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Bolt Beranek and Newman Inc. T Technical Memorandum No. 457 I ~~ Results of Air•Gun Tests at St. Croix , April 1978 J.E. Barger, C.M. Gogos, and W.R. Hamblen August 1978 I Prepared for: Defense Advanced Research Project Agency I T ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I ~ c ~ sak~; is C..) t ~ — —~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ r ~~~~~~~~~ ~~~ _ _ _ _ ‘I I Tech Meao No. 457 (p RESULTS OF AIR-GUN TESTS AT ST. CROIX9 APRIL 1977 ~~~~~~~.. - ~~~~.- ~~~~. ~~~~~~~~~~~~~~~~ 7 ~~~~~~ 1~~ oI7 4 Prepared by: Bolt Beranek and Newman Inc.7 50 Moulton Street Cambridge, Massachusetts 02138 ~~~~~~~ (C~~H1LL Sponsored by: Defense Advanced Research Agency Monitored by CDR Wesley Jordan r AISN—~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Under Contract ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - The views and conclusions contained jn this doci~~ent are those of I the authors and should not be interpreted as representing the 1. official policies, either expressed or implied , of the Defense Advanced Research Projects Agency or the U.S. Government. ~~~~ B tV-Tft ~~~~1 J~ II OIDO ~LOO ~~~~I~~Jj~~J ~~1~~~~~~-LI - ~~~~~~~~ ‘I Tech Memo No. 457 Bolt Beranek and Newman Inc. TABLE OF CONTENTS page I i. INTRODUCTION • . . 1 2 • PROCEDURE 2 Ii. . 3 . RESULTS . • . • • • • . • . • . 8 4. ANALYSIS OF RESULTS • 12 5. SUMMARY AN CONCLUSIONS 23 APPENDIX A. CALCULATION OF RADIATION EFFICIENCY A-i APPENDIX B. DEPENDENCE OF AIR DISCHARGE ON AIR GUN PARAMETERS • B—l / H - I ~9 Ui b ih ~~~~~~~~~ — L______ _ _ _ _ _ _ _ _ _ Tech Memo No. 457 Bolt Beranek and Newman Inc. LIST OF FIGURES page FIGURE 1. BLOCK DIAGRAM OF EX PERIMENTAL APPARATU S 3 FIGURE 2. EXAMPLES OF ON—LINE DATA ANALYSIS.... ..... 9 FIGURE 3. EFFECT OF EXTENDED PORTS ON ENERGY SOURCE LEVELS OF 1500C AIR GUN WITH 1000—cu in. CHAMBER . 13 FIGURE 4. EFFECT OF THROAT NOZZLE ON ENERGY SOURCE LEVELS OF 1500C AIR GUN WITH 1000—cu in. CHAMBER .• 15 FIGURE 5. EFFECT OF EXTENDED PORTS AND THROAT NOZZLE IN ENERGY SOURC E LEVELS OF 1SOOC AIR GUN WITH 1000—cu in. CHAMBER. . . . . . . . 16 FIGURE 6. EFFECT OF MODIFIED UPPER SLEEVE ON ENERGY SOURC E LEVELS OF 1500C AIR GUN WITH 300—cu in. CHAMBER 17 FIGURE 7. EFFECT OF CHAMBER VOLUME ON ENERGY SOURCE LEVELS OF 800C AIR GUN WITH STANDARD SHUTTLE.... 18 FIGURE 8. EXAMPLE OF GUN REPRESSURIZATION DUE TO BUBBLE COLLAPSE: 800C AIR GUN WITH 2000—cu in. CHAMBER AND STANDARD SHUTTLE AT 300 psi 20 FIGURE 9. EFFECT OF LARGE SHUTTLE ON ENERGY SOURCE LEVELS OF 800C AIR GUN WITH 2000—cu in. CHAMBER. 22 I _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ‘ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tech Memo No. 457 Bolt Beranek and Newman Inc. L LIST OF TABLES page TABLE I. SUMMARY OF AIR-GUN CONFIGURATIONS TESTED 10 TABLE II. ENERGY SOURCE LEVELS AT FUNDAMENTAL FREQUENCIES FOR AIR-GUN WAVEFORMS (AVERAGE OF TWO - WAVEFORMS ) 11 a • I. I 21 1 - vi — ~~__ __ ‘__ ___ 7_ — —--- -~ —-- — -. - ~~ --~~-- . - -. -~ • - . -~ - Tech Memo No. 457 Bolt Beranek and Newman Inc. L. 1\~INTRODUC TION This technical memorand um presents the results of source level measurements made on two Bolt Associates air guns — with and wi thout various modifications . The objective of the tests was to determine the air—gun configuration that would best meet the requ irements developed in Ref. (1] for an array element. The energy source level requ ired from each array elemen t is 223 dB Cre ~jpa2’ sec in the f undamen tal frequ~~~~y b and~~~ ~~~~~ • - ~~ -, Earl ier test results (2] showed that this source level can be ach ieve d — bu t only at low fun damen tal fre quenc ies (shallow depths) . The ener gy source levels were seen to decrease w ith incre asin g fun damen tal frequency obta ine d at increase d source depths. An additional test objective , therefore , was to study the causes of this decrease in source level. Diagnostic measurements were made , us ing a pressure trans ducer mounte d insi de the a ir gun , and var ious modifications were introduced to the two basic air guns in an attempt to improve the ir ac ous ti c perf ormance at lar ge depths. [1 1 . , -~ - .