THE UNIVERSITY OF MICHIGAN COLLEGE OF ENGINEERING Department of Nuclear Engineering Laboratory for Fluid Flow and Heat Transport Phenomena Technical Report Noo I ACOUSTIC NOISE FROM A CAVITATING VENTURI RO Garcia,.... -.aHammiitt: - J "' C O. ~"M J, Robinson. 1 Finan ial Support Providded by.. NATIONAL AERONAUTICS'-AND SPACE ADMINISTRATION (Grant NsG 39,60)': and ATOMICS INTERNATIONAL (Contract N4i$6BA-002006) administered throughOFFICE OF RESEARCH ADMINISTRATION ANN ARBOR June 1, 1964

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ACKNOWLEDGMENTS Financial support for this investigation was provided by the Atomics International Division of North American Aviations Ince, and the National Aeronautics and Space Administration0 Special thanks are also due Mro Glenn Mo Wood, Project Engineers Pratt & hitney (CANEL)X for arranging for the processing of data on their automatic harmonic analyser; Mr. Willy Smith, graduate student in the Department of Nuclear Engineering, for his many suggestions and for the use of his tape recorder; Mr. Richard Jamron, Research Engineerq University of Michigan Willow Run Laboratories' for arranging use of the Hewlett-Packard harmonic analyser; and Messrso Richard Ivany and David Ericson, graduate students in the Department of Nuclear Engineering, for many helpful suggestions0 ii

ABSTRACT A stainle ststeel acoustic probe for detecting cavitation incipience, degree, and intensity in a flowing fluid has been developed and tested in both mercury and water tunnel facilities at the University of Michigano The barium titanate crystal attached to one end of the probe is used to observe the sound pattern generated by the collapsing bubbles in the venturi test section contained in each loopo The voltage generated by the crystal gives definitive information as to the degree of cavitation present. The probe is suitable for use in water-Plexiglas, mercuryPlexiglas, and high temperature liquid metal-steel systemso It was found for both mercury and water that the signal from the probe increases as the degree of cavitation becomes more intenseo For mercury the cavitation signal is proportional to the velocity raised to the 1o7 power, while in the case of water the appropriate exponent is 15o For mercury the cavitation signal decreased by 15% when the temperature was raised from 70 OFo to 500 OF. The frequency of the observed voltage waveforms is approximately 2 KCo, somewhat higher than the frequency of the loop and pump noises presento This frequency is indicative of the frequency of the noise produced by the collapsing bubbleso The probe is operational and can be easily and quickly installedo iii

TABLE OF CONTENTS Page ACKTOWLEDGo TSo o o o o o o o o o o o o o o o o o o o o o o ii ABSTRACT A 0 0 0 0 0 0 0 0 0 0 o 0 0 0 0 00 0 O0 0 0 0 0 0 0 0 0 i LIST OF FIGURESo o o o o o o o o o o o 0 0 0 0 0 0 0 0 0 0 v Io INTRODUCTION 0.0 o 0..0 0.. o o o... o o o o o 00 1 Ao Importance of Detection and Measurement of Cavitation Noise 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 Bo University of Michigan Cavitation Facilities o o o 0o 1 Co Proposed Acoustic Studieso o o o o o0 o o o 1 IIo RECORDING AND ANALYSIS OF AIR-BORNE NOISE o.0 0 o 0 3 Ao Tape Recorder System o 0 0.........o o 3 Bo Hewlett-Packard Harmonic Analyser o o o o o o o o o o 5 Co Pratt & Whitney Automatic Harmonic Analysero o o o o 5 IIIo RESULTS OF ANALYSIS OF RECORDED AIR-BORNE NOISE 0o o o o Ao Hewlett-Packard Harmonic Analyser Results, Mercury Loopo 0 0 0 0 o o 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Bo Hewlett-Packard Harmonic Analyser Results' Water Loop. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 Co Pratt & Whitney Harmonic Analyser Results, Mercury Loop & Water Loopo. o o o o 0 0 o o o o o 9 Do Summary and Discussion of Resultso o o o o o 0 o o o o 18 IVo DESIGN OF A SUITABLE ACOUSTIC PROBE 0 0 0 0 0 o 0 0 o0 23 Ao Theoretical Aspects o oo0o0oooooooooooo 23 Bo Details of Probe Design and Detecting System o o o 24 Vo ACOUSTIC PROBE DATA o o 0 0 0 o o o o o 28 Ao Oscill:oscope Data)o * o o o o o o o o o o o o o o o 28 Ao Oscilloscope Data0 0000000000000 0000 28 Ba Vacuum Tube Voltmeter Data 0 o........... 33 Co Summary and Discussion of Results o o o o o o 37 VIo RECOMMENDATIONS FOR AN OPTIMUM SYSTEM o o o o o o o o 40 BIBLIOGRAPYo o o o o o o o.0 0 o 42 iv

