Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Acoustic Gel
Field of Invention
This invention relates to a method for reducing
cavitation around underwater acoustic projectors, and to a
composition for use therein.
Background of Invention
When underwater acoustic projectors are driven at
levels at which peak acoustic pressures exceed the ambient
hydrostatic pressure, a phenomenon known as cavitation can
occur. This is manifested by bubbles appearing on or near
the surface of the projector and by a sudden reduction in
the acoustic loading of the device. Acoustic energy cannot
be transferred through the gas bubbles created and
consequently the projector ceases to radiate the desired
acoustic signal. When the acoustic loading of the projector
is reduced, catastrophically high vibrations of the
projector can occur with resultant damage to the projector.
Further, insidious low level cavitation can cause rapid
erosion of the projector face. It is, therefore, highly
desirable to avoid cavitation whenever possible.
Traditionally this has been achieved by operating the
projector at depths where the hydrostatic pressure is high
enough to prevent cavitation or by enclosing the projector
in an acoustically transparent pressurized container.
tJnfartunately, there are many instances where the operating
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depth of a projector is dictated by considerations other
than cavitation prevention. V~or example full power
operation at shallow depths may be an operational
requirement in order to achieve long distance sound
propogation. In the case of hull mounted projectors, such
as sanar domes and echo sounders, deep operation obviously
is not possible. Enclosing the projector is usually
impractical or expensive. Oontainers capable of
withstanding the pressure at, say, 100 m (approx. 1000 k Pa)
would have to be made from enormously strong materials.
Acoustically transparent materials are not generally strong
structural materials. There is, therefore, a need for an
alternative method to prevent cavitation around an acoustic
projector.
Object of Invention
Thus, it is one object of the present invention to
provide a method far preventing cavitation around acoustic
projectors.
Another object of the invention is to provide a
composition of matter suitable for application to acoustic
projectors to prevent cavitation.
Brief Statement of Invention
By one aspect of this invention there is provided a
method for reducing cavitation around an underwater acoustic
projector, comprising encapsulating said projector in an
aqueous gel comprising a polysaccharide polymer, a
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hydrophilic stabilizer and a non-gel inhibiting and soluble
biocide, and curing said gel around said projector.
By another aspect of this invention there is provided
an aqueous polymeric gel for encapsulatiang underwater
acoustic projectors, comprising 0.5-1.0% by weight of a
polysaccharide polymer cross linked with about 5-25% by
weight of a hydrophilic stabilizer selected from the group
comprising ethylene glycol and glycerol, 0.024 - 4.48% by
weight of a cross linking agent and containing about 1 ppm
of a non-gel inhibiting and gel-soluble biocide.
Detailed Description of Preferred embodiment
Tn order to operate sonar projectors at high power
levels in shallow water it has been found that cavitation
can be reduced or eliminated if the projector is Surrounded
by a fluid which is more resistant to cavitation than water.
Tt has been found that the required acoustic properties and
equipment adhesion properties are provided by derivatives of
chitasan based gels to which a biocide has been added.
chitosan , a deacetylated chitin, is a proprietary
polysaccharide, available from Nova Ghem l.td., Ganada and is
made from naturally occuring materials such as lobster and
crab shells. Preferably the derivative of chitosan is an
aliphatic substituted derivative such as a carboxyalkyl,
and most particularly is N,O-carboxymethylchitosan. The
polysaccharide is cross linked with a cross linking agent in
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an aqueous ethylene glycol solution, preferably containing a
biocide. The polysaccharide is generally in the range 0.5-
1.0% by weight polymer, preferably about 0.85% by weight.