- - •-~~-~ . r — t - ~~~~~~~~~~~~ —“-~ - —— -- ~ ——- _ _ _ _ _ _ _ _ _ _ _ Tech Memo No. 457 Bolt Beranek and Newman Inc. 2. PROCEDURE The measurement program was conducted dur ing the period from 3 to 21 April 1978 in the deep ocean channel 6 miles north of Christiansted , St. Croix , USVI . The measurements were staged from the laboratory barge YFN1I26 , which is operated by the Key West Detachment of the Naval Air Development Center. The barge was outfitted with the experimental apparatus shown schematically in Fig. 1. Th e a i r guns were deploye d , one at a tim e, from a 2000—ft 1/2—in, wire rope fitted to an oceanographic winch on the 01 deck amidships via an A—fram e and sheave on the starboard side. The 3/8—in, air hose , rated at 4500 psi working pressure , was taped to the firing line and the pressure transducer signal cable. This 1700—ft—long taped bundle was flaked out on the 01 deck next to the winch. A small platform on the main deck provided working space for lashing the taped bundle to the wire rope as the air gun was lowered to the test depth . Returning the air gun to the deck involved use of a Pettibone hydraulic crane. The oceanographic winch raised the air gun/accumulator to just below the water surface (to prevent swing ing due to ship motion) . At this point , the Pettibone cable was attached to a bridle on the accumulator , and the air gun assembly was hauled aft onto the main deck. The air compresso r was a multistage Ingersoll Rand Model D4R1SMX2S , having 40 SCFM capacity at 5000 psi. The final stage • outpu t v ia a small accumula tor was p iped to a control man if ol d where precision gauges measured air—compressor pressure on one side and air—gun pressure on the other. Manual control valves allowed a i r connec tions to be ma de amon g the a i r compress or , the a i r gun , and the a tmosphere , thus providing the ability to make fine , 1 2 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - Tech Memo No. 457 Bolt Beranek and Newman Inc. _ _ 1 L 11r U I 1!~ 1 L~J _ _ w I-’ w IJJ 2i~ 4 __ - • 4~~~~~I I~~~I I~~~~ 1 • I p. S - S S _ _ _ _ _ S S S ‘- ‘1~ ø~5i ~ ~~~~~~~~~~~~~~~~~~ ~~~~~~~ S i _ 3 __________ tSr- — -~~~~~~ --c- Tech Memo No. 457 Bolt Beranek and Newman Inc. air—pressure adjustments . The output of this control manifold was connected to the 1700 ft of 3/8—in, high—pressure air hose. A large 10—gal . accumulator was connected to the deep end of the air hose , providing storage of high—pressure air to enhance gun sealing at the deepe r depths. A manually operated shutoff valve was installed at this accumulator output. The air gun was charged throug h a 22—ft air hose just before immersion. Charging was able to be accompl ished away from the r iggers , because the m anual valve was opened by use of a lanyard. A dynamic pressure transd ucer , PCB Model 11lA22 , was installed , usually at the bottom of the air gun , to measure the air—discharge history in the lowe r air chamber. The electrical cable of the pressure transducer provided dc powe r to the transd ucer amplifier via a power supply topside and the signal line up to the measurement equipment . The nominal sensitivity of the pressure transducer is I mv /psi with a full—scale range of 5000 psi . The air