Vibration and towing characteristics of surface-suspended hydrophone systems

VIBRATION AND TOWING CHARACTERISTICS
OF SURFACE-SUSPENDED HYDROPHONE SYSTEMS

HYDROMECHANICS

by

Chester O. Walton and Mervin M. Merriam

AERODYNAMICS

STRUCTURAL
MECHANICS

HYDROMECHANICS LABORATORY
RESEARCH AND DEVELOPMENT REPORT

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VIBRATION AND TOWING CHARACTERISTICS
OF SURFACE-SUSPENDED HYDROPHONE SYSTEMS

by

Chester O. Walton and Mervin M. Merriam

AUGUST 1961 REPORT Alb ois

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TABLE OF CONTENTS
Page
ABSTRACT
INTRODUCTION
GENERAL CONSIDERATIONS
EXPERIMENTAL PROGRAM
SHALLOW-WATER TOWING TESTS
OPEN-WATER TOWING TESTS
EVALUATION TESTS AT SEA
PRESENTATION AND DISCUSSION OF RESULTS
CONC LUSIONS AND RECOMMENDATIONS
REFERENCES

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LIST OF FIGURES

Schematic Diagram of a Hydrophone Array
Karman Vortex Trail
Sketch of Hydrophones Used in Basin Tests

Towing Configuration Used in Basin Tests

Diagram of Towing Configuration Used in Open-Water

Tests
Instrument Housing and Fairing Assembly

Fairing and Hydrophone Assembly Used in Acoustic
Tests

Diagram of an Experimental Directivity Hydrophone
System

Diagram of the Modified Experimental Directivity
Hydrophone System

A Comparison of the Relative Noise Levels
Received from an AX-58 Hydrophone Tested
in the Basin under Various Conditions

Comparison of Computed and Actual Configuration
of the Experimental Array Used in Open-Water
Tests

Strouhal Number as a Function of Reynolds
Number

Comparison of Computed and Experimental
Frequencies for System Used in Open- Water
Tests

Comparison of Measured and Computed Cable
Configurations for the Experimental Directivity
Array

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ABSTRACT

An experimental investigation was conducted to determine the sources
and methods of reducing cable vibrations in acoustic measuring systems, to
provide information concerning full-scale towing behavior, and to accurately
define the towing configuration of such systems. The results of this investi-
gation including comparisons with theory and recommendations for improving
towed acoustic systems are given in this report.

INTRODUCTION

The use of towed hydrophone systems to measure radiated noise from
submarines has led to many problems which must be alleviated if such
systems are to fully serve their purpose. Specifically, the vibration of the
cables is believed to be one of the major sources of the high level of back-
ground noise in the low frequency bands which has been associated with such
measurements. Furthermore, insufficient data concerning the resistance of
these kinds ofarrays have made it difficult to determine the configuration of
the array and, consequently, the orientation of the hydrophones.

Accordingly, an experimental program was established under the Funda-
mental Hydromechanics Program at the David Taylor Model Basin to study
the cause and effects of these flow-created problems as they pertain to typical
surface-suspended hydrophone systems. The specific objectives of the pro-
gram were: to investigate the capabilities of such systems with regard to
speed, depth, and steadiness of tow; to determine how well the behavior of
full-scale systems can be predicted for a range of operable conditions; and
to provide information which is required to accurately define the configuration
of a given system.

The facilities which are necessary to carry out tests of a complete system
under highly controlled conditions and at a large enough scale required for
accurate representation are not available. Consequently, the approach used
was to carry out the program in the following three phases: shallow-water
towing tests in the towing basins at the Taylor Model Basin to determine sources
and magnitudes of low-frequency noise components in the acoustic system, open-
water tests in the Chesapeake Bay to provide data on the effects of cable scope
and fairing on vibrations and towing attitude of the system, and tests at sea to
evaluate the characteristics of a full-scale system proposed for submarine
radiated-noise measurements.