LIST OF FIGURES Figure Pa 1 CCavitating Venturi Test Section o o o o o 4; 2 Frequency Spectrum of Cavitation Noise in Mercuryo 0 o 6 3 Frequency Spectrum of Cavitation Noise in Water o o0 o 8 4-7 Frequency Spectra of Cavitation Noise in Mercury for Various Degrees of Gavitationo o o o o o 0o oo107 899 Frequency Spectra of Cavitation Noise in Water for Various Degrees of Cavitationo o o.. o.. o o 19-22 10 Schematic Diagram of Acoustic Probe o o o0 0o o o 0 26 11 Block Diagram of Sonic Detection Systemno o o o 27.213 Acoustic Probe Voltage Representation of Cavitation Noise From Collapsing Bubbles in Mercury o o o o 29=32 14 Effect of Cavitation Condition and Velocity on Sound Amplitude in Mercury o0 0 o o.. o o o o0 34 15 Effect of Cavitation Condition on Sound Amplitude in Mercury at 500OF0 0 0 0 0 0 0 0 35 16 Effect of Cavitation Condition on Sound Amplitude in Mercury With Pin-Type Specimen o 0 0 o oo 0 00 36 17 Effect of Cavitation Condition on Sound Amplitude in Water at 100 feet/second, 0 o o o o o o o o 38 18 Effect of Cavitation Condition on Sound Amplitude in Water at 200 feet/second o o o o o o o o o o o o0 o o0 39.T

ACOUSTIC NOISE FROM A CAVITATING VENTURI Io INTRODUCTION A i. mportance of Detection and Measurement of Cavitation Noise The use of mercury as the coolant and heat-engine working fluid in many SNAP-type reactor power plants has focused attention on the important problem of cavitation in liquid metalso At the same time cavitation is also a problem encountered in the large central station water-cooled reactor power plantss resulting in a reduced efficiency of the pump system and in some cases rendering the pump inoperableo When conducting cavitation tests in an experimental system, it is necessary to know the nature and degree of the cavitation produced under a given set of conditions One method of determining the inception and degree of cavitation activity is to analyse the noise produced by the collapsing bubbleso Bo University of Michigan Cavitation Facilities At the University of Michigan both mercury and water loops containing a venturi test section are available for cavitation researcho These facilities have been described elsewhere. (l2) For low temperature tests a transparent Plexiglas venturi is suitable and, indeed, desirable in that degree of cavitation present can be visually observed.0 However, high temperature tests are of great interest, and such investigations require a stainless steel venturi for loop operation0 Hence visual observation of the cavitation cloud is impossible9 and some other means of determining cavitation inception is necessaryo C0 Proposed Acoustic Studies t The technique described in this report to detect cavi

tation incipience, degrees and intensity consists of observing the sound pattern generated by the collapsing bubbles in the venturi test sectiono The detection of cavitation through sonic techniques has been utilized considerably and with success in the past(3o8)0 Many techniques are available, but the one chosen consisted of generating standing waves along a stainless steel rod of suitable length, one end of which was placed in close proximity to the cavitating venturi while the other end accommodated a BaTiO3 (barium titanate) piezoelectric crystalo Collapsing bubbles generate sound waves which are propagated along the length of the stainless steel acoustic probe The BaTiO3 crystal is displaced slightly from its equilibrium positions and hence a small voltage is generatedo This voltage is fed.to an oscilloscope and a suitable high-~ sensitivity vacuum tube voltmeterr for inspection and recording0 The degree of cavitation determines the amplitude of the standing waves set up in the stainless steel probe0 More severe cavitation results in greater aplitude of the sound waves and hence greater output voltage from the piezoelectric crystalo If the probe is of suitable length. it will act as a filter for sound waves of frequencies other than the resonant frequency of the rod.o Hence the first step is to determine the frequency range of the sound waves generated by the collapsing bubbles and the corresponding amplitudeso This can be determined by recording directly the air-borne noise from the loop when it is operating in cavitating and non-cavitating modeso The recording must then be subjected to a harmonic analysis so as to determine an amplitude versus frequency relationship for both cona ditlonso Comparison of the data will yield the range of frequencies

