A WAVE DAMPING DEVICE FOR MODEL SEA-MOTION TETS

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-04861A000400030002-4
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 2
• Document Creation Date: December 20, 2016
• Document Release Date: May 25, 2006
• Sequence Number: 2
• Publication Date: September 1, 1999
• Content Type: REPORT

Approved For Release 2007/10/23: CIA-RDP78-04861A000400030002-4
25X1
Schiff und. Haf en, . 6 12 (December, 1950 771
Fran German
Investiations on the behaviour of ships in sea motion can be carried
out more cheaply and rapidly by model tests than at sea. Such tests also
have the advantage that the desired wave can be produced at any time. In
order to provide reproducible conditions, it is very important to be able to
doshxy the ?va3reN z..t te end of the tali'.:. With the means. so Tar. ettployed
(beach installations, damping screens., bars, etc.), however, the effect
obtainable is inadequate, since such obstacles refleot alarge proportion
of the energy, Consequently, there is little agreement between the results
of different 'authors. Allowing the waves to run out into a larger tank is
likewise merely an expedient, since damping is no better, the waves are
stored i n the l a r g e r t a n k to a aonsiderable.extent , a n d after soW4 tame
return to the experimental tank. In addition, there is reflection at the
mouth of the experimental tank into, the large tank1
The ' problem of reflaoti4n4ree absorption ~ of .waves ' can, ihowevgr, be
solved in a tt c ,y' a act and. practically satisfactory manner. For this
purpose, the . experimental tank should be closed by a mechanin which with
respect to the impinging waves behaves like the tank extended to. infinity.
("matching"). In the hydrodynamic respect, therefore, this mechanism must
have a purely dissipative reaction and in addition it must be waterti it
so that no water can run out,
The simplest construction of such a mechanism consists of a transverse
bulkhead mounted on the bottom of the experimental tank and arranged to
tilt to-and--fro, the top of the bulkhead being connected to appropriate
mechanical elements (springs,. weights, damping means). Since the bulkhead
zm~
cannot be made watertight at the sides without giving rise to troublesome
friction, a stationary wall must be fixed at some distance behind it.
Adjustment to the incident wave is to be effected as follows:
First the natural frequency of the mechanism (resultant of all inertias
and elasticities of the bulkhead, oscillating mass of water, volume of
water between bulkhead and back wall and the arbitrarily selected springs
and wei opts) must be made equal to the wave frequency. The sum of all the
conservative forces thus disappears and the effect of the system for the
wave in question is purely dissipative. Secondly, the damping must be of
such proportions that the movement uf the bulkhead consumes just as much
energy as would be required for wave production in an infinitely long tank1
Such a mechanism (called "wave damper") has been built in the
Shipbuilding Listitute of Hamburg University in an exi~erimental tank or
trough 1 metre wide with a depth of water of 40 cm. The distance between
the s g bulkhead and the back wall is 16 cm. Frequency adjustment is
effected by hooking cylindrical springs to the bulkhead fixing the frequency
of the wave the absorption of zwhioh will be optimum. This wave is then
adjusted on the wavenaker by means of a stepless gear. Damping is by
means of an eddy-current brake consisting of a few copper plates connected
to the bulkhead and swinging in the gap of a. permanent magnet. The
engagement in the magnet gap is adjustable.
Approved For Release 2007/10/23: CIA-RDP78-04861A000400030002-4
 Approved For Release 2007/10/23: CIA-RDP78-04861A000400030002-4
The function of this wave damping device has been verified-by measure-
ments and 1 ydrodynau>ic calculations, the details of which may be passed
over, The absorption of waves between 1.13 and 1,50 metres long; is more
than 98,x, i,e, due to inaccuracies of a mechanical nature, a wave of less
than 20 of the original height is reflected. To improve on this value
would require considerable `effort not only in the precision of the entire
installation but also in the measuring technique.
The physical operation of the wave damping device having now been
confirmed experimentally and :theoretica.ll,y,- it is possible to proceed to the
technical development of. an experimental tank for model sea-motion tests and
the lixce. As compared with the,provisional damping arrangements mentioned
at the commencement, which are theoretically vrong on account of, defective
matching, three advantages are obtained, namely short experimental tank,,
any length of test and short adjustment interval between tests, En the.
practical development, the following will. have to be borne in mind:
1. There should be a wave-damping device not only at the end.of the
tank but also at the begrinn.ing, since secondary waves are, radiated on both
sides from the test object, It is therefore necessary to use a wave- nicer,
which at the same time will also a1 sorb incident waves. This can be formed
from a wave damping device if, for example, the fixed damping magnets are
set in periodic motion.
2, In practical ?gperation, not only must the adjusted rave be.
satisfactorily absorbed, .but .also waves of adjacent frequency, so that the
accuracy of adjustment of wave er and damper need not be too high, and
so that the long waves always associated with the starting and stopping of
the wave-maker will disappear rapidly enough. The method of doing this is
to avoid all unnecessary inertias.
SJC/CCB.21k,
Approved For Release 2007/10/23: CIA-RDP78-04861A000400030002-4