EXPERIMENTAL INVESTIGATION ON NONLINEAR REFLECTION OF SHORT IMPULSIVE UNDERWATER SOUND AT WATER SURFACE

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EXPERIMENTAL INVESTIGATION ON

NONLINEAR REFLECTION OF SHORT IMPULSIVE UNDERWATER SOUND AT WATER SURFACE

Yasumasa Urabe, Takahiko Otani, Keiichi Komatsu

To cite this version:

Yasumasa Urabe, Takahiko Otani, Keiichi Komatsu. EXPERIMENTAL INVESTIGATION ON NONLINEAR REFLECTION OF SHORT IMPULSIVE UNDERWATER SOUND AT WATER SUR- FACE. Journal de Physique Colloques, 1979, 40 (C8), pp.C8-315-C8-318. �10.1051/jphyscol:1979856�.

�jpa-00219562�

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E X P E R I M E N T A L INVESTIGATION ON N O N L I N E A R R E F L E C T I O N OF S H O R T IMPULSIVE UNDERWATER S O U N D A T WATER S U R F A C E

Yasumasa URABE , Takahiko OTANI and Keiichi KCMATSU Faculty of Engineering, Doshisha University, KartAgyo-Ku, Kyoto 602, JAPAN

Abstract. - The nonlinear phenomena in the vicinity of free surface of water, induced by the inciden- ce of high intensity underwater compressional pulse, are experimentally investigated. The behaviors of cavitation bubbles and water surface are observed by photographical visualization. It is conclu- ded that the transmission and the reflection at the free surface are linear, and the limitation of negative pressure observed in the reflected pulse must be due to the onset of cavitation. Utilizing the phenomena, the cavitation threshold of water for negative pressure pulse of about 10 us duration is measured. Obtained threshold value of air-saturated tap water is about 10 atm., and after water is degassified to 20% of saturation it becomes about 20atm., at 20°C. The dependence of the threshold on temperature is also investigated.

1. Introduction. - A nonlinear phenomenon was observed In the experiments of underwater impulsive sound generated by electrically exploding wire /l/.

When the wire was exploded under water nearby the free surface of water, the negative or tensional pulse reflected at the surface was observed to be limited and extremely small comparing with the incir dent positive or compressional pulse. To investi- gate the cause of this phenomenon, additional expe- riments was performed and the following conclusion was obtained ; this phenomenon can not be caused at free surface, but is due to onset of cavitation under water by a negative pressure of reflected pulse. Finally the vicinity of the water surface was observed by photographical visualization to confirm the conclusion. The cavitation threshold of water for an impulsive negative pressure was measured by Sedgewick and Trevena / 2 / . The same was measured also by the present authors and some dif- ferent results from those by Sedgewick and Trevena were obtained.

2. Photographical observation. - The behavior of phenomena occurring nearby the free surface of wa- ter, at various timing relative to the instance of

pulse reflection,.was observed by moment photo graphs. The compressional pulse of about 10 ys du- ration is generated by discharging a charged capa- citor through a thin copper wire placed under water.

The peak pressure of the incident pulse at the sur- face is varied by changing the dimension of the wire or the charging energy of capacitor. The shut- ter of a camera is hold open, and a back light pul- se of a duration shorter than 10 \xs is flushed, by a trigger pulse with adjustable time delay relati- ve to the start of current through the wire. Only single frame of moment photograph is obtainable for each shot of the sound pulse. Then, the progress of phenomena is observed by a series of pictures taken for various time of delay. The waveform and peak pressure of pulses are monitored by a hydro- phone and an oscilloscope.

Examples of the photographs, in a case that the negative pressure of reflected pulse is limi- ted, are shown in Fig. 1. After the pulse is re- flected at the surface, small bubbles stari to appear in a region under water and progressively grow up to their maximum, then decay and finally disapear. The life time of the bubles depends on JOURNAL DE PHYSIQUE Colloque C8, supplément au n" 11, tome 40, novembre 1979, page C8-315

Résumé. - On étudie le phénomène non-linéaire produit, au-dessous d'une surface d'eau, par une im- pulsion de pression de très forte intensité. Les comportements des bulles de cavitation et de la surface libre sont étudiés au moyen de photographies. On en conclut que la transmission et la reflection s'effectuent de manière linéaire, tandis que la limitation de la pression négative des on- des réfléchies (un phénomène non-linéaire) est sans doute due à l'existence de cavitation au-dessous de la surface. Le seuil de cavitation de l'eau est mesuré en observant la limitation de la pression négative au moyen d'impulsions de pression négative au moyen d'impulsions de pression d'une dizaine de microsecondes. Les valeurs obtenues sont (à la température de 20°C) d'environ 10 atmosphères pour de l'eau saturée d ' a i r à la pression atmosphérique et d'environ 20 atmosphères pour de l'eau p a r t i e l - lement degazëe "$••-. = 20 %). L'influence de la température sur le seuil de cavitation est également

étudiée. a

Article published online by EDP Sciences and available at

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979856

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~ 8 - 3 1 6 JOURNAL DE PHYSIQUE

over t h e range o f d i s t a n c e from t h e exploding wire.