of the sound waves generated by the collapsing bubbles as the background noise due to the pump and loop are present in both cases. Then it is possible to compute the required length of a stainless steel sonic probe, so that it will be resonant in the frequency range of the sound waves generated by the collapsing bubbleso The probe is equally applicablefor use in water-Plexiglas$ mercury-Plexiglas, and high-temperature liquid metal-steel systems0 IIo RECORDING AND ANALYSIS OF AIR-BORNE NOISE A1 Tape Recorder System For purposes of arriving at a suitable length for the sonic probe it was necessary to first obtain some indication of the amplitude versus frequency relationship for the sound waves generated by the collapsing bubbles0 In this investigation a Roberts Model 1057 four track stereo tape recorder was utilizedo Tape speeds of 3 3/4 and 7 1/2 inches/seco are possible and the frequency response shows no more than a 2 db0 variation in the range 4015000 CPSo The microphone was supported in a ring stand and placed approximately 10" 2" from the cavitating venturio In all cases a two minute recording of air-borne noise was madeo This was done for a Zero cavitation condition and for three different degrees of cavitation referred to as Visible, Standard, and First Marko A cross-section of the venturi test section is shown in Figure I along with an indication of the extent of the various degrees of cavitationo Investigations were made in the mercury loop for both the Plexiglas and stainless steel venturis at velocities of 34 and 48 feet/seCo Also in the water loop studies were made for the Plexiglas venturi at velocities of 100 and 200 feet/sec. The data

~1251 ~^ x3 STUD B SPECIMEN HOLDER - x 3 STUD V, / AV TO NOSE % l WEAR SPECIMEN X / STANDARD CAV TO BACK CAV TO IST MK 3 015-~ CAV TO 2NDMK SECTION B-B VISIBLE INITIATION AV O 2 MK -- 15510"1 r f b- K -, ~-20 - 11. )0 2.247 X//5. ~ 2.247 3.013" l 5.361" 504 ~5773".SECTION A-A 2 = 4, - 6.522" ~~ 14 578".412"- z AXIS -786- 1 1001 ~ —1.75~ Figure 1. Cavitating Venturi Test Section

consists solely of two minute recordings of air-borne noise for the non-cavitation case and for three degrees of cavitationo B0 Hewlett-Packard Harmonic Analyser The several recordings were subjected to a harmonic analysis to determine an amplitude versus frequency relationship0 Various harmonic analysers are available for this worko The one utilized was a Hewlett-Packard Model 302A Wave Analyser having a frequency range from 5050,000 CPS with a bandpass of 7 CPSo In analysing the tapes it was necessary to manually vary the frequency from 200 CPS to 20,000 CPS in convenient. intervals and note the amplitude of the sound waves present in the narrow frequency interval0 Then it is possible to compute the signal-to-noise ratio for a given degree of cavitationo The noise signal is taken to be that response obtained from recordings of air-borne noise when the loop was operating in a non-cavitating mode at the same velocity~, Pratt & Whitney Automatic Harmonic Analyser The recordings were also analysed by personnel at Pratt & Whitney Aircraft (CANEL) using an automatic harmonic analyser which immediately generated plots of amplitude versus frequency for the various conditions investigatedo IIIo RESULTS OF ANALYSIS OF RECORDED AIR-BORNE NOISE A. Hewlett-Packard Harmonic Analyser Results' Mercury Loop As indicated previously recordings were made in the mercury loop for both Plexiglas and stainless steel venturis at throat velocities of 34 and 48 feet/seco Figure 2 is a plot of Signal/Noise ratio for the mercury loop with the Plexiglas venturi

40,: ~ — 1..........1.11 MERCURY LOOP - 70~ F __ O | | NO SPECIMEN 20_ PLEXIGLAS VENTURI QI: 34 FEET/SECOND oLJ I I I I \ 1 L/ o 1 1 LU0__ _____ 0 8b 10 ____________ ______ Z7 6 0 11~a 6-....____.____0. — -_ _ F4 I~~~~~~~0, 100 1000 10000 FREQUENCY (cycles per second) Figure 2. Frequency Spectrum of Cavitation Noise in Mercury Figure 2. Freqiuentcy Spectrum of Cavitation Noise in Mercury