This report describes the various experimental investigations, presents
the results of measurements to determine vibration characteristics, and in-
cludes pertinent observational data. The towing configuration of a proposed
system is briefly described,and curves and sketches are provided to define its
towing configuration. Recommendations are made on how to improve such
systems as well as for future studies which are necessary for further devel-
opment.

GENERAL CONSIDERATIONS

. . ° 2 : :
Submarine radiated-noise measurements’’ “ are presently being obtained

with a hydrophone array in which the cables are bundled,and the system is
allowed to drift with the listening ship in the manner shown in Figure 1. Be-
cause of ocean currents and winds, the system is set into motion and the
cables move relative to the water which results in the formation ofa'Karman
Vortex Trail.""* Above certain velocities, eddies break off alternately on
either side of the cable in a periodic fashion, as indicated in Figure 2. Thus
a staggered, stable arrangement or trail of vortices is formed behind the
cylinder. This alternate shedding produces periodic forces normal to the un-
disturbed flow which act first in one direction, and then in the opposite direc-
tion. The alternating forces cause the hydrophone cables to vibrate and the
cable vibrations are either received directly by the hydrophones.as sound
waves or cause an actual acceleration in the sensitive hydrophone elements
which also results in noise. The resulting signals are of high amplitude and
tend to mask out lower-level noise components present in the low frequency
portion of the spectrum. Attempts to reduce the vibrations have been made
by sliding loose plastic tubing over the single cable (see Figure 1) to break
up the flow around the cable. This technique has been partially successful, pun
not to the degree necessary for accurate sound analysis.

The motion of the system through the water also causes the hydrophone
array to tow in a catenary so that the hydrophones are neither at desired
depths nor in a true vertical plane with respect to the noise source. Since the
depth and configuration of the present type of array is difficult to predict, the
assumptions made with respect to the position of the hydrophones in the anal-
ysis of data are sometimes far from accurate.

EXPERIMENTAL PROGRAM

As mentioned in the Introduction, the experimental program was restricted
by limitations of test facilities at the Taylor Model Basin as to size, depth,
and background noise. Therefore, this investigation was conducted in three
phases:

1. Shallow-water towing tests to determine the magnitudes and
sources of vibrations or low-frequency noise components in the acoustic
system,

2. Open-water tests to determine the effects of cable scope and
fairing in the reduction of vibrations and to obtain information relative to the
towing attitude of the system, and

3. Evaluation tests at sea to determine the towing behavior and con-
figuration of a proposed system for submarine radiated noise measurements.

1References are listed on page 18.

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SHALLOW-WATER TOWING TESTS

The shallow-water tests were conducted in the towing basins at the
Taylor Model Basin primarily to ascertain whether the interference in the
acoustic measurement systems was due to cable vibration or to hydrophone
oscillations. This preliminary investigation was intended to set the basis for
possible solutions of the problems affecting acoustic measurements.

The initial tests conducted in the basin were made using AX-58 type
hydrophones, as shown in Figure 3a. A shroud-ring tail similar to that shown
in Figure 3b was attached to the hydrophone to minimize oscillatory motions.
Each hydrophone was towed ona 9/16-inch diameter, rubber covered, elec-
trical cable, as shown in Figure 4. The cable had a weight of approximately
0.1 pound per foot in water and served as a conductor for the hydrophone signal.
The units were towed over a speed range of 0 to 3 knots.

Standard type cable fairing was not available for the size cable being used
in the basin tests. As a substitute measure, a simulated fairing made from
2-inch plastic tubing, was used for some of these tests. The tubing was placed
over the cable so that it was free to align itself with the stream and was tested
using the hydrophone with and without the shroud-ring tail. In an attempt to
further break up the flow around the cable and thus reduce vibrations, the
plastic tubing was coated with a cork mixture to roughen the surface. The system
with the coated tubing was also towed over the 0-to 3-knot speed range. During
each run, noise measurements were made in 1/3-octave bands using a spec-
trometer and a sound-level recorder.

