The battelle anechoic tank with hydrodynamic test section for acoustic measurements

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THE BATTELLE ANECHOIC TANK WITH HYDRODYNAMIC TEST SECTION FOR ACOUSTIC MEASUREMENTS

Wolfgang Burgtorf 1 Summary 2 Introduction 3 The Batteile 4 Hydrodynamic for Acoustic 5 References Anechoic Tank Test Section Measurements

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1 Summary

The present paper reports on the design, the properties and

the applications of the anechoic tank o Battelle-Institut e.V., Frankfurt am Main. This tank permits absolute

measure-ments in the frequency range between about 1 kHz and 100 kHz (reflection loss 10 dB) and relative measurements for even

frequencies below 1 kHz.

An extension to this facility consisting in a hydrodynamic

test section to investigate the noise of cavitating pro-pellers under acoustic free-field conditions is described; its desig and properties are outlined, and the potential

applications of the facIlity are illustrated by an examplé.

2 Introduction

In acoustic measurements, any influence of the measuring

equipment on the results must be excluded. Therefore,

hydroacoustic measurements are normally carried out in exactly defined sound fields which exist, e.g. in

- rigid or sound-absorbent impedance theasuring tubes, - pressure chambers,

- ànechoic tanks for simulating the acoustic free field, sound-absorbent shallow tanks,

- echo tanks.

These are measuring facilities with defined measuring and,

boundary conditions, which permit absolute measurements without disturbing influences. If interfering noise exists or if reflections affect the measuring result, exact

measurements are no longer possible. In many cases, it is

still possible, however, to perform relative measurements.

3 The Battelle Anechoic Tank

For hydroacoustic measurements, Battelle-Institut e.V., Frankfurt am Main, has a large anechoic tank (Fig. 1).

Its inside dimensions are 10 m (length), 5 m (width) and

4 m. (depth). It i coated with broad-band absorbers, ana its surface can be covered with floating absorbers.

In this tank it is possible to simu1te unimpeded

three-dimensional ôund propagation in a specific frequency range under laboratory conditions, ie. free from, e.g

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-3-weather influences. The.field of application Of the

an-echoic tank is determined by its dimensions and by the coating.

The coating of the tank consists of cornmerOial wedge-shaped absorber material The individual wedges are com-posed of three. rubber layers. The middle layer contains holes of defined size. Together with the surface layers,

these holes form tuned resonance absorbers. The degree of absorption is determined essentially by the

dissipa-tion factor of the rubber material In the frequency range from 6 to 60 kH.z, the absorbers. have, a reflection

loss of more thati 20 dB (Fig. 2).

This means that no more than 1 percent. of the incident

sound energy is reflected at the boundary surfaces and

that no less than 99 percent is absorbed. The reflection

coefficient for amplitudes is smaller than 10 percent For many investigations, reflection losses as low as

10 dB, corresponding to a sound absorption coefficient of 90 percent, are sufficient, is then possible to

perform measurements free from disturbing reflections

in the frequency range from about 1 kHz to above 100 kflz.

In addition, the frequency range for which the anechoic

tank is suitable is determined, by its dimensions.

Un-damped sound propagation between parallel, absorbing surfaces is only possible if their spacing exceeds the

wavelength, of the sound. For uncovered water surface,

the tank width of 5 m results in a lower limiting

fre-quency of 290 Hz. The frequenc.y is further limited by

the mirror effect Of the water surface if it is not

:covered with absorber material. The latter effect is insignificant if the water depth is below about 1 m.

Potential applications of a measuring tank of this type include the performance of reciprocity calibrations of hydroacoustic transducers and measurement of their directional characteristics, investigations on under-water'sound sources and investigations into the radia-tion behavior and the reflecradia-tion or absorpradia-tion

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4 The Hydrody-namic Test Section

for Acoustic Measurements

To perform experimental investigations oh cavitating pro-pellers under defined hydrodynamic and hydroacoustic

con-ditions, the anechoic tank was provided with a free-jet section /1/. This permits measurement of cavitation noise at defined incident flow against the propeller and under acoustic fre-fie1d conditions. This measuring section is suitable for investigating propellers in the range of advance ratios between 0.3 and 0.8 and at cavitation

numbers of 6 2.

Fig. 3 shows the principle of design of the facility. Its main dimension, i.e. the diameter of the measuring section

in which the propeller is placed, is determined by the

dia-meter of a porthole in the tank. The measuring section per-mits propeller models of diameters up to 0.2 m to be in-vestigated. It consists of plexiglass, ie. it is

sound-transmitting (Fig. 4).