at a velocity of 34 feet/seco No damage specimens were in the loopo The signal was that obtained from air-borne noise generated during First Mark (severe) cavitationo The first peak at approximately 600 CPS is possibly due to machinery vibrations excited by cavitationo The loop and pump noise is expected to be in the range 200-800 CPS, but should be present during both the non-cavitation and cavitation runso Hence one would expect a Signal/Noise ratio of )lo The second peak at approximately 5000 CPS indicates the presence of a strong signal at this frequency during First Mark cavitation which was not present in the nonr cavitating mode, This large Signal/Noise ratio definitely suggests that sound waves.in the frequency range 4-6 KC are being generated by the collapsing bubbles0 The data obtained at a velocity of 48 feet/seco were not significantly different from that presented in Figure 2 for a velocity of 34 feet/seco The data obtained with the stainless steel venturi in place with no damage specimens at velocities of 34 feet/seco and 48 feet/seco were inconclusive in that the Signal/ Noise ratio did not differ appreciably from unity over the frequency range investigatedo It is felt that this is due to absorption by the stainless steel of the sound waves generated by the collapsing bubbles0 Hence only loop and machinery noise was recorded for all the runs B. Hewlett-Packard Harmonic Analyser Results, Water Loop In the water tunnel facility recordings were made with a Plexiglas venturi in place at velocities of 100 feet/seco and 200 feet/seco with standard damage specimens in placeo Figure 3 is a plot of Signal/Noise ratio for the water loop at a velocity of 200 feet/seco

40 30o_ WATER LOOP- 700 F ______ _ ____ STANDARD SPECIMEN 0 20~1 PLEXIGLAS VENTURI 200 FEET/SECOND 00 LU) 10 __ — - z~ i^-^ ~ ~ H _ 0~D " ~ ^^^ ^ ~ ^^^^^: ~==L^a~~~~0 C) 0()~~~~~~~~~~~~~~~00 r rI. 7 1 I I I / o, o z0 _______________________ ___ _ _ ____________________________ _ _ I_ _________ _ _____.... I i...... \oo 100 1000 10000 FREQUENCY (cycles per second) Figure 3. Frequency Spectrumn o'f Cavitation ioise in Water

The signal was that obtained from air-borne noise generated during First Mark (severe) cavitationo Only one large peak is in evidence, and this occurs in the 416 KC ranges similar to the peak obtained in the mercury tunnel facilityo Once again one is led to the conclusion that sound waves in the frequency range 4-6 KC are being generated by the collapsing bubbles Hence it might be concluded further that the proposed sonic probe should be made of such a length so as to be resonant in the 4-6 KC frequency rangeo The data obtained at a velocity of 100 feet/seco were not significantly different from that displayed. in Figure 3o C Pratt & Whitney Harmonic Analyser Results' Mercury Loop and Water Loop All of the recordings of air-borne noise were also analysed utilizing an automatic harmonic analyser at Pratt & Whitney (CANEL)o The output is displayed as a plot of amplitude (in dbo versus frequency0 Figure 4 presents the results obtained in the mercury loop at a velocity of 34 feet/seco with the Plexiglas venturi in placeo It is clear that the signal increases in the l-6 KC range as one proceeds from Zero cavitation to Visible cavitations and then increases still further as the cavitation degree is increasedo Figure 5 presents similar results at a velocity of 48 feet/sece The dependence of signal on cavitation condition is clear, The effect of velocity on signal appears to be somewhat obscured as Figures 4 and 5 are quite similaro A slight increase in signal is noted at 48 feet/seco for some sections of the spectrum, principally beyond 6 KCo The results obtained with the stainless steel senturi in place in the mercury loop at velocities of 34 feet/seco and 48 feet/se@ are presented in Figures 6 and 7,

1350 (a) Fiure 4(a). Zero Cavitation Figure 4(b). Visible Cavitation Figure 4. Frequency Spectra of Cavitation Noise in Mercury for Various Degrees of Cavita'tion Plexiglas Venturi & Velocity of 34 feet/secon6d -10

1350 (c) Figure 4(c). Standard Cavitation 1350 (d) Figure 4(d). First Mark Cavitation Figure 4 (Continaed) -II-l

Figure 5(a). Zero Cavitation 1351 (b) Figure 5(b). Visible Cavitation Figure 5. Frequency Spectra of Cavitation Noise in ilercury for Various Degrees of Cavitation Plexiglas Venturi & Velocity of 48 feet/second -12

1351(c) Figure 5(c). Standard Cavitation Figure 5(d). First Mark Cavitation Figure 5 (Continued) -13