gun was fired from an AG Series air—gun fire control , which provided a 90—V electrical pulse down the firing cable to the firing solenoid mounted on the air gun . The measur ement hydrophone , Type F—SO Series No. 21 , was lashed to a weighted nylon line that was run through a block on a davit located near the stern on the j~ort side. The hydrophone sensitivity, based on calibrations made in the BBN hydrophone calibration facility before and after testing , was —222.0 dB re • v/u Pa. Frequen t checks on this c~a1i~ ration were made during the test program with a ‘G—19 hydrophone calibrator. The hydrophone was lowered to a depth of about 300 ft and about 80 ft aft of the air—gun deploying rope . The gun—to—hydrophone distance was obtained by measuring the elapsed ii P Tech Memo No. 457 Bolt Beranek and Newman Inc . time between the pressure—transd ucer air—discharge pulse and the d irect acoustic arrival. Both the hydrophone and the pressure signals were displayed on a 2—channel oscilloscope and tape recorded . The hydrophone signals were spectrum—analyzed upon arr ival. Each shot was analyzed on—line , in addition to being tape recorded . Each source— level data point required the following processing: Capture of the pressure wav eform Four ier transform of each waveform Averag e of two transfo rms per condition Summation of the energy in all analysis band s that comprise the fundamental—frequency band Measurement of source—to—receiver acoustic transient time and computation of distance Calculation of transmission loss on the basis of spherical spreading Calculation of the energy source level in the fundamental—frequency band . In addition , the chamber—pressure amplitud e time history was also recorded and photographed on—line for both chamber pressure wavefo rms and the acoustic transit timing . The test program was conducted in accordance with the test plan (31. Two basic Bolt Associates air guns — a PAR 800C and a PAR l500C — and various configurational mod ifications of these basic guns were tested in an effort to maximize the acoustic output . The PAR 800C was fitted with three air—chamber combinations — 400, 1000, and 2000 Cu in.; the PAR l500C was fitted with two—300 and 1000 cu in. These air chambers were used to study the effects of charge volume on radi ated source level for var ious charge and amb ient pressures. Other modifications to the air guns were made • for the following reasons. L A _ _ _ _ - . _ - - ~~~~~~~~~~~ -,~~~~~r~~~~~~~~ - iS - - - _ _ _ - Tech Memo No. 457 Bolt Beranek and Newman Inc . The maximum ambient pressure differential for the 800C gun was limited to 3500 psi , to aioid structural damage to the gun . Bolt had indicated there might be problems with gun sealing at our highest test pressure of 4000 psi . In an attempt to correct this problem , a special shuttle was provided with a large diameter shaft and thicker sealing flanges to reduce distortion under pressure. Previous testing on the 1500C gun (2] indicated that the charged air might not be full y discharged either because of exhaust—port constriction or because of turbulence generated by sharp edges at the throat of the air chamber . Two hardware modifications were impl emented to investigate this air—discharge problem : extended exhaust ports and a streamlined throat nozzle. Reference 2 also indicated that the shuttle was not staying open long enoug h , thus prematurel y cutting off the air discharge. An additional hardware modification was therefore introduced to delay shuttle closure: An upper chamber sleeve was Cut down to allow the shuttle to travel higher into the upper chamber (increasing shuttle throw) before trapping the air necessary to reverse shuttle