Experimental investigation of cavitation noise

INVESTIGATION OF CAVITATION NOISE

AND G. SEBESTYÉN ,',',

Introduction

Cavitation phenomena due to shock 'Naves pro- duced by the collapse of bubbles is suitable to make investigations by acoustic methods. Therefore more research institutes would be expected to deal with the noise spectrum of cavitation [1

J,

however, only a few of them performed such experiments up till now.

Under specified conditions, cavitation noise spectrum characieristics render a reliable basis for the evaluation of cavitating now features and cavi- tation erosion damage. Led by this consideration, the Department of Fluid Machinery, Technical Uni- versity Budapest, supported by the Hungarian Aca- demy of Sciences supplemented their CUITent cavi- tation damage experiments [2J, [iq, [4] ,vi th inves- tigations on cavitation noise spectra.

In the course of these experimental investiga- tions the cavitation noise speetrum has been studied with its visual appearance and with its erosion cha- racteristics. The results of these investigations will he reported in the following paragraphs.

Test equipment

The experiments were performed in a closed cir- cuit hydrodynamic tunnel. There was a 48 mm

(0) Professor, Department of Hydraulic l\Iachinery, Budapest Teehnical University, Hungary.

(H) Hesearch Engineer, Department of Hydraulic l\Iachi- nery, Budapest Technical University, Hungary.

diameter bronze circulaI' cylinder Iocated horizon- tally in the working section with the dimensions 48 X 200 mm, perpendicular to the How direction, and the acoustic behaviour of the cavities produced beyond this cylinder was then studied. One side or' the workiI~g section was constructed of steel, wh creas the other three of plexiglass.

For frequency and sound pressure Ievel measure- ments th cre ,vas a Brüel and Kjaer electrostatic microphone mounted normally to the sidewaIl, dis- placed with 100 nUll l'rom the cylinder end, in the horizontal plane containing the axis of the cylinder.

The microphone indications were connected to a Brüel and Kjaer frequency analyzer to study sound pressure level values within the frequency range of from 20 to 20 000 cps. Connecting the frequency analyzer to an automatic level recorder, measure- ment results could be continuously registered.

Cavitation noise

The noise of cavitation could he observed in a natural auditory manner although, in the course of the joint perception of ail sound frequencies, there is a possibility existing that the higher sound pressure level frequencies would suppress the less intensive ones. 'Vith some periodical shedding of cavities produced in the system, due ta the cavita- ting now, the sound pressure level generated along the collapse of hubhles could readily he ohserved as a vibration occuring at the same time. Madel experiments revealed that the shedding of the cavi- tics from the circulaI' cylinder of 48 mm diameter in 905 Article published by SHF and available at http://www.shf-lhb.org or http://dx.doi.org/10.1051/lhb/1966057

the aforesaid working section hegins at a c:l\'ita- tion numher of about (5

=

2.4, .. , 2,5. The cavita-

tion number is defined hy: ^À

The frequency spectrum of cavitation noise

where !:lIlI' is the sound pressure levcI differenee exprcssed in dccihels, Il]!! is a sound pressure lcvcl II HelalÎ\'e eavity length (i.) in funetion of the eavitation

number (a).

Longueur de ]Joehe rell//ive 0,) en fone/ion du coen;cien/

de CIlvi/l/tion (a).

The automatic level reeorder conneeted to the frequency analyzer registers sound pressnre level

(Ilp ) as funetion of frequency

(n.

The dilIerence hetween the cavitation noise level and arhitrarv referenee sound pressure level may he expressed i;l the following way:

dB

2,5 6

10

2,0

+

10

1,5

!:lIl_p = 10 log

\

A' 48 x 200 mm^'\°.\(6) 2

~

^d04Bmm

~

_i"..

~~ ---

2 3 5

4

(5

= -:'---

P 0,5 V2 P

cr.0

wh cre P", is the static pressure measured at the sidewall in place of the cylinder, P", is the vapour pressure referring to the given liquid temperature, p is the liquid density, and !J",o indicates the maxi- mum velocity of undisturhed now.