OPEN-WATER TOWING TESTS

The shallow-water tests did not provide adequate information for deter-
mining the towing configuration of the hydrophone array as well as the effects
of the use of greater cable scopes and standard cable fairing on cable vibration.
Consequently, in an attempt to obtain the additional information in an environment
having a minimum of background noise, tests were conducted in open water in the
Chesapeake Bay. A secondary purpose of these tests was to obtain design in-
formation relative to a full-scale hydrophone array.

The tests in the Chesapeake Bay were conducted with the configuration
similar.to that shown in Figure 5 using a motor boat as the towing vessel. Two
50-foot sections of fairing were used as the main towline. The fairing was of
an airfoil shape (TMB No. 7)* made of a two-durometer rubber which normally
would enclose the towcable. The 50-foot sections were joined together by
junction boxes,as shown in Figure 6. Both at the extreme end and at the junc-
tion of the two sections an instrument housing was attached which contained two
pendulum angle indicators {one for longitudinal and the other for lateral meas-
urements). A 100-pound faired towing weight was attached at the deepest end
of the array to provide directional stability.

The initial tows were made over the stern, but satisfactory measurements
could not be obtained because of propeller wake. The towing arrangement was
then modified to permit over-the-side towing. It was then possible to tow the
configuration and make pressure and angular measurements over a speedrange

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Figure 7 — Fairing and Hydrophone Assembly Used in Acoustic Tests

of 0 to 4 knots. At 4 knots, the fairing tended to tow in towards the propellers
and it was not feasible to tow above this speed.

To carry out the tests to determine the vibration characteristics of the
faired system, the instrumented housings were replaced with hydrophones, as
shown in Figure 7. Two similar hydrophones were attached to an unfaired
5/8-inch (not shown in Figure 5) weighted line. to measure the vibrations for
comparison with the faired system. The hydrophones were located at depths
of 50 and 100 feet in each system. Tests were made over a speed range of
0 to 4 knots and the hydrophone signals were recorded over the full speed
range.

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EVALUATION TESTS AT SEA

‘The tests conducted in Chesapeake Bay resulted in information which was
applicable to full-scale arrays. Accordingly, an experimental full-scale array
for studying submarine radiated-noise patterns was first constructed and then
tested at sea off Key West, Florida. The purpose of these tests was to deter-
mine stability, towing characteristics, configuration, and acoustic performance
ofthe array. It was also desired to obtain information required for design
modifications for future arrays.

The array used in the first sea tests is shown by the sketch in Figure 8.
It is composed of a 100-pound faired towing model, pressure gages, hydro-
phones, buoys, float material, and a network of cables. All three legs of the
system (horizontal and two vertical legs) are composed of 0.7-inch diameter
cable with 26 twisted pairs of conductors and a strength member. The inter-
mediate cables suspended from the horizontal leg are 0.3-inch in diameter.
The horizontal leg is supported by flotation material. Hydrophones and depth
gages were located at the points indicated in the sketch. Junctions were pro-
vided for additional hydrophones and depth gages to be located every 100 feet
along the vertical legs. The system was towed over a speed range of 0 to
3 knots while pressure measurements and hydrophone signals were recorded.

On a subsequent sea trial, the array shown by the diagram in Figure 9
was used. The added 500-pound faired towing weight in the second system was
intended to provide greater depth and more vertical area in the loop formed by
the array.

PRESENTATION AND DISCUSSION OF RESULTS

The results of the shallow-water tests are presented in Figure 10 as dif-
ferences in relative noise level versus frequency for three of the test con-
ditions. Since the results with the fourth condition (simulated fairing with a
roughened surface) were approximately the same as those with a smooth sur-
face they are not presented. It may be seen from Figure 10 that, for the very
low speeds (0.1 to 0.25 knots), there are no significant differences in noise

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levels among the conditions tested. However, observations made during the
tests indicated that the bare cable vibrated at these speeds and that these vi-
brations appeared to influence the motion of the hydrophone as well as its
signal as seen on an oscilloscope. The vibrations at these speeds are of
such low frequency that they do not appear in the analysis, since the lowest
1/3-octave band on the spectrometer is centered at 16 cycles per second.