1352 (a) Figure 6 (a). Zero Cavitation 1352(b) Figure 6(b). Visible Cavitation Figure 6. Frequency Spectra of Cavitation Noise in Mercury for Various Degrees of Cavitation Stainless Steel Venturi & Velocity of 34 feet/second -1L

1352(c) Figure 6(c). Standard Cavitation 1352 (d) Figure 6 (Continued) -15

1353 (a) Figure 7(a). Zero Cavitation 1353 (b) Figure 7(b). Visible Cavitation Figure 7. Frequency Spectra of Cavitation Noise in Mercury for Various Degrees of Cavitation Stainless Steel Venturi & Velocity of 48 feet/second -16i ~ ~ ~ ~ ~ ~ ~ 00

1353(O.) Figure 7(c). Standard Cavitation 1353 (d Fi gure 7(d). First Mark Cavitation Figu re 7 (Continued) -17

-18respectivelyo The results are inconclusive in that neither degree of cavitation nor velocity appear to significantly affect the signal obtainedo Once again this is probably due to absorption of the sound waves by the stainless steel venturio The results obtained for the water loop are presented in Figures 8 and 9o At a velocity of 200 feet/seco the signal does not appreciably increase until Standard and First Mark cavitation are achieved, The signal at 100 feet/sece is considerably reduced from that obtained at 200 feet/ sec. for frequencies greater than 6 KCo The data from both the Hewlett-Packard harmonic analyser and the Pratt & Whitney harmonic analyser appear to be in very good agreement0 Do Summanr and Discussion of Results The following statements summarize the results of the air-borne noise studys lo For either mercury or water the signal increases with increasing velocity for frequencies above 6 KC, In the case of mercury this is true for both the Plexiglas and stainless steel venturise 2o The mercury and water spectra are very similar in form0 The signal does not appear to be a strong function of the test fluid0 3o In the case of mercury the signal appears to be considerably attenuated when the stainless steel venturi is in placeo Apparently the sound waves are absorbed more completely in the stainless steel than'-, in he Plexiglas0 As a results the Signal/Noise ratio is ~r unity with the stainless steel venturi in place

1354(a) Figure 8(a). Zero Cavitation 1354(b) Figure 8(b). Visible Cavitation Figaure 8. Frequency Spectra of Cavitation Noise in Water for Various Degrees of Cavitation Plexiglas Venturi & Velocity of 200 feet/second -19

1354(c) Figure 8(c). Standard Cavitation 1354 (d) Figure 8(d). First Mark Cavitation Figure 8 (Continued) -20

1355(a) Figure 9(a). Zero Cavitation 1355 (b) Figure 9(b). Visible Cavitation Figure 9. Frequency Spectra of Cavitation Noise in Water for Various Degrees of Cavitation Plexiglas Venturi & Velocity of 100 feet/second -21-.

1355(c) Figure 9(c). Standard Cavitation 1355 (d) Figure 9(d). First Mark Cavitation Figure 9 (Continued) -22

-234o For both mercury and water the signal increases as the degree of cavitation becomes more intenseo In the case of mercury this is true for both the Plexiglas and stainless steel venturiso 5o All signal seems to end at about 8-9 KC in Figures 419o This apparently is due to frequency cut-off of the Pratt & Whitney harmonic analyser sinea a signal was obtained beyond 8-9 KC when the Hewlett-Packard harmonic analyser was usedo 6o Considerable high frequency signal is noted for the case of Zero cavitation in water especially at a velocity of 200 feet/seco This may indicate presence of cavitation on an invisible scale0 7o Generally for either fluid cavitation noise is approximately "white" from about 200-6000 CPS (at least)o 8o Since the Signal/Noise ratio is maximum in the 44- KO frequency range for both mercury and water, the length of the proposed sonic probe should be such as to make it resonant in this frequency rangeo IVo DESIGN OF A SUITABLE ACOUSTIC PROBE Ao Theoretical Aspects It is desired to generate standing waves along the length of a stainless steel rod, which is fixed at one end and whose other end is free According to Den Hartog(9) the resonant frequency of a cantilever which is set in longitudinal vibration is given bye f (n1/2) n \2 L