In case of cavitation numhers exceeding the aforesaid values, only a low ,vhisper will be heard ehanging into an increasing rumble with the redue- tion of the eavitation number. The thin vortex filaments visible at the heginning of cavitation change into a staple l'rom of a few mm length, pro- duced along the top and boLtom generating line of the cylinder, with the reduction of the eavitation numher. Bv the further reduction of the cavita- tion numbe;:, this will develop into shedding vor- Liees having a visu al appearance of short cavitation zones. Hedueting the cavitation number even more the cavitation zone increases.

According to the results of the frequency amtly- sis to he presented later, the short cavitation zone 1.:= 1.5^d where 1., indicates cavity length measu- red by visuaI observation l'rom the center line of the cyIinder generating the highest sound pressure level whieh will graduaIIy decrease with the increasc of the cavity length, lhat is, with the reduction of the cavitation number. The functio- naI correlation hetween the relative--nondimensio- nal-eavity length À = ljd and the cavitation num- her((5) is shown in Figure 1, [5]. Figures 2, :3 and 4 present short exposure lime pietures of various cavitation conditions.

2/

Photograph of the wakes shedding from the circulaI' cylinder model where d

=

48 mm, exposure time /

=

2.10-(; see., and 0=2.18.

Détl/chement des silil/ges du modèle cylindrique de section circulaire d = .~-8mm; tem]Js de ]Jose t 2.10-fis; 0=2,18.

3) Photograph of the wakes shcdding t'rom the cylinder model at 0= 1.80.

Détl/chement des sil/I/ges du modèle cylindrique à 0= 1,80.

hy a given t'requeney, and Il_p2is the referenee sound pressure level at the same frequeney.

If the sound levels equal _{(Il p1} = _{Il ll2)'} the eOll1bin- ed total sound pressure level would exceed the sound level of one of the noise sources by 3 dB. If the difTerence between the two sound pressure levels is 8 dB then, according to unavoidable read- ings e!Tor limit the total sound pressure levcl is equal to the higher component level.

Investigations of the frequency spectra of various cavitation conditions shows that, within the fre- queney range of 20, ... 50 eps, the motor and pump noise is intensive enough to mask any cavitation noise. Above this range, however, the machine noise level can be neglected. Thus, for example, the

ÂIl_p value between the sound pressure level selected as cavitation test referenee level determined by flow conditions (O'= 2.8), and the sound pressure level exhibited by the electric motor driven equip- ment in case of

l' > ]

01, was more th an 20 dB and l'ven within the frequeney range of 400, ... 10 000 cps

was not less th an ] 0 dB.

In order to investigate the development of noise level conditions, sound pressure level-frequency eurves for constant flow veloeities but variable cavitation conditions have been plotted. Of these cavitation noise speetra Figure 5 presents two

II_p

=

^IIp

(n

^{eunes referring to difTerent cavita-}

tion numbers but to identical now veloeities (v",

=

12m/s). The l'unes plotted by using the values registered by the automatic level recorder show that there is a significant dill'erence existing hetween the sound pressure level of the cavitating state and the cavitation-l'rel' condition seleeted as reference leve1. It was found that above the fre- quency of

l' = ]

0⁴eps, due to the significant del'rease of the sound pressure of external sound sources, sound pressure level comparisons for various cavitation conditions mav be safel" and

reliably performed. . . . .

41

Photograph of the wakes shedding fl'om the cylinder mode] al Ci

=

^1.4;;.

J),>tacllement des sillages dll modèle cylindriqlle li Ci

=

^1/15.

Figure G presents curves illustrating sound pres- sure level values obtained with dill'erent 110w velo- ci tics, in funetion of the relative lenglh of the cavitation zone. The Figure reveals that the cur- ves have a peak value at À= 1.5 with steep varia- tions in either direetion. The sound pressure level values decreasing with reduced 110w velocities.