direction . Each of these modifications were tested independently. In all , the testing involved 6 configura tions for the 800C and 16 for the l500C . Two problems encountered in these preliminar y measurements extended the testing time and , to some extent , limited the data • acqu isition. First , the air compresso r developed a loud knock in its left side during the third day (13 April) of testing . The air compressor was judged unsafe , - and testing was temporarily suspended . Action was taken to repair the air compressor and to acqu ire a backup machine. Hoffart Marine Inc. rebuilt the left section in time to start testing by 17 April. A Worthington Model L 6 - - Tech Memo No. 457 Bolt Beranek and Newman Inc. SABC 20 SCFM air compresso r rated at 5000 psi was rented from Innerspace Research for the backup . This machine arrived at St. Croix on 17 April. On 19 April , the 40 SCFM a ir compressor froze , necessitating the use of the smaller capacity Worthington. Little testi ng time was lost in implementing this ai r compr essor but because of its 20 SCFM capacity, the time to recharge the air gun system was sign i fican tly increased , thus slowing the data—acquisition process. The second problem encountered was the physical deforma tio n of the PAR 800C air gun . The gun body was defo rmed out—o f—round , causing the air seals to leak at charge pressures g rea ter than about 2200 psi. This problem prevented the acquisit~: of the l600—ft , 4000—psi data point. i_ -— .__ -__a____ -- ~~~~~~~~~~~~~~~~~~~~~~~ • • • ~ •• — rnn--ur~ar_ _ _ _ _ _ _ _ _ _ _ _ _ _ - - • - ‘-I Tech Memo No. 457 Bolt Beranek and Newman Inc. ~ I 3. RESULTS Each shot was analyzed on—line as described in Sec. 2. An example of the results obtained with this on—line processing is given in Fig. 2 for 800C air gun configured wi th the large shuttle and the 1000—cu in. air chamber. The gun was charged to 3000 psi and detonated at a depth of 400 ft. The pertinent features of this on—l ine analysis are: . The wavefo rm resem bles a dam~ ed sinusoid with a bubble pulse per iod of about 32 msec as seen in Fig. 2a. • The fundamental energy component is centered at 30 Hz with about a 5—Hz bandwidth (Fig. 2b) • The lowe r chamber discharged 2700 psi of the 3000 psi static pressurization in about 5 msec (Fig. 2c • The acoustic transit time is about 27 msec , reckoned from the time of the sharp rate of the pressure discharge to time of reception of the acoustic shock wave. The following table lists the air—gun configurations tested. A number of possible configurations were not tested for a variety of reasons , the most common being tha t the source levels ex trapola ted from the test data would not be high enough to be of interest and/or other configurations showed greater promise . There were cases , however , that provided significant information about how air guns work for these specific environmental conditions and how to improve their performance in spite of rather low source levels. For these cases , sufficien t d ata were acquir ed to establish perfo rmance trends that suggested specific configurations or gun modifications to be tested . I. 8 - ________________ - - _ _ _ Tech Memo No. 457 Bolt Beran ek and Newman Inc. - :~ - c~ W ’ ~4 z 4 4(1, I- LU ~,, ~~~ CD CD LU -I 00 z LU — LU LU I I I I I I I I I I ~ 2 O ~~~~~~~~ ~~~~~~~~~~~~~~~~ — — ~~~~~~~ OLU 4W (3~~ Dd’1(//9P)AjjSN3O ADèI3N3ONV9 ‘H-c 0 -o -~~ c’J LU>- 2 ~~ ~~~~~~~~~~~~~~~~~~~ 4 E c~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ O~~ ~ _~~~~~W ~~ U LU 0 U w 4 4 -J _ _ o I I I I ~~ I I I 0 0 Q 0 • w ~~~ (!