Ethylene glycol should preferably be in the range 5-25% by
weight and more preferably 15% by weight. The cross linking
agent, such as glyoxal should be in the range 0.024 -
0.48% by weight, and preferably about 0.048%. It has been
found that aqueous gels containing about 0.85% N,O-
carboxymethylchitosan, 15% ethylene glycol, balance
substantially water, develop fungal growth when stored in
daylight at room temperature over a period of time (2-3
weeks) causing deterioration of the gel. It is, therefore,
advisable to incorporate about 1 ppm of a biocide, such as
Quaternary Ammonium Salt {QAS) or hexadecyltrimethyl
ammonium bromide which are soluble in the gel mixture and
which do nat inhibit gel formation. Biocides such as
CaptanR and BenlateR are not suitable as they are not
soluble in the gel, and sodium metabisulphite is equally
unsuitable as it inhibits gel formation. The ethylene
glycol serves as a hydrophilic additive to stabilize the gel
and minimize the release of water therefrom particularly
when the gel is subjected to a series of freeze-thaw cycles.
Other hydrophilic stabilizers such as glycerol may also be
incorporated. 15% by weight ethylene glycol as the
stabilizer is preferred as this reduces the freezing point
of the gel to about -7.1°C which is well below the freezing
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point of sea water.
Before the gel is applied to the radiating surface of
the projector, it is preferable that the surface should be
pretreated to ensure maximum wettability and adhesion of the
gel. Pretreatment with a surfactant such as JoyR, TweenR
20, 60 or 80, Agra1R90, Triton N-57, Triton X114, Atsurf
241, Atsurf 249 detergents, increase wettability and gel
adhesion depending to some extent upon the nature of the
projector face. The preferred surfactant is NAJ which is a
blend of equal vo3umes of 1% polysaccharide polymer in
water, 1% Aerosol OTP solution and 10% JoyR detergent.
Aerosol OTP is sodium dioctyl sulf osuccinate, and JoyR
detergent is a mixture of saponified fatty acids. Example 1:
Acoustic Testing
The cavitation strength of the various gel formulations
was determined by means of a resonant system comprising a
hollow glass or aluminum sphere filled with the gel being
tested, and a piezoelectric driver. A small region of high
acoustic pressure was created at the centre of the sphere
when the system was driven at one of its resonances. The
mechanical Q (fundamental frequency in Hz) of the system was
high (generally between 2000 and 3000) and sufficiently high
pressures to cause cavitation were possible with moderate
input power to the driver. The acoustic pressure at the
centre of the gel was indirectly measured by means of a
sensor bonded to the exterior surface of the sphere.
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A glass or aluminum sphere having a diameter of 25 cm
and wall th i ckness of 0 . 6 cm was f i 11 ed w i th the ge 1 be i ng
tested. A piezoelectric driver in contact with the gel
drove the system at one of its resonance frequencies. The
acoustic pressure at the centre of the gel was determined by
measuring the electrical output of the sensor bonded to the
sphere. The sensor was previously calibrated by means of a
calibrated probe hydrophone situafi..ed at the centre of the
gel.
The test routine consisted of applying a low drive
voltage to the driver arid monitoring the output voltage and
waveform of sensor. The drive voltage was gradually
increased until the onset of cavitation. Cavitation was
manifested by a sudden drop in the output voltage of the
sensor, a distortion of the waveform of said sensor, and the
simultaneous increase of drive voltage of driver. With the
glass sphere there was usually visual evidence of cavitation
manifested by very small gas bubbles dancing in the central
region of the sphere. There was also very definite audible
evidence of cavitation.
The initial studies were conducted using the glass
sphere. However, since this container was not truly
spherical in shape, and the wall thickness was not constant
throughout, there was some concern that flexural resonances
might be generated that would interfere with the
measurements. An accurately machined aluminum sphere was
therefore prepared and used for subsequent tests. This also
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afforded an opportunity to assess the gel when in contact
with two different surfaces. The aluminum sphere was
pretreated with a coating of QAS and the glass sphere was
rinsed with the NAJ mixture referred to above. The spheres
were filled with the preferred gels containing 0.85% N,O-
carboxyrnethylchitosan polymer, 0.048% cross-linker, 15%
ethylene glycol, 1 ppm QAS, and the various parameters were
measured from which the cavitatian threshold pressure was
calculated.