For completeness' sake, the investigations des- cribed above have heen cond ucted also hv means of vibrometer heacls mounted onto the steel and plexi- glass wall of the test section where the acceleration

le\'(~1 induced bv difTerent cavitation conditions was mcasured. Figt;re ï illustrates the acceleration level data obtained at the plexiglass (a) and steel (b) wall, respectively, of the test chamber, in function of the cavitation number, for constant frequeney value, and with two dill'erent llow veloeities, each.

The aeceleration level values obtained with iden- tical 110w velocities hut dilrerent frequencies are illustrated hy Figure 8. The aceeleration level values decrease with inereasing frequencies without, however, leading to any changes in curve eharac- teristics. lt _deser\'(~s attention that there may he such a frequency value found (in our case

l' =

12

non

cps) where the acceleration level curves ohtained at the steel and plexiglass walls, respecti- vely, would coincide [G].

Cavitation damage intensity

Parallel to the frequency analysis, in orcIer 10 support the l'l'suit described above, the variations of cavitation damages in function of the cavity length have also been studied. Thcse experi ments ,vere conducted in the same 110w deviee outIined pre- viously. As test specimen lead plate of 8 mm thick- ness ,vas used. Flow velocity was identical to one of those employcd for noise Icvel studies (v",

=

12mis). These test results are illustrated

907

1

ni"

--+~+-+-+-"...+~+-l--i+!-~!--+-+-+--+-+-+++++-~-1-_' !_-+~+-+-+-'f-+++._--- ^+---1

n_p=n_p

ff

^{1 - - \}

l "'.

n_p [dB]

80

70

60 ^-1-

50

1 .k ~^..~ ¹

! • _

'l-r ^1-.

^{1 1}

. " . - 1 t--.

i ' .

^Œ"=^{1, 84}

l/\f/i\1/ :: r

vVVK

~C-~ltE;lr

40"1O:-;1~-'----2-'--'-3-'--'4,---,--1-6-'--"8...L1OL2;;-~--'2---'--'3~4-'---'6-'-L8--'.J.'O-';3~-'---'2----'--3l--4L--l-6LJ.-8L.Ll.L0-;-4~l--2-'-L--3~f

5/

Cavitation noise analysis eurves. Sound pressure level (n_p) as funetion of frequency (v oo

=

^12mis).

Courbes d'analuse acoustique de la cavitation: pression acoustique (nI) en fonction de la fréquence (v00

=

^{12 mis).}

62

60

np

[dB]

58

56

52

50

46

42

I\

A-It8.âOOmm

d=4I3mm

17f\. f"'S000cps

• V_-12,0 mIs

r \\

^aV.."ro,76m/s

av.... 9.68mfs

t' \\

'1 ~l\

~

'1 ^~

~

Il

il'"

-30+-t-~----r"-+~~~_..L.~~~,---I- ng

[d8]

-So+-+--I~'+--+~;----+--~--+-

-60t-H!--~-+~~~~+-~-'-~----1!.:...

ng

ltiil

-70l4--~+-~--J

1Jnp

•

[dB]

5

la

15

a/

Vibration acceleration (1\,) as function of the relative cavity length 0.. ).

Accélération des vibra- tions (n,,) en fonction de la _101~lueurde poehe relative (U.

("\

v..= 12 mis d=48mm a"8mm

1\

{=15000cps

•

1

\

^!

a

^• l~

^/---

100 150

o 1JV

f~m3J

10/ Eroded material volume pel' unit ~ 5 time (~V) and sound pressure level differences (~n/,) as function of relative cavity length OJ.

7/Vibration aeeeleration level (Il,) as funetion of the cavitation number (cr).

Accélération des vibrations (n) en fonction du coeflicient de cavitation

(a) .

... 9/ Eroded material quantity (G) as fune- tion of the test period (,), at flow

velocity ^Voo

=

^12mis.