2h4wheres A cross-sectional area of cantilever E modulus of elasticity of material /UL = mass per unit length of rod L length of rod - n 0 91,29,3, —- = number of nodeso In the case of stainless steel the expression becomesfn (2n + ) L),~ CPS L Li in inches For n 0~ we have6 L Choosing a value of 5000 CPS for f, we haveL: 59 0000 Hence it appears that the proper length of the probe should be approximately 10:o Two probes were fabricateds one of length 10" and the other of length 241 o In the ensuing measurements it appeared that the signal obtained with the 24k probe was maximumo Hence it was adopted as the standard lengtho This discrepancy in length calculation is unexplained at this time0 B0 Detai s of Probe De n nd Detecting System As mentioned previously it is desired to detect cavitation incipiences degree, and intensity by means of observing the sound pattern generated by the collapsing bubbles in the venturi test sectiono This can be done by generating standing waves along a stainless steel rod of suitable length, one end of which is placed:in close proximity to the cavitating venturi while the other end accommodates a suitable piezoelectric crystalo Piezoelectric crystals

of adequate sensitivity are highly temperature-sensitive and cannot be used above rather moderate temperatures Also the design should be applicable to all fluids to be testedo It seems evident that an arrangement suitable for high-temperature liquid metals must isolate the crystal sufficiently from the system to allow the maintenance of low temperature0 Also, the probe should not penetrate the venturi wall because of possible sealing problems. Hence the following are the major features of the sonic probe designs lo A stainless steel rod of circular cross section having a diameter of 1/41 and a length of 24'1 was adopted as a standard0 2o BaTiO3 was selected as the piezoelectric crystal despite its severe temperature limitation (200 0Fo) because of its high sensitivity0 The crystal is 3/16" diameter by o02" thick and was cemented to one end of the stainless steel rodo One side of the crystal was grounded directly to the stainless steel rod; the other side was connected to a modified banana jack which was spring-loaded against the crystal0 This arrangement makes it simple to monitor the output from the crystal with either a cathode-ray oscilloscope or suitable low-level vacuum tube voltmetero The probe must be shielded to minimize 60=cycle pick-up which can be a problemo 3o The other end of the rod was fitted with a 1/41 - 28 thread. This made it possible to screw the probe into any one of several venturis, each of which had been fitted with an adapter plate. The details of the probe design are shown in Figure 10o Figure 11 is a schematic diaeram of the sonic probe and the detecting system0 The signal from the crystal can be monitored either with the oscilloscope or the vacuum tube voltmeter0

ADAPTER SLEEVE -CONTACTOR \ \ N~BANANA JACK v.-28 /~ROD " PIN /-SPRING 24" Ba Ti 03 CRYSTAL INSULATOR F356 Fipure 10. Schema~tic Diagrwja of ACcoustic Probe

VENTURI OSCILLOSCOPE Ba Ti Os CRYSTAL'7 -- ACOUSTIC PROBE I ^A ADAPTOR PLATE RMS VOLTMETER 1357 Figure 1. Block Diagram of Sonic Detection System

-28It was found that the output from the crystal generally varied from 050 mvo Hence a sensitive low-level vacuum tube voltmeter was neededo A Ballantine Model 300-D vacuum tube voltmeter with a voltage range of 1 mv 1000 V, a relatively flat frequency response from 20 CPS 250 KC, and a scale accuracy of 2 2% was utilized for the investigation~ Shielded cable for the leads is also necessary0 Any good general purpose oscilloscope would be satisfactoryo V. ACOUSTIC PROBE DATA A. Oscilloscope Data Several oscilloscope photographs of the voltage waveforms from-the crystal were taken with a Polaroid cameras Figure 12 consists of two such photos0 Shown are waveforms for the mercury loop with the Plexiglas venturi in place (containig special damage specimens) at a velocity of 34 feet/seco for the case of Zero cavitation and Standard cavitationo In each case the oscilloscope probe was held directly against the crystal. There is a large increase in signal as the degree of cavitation is increased from Zero to Standardo The frequency of the waveform obtained for the Standard cavitation condition is approximately 29000 CPS, which is consistent with the rod length as previously discussed0 Figure 13 indicates a sequential progression from Zero cavitation to First Mark cavitation in the mercury loop with the stainless steel venturi (standard specimen in place) at a velocity of 34 feet/seco The observed frequency is much higher than that noted for the case of the Plexiglas venturi and is very difficult to estimate, In each case

100 mv. /cm. 1358 (a) 2 milliseconds/cra Figure 12(a). Zero Cavitation 100 mv./cm. 1358( (b) 2 milliseconds/cm. Figure 12(b) Standard Cavitation Figure 12. Acoustic Probe Voltage Representation of Cavitation Noise From Collapsing Bubbles in Mercury Plexiglas Venturi & Velocity of 34 feet/second Probe Touched Directly to Crystal -29

