What is the difference between the speed of sound in the atmosphere and under water?
What would happen if a boat travelling on the surface of the ocean broke the sound barrier? Would the parts of the boat that are travelling beneath the surface break the sound barrier later, due to the fact that sound travels faster underwater and hence the barrier is harder to break?
What would happen if a submarine broke the speed of sound while completely immersed in water? What would the shockwave do?
As an avid scuba diver I can tell you one side effect of the speed of sound underwater…you can’t tell which way sound is coming from. The time it takes for the sound to travel from one ear to the other is too short for your brain to distinguish which ear heard the sound first. This has the effect of making the sound seem as if it is coming at you from all directions at once.
Big deal you might say but this is of great importance to scuba divers. During my training classes we saw pictures of people who had been run over by boats. It is not a pretty sight. Unfortunately, since most boaters need do no more than send their registration fee to the state, many boaters don’t know to steer clear of dive flags. As a diver if you hear a boat in the vicinity you either stay underwater or come up on shore or next to your own boat.
Hypothetically? As a WAG I’d say the same thing that happens in air. Fluid dynamics treats both air and water the same just with different densities.
As the sub approached the speed of sound a stronger and stronger pressure wave would form in front of the sub. As the sub passed the sound barrier it would pass through this pressure wave eventually leaving a bunched up sound wave trailing behind the boat. The ride would be VERY rough till the sound barrier was breached at which point the ride would become considerably smoother.
As for the wave I imagine that much energy would travel quite a distance on its own (even after the boat slowed down) and may form a few considerable waves on the first beach it met.
Before nitpickers jump all over this I think we are all clear that anything man could build would almost certainly be torn to pieces long before getting that fast in the water not to mention finding a powerplant that could provide that much energy.
I would say most definitely but you’d only hear it for the instant before your eardrums collapsed. I seriously doubt any creature’s ears could withstand that kind of overpressure.
You might as well ask what would happen to an object moving through steel at the speed of sound. It’s not much different.
There are substantial differences between moving through water and moving through air, the primary one being that water is, for all intents and purposes, incompressible. Density is not the major consideration.
Incompressiblity means the fluid must get out of the way of the oncoming object, it can’t just bunch up in front of it like a compressible fluid would. The effect of the sound barrier in air is the result of not being able to compress fast enough, so that the compression occurs as a shock wave. An incompressible fluid faces an entirely different set of problems – it has to actually move without any significant change in its physical dimensions.
A boat moving at what would be the speed of sound in water would be skipping across the surface. Hell, it’d be flying! There would not be time for normal buoyancy to operate. The water’s surface would be indistinguishable from a solid.
A completely submerged object moving at the speed of sound through water would be expending an enormous amount of energy. Not only would the water be boiling, the object itself would probably boil away as well. The X-15 topped out at about 3000 mph and they had trouble keeping it cool enough to operate moving at that speed through very rarefied air.
Given enough energy you could drive something through water that fast. A much bigger problem would be to keep it together.
I suppose something like this has actually happened at some point in time. A meteor like the one that killed the dinosaurs would presumably hit the ocean at a high enough speed to exceed the local sonic velocity. Some interesting physics must have occurred, and there was definitely a shock wave or two, but words like detonation and vaporization would be the way to deal with it. I doubt the physics would be much different whether it hit land or sea.
No answer for you, but as I was researching the question, I came across one of the neatest picture I have ever seen. An F-18 is just crossing the sound barrier and a shock wave has caused the water vapor in the air to condense.
If you want me to look into what some fluid dynamics textbooks say about this, I will be back at college and able to look into it on August 21st or so. If you let me know, I’ll try to remember to bump the thread up with an explanation shortly thereafer. I suspect that pluto’s got it pretty much nailed, but might be able to provide some more detail.
I recently read something about “cavitation” in water which would let a sub reach extremely high speeds. However, this process seems to require some sort of large bubble that contains the ship, so I’m not sure how that would affect the physics involved in the original problem. Anyone have any more info?
Cavitation does not allow a sub to reach high speeds. Cavitation is what happens when a sub tries to reach high speeds…important difference.
Basically cavitation is the formation of ‘bubbles’ on the edge of a propeller.
Cavitation is always bad as regards boats and propeller performance. In a sub its TERRIBLE. A sub’s main goal is to remain silent. If they spin their propeller fast enough tiny pockets of vapor form then collapse. This collapsing makes noise. When you have hundreds of thousands of bubbles doing this at once the noise is considerable…MORE than enough to pinpoint a submarine for anyone listening within miles of the sub.
Cavitation only occurs at higher speeds. US subs have an extremely well milled and designed propeller to allow them higher speeds before cavitation occurs. A normal boat propeller cavitates practically all the time except at idle speeds (why spend a fortune refining power boat propellers…they don’t need to be quiet).
In addition, cavitation decreases performance of the propeller. Each little bubble adds drag to the spinning propeller. Again, this is generally not enough to be of concern to anyone but sub drivers and extremely high-performance (read racing) boats.
Finally, the speeds a sub can reach today are FAR FAR FAR below the speed of sound in water. Maybe 40-50 mph for the fastest subs (Russian Alpha supposedly could reach 60 mph but was noisy as all hell when it did so) compared to 3,000+ mph needed to break the sound barrier in water.
Thanks for the information. The article that I read proposed that cavitation, although normally an obstacle to underwater travel, can be harnessed to increase speed. The article mentioned research being done that will allow topedoes, and eventually entire vessels, to travel through water enclosed in a bubble that apparently is a side-effect of cavitation. The article mentioned that such a process would require the vessel to be shaped in a certain way to take advantage of cavitation and that instead of a propeller some alternate means of propulsion would be used.
Unfortunately I couldn’t find a link. I was hoping someone on the board had heard about this, but I guess it’s a little too obscure. Tried different searches with no success. I wish I could remember where I had read it…
BTW, underwater sound barrier has already been breached by man-made machines: Atomic/hydrogen Bombs. When they detonate, the shockwave they cause is a lot faster than underwater speed of sound.