$d)3bjfl~~3~Jd 398YH3S10 • 9 _ _ _ _ _ _ _ _ • - -- • -~~~~~ I •. ~• ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ • - • ~~~-~~-- ~~~~~~~~~~~~~~~~~~~~~ - Tech Memo No. 457 Bolt Beranek and Newman Inc. TABLE I. SUMMARY OF AIR-GUN CONFIGURATIONS TESTED Model: PAR 8UC 400—cu in. chamber standard configuration 1000—cu in. chamber large shuttle 2000—cu in. chamber standard configuration 2000—cu in. chamber large shuttle Model : PAR 1SUC 1000—cu in. chamber standard configuration 1000—cu in. chamber extended ports - . 1000—cu In. chamber extended ports and ring nozzle 300—cu in. chamber extended ports and ring nozzle 300—cu in. chamber extended ports, r ing nozzle, and modified upper sleeve Table II lists the energy sourc e levels obtained at differen t depths using the air—gun confi gurations described in Table I. I ‘ a ~ ii~ 10 - - -r - - ~~— -- — —— — .—— Tech Memo No. 457 Bol t Beranek and Newman Inc. TABLE II. ENERGY SOURCE LEVEL S AT FUNDAMENTA L FREQUENCIES FOR AIR -GUN WAVEFORMS (AVERAGE OF TWO WAVEFORMS). Energy Source Level In dB re UPS2 SIC at im at Depths of: Air-Gun Configuration 200 ft 400 ft 800 ft 1600 ft 800C/1.00—cu in. standard shuttle 2000 psi 111..5 at 32 Hz 110.1. at 50 Hz 106.6 at 87.5 Hz 3000 psi 113.3 at 27.5 Hz 111.8 at 1.5 Hz 109.8 at 80 Hz .000 pit ———— SOOC/2000—cu in. standard shuttle 2000 psi 120.2 at 17.5 Hz 118.9 at 30 Hz 111.1 at 62.5 Hz 10l. .1 at 100 Hz 3000 pat 122.6 at 15 Hz 112.8 at 25 Hz 113.9 at 50 Hz 108.5 at 90 Hz 1.000 pet ———— 800C/2000—cu in. large shuttle 2000 psi ——— — 121.1 at 30 Hz 115.2 at 27 Hz 110.7 it 62.5 Kz t 2500 psi —— — — 122. 1 at 30 Hz 115.0 .~t 69 Hz 5 ll1.J. at 75 Hz t 3000 psi ———— 122.7 at 27 Hz 118.6 at 1.h.6 Hz 1.000 psi ———— ———— 120.1 at 50 Hz —— 300C/1000—cu in. large shuttle 2000 net 118.0 at 20 Hz 116.3 at 38 Hz 112.1. at 73 Hz 10L5 at 125 Hz 3000 psi 120.3 it 18.1 Hz 118.3 at 37 Hz 115.5 at 62.5 Hz 108.9 at 101. Hz .000 psi -——— ———— -——— 110.8 at 100 Hz 1500C11000—cu in. stindard contiguratlon 2000 psi ———— 109.1. at i~ Hz 105.8 at 75 Hz 3000 pci ———— 112.6 at 1.0 Hz 109.0 it ‘0 Hz 101.1. at 125 Hz 1.000 pat --—— -——— 111.7 at 62.5 Hz 105.0 at 112 Hz l500C/1000—cu in. extended ports 2000 psi 112.2 at 22.5 liz 1)9.5 at .5 Hz 103.6 at ‘5 Hz 3000 psi 116.2 at 20 Hz 1l2.~. at 1.0 Hz 108.3 at 7) Hz 1.000 pet ———— -——— 111.3 at 62.5 Hz 1500C/1000—cu in. ports and ring nozzle 2000 psi 112.1 at 22.5 Hz 109.1 at 1.5 Hz 103.5 at 75 Hz 3000 psi 116.7 at 20 lIz 112.2 at 1.0 Hz 107.8 at 68 Hz 3800 psi ——— -——— Ll1 .3at 62.5 Hz 15000 / 300-cu to. port s and ring nozzle 2000 psi 110. 1. at 35 H: 109.2 at 62 .5 H: 105.6 at 100 Hz 3000 pet 113.3 at 32.5 Hz .12.2 at 51. Hz 108.3 at ~1 Ix 3800p11 ———— ———— 110.O at 62 Mz 1500C / 300—cu in. ports , nozzle • and sleeve 2000 psi 110.7 at 35 Hz 109.1. at 62.5 Ii 10l. .9 at 105 Hi 3000 psi 112.9 at 30 5: 111.6 at 50 Hi 108.1 at 90 ~i 3700 psi -——— --—— 109.0 at 82 Ix ‘isaaured at 1000—ft depth. • . t$esaursd at 1200—ft depth. — •~~~~~~~~ . -- — _• - -- __ ~~~~~___ ~~~~~~~~~~~~ .• —- _._•—----.- — - - Tech Memo No. 457 Bolt Beranek and Newman Inc. 4. MIALYSIS OF RESULTS An objective of these tests was to acquire a better understanding of the dynamics of these air guns in an effort to optimize their energy source level throug h selective configuration changes . As men tioned in Sec. 2, results from the Key West tests suggested that significant amounts of air were unavailable for acous tic sourc e level genera tion : Either the shuttle ac tio n was too fast (thus cutting off prematurely the air discharge from the lowe r chamber and resul ting in red uced source levels ) , or the air flow was unduly constricted . Consequently, these St. Croix tests were designed (1) to extend the shuttle motion to give the charged air more time to exit and (2) to expedite the air flow from the lower