MadSci Network: Physics |
Greetings Viktor:
References:
1. Landolt-Bornstein , Oceanography, New Series, group 5,
Volume 3, part A.
2. Cureio, Petty, Optical absorption of pure water,
Journal of the Optical Society of America, (JOSA) V41, pp 302-304
Your question revolves around the basic concepts of the science and
laws of
THERMODYNAMICS and the transfer of heat.
There are three ways heat can be transferred form a submarine; by
conduction,
by convection and by radiation. For a submarine operating in the sea,
all three
of these methods of heat transfer must be taken into account.
Conduction: If one end of a metal rod is placed in a flame
while the other
end is held in your hand, you will feel the rod becoming hotter and
hotter
even though your hand is not in direct contact with the flame. Heat is
said
to flow along or through the material by conduction.
Convection: The transfer of heat from one place to another by
the actual
motion of material is called convection. Examples are water cooling or
air
cooling of a hot engine.
Radiation: When we hold a hot body near our hand in still air
or a vacuum,
we feel the heat by radiation. In the design of satellites operating
in the
vacuum of space, convection and conduction cannot be used to remove
heat
from the spacecraft, only radiation can be used to dissipate waste
heat.
The data in References 1 and 2 for clear fresh water and clear sea
water show
that the water is most transparent at visible blue - green wavelengths
between
0.4 to 0.5 micrometers (micrometers are 1/1,000,000 of a meter) where
the
attenuation (loss) of electromagnetic energy is about - 0.1 dB per
meter
(about 2% loss, 98% transmission). However, the attenuation rapidly
increases
for both longer and shorter wavelengths.
A submarine's propulsion system operates at very high temperatures of
several
thousand degrees C and all three methods of heat transfer must be
taken into
account. However, convection and conduction of the waste heat to
seawater is
the primary way of cooling a submarine. However, remote sensing
usually relies
on monitoring the heat radiation from an object including it's heating
of the
surrounding environment, which in this case is the seawater around the
submarine.
Let us assume that the submarine is expelling boiling water at 100
degrees
C (Celsius).
The wavelength for the peak of the thermal radiation spectrum (Black
body
radiation) for a material at a temperature of 100 degrees C is
determined by:
Max Wavelength (in micrometers) = 2897 divided by the temperature in
degrees K
0 degrees C = 273.16 degrees K, thus boiling water is
at 273.16 + 100 = 373 degrees K.
Max Wavelength = 2897 / 373 = 7.8 micrometers in the infrared.
From References 1 and 2 we find that at 7.8 micrometer wavelength the
attenuation
of water is greater than -30 dB per meter. This means that for a given
amount
of energy
entering a meter of length of water only 0.1% passes through the meter
and the
99.9% of the energy is absorbed or scattered. The second meter of
water will
attenuate 99.9% of the original 0.1% entering and an insignificant
amount of
thermal radiation would remain. Thus we conclude that we will only be
able to
detect a submarine by thermal radiation only a few meters under the
water.
If the submarine were traveling on or near the surface, the surface
radiation from
the water heated by the submarine might leave a thermal wake which
might be
detectable. However, detecting thermal radiation directly from the
submarine at a
depth greater than a few meters would be very difficult if not
impossible and
submarines usually travel at much greater depths.
We know that in clear water we can visually see a submarine at a much
greater depth
than a few meters because of the blue - green "window" in water;
however, the
thermal radiation from the submarine at these wavelengths is
negligible.
After the invention of powerful blue - green lasers during the 1960s
there has
been a great deal of research in using LIDAR (laser radar) to detect
submarines
at depth; however, information about how well this works is not
pubically available.
Best regards, Your Mad Scientist
Adrian Popa
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