At speeds between 0.5 and 2.0 knots, the noise levels for the bare cable
condition are from 5 to 20 decibels higher in the low frequency bands (16 to
125 cycles per second) than for the faired cable condition. All test conditions
produced high-noise levels for speeds above 2.0 knots.

The addition of plastic tubing (fairing) reduced cable vibration at allspeeds
and seemed to completely eliminate the vibrations at the low speeds. The
shroud-ring tail on the hydrophone greatly reduced the very low-frequency os-
cillation of the hydrophone at all speeds. The effect of the reduction does not
appear in the analysis because the frequency of the oscillations is below the
frequency range of the instrumentation. The addition of the shroud-ring tail
to the hydrophone had no effect in reducing the higher frequency cable vibra-
tions.

It should Se noted, however, that the results shown in Figure 10 may be
influenced by background noise in the basin, and noise and vibration of the tow .
carriage. Nevertheless, the data indicate that further investigation into the
effects of more refined fairing, greater cable scopes, and other towing con-
figurations is warranted.

The experimental results obtained from the open-water tests conducted
in Chesapeake Bay are presented in Figure 11. The results of theoretical
calculations, using the method outlined in Reference 5, are superimposed for
comparison. It may be seen that the computed position of the array does not
agree very well with the measured portion. Lack of agreement is attributed
mainly to the fact the system towed to one side. Using a constrained, flexible,
faired section that is not free to swivel, the tow member will cause the towline
to develop side forces which deflects the systemtoone side. This occurrence
is indicated in this system by lateral angular measurements which approached
45 degrees at 4 knots. The angular records and observations showed that a!l-
though the array towed to one side, it remained reasonably steady over the
speed range.

A qualitative narrow-band frequency analysis was performed on the hydro-~-
phone signals recorded during the open-water tests. It was found that for the
condition with the hydrophone at 100-foot depth suspended on the faired cable,
the interference which might be attributed to cable or fairing vibration in the
0-to 50-cps frequency range was negligible. Thus, the broad-band signal could
be amplified so that other noise components such as the firing rate of the boat's
engine, signals from a passing tanker, random background noise, etc., could
be identified. However, the records from the two hydrophones secured to the
unfaired weighted rope were quite different. In particular, the record for the
100-foot depth hydrophone in the latter system showed many interfering high-
level noise components in the very low frequency range. The hydrophone was
taped directly to the rope which was the vibrating member in this case.

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Figure 11 — Comparison of Computed and Actual Configuration of the Experimental
Array Used in Open-Water Tests

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by using the equation
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f is frequency of vibration,
N is Strouhal Number,
V is velocity of fluid, and

d is diameter of cable.

Figure 12 shows the relationship of Strouhal Number and Reynolds Number and
can be used to compute the frequency for a series cf speeds and cable diameters.
It should be noted that these data refer to rigid cylindrical sections and should
be applied with caution to cables. Once an elastic body has been excited, the
motion of the body modifies the frequency.

Figure 13 compares the frequencies measured on the system used in the
open-water tests with values computed by the foregoing method. It may be
seen that, in spite of the fact that the computed value is based on rigid cylinders,
it compares reasonably well with the low frequency components that were meas-
ured. The interfering components seemed to be composed of fundamental fre-
quencies and a number of related harmonics. These frequencies changed with
towing speed. The noise components were not present for the portion of the re-
cord taken when the boat was not moving. When these components were present
on the record, they were of such a high level that the broad-band signal could
not be amplified to allow the identification of other signals without overloading
the analysis instrumentation.

The results of the evaluation tests at sea are shown in Figures l4a and 14b,
the corresponding theoretical computations are superimposed for comparison.
It may be seen that, in these two cases, the theory and experiment are in very
good agreement. The system without the weight (Figure 14a) was observed io
tow in a reasonably steady manner.

A qualitative frequency analysis of samples of hydrophone signals for th1-c
towing speed conditions was made. The records showed that there was con-
siderable interference due to cable vibration in the low frequency region at all
speeds. The frequency of vibration in these cases is proportional to the flow
velocity divided by the diameter of the cable. This was substantiated by the
records which showed that the frequencies received by the same hydrophone
became higher both with increased towing speeds for fixed diameter, and with
decreased cable diameter for fixed speed. The signals from hydrophones on
the weighted leg were higher in amplitude than those from the non-weighted leg.
This was attributed to the fact that the weighted leg was a better carrier for
the flow-induced vibrations than was the non-weighted leg.