Yolume de matériau érodé (G) en fonc- tion du temps d'essai (,). Yitesse d'écoulement Voo

=

12mis.

mm

,

m mm

mis

' j

1 1

1 1

^A'48x200

1

1

^d-4BV_'12

Il

^Pb

I~ ^a-Bm

1--' A-t

A.j

/1

ⁱ

1

, 1

Il /

1

¹

il

2 GJO [mg]

B

5

6/

Sound pressure level (n_p) in fUl1etion of relative eavity length 0,) with different flow veloeities.

Pression acoustique (n) en fonction de la longueur de poche relative (U et de la IJitesse de l'écoule- ment.

hy Figure H where the erosion weight los ses (G) versus time (1:) for difl'erent relative cavity length values O.) are presented.

\Vhen, according to Figure \) the eroded volume loss pel' unit time .<lV

=

G/Y1: (where y represents the specifie weight of the test specimen) is plotted as function of the relative cavity length (À), for constant eroded material quantity (for example G= 8 000 mg), then the curve illustrated by Figure] 0 could be obtained. The diagram ineludes, in a suitably selected scale, also the sound pres- sure level values for identical now velocity. Taking the measurement scatter into consideration, the points of the curve of eroded volume loss pel' unit ti me, and those of the sound pressure level difl'eren- ces show good agreement. TÎ1e maximum intensity of the cavita tion damages is at À = 1.5 whcreto also the maximum sound pressure level pertains.

In our previous papers [7], [8], [H] reference had been made to the faet that the extent of cavi- tation erosion damage would be jointly governed by the freqllencies of wakes shedding from the model, and the pressures prevailing in the novv. The bubble, starting to move away from its place of origin may grow to the size determined by the now conditions as its growth capaeity is in correlation with the cavity length. The energetieal maximum of the conditions detennined by the double fune- tionality, as interpreted from erosion aspects, is arrived at in the vicinity of À

=

1.5 or so. It seems, however, that the m:ühematieal description of this problem requires further experiments to be conducted.

A proportional part of the collapse enel'gy of the bubbles emitted as vibration and sound energy hased on the experimental results referred to above, will characterize of erosion intensity in reliable nUlnner.

Shalnev [] 0], []] ] conducted similar exper- iments on the determination of erosion intensity, and fOlll1d that the intensitv of erosion varies t~S function of the cavity lengtl~, arriving at its max- imum value at around À

=:30

The comparison of Shalnev's and the authors results, respectively, rel1eets the fact that, among others, the Heynolds number also affects the position of peak intensity values. Shalnev has condueted his experiments ,vithin the critical Heynolds number range (0\.

= ]

(fi - 2.:3 . ] (F) whereas our investigations took place in the range ahove the critical Heynolds number (Oi..

>

2.3 . ](F').

Conclusions

In case of model tests, according to the afore- mentioned findings, cavitation conditions can be readily followed and surveyed by lneans of noise test methods. On grounds of visual observations

and cavitation erosion measurements, a close corre- lation may be demonstrated hetween the cavitation condition, erosion intensitv, and the character of the noise associated with· cavitation, respectively.

According to the resuIts of the ahove experimental investigations, noise pattern of difIerent cavitation conditions gives an unequivocal correlation on cavitation damage intensity as vveIl , which appears sufficientlv verified by the resemblance of the curves cht,raeterizing s(;und pressure level and ero- sion intensity, respectively, as weU as by the posi- tion of the peak val ues in function of À.

On grounds of the results thus ohtained, the cavitation noise test of hydraulic machinery sec ms suitable to give information on the cavitation phe- nomena taking place in the equipment permitting, at the same time, the elimination of cavitation, or, at least, to reduce their intensity.

References

1J BATA 01.1. -- Beeensement et examen (Titique des méthodes d'ohservation de la eavitation par voie acoustique. La /1ollille BlaIlc1le, n" G (1DG;l).

[2] VAHGA (.1.) und SEBESTYJ\N (Gy.). - Beitl"iige zur Hydro- dvnamik der Kavilationsel'osion und des Skalenef- f~l{]es. Mill. der [{Oll/' jïir Wasserkrafllllllsc1linell.

Timisoara (sept. 1!)(i4).