2 milliseconds/cm* FIgI 11/1 H Ci Figre 13(a). ZeroVi Cavitation -30e......EE(b)............ Figure 13. Acoustic Probe Voltage Representation of Cavitation Noise From Collapsing Bubbles in Mercury Stainless Steel Venturi & Velocity of 34 feet/second Probe Inserted into Banana Jack -30

50 mv.,/cm. bim 77 ii, 1359 (c) 1359(d) 2 milliseconlnd/cm. Figure 13(c) >Vios be Cavitat ion Fi50re 1 (Continued) -31-31

50 mv./cm. 1; E,,i-,.lEE ~ ~ ~ ^ ~ ^^^^^- 1359(e) 2 milliseconds/cm. Fiigure 13(e). Standard Cavitation 50 mv./cm.,Ill_ U, I I..l.l.. 1359(f) 2 milliseconds/cm. Figare 13(f). First Mark Cavitation Figure 13 (Contirmed) -32

-33" the oscilloscope probe (banana plug) was inserted into the modified banana jack which was spring-loaded against the crystal0 This arrangement results in a complex path for the sound waves and is probably responsible for bh-. higher frequencies noted in Figure 13o No photographs of voltage w.aveforms for the water loop investigations are available but tae waveforms were noted as being similar to those obtained for the mercury loopo Bo Vacuum Tube Voltmeter Data Readings of RMS voltage versus pressure drop across the venturi (which is related to degree of cavitation) are available for both the mercury and water loopso Figure 14 is such a plot for the mercury loop at room temperature, The stainless steel venturi was in place and contained a standard damage specimen0 Investigations were made at velocities of 26 feet/sec, 34 feet/seco0 and 37 feet/seco It is clear that the RMS signal increases as the venturi \ p (extent of cavitation) increases until a plateau value is reached. Thereafter the signal is about constanto For a given cavitation condition the RMS signal is proportional to velocity to the P 1.7 power0 Figure 1r presents data obtained at a temperature of 500 ~Fo and at a velocity'. -f 34 feet/seco A standard damage specimen was in placeo The general shape of the curve is similar to that obtained in Figure l40 However, the plateau at 500 Fo, appears to be about 3 mv0 less than at room temperature. This might be due to the fact that the incr~ —> mercury vapor density at OO 0F0 is cushioning the collapse of rnobles and, hence absorbing a fraction of the sound energyo Hence c.,^e - uld expect a reduced signal at the elevated temperature0 Figure 16 presents additional data for the

37 FEET/SE3 FCOND El 24 __/________ 16 _E | I i /I / I-34 F FEET/SECOND 28 1 I 1 711 / tMERCURY LOOP - 70 F 37 FEET/CO 34 FEET/SECOND 20 40 60 80 100 120 iiA. FiCure 14. Effect of Cavitation Gond-ion an2d VelociyOND on Soud DlApli-ude in MercurN -3-3)4

32 MERCURY LOOP - 500~ F STANDARD SPECIMEN 28 STAINLESS STEEL VENTURI 34 FEET / SECOND 24 ~ _ _ - _ 0p'AL I U) I ~CI - /20 1 6 12I7 STANDARD CAVITATION /II I I 1 1361 20 40 60 80 100 120 PRESSURE DROP ACROSS VENTURI (psi) Figure 15. E'ffect of Cavitation Condition on Sound Armplitude in Mercury at 5000F. -35~~~i........

22 __~~~_ MERCURY LOOP - 70~ F __20 _ PIN-TYPE SPECIMEN __ STAINLESS STEEL VENTURI 30 FEET/SECOND 18 ~.1IS0 1 / -JJ i____ 1362 40 60 80 100 120 140 160 PRESSURE DROP ACROSS VENTURI (psi) Figure 16. Effect of Cavitcation Condition on Sound Amplituade in Mercury with Pin-Type Specimen -36