A pumping. set mounted directly above the bottom of the

an-echoic tank sucks water through an inlet section provided with a protecting screen, through a bend, the contraction section, the flow-straightening section and a second con-tractiOn section and finally as parallel incident flow

against the propeller in the measuring section. The -free

jet ejected from the nozzle mounted at the outlet side of

the measuring section mixes with the water in the tank.

In the mixing zone a vortex system develops, into which the kinetic energy of the jet is transferred. At the rear wall of the tank the flow is reversed. Thus, a reverse

flow is generated, which is intensified by the continuous sucking action of the pump. To reduce the interfering noise level in the tank, the pumping set is resiliently mounted and the piping is connected via compensators

(Fig. 5). he flow rate of the pumping set can. be varied

from 400 m /h to 2400 m3/h; the advance rates may range between 0.3 and 0.8. The maximum face velocity thus is w = 10 rn/s. The propeller speed is continuously variable

from 50 rpm to 4000 rpm. The measuring section which con-tains the propeller model is mounted 2 m below the water surface. The smallest possible cavitation number (o2) is determined by the maximum face velocity. The pressure can be varied only within very narrow limits by lowering

the water surface. The measurements are carried out

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-5-The hydrodynamic parameters that can be measured are, the flow rate of water, i.e. the face velocity, and the pro-peller speed, the propro-peller thrust and the propro-peller torque.

The speed is determined via an inductive receiver. The

'thrust measuriig unit can measure thrusts between 0 and 1000 N.. It uses strain gages and is mounted n .a special

oil-cooled thrust bearing of the propeller shaft. The

torque is measured by means of strain gages via a torsion rod attached to the propeller Shaft.

The propelling machinery as well as slowing down, returning

and mixing of the flow cause 'interfer'ing noise. The

spec-tral distribution of the hydrodynamic noise that increases

with increasing flow velocity is shown in 'Fig. 6 for three face velocities. Fig. 7 shows both the spectral

distribu-tion of the hydrodynamic noise for the face velocity

- w = 4 rn/s (curve b) and the level distribution of the

noise produced by the propelling machinery (curve c).

The sum of the two types of noise limits the dynamics

(curve a). The background noise level in the anechoic tank is negligible (curve d).

Finally, the spectral distributions of propeller noise

levels at different states of c'avit'ation adjusted by variation of :the advance ratio will be discussed as an

example of the potential applications Of the facility

(Fig. 8) In these measurements the face velocity was

kept constant at w . = 4 rn/S. The various advance ratios

were adjusted by Varying the propeller speed.

For J = 1 it was 1440 rpm, for J = 0.54 2600 rpm and

for J = 0.46 3050 rpm. At the lowest propeller speed,

the region of flow around the propeller is free from cavitation (curve c in Fig. 8). The interfering noise level is only slightly exceeded. At J = 0.54, hub vortex

cavitation occurs. The level of cavit'ation noise increases

markedly (curve b in Fig. 8), in particular in the

fre-quency range above 500 Hz The maximum of noise at the

frequency of 1630 Hz and the high 'noise level values in

the frequency range above 2 kHz appear to be typical of

the noise of hub vortex cavitation. Curve a in Fig. 8

presents the spectral noise level distribution for the

advance ratio J = 0.46. The, cavitation that occur,s at this advance ratio is both hub and tip vortex cavitation.

The higher intensity and the larger extension of

cavita-tion results in a further increase in the noise level.

The maximum is shifted towards lower frequencies. The

dynamics values obtained for examples a and b are shown.

in Fig. 9. Depending on the state of cavitation, it is possible to reach dynamics values of 20 dB and more

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-6-5 Ref erencës

/1/ KR. Simhan, R. Schmitt, H. Sudhof

Experimentelle Untersuchungen uber das Schailfeld eines kavitierenden Propellers in einem Freistrahi Report for the Bundesministerium der Verteidigung,

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1 Pumping set 2 Propelling, machinery 3 Propeller model 4 Driving shaft 5 Stern tube 6 'Inlet section 7 Protecting sçreene 8 Compensator 9 Bend _{10' 'Contraction section.} _{11 Flow-straightening}

The battelle anechoic tank with hydrodynamic test section for acoustic measurements

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F g. 7: Interfering and background noise

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Fig. 8: Cavitation noise

hub vortex cavitation and

tip vortex cavitation

b hub vortex cavitation

_measurements