14

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Frequency of Generation of Eddies
Diameter of Cylinder
Speed of Cylinder

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Figure 12 — Strouhal Number as a Function of Reynolds Number

24

Computed

20

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Frequency in cps

(0) | 2 &) 4
Speed in knots

Figure 13 — Comparison of Computed and Experimental Frequencies for System Used in
Open-Water Tests

15

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Distance Aft in feet
1000 800 600 400 200 te)

| 200
400 vey
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Distance Aft in feet
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2.6 knots L 400 2
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= = §
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1.3 knots
eal 1 — 800
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1000

Figure 14b — With 100-Pound Weight

Figure 14 - Comparison of Measured and Computed Cable
Configurations for the Experimental Directivity
Array

16

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CONCLUSIONS AND RECOMMENDATIONS

On the basis of experimental and theoretical investigations of typical
surface-suspended hydrophone systems, it is concluded that:

1. Any movement of sensitive non-acceleration cancelling hydro-
phones such as the AX-58, whether due to cable vibrations or to hydrophone
oscillations, affects the acoustic signal in the low frequency range.

2. By stabilizing the hydrophone with the addition of a shroud-ring
tail the very low frequency oscillations are substantially reduced for the range
of towing speeds which are usually encountered during submarine radiated-
noise measurements.

3. Single hydrophone cables in a flow environment tend to vibrate
due to vortex shedding of the Von Karman vortex street type. The vibrations
of the single cables interfere with acoustic measurements in the lower end of
the frequency range of interest. In general, bundling a number of cables,
thereby increasing the effective size of the cylinder, decreases the frequency
of vibration below the range of interest.

4. Fairing hydrophone cables reduces cable vibration, aids in ob-
taining greater operating depth and speed, and improves the stability of the

system.

5. The configurations of arrays similar to the ones investigated
can be predicted with reasonable accuracy.

6. To obtain maximum depth at speeds above one knot, vertical
cables in array systems must be weighted. However, this may increase the
acoustic interference.

7. The overall towing attitude of the full-scale array is satisfactory.

Based on the foregoing conclusions, it is recommended that:

1. Hydrophones in surface-suspended array systems be stabilized
to reduce the oscillations and vibrations.

2. The vibratory motions of the array lines be reduced by fairing
methods.

3. Further tests be made to determine the type of cable fairing most
suitable for these kinds of arrays.

4, The amount of fairing required to eliminate or reduce cable
vibration should be determined either experimentally or theoretically.

5. Techniques for improved fabrication, launching,and storage of
complete array systems should be investigated.

17

iain, t

REFERENCES

David Taylor Model Basin Report C-987, SECRET.

David Taylor Model Basin Report C-1153, SECRET.

Binder, R. C., PHD, "Fluid Mechanics," Prentice-Hall, Inc. , (1955),
Fehlner, Leo F. and Pode, Leonard,"'The Development of a Fairing

for Tow Cable," David Tayler Model Basin Report C-433 (January 1952)
CONFIDENTIAL.

Pode, Leonard, ''Tables for Computing the Equilibrium Configuration of
a Flexible Cable in a Uniform Stream, '' David Taylor Model Basin
Report 687 (March 1951).

Relf, E. F. andSimmons, B. A., 'The Frequency of the Eddies

Generated by the Motion of Circular Cylinders Through a Fluid,"
A.R.G.; R& M No. 917 (June 1924).

18

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Director

Columbia University
Hudson Laboratories
145 Palisades Street
Dobbs Ferry, N. Y.

Woods Hole Oceanographic Institution
Woods Hole, Massachusetts

Stanford Research Institute
Menlo Park, California

Attn: Dr. Vincent Salmon

Chesapeake Instrument Corporation
Shadyside, Maryland

ASTIA

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