Underwater Explosions

from a post by Pacomartin from the "are we doomed" thread

Quote:

A man, unharmed by an air explosion of a hand grenade at 5m (out of shrapnel line) would be killed by the same explosion underwater. Float on back on surface.. Lift the chest & abdomen out of the water, on a solid support. Face away from explosion.

I know Mr. Martin [and probably others] know why an explosion is more effective underwater, but in the 19th century the reason was put very concisely in a certain expression when they first realized how effective torpedoes would be. What they had discovered was a wooden hull underwater next to a respectable explosion would receive nearly the full effect of it. What they laid this effect to they called the [*] of water.

* fill in the blank

Quote: Pacomartin

technical term

Incompressibility

correct

Well, DUH! Of course Paco would know that one...

Everything gets more dense as it cools. Gas>liquid>solid. One of the only things that doesn't is water, and we see proof of it all the time. A water bottle exploding in the fridge. A pothole appearing in the road. An engine block cracking when it isn't winterized. The liquid form of dihydrogen monoxide is its densest form.

I thought that was common knowledge =/

Quote: Face

I thought that was common knowledge =/

It should be fairly common knowledge that ice is lighter density than water. If it wasn't, we probably wouldn't have life as we know it on Earth, as the sinking ice would freeze most of the bodies of water.

But the OP is talking about a different characteristic of water. There are articles in wikipedia about compressibility, but they require a working knowledge of partial differential equations to understand them.

Incompressible flow
Compressibility


The most common popular phrase associated with compressible and incompressible flow was the "sound barrier".

Quote: Face

Everything gets more dense as it cools. Gas>liquid>solid. One of the only things that doesn't is water, and we see proof of it all the time.

Not exactly.

Water contracts as it cools, but only down to a certain temperature and then it expands, then past a certain other temperature it contracts again. Certainly steam becomes more dense as it cools into liquid water.

Simply put:

Steam = hot water = less dense than water below 100 Celsius

Liquid water = contracts as cooled down to around freezing

Ice = solid water = expands as it freezes and is less dense than liquid water.

Very cold ice does contract as compared to warmer but still frozen ice. I don't recall when and how it happens, I mean at what temperature, but it does. In any case the Earth never gets too cold. Nitrogen boils at around minus 40 C, and no place in the world, as far as I know, ever gets so cold that the largest part of the atmospehre becomes liquid.

Quote: Nareed

Very cold ice does contract as compared to warmer but still frozen ice. I don't recall when and how it happens, I mean at what temperature, but it does.

Yes, that is true, but it is a relatively small effect, and it occurs gradually as you lower the temperature. Note that in the image below there is a big chunk missing from the scale.

Quote: Nareed

Nitrogen boils at around minus 40 C, and no place in the world, as far as I know, ever gets so cold that the largest part of the atmospehre becomes liquid.

Nitrogen boils around minus 200 C. No place on earth gets colder than that , but temperatures of minus 90-100 C have been recorded in Antartica.

Carbon Dioxide has a boiling point of minus 57 C. Could that be what you are thinking of?

Quote: Pacomartin

Nitrogen boils around minus 200 C. No place on earth gets colder than that , but temperatures of minus 90-100 C have been recorded in Antartica.

Carbon Dioxide has a boiling point of minus 57 C. Could that be what you are thinking of?

No. I was thinking about nitrogen. When I worked at a cord-blood bank, the blood deposited was kept at around -40 C in in a freezer, with liquid nitrogen as a backup in case of loss of power. Allegedly the liquid nitrogen would keep the samples at around -40 C for up to 48 hours in a sealed container. I assumned therefore that was how hot nitrogen needed to be in order to boil.

Quote: Pacomartin

they require a working knowledge of partial differential equations to understand them.

A math-less understanding of that incompressibility is still pretty easy.

Basically, a given amount of water cannot be changed from a larger volume to a smaller one by applying pressure, at least not in any practical manner. If you want something like a garden hose to have pressure, you can use a water tower, or use air pressure. But if you try to keep the pressure in that hose, but merely turn off the spigot, the pressure vanishes. The water in and of itself cannot hold it. There are other examples.

In the case of an underwater explosion, the water contains the expanding gases due to the incompressibility; should a ship's hull* be next to that force, it gives way readily as it represents no resistance compared to the water. A path of least resistance I suppose.

* or a person's body as in the original thread