37mercury loop with a pin-type specimen in the stainless steel venturio Here the velocity is 30 feet/seco The curve shape is similar to the others except that the plateau region is not nearly as apparent. Since the rate of damage is much greater with the pin, it might be expected that -he noise level would be considerably greater0 However, this is apparently not the case0 In the case of the water loop data is available at velocities of 100 feet/seco and 200 feet/sec. with the Plexiglas venturi in place and containing standard stainless steel specimenso The temperature was ambiento Figure 17 is a plot of RMS signal versus venturi p at a velocity of 100 feet/seco Figure 18 is a similar plot at a velocity of 200 feet/see, In both cases a plateau is evidentg as was the case in the mercury investigations, For a given cavitation condition the S signal is proportional to velocity to the J 1o5 power Once again this is similar to the mercury loop resultso Co Summary and Discussion of Results The following statements summarize the results of the acoustic probe study' lo For mercury the cavitation signal is proportional to velocity to the t^l17 power. 2o For water the cavitation signal is proportional to velocity to the aJ 1o5 power, 3o The RMS signal pppears to be stronger for mercury at 34 feet/seco than for water at 100 feet/sec. for the same cavitation condition by a factor of abotP 3o 4o For mercury only data with the stainless steel venturi

,0 _ >|WATER LOOP - 70~F ____ STANDARD SPECIMEN 9 L_ PLEXIGLAS VENTURI -100 FEET/SECOND (I 8 1 0 6 3 -J2 t STANDARD CAVITATION.... I....... I -38I 1363 ~~~Mon. i m ot m -..

22 1 20 1' ~ (nI16 I~__ __ _ I 16 2 LJ / 6__ IJ \PLEXIGLAS VENTURI I 4 I ____ 200 FEET/SECOND STANDARD C VITATION 1364 76 78 80 82 84 86 88 90 92 PRESSURE DROP ACROSS VENTURI (psi) Figure 18. Effect of Cavit-ation Condition on Sound. Amplitude in Water at 200 feet/second -39

40are available~ For water only data with the Plexiglas venturi are available0 Hence no conclusions can be drawn as to the effect of venturi material on the cavitation signal0 50 For both mercury and water the signal increases as the degree of cavitation becomes more intense (venturi A p increases)o This is to be expected. 6o For mercury the signal decreases by 15% as the temperature is raised from ambient to 500 OFo 7. The frequency of the observed voltage waveforms from the crystal appears to be about 2 KC when the oscilloscope probe is held directly against the crystal. This frequency seems consistent with the 4-6 KC range of frequencies observed in the air-borne noise investigations, HQwever, when the oscilloscope probe (banana plug) is inserted into the modified banana jack which is spring-loaded against the crystal9 then the frequency of the observed voltage waveforms from the crystal is increased into the high KC range, This might be due to the complex sound, path through the venturi wall, along the length of the stainless steel rod, and then through the modified banana jack to the oscilloscope probe. 8o The sonic probe technique was checked for reproducibility of results, and generally this was achieved admirablyo VI. RECOMMENDATIONS FOR AN OPTIMUM SYSTEM Thia:acoustic probe described in this paper appears to be satisfactory for detecting cavitation incipience, degree, and intensity by means of observirg the sound pattern generated by the collapsing bubbles in the vent-i A test section. The sound path from

bubble to crystal is a complex one and appears to obscure needed frequency information. Perhaps threading the probe directly into the venturi would be an improvement. However, sealing problems at high temperature may then be prohibitive.

-42BIBLIOGRAPHY 10 Hammitt, Fo GOF9'cavitation Damage and Performance Research Facilitie$s', ORA-Technical Report Noo 03424-12-T, Department of Nuclear Engineering, The University of Michigan, November, 19630 2, Hammitt, Fo G., "Observations on Cavitation Damage in a Flowing System'" Journal of Basic Engineering, Transactions, ASME, Series D, Volo 85 (1963) ppo 347-359o 3o Hueter, T- Fo, and Bolt, R. Ho,, Sonics, John Wiley and Sons, Inc, 19550 4. Peterman, Lo Ao, "Cavitation Attack in Ultrasonic Equipment", Product Engineer, September, 1955o 50 Fitzpatrick, Ho M,, and Strasberg, M,, "Hydrodynamic Sources of Sound", Naval Hydrodynamics, Publication 5159 NAS-NRCG 1957 6o Harrisonr, Mark, "An Experimental Study of Single Bubble Cavitation Noise'" D.To.lBo. Report 815, November, 1952. 70 Mellen, Ro H,, "An Experimental Study of the Collapse of a Spherical Cavity in Water', Journal of the Acoustical Society of America, Volo 28, Noo 3, May, 1956. 8o Mellen, R. Ho, "Ultrasonic Spectrum of Cavitation Noise in Water'" Journal of the Acoustical Society of America, Volo 26, Noo 3, May, 1954o 90 Den Hartog, Jacob, Mechanical Vibrations, Fourth Edition, McGraw-Hill Publishing Company, 1956.