7

air

- - . . .- -

water

s DjH

measuring tank

\ /

Fig. 1.

-

Examples o f moment photographs, a t ( a ) Fig. 2.

-

Experimental arrangement o f hydrophone 30 ps, ( b ) 90 us, ( c ) 150 us and (d) 250 ps a f t e r (H) and sound source ( S ) .

r e f l e c t i o n .

t h e peak pressure o f t h e i n c i d e n t pulse r e l a t i v e t o a t h r e s h o l d value. The bubbles seem t o be generated by a negative pressure o f t h e r e f l e c t e d pulse. The c l o s e c o r r e l a t i o n i s recognized between t h e l i m i t a - t i o n o f t h e r e f l e c t e d negative pressure and t h e ge- n e r a t i d n o f c a v i t a t i o n bubbles.

No disturbance o f t h e water surface i s observed through these processes. The disturbance o f t h e wa- t e r surface, which i s o f t e n v i s i b l e w i t h unaided eye, occurs a f t e r a r e l a t i v e l y l o n g period. The t r a n s m i t t e d p u l s e i n t o a i r was a l s o observed by a microphone placed over the surface and the l t i n e a r transmission, conforming t o t h e theory, i s recogni- sed.

From these r e s u l t s , i t i s concluded t h a t the transmission and t h e r e f l e c t i o n a t t h e f r e e surface o f water a r e l i n e a r , and l i m i t i n g o f t h e n e g a t i v e peak pressure i n t h e r e f l e c t e d p u l s e i s due t o t h e onset o f c a v i t a t i o n under water.

3. C a v i t a t i o n t h r e s h o l d o f water.

-

The experimen- t a l arrangement f o r t h e measurements o f c a v i t a t i o n t h r e s h o l d i s i l l u s t r a t e d schematically i n Fig. 2.

The d i r e c t and t h e r e f l e c t e d pulse are r e c e i v e d by a

B&K

Type 8103 hydrophone, placed a t a s u i t a b l e p o s i t i o n under water, and observed by an o s c i l l o s c o - pe. I n F i g . 3. an example o f the observed waveform i s shown. The observed peak pressures o f d i r e c t po- s i ti v e and r e f l e c t e d n e g a t i v e p u l s e are converted i n t o t h e values and P~ a t t h e water surface, by i n t e r p o r a t i o n c o n s i d e r i n g s p h e r i c a l spreading o f t h e wave f r o n t which i s i n v e s t i g a t e d e x p e r i m e n t a l l y

Fig. 3.

-

An example o f observed waveform.

V : 6.8 atm/div., H : 1 0 0 p s / d i v .

Fig. 4.

-

R e l a t i o n between Pi and Pr

,

f o r o r d i - nary t a p water a t 17.5OC.

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Yasumasa URABE

The r e l a t i o n between Pi and pP i s p l o t t e d f o r var$ous values o f pi as shown i n Fig. 4. The l i n e a r r e l a t i o n between Pi and Pp

,

w i t h t h e cons- t a n t r e f l e c t i o n c o e f f i c i e n t o f -1, up t o a c e r t a i n c r i t i c a l value PC i s obtained. For values o f

pi h i g h e r than pc

,

t h e p l o t s depart from l i n e a r r e l a t i o n and pr tends t o be l i m i t e d and f l u c t u a t e . The c a v i t a t i o n takes p l a c e c l o s e l y nearby t h e surface, so t h e value o f pc i s considered as t h e c a v i t a t i o n t h r e s h o l d o f water f o r the i m p u l s i v e n e g a t i v e pressure o f about 10 ps d u r a t i o n .

By repeated e x p l o s i o n o f wires, t h e water i n t h e measuring tank g r a d u a l l y becomes d i r t y by t h e c l o u d o f explosed copper wires, b u t i t s i n f l u e n c e i s n o t appreciable i n t h e experiments. The water i s n o t changed f o r about 50 shots o f pulse. Usual- l y f r e s h tap water containes oversaturated a i r , so t h e excessive a i r i s removed by b u b b l i n g o r by long- time standing i n t h e measuring tank before e x p e r i - ments.