Initial studies showed that the cavitation threshold
was greatly influenced by the amount of dissolved air in the
gel - the higher the dissolved air content, the lower the
cavitation threshold. Various methods of removing the
dissolved air were tried including vacuum pump degassing at
room temperature, vacuum pump degassing at an elevated
temperature, helium sparge followed by vacuum degassing, and
oxygen sparge with sulfite treatment. Vacuum pump degassing
at elevated temperatures was used in these studies.
The cavitation pressure was monitored over extended
periods of time and the results shown in Table 1 are typical
of degassed gels (dissolved air content of 2 to 3 percent).
Gels having higher dissolved air contents had significantly
lower cavitation pressure thresholds; gels not degassed
typically had cavitatian pressure thresholds that did not
increase above 0.8 atm. The increase of threshold pressure
with time corresponds with the curing times of the gels.
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Table 1 also shows that there was no significant difference
between the results obtained with the two spheres.
In order to evaluate the effectiveness of the gels as a
medium for increasing the operating cavitation level of
projectors, the cavitation threshold of water that was not
degassed (the medium in which projectors normally operate)
was measured in the spheres and found to be approximately
0.5 atm.
Table 1
Cavitatior~ Threshold Pressure of Acoustic Ge7
Glass Alueinue
Sphere Sphere
DaysFrequencyCavitationDays FrequencyCavitation
(Ht) Pressure (Hz) Pressure
(ate) (ate)
2 12,474,11.2 1 12,821.2 4.7
12,451.01.1 2 12,824.6 1.0
12 12,438.41.1 6 12,821.5 0.7
19 12,475.41.7 9 12,811.6 0.9
26 12.,495.92.3 13 12,815.8 2.2
29 12,525.43.0 15 12,815.7 2.6
34 12,521.62.9 19 12,794,3 2.9
22 12,792.2 3.0
25 12,785.5 3.0
28 12,793.5 3.0
32 12,797.3 3.0
36 12,793.1 3.1
39 12,798.5 2.8
42 12,805.6 3,0
46 12,811.1 2.9
47 12,814.5 3,0
48 12,822.9 3.0
49 12,828.6 2.9
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Example 2: Ability of the gel to repair itself of ter
cavitation
In order to determine if the gel has the ability to
repair itself after cavitation, the gel was cavitated
continuously for 15 min. A comparison of the cavitation
levels immediately before and after the cavitation period
showed no change in the cavitation level. This suggests
that the gels are either not damaged or are capable of
repairing themselves after they have been cavitated for
short periods of time. Table 2 shows the cavitation level
immediately before and at various times after the test.
Table 2
Cavitation ~.eve1 of Gel Before and After Cavitation
Time fo Pc
(Hz) (atm)
Before cavitation 12,814.5 3.0
0 min after cavitation 12,813.2 3.0
min after cavitation 12,814.0 3.1
25 min after cavitation 12,814.2 3.0
180 min after cavitation 12,816.3 3.4
Example 3: Durability of Gels
The "durability'° of the gel was measured after 48 days
of storage. The term "durability" is defined as a measure
of the performance of a gel subjected to a relatively high
power level for a given period of time. The gel was driven
for 90 min. at three-quarters of the power required to
achieve cavitation. Table 3 shows resonance frequency and
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cavitation pressure measured at 0, 15, and 930 min. after
the durability drive. The cavitation pressure was not
affected by the test.
Table 3
Durability Test
Time fo . Pc
(Hz) (atm)
Before test 12,822.9 3.0
0 minutes after test 12,822.5 3.2
15 minutes after test 12,823.4 3.1
930 minutes after test 12,8'8.6 2,9
From the above it can be seen that substantial
improvements in cavitation threshold (14-15d8) relative to
aerated water can be achieved by encapsulatiang acoustic
projectors with the gels of the present invention. The
performance of these gels is not affected by either driving
the gel at cavitation for 15 mins. or subjecting the gel to
a relatively high acoustic power for 1.5 hours.