[:lI Bapra (VI.VI.I, I1la.ITbHeB (1(.K.) II LJepIHlBCKIlH Œ. A.l. -

o

MeTOI!e IICC.nel!üBaIHlfl ~WCIIJTa6HOro3qJ(jJel<'ra KaBII- Tal\IIOHHOH 3p03111I. )KrJMT<p. No. 3 096:3).

VAIH;A O,L), ~'AI:"EV (l{,lU i CSEHN.IAVSZKI.I (B.A). --

o metode isszledovanija maszslabnogo effekta kavita- eionnoj erozii. ZsPMTF, n" il (1nG:l).

[4J VAHGA (.1.), SEBESTY1\N (Gy.'!, SCHALNEW (K.K.) und TSCHEHNAWSKI.I lB. A.). - Untersuchung des 1I1ass- staheffektes der Kavitationserosion. Aela Tec1lnicll Hllny. ;'1, NI'. :l-4 (U)(i;').

[;,! VAHGA (.J.) and SEBE5TY1\N (Gy.). - Expel'imental Inves- tigations of Some Properties of Cavitating Flow.

l'eriodica l'olylecllIlica. EllY., Vol. D, No. il.

[G! VAIH;A (,1.) und SEBESTYI\N (Gy.). - Das Geriiuschspek- trull1 der 1\avilation und der Kavitationserosion. Vo]'- Irâye der Il. J{onf. fiir Slri51ll11Ilys111IlSchiIlen. Budapest, X, 24-2D [lDGG).

[7] VAnGA (,1.) and SEBE5TYI\N (Gy.). -- Observations on Cavitation Velocity Dnmage Exponent in a Flowing System. [Jeriodicll Polylechnicll. Eng., Vol. 8, No. il.

[8] VAll<;A [.1.) and SEBESTYJ\N (Gv). - Determination of the Frequeneies of Wakes SheZlding l'rom CirculaI' CyJin- del·s. Aelll TechIl. HllIlY, ;,:l, No. 1-2 O%G).

Ln] VAnGA (.J.) und SElmSTYl\N (Gy.), - Beitriige zu!' Unter- suchung dei' Intensitüt dei' Kavitationserosion. Vor- Irüye der Il. ]{onf. j'iil' SlriJ11111nys111aschineIl. Budapest, X, 24-2D (lDGG).

101 llla.TIbHeB (1(.K.l. - YC!IOBllfl 11ilTeHCIIBHOCTH IŒBIlTaIlIl- OHHOH 3p0311I1.- V13B. AH.CCCP. OTH. (]956), No. 1.

SAI:NEV Il{,]\.), - Uszlovija intenszivnoszlikavita- cÎonnoj erozii. f::v. AN. S.S.S.R. 01'71'. (1!J;,G), No. 1.

[11 [ SHAL'NEV (K.K), VAHGA (1.L) and SEBESTYJ~N (D.). - Investigation of the Scale Effects of Cavitation Ero- sion. Phil. Trans. of the ROYIlI Society of London, A, vol. 2GO (1!J(jG)'

909

Résumé

Etude expérimentale du bruit de cavitation par J. Varga * et G. Sebestyén

* *

Les auteurs - en partant de l'hypothèse que les coups de bélier provoqués par les bulles s'écrasant après l'appari- tion du phénomène de cavitation, rendent possible l'examen des phénomènes de cavitation au moyen de méthodes acous- tiques - ont efrectué des expériences dans le cas d'un obstaele cylindrique placé dans la veine d'essais d'un tunnel hydro- dynamique fermé, pour établir les rapports entre le niveau de bruit de la cavitation d'une part et l'érosion de cavitation d'autre part. Les mesures de fréquence et de niveau de pression acoustique ont été e!rectuées dans la gamme de fréquen- ces de 20-20000 Hz avec un microphone électrostatique, puis avec l'accéléromètre relié à un analyseur de fréquence.