The c a v i t a t i o n t h r e s h o l d o f o r d i n a r y t a p water (100% a i r - s a t u r a t e d ) i s measured i n t h e tem- p e r a t u r e range from Z°C t o 26OC. The r e s u l t s a r e shown by the s o l i d curve i n Fig. 5. The measurements f o r d e g a s s i f i e d t a p water i s a l s o performed i n a smaller tank. B o i l e d t a p water i s l e t t o be n a t u r a l l y cooled i n t h e tank, u n t i l t h e temperatu- r e drops t o a measuring p o i n t . The a i r concentra- t i o n o f water increases w i t h t h e time r e q u i r e d f o r cooling, and percentage a i r - s a t u r a t i o n v a r i e s w i t h t h e measuring temperature. The a i r c o n c e n t r a t i o n i s measured by a h e a t i n g method /3/, i n which sam- p l e of water i S g r a d u a l l y heated up i n a beaker and t h e temperature, a t which a bubble j u s t begins t o appear, i s measured. This temperature i s consicie- r e d as t h e p o i n t o f water being a i r - s a t u r a t e d s t a t e and the percentage a i r - s a t u r a t i o n a t t h e tempera- t u r e o f experiment i s derived. The r e s u l t s o f t h r e - shold measurements f o r d e g a s s i f i e d water i n t h e temperature range from 10°C t o 50°C a r e shown by t h e broken curve i n F i g . 5. The percentage a i r - s a t u r a t i o n i s about 30% a t 10°C, 25% a t 15°C and 20% a t 20°C. I f the percentage a i r - s a t u r a t i o n i s h o l d constant, h i g h e r values o f t h r e s h o l d i n t h e lower temperature range might be-expected.

The c a v i t a t i o n t h r e s h o l d obtained a t 20°C f o r d i f f e r e n t percentage a i r- s a t u r a t i o n a r e summarized

i n

Table I.

0;

1 0 2 b 3 b 4 0 5 b

Temperature (Oc )

I

Fig. 5.

-

Temperature dependance o f c a v i t a t i o n threshold. ( a ) Ordinary t a p water.

( b ) Degassified t a p water.

Table I.

-

C a v i t a t i o n t h r e s h o l d o f water a t 20°C.

4. Discussion.

-

The c a v i t a t i o n t h r e s h o l d o f water f o r t h e i m p u l s i v e negative pressure o f 250-450 us d u r a t i o n and i t s dependence on temperature was measured i n a s t e e l p i p e by Sedgewick and Trevena /2/.

Comparing t h e present r e s u l t s w i t h those by Sedgewick and Trevena, pc a t 20°C f o r o r d i n a r y t a p water agrees q u i t e w e l l and the dependence on tem- p e r a t u r e shows a s i m i l a r tendency. But f o r t h e de- g a s i f i e d water and i n t h e lower temperature range, t h e present r e s u l t s show much h i g h e r values.

A i r concentration

Saturated

60% o f s a t u r a t i o n 20% o f s a t u r a t i o n

The avoidance o f t h e e f f e c t o f w a t e r - s o l i d i n - t e r f a c e and t h e s h o r t e r p u l s e d u r a t i o n i n t h e experiments a r g considered as t h e p o s s i b l e causes o f t h e d i f f e r e n c e mentioned above. The existence of w a t e r - s o l i d i n t e r f a c e i n t h e measuring f i e l d may

C a v i t a t i o n t h r e s h o l d (atin)

10 13

20

-

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JOURNAL DE PHYSIQUE

cause lowering o f t h e measured t h r e s h o l d value by t h e e f f e c t o f c a v i t a t i o n n u c l e i attached t o t h e s o l i d surface, e s p e c i a l l y i n t h e cases o f a h i g u e r t h r e s h o l d value. A l o n g e r d u r a t i o n o f n e g a t i v e pressure can be a cause o f g i v i n g enough time t o microbubbles o r c a v i t a t i o n n u c l e i f o r growing up t o t h e " c a v i t a t i o n " and lowering t h e t h r e s h o l d value.

I n t h e present study, f i n e p a r t i c l e s o f t h e explosed w i r e shows no appreciable e f f e c t , b u t f o r measurements i n more h i g h l y d e g a s s i f i e d cases, an improvement o f t h e experimental system may be r e - q u i red.

REFERENCES

/1/ Matsuda, M., and Urabe, Y., U l t r a s o n i c s I n t e r n a t i o n a l Conf. Proc. (1973) 155.

/2/ Sedgewick, S.A., and Trevena, D.H., J. Phys. D : Appl. Phys.

2 ,

(1976) 1983.

/3/ Saneyoshi, J., and Okushima, M., J. Acous. Soc. Japan

2,

⁽¹⁹⁵⁸⁾ ^310.