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Hot bodies get super-slippery when wet
Posted: Fri Jun 03, 2011 2:21 pm
by Sabre
The Reg
An Australian boffin says he has come up with a novel method for making things such as ship's hulls or torpedoes slip through water more easily.
Professor Derek Chan of Melbourne uni suggests that it would be practical for ships to exploit the "Leidenfrost effect", named after its discoverer in 1756. This refers to the behaviour of liquids when they encounter a surface significantly hotter than their boiling point: the portion of the liquid in contact turns into a cushion of vapour, keeping the remainder of the liquid away from the hot surface.
An everyday example of the Leidenfrost effect in play is that of a drop of water hitting a hot skillet and skittering about. As the vapour cushion is poorer at transmitting heat than direct contact, the droplet will then actually take longer to boil away than it would have if the pan were cooler. The same effect can also allow a person to put their hand into a bucket of liquid nitrogen without harm.
The Leidenfrost effect is poorly understood, however: it's quite hard to predict at just what temperature it will set in. But Professor Chan has carried out detailed experiments which involved dropping hot, polished balls into various liquids and viewing their interactions on high-speed video. The prof believes he's gained enough of a handle on the effect that it could be used in practical applications.
My only question is how much energy would it take to actually make this effective on a ship... especially a large one!
Re: Hot bodies get super-slippery when wet
Posted: Fri Jun 03, 2011 3:07 pm
by complacent
Re: Hot bodies get super-slippery when wet
Posted: Fri Jun 03, 2011 3:12 pm
by Sabre
Re: Hot bodies get super-slippery when wet
Posted: Sat Jun 04, 2011 3:49 am
by Mr Kleen
Thank goodness for the thin protective layer around them.
Re: Hot bodies get super-slippery when wet
Posted: Sat Jun 04, 2011 12:03 pm
by avriette
This is exactly what the shkval (a Russian/Chinese torpedo) does, although it uses a sort of bubble-generator rather than temperature. I think temperature is kind of an inefficient way of doing this. What might be really interesting (because air is a fluid after all) is very hot flying bodies. There'd be less resistance for very fast objects (e.g., objects which no longer require lift to stay aloft). At any rate, I know the US is actively researching supercavitating torpedoes, but it's acknowledged that the Russians and Chinese are a bit ahead. To give some context: the theoretical speeds of supercavitating torpedoes (which resemble underwater rockets rather than traditional, screw/prop driven torpedoes) are at least Mach 1. The interesting thing that happens at these kinds of speeds is you no longer need an explosive: your projectile can just slip right through the hull of something as big and ugly as an aircraft carrier and effectively break it in half.
They don't have a lot of guidance, because they're very fast, but because the targets they're tracking are so (relatively) slow, it matters very little.
I digress a bit, but I am not sure I see the value in hot or superhot hulls being used to create an envelope around an underwater or surface body.
I'm sorry, I don't get it...