En premier lieu, on a établi le rapport existant entre la longueur l, observable de la zone de cavitation naissant der- rière l'obstacle cylindrique, comptée à partir de l'obstacle cylindrique et le coefIieient de cavitation (cr). La figure 1 montre la variation de la longueur sans dimension ), = loi d, où d est le diamètre de l'obstacle cylindrique. On a constaté ensuite que n'importe quelle fréquence supérieure à Hl 000 Hz permet de comparer des niveaux de bruit entre différents états de cavitation. La figure 5 représente deux courbes de niveau de bruit correspondant à deux coefIicients de cavitation dif- férents en fonction de la fréquence, à vitesse constante. De ces courbes, on peut déduire qu'il y a une différence impor- tante du niveau de bruit selon l'état de cavitation et le niveau de bruit choisi comme valeur de base pour l'état exempt de cavitation. Après avoir établi les valeurs du niveau de bruit pour des longueurs relatives différentes de la zone de cavi- tation, on a obtenu les résultats présentés sur la figure G qui montrent que le bruit atteint un maximum prononcé pour la valeur de ), = 1,5. Ce résultat a été également confirmé par des mesures de niveau d'aeeélération (fig. 7 et 8).

Les auteurs ont établi pour une vitesse constante de courant, en meS'Irant le niveau de bruit avec des éprouvettes en plomb, les courbes de perte de poids des éprouvettes en fonction du temps, pour différentes longueurs relatives de zone de cavitation (fig. 9). Les résultats de mesure obtenus de cette façon ont permis de calculer les volumes érodés par unité de temps pour différentes longueurs de la zone de cavitation. Les résultats sont portés sur la figure 10, avec les résultats des mesures de niveau de bruit. L'évolution identique de la perte en poids par unité de temps et des différences du niveau de bruit ainsi que la valeur maximale qui se manifeste au mème endroit donnent une signification satisfaisante aux Iuesures de bruit.

Les résultats présentés permettent de constater qu'il y a une relation étroite entre l'état de cavitation, l'intensité de l'érosion et le bruit accompagnant la cavitation, et que le niveau de bruit pour divers états de cavitation fournit une information sur l'intensité de l'érosion de cavitation. Les résultats présentés permettent d'espérer que cette méthode soit appliquée frnctueusement à l'examen de la cavitation dans les machines hydrauliques.

Résumé

Etudes expérimeniaRes de l'érosion de cavitation pendant la période d'incubation par J. Varga * et G. Sebestyén

* *

Les auteurs exposent les résultats d'essais e!rectués dans un tunnel hydrodynamique avec un obstacle cylindrique. Ces essais avaient pour but d'étudier la «période d'incubation» de l'érosion de cavitation. Les courbes représentant la perte en poids par érosion en fonction du temps peuvent ètre divisées en deux branches bien distinctes (fig. 1). La première branche représente la période d'incubation suivie d'une période de destruction totale. Le point d'intersection de ces deux branches est un point critique caractérisé par un temps critique et une perte en poids critique. Les auteurs montrent que seule la première branche correspond à une véritable érosion de cavitation, car au-delà du point critique apparaissent déjà divers effets cumulatifs. La limite de rupture du matériau est déjà atteinte au point critique.

Les auteurs exposent une méthode permettant de déterminer le point critique. Le temps critique se détermine avec une précision satisfaisante en calculant l'accélération de l'érosion de cavitation (fig. 5). Les auteurs montrent que, dans le cas d'un coefIieient de cavitation constant, la perte en poids critique est indépendante de la vitesse d'écoulement et ne dépend que des dimensions de l'obstacle et de la veine d'essai. On constate également que la relation déjà proposée par les auteurs, 'rU" = Cte entre la durée de l'essai et la vitesse. à perte en poids constante, reste valable au point critique (fig. 7).

Les résultats des essais montrent que la durée critique diminue avec la dureté superficielle (fig. 8, 9, 1G), alors que cette dernière n'influence pas considérablement la perte en poids dans la deuxième période de destruction.

Les auteurs démontrent enfin que l'effet d'échelle dû aux dimensions géométriques peut se déterminer à partir des per-