing a sonic boom as a result.

Now my question is, is it theoratically possible for the sound barrier to be broken underwater as well? Since water cannot be compressed like air, is it still possible?

2. Originally Posted by dzerzhinsky
I understand that as an aircraft flies breaks the sound barrier, the air cannot move aside fast enough for the aircraft to pass though. When this happens, the aircraft pushes though the air, compressing it and producing a sonic boom as a result.

Now my question is, is it theoratically possible for the sound barrier to be broken underwater as well? Since water cannot be compressed like air, is it still possible?
Considering that the speed of sound underwater is about 4 1/2 the times faster than in air, it would be pretty difficult to do. There have been some rumors that Russia developed a rocket-powered torpedo that could do it.

In any event, yes, it's possible and the major effect it would cause - by extreme cavitation - would be very similar to a sonic boom in air.

3. Yes, it is possible through something I think is called supercavitation. Basically, the shape of the torpedo creates a shell of cavitation around it, enabling it to obtain extreme speeds.

Ah, yes, wikipedia to the rescue,
http://en.wikipedia.org/wiki/Supercavitation

4. from wiki: "the VA-111 Shkval ("Squall") torpedo. This can travel at 230 mph (100 m/s) underwater"

so far it doesn't look like we've broken it with a torp, even the air-speed sound barrier underwater, but i'm pretty sure a bullet striking water maintains a supersonic velocity at least part-way into the water. maybe not the 4 and 1/2 mach water-sound-barrier, but wouldn't that just need a really big gun and a pool of water to test? how about a railgun? pretty sure they have ones that'll get something small to low-earth-orbit velocity in a lab environmen.

5. But I don't understand, where does the gas bubble come from in supercavitation? Wouldn't it be more like a vacuum?

6. I do not know, but I suspect it results from a localised massive reduction in pressure, which either causes dissolved gases to come out of solution, or makes water vapour the stable state. Or both.

7. Originally Posted by Ophiolite
I do not know, but I suspect it results from a localised massive reduction in pressure, which either causes dissolved gases to come out of solution, or makes water vapour the stable state. Or both.
Correct. With the greatest contribution coming from the former.

Edit: too quick on the 'post' button there, I was distracted by a conversation here in the room.

There's also a substantial amount of partial vacuum created. Those "vacuum bubbles" collapse very rapidly and are responsible for the abrasivness caused by cavitation. Also, the noise produced by them is what's important in underwater detection methods.

8. Originally Posted by dzerzhinsky
I understand that as an aircraft flies breaks the sound barrier, the air cannot move aside fast enough for the aircraft to pass though. When this happens, the aircraft pushes though the air, compressing it and producing a sonic boom as a result.

Now my question is, is it theoratically possible for the sound barrier to be broken underwater as well? Since water cannot be compressed like air, is it still possible?
It might be theoretically possible, but I don't think the sound barrier in water would be as significant. In air it was such a problem because the compression ahead of the aircraft made a "wall" of extra dense air. In water, the differences in density would be very small (water is essentially incompressible). Also, water is much more viscous than air, so it's likely not possible to reach the speed of sound due to all that friction.

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