chamber by increasing the exhaust areas and decreasing air flow turbulence. The extended—port modification provided more area for the di scharg ing a ir , while the streamlined throat nozzle effectively increased the cross—sectional area at the exit of the lowe r chamber by decreasing turbulence. The modified upper sleeve provided an opportunity for the shuttle to extend its upward travel before reversing its direction , thus staying open longer. In testing the effects of the ext-ended ports , we looked at both the radiated source levels and the radiation efficiency, wh ich is the ratio of acoustic energy to the work expended on the ocean in expanding the bubble to its maximum size. The detailed development of radiation efficiency is given in Appendix A. Figure 3 shows th e effec t on source level and ra di a ti on efficiency of extending the exhaust ports on the l500C air gun. The effec t is seen to be insignificant , indicating either that the • or ig inal por ts were alrea dy lar ge enough or that the flow is be ing restricted elsewhere — such as at the lower chamber/throat. The 12 - r ~ - - __________ • Tech Memo No. 457 Bolt Berane k and Newman Inc. — I I I~~~• 1 1 - - . - - p.. - -ID = :“ 000zIIJ - OOO 4~- o Oi- * tO - 10. 11 - N~~~~ (Wt 40 3S$~ Dd Tl(//Sp) (9~) 13A31 33W~OS A983N3 A3N313U.13 NOI1VI~Vè1 LI 13 I . - --- ~- — Tech Memo No. 457 Bolt Beranek and Newman Inc . effect of the throat nozzle and the throat nozzle and extended ports together is given in Figs. 4 and 5, respectively. Again , there is apparen tly no improvem ent in energy sour ce lev el perf ormance , indicating that the air flow was not impeded by the ports or throat but rather must be limited by the speed at which the rarefaction wave propagates in the gun structure. The eff ec t of the modi f ied upper sleeve is shown in Fig. 6. The source levels are seen to be unaffected by this modification , indicating either that the modification did not increase the turn—around time of the shuttle or that the air discharge , as was prev iously assumed , was not being cut off prematurely. Comparison of the pressure waveforms shown in Figs. 5 and 6 indicates the former condition to be more likely, since the motion of the shuttle was not significantly changed by this modification. Finally, the pressure waveforms for the l5øOC gun , for deep conditions , show evidence of gun repressurization at a time shortly after the maximum air discharge. This result indicates that the shuttle is still open , allowing for partial repressurization of the lower chamber by the bubble collapse. Further , the bubble collapse could be cushioned by the remaining outgoing air discharge. This mechan ism woul d expla in the observe d decreas e in source level w ith increasing fundamental frequency. This point is discussed further below. The energy source levels for the 800C air gun with the 2000—cu in. air chamber show a strong decrease with increasing frequency, as seen in Fig. 7. At the lower frequencies (shallow depths) , the decrease in source level is seen to be a moderate 1—1/2 dB per • octave. As the depth Increases , the source—level slope is seen to decrease much more significantly — about 8 dB per octave. The 800C gun with the 400—cu in. chamber does not exhibit this sam e trend. I 14 - •~ -•- . — - - ~~— - . ~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~ ‘— -. - — ~~~~ .__ -i.-_ -. _ —~ -.- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ — - —-- - ~ _-~~ • — Tech Memo No. 457 Bol t Beranek and Newman Inc. — I I I~~ ’ • ’ I I ~~~~~~~~ - - = I- - _ _ _ _ - p - • - - I,, o o o x x~ - 1 0 . 1 1 - N _ ~~~a I C? ~~~~~~~ L I / I I _ _ _ _ _ _ _ _ _ _ (Wt 4D 3S$~Dd Tf (//9p) (8P) 13A31 33~flOS A9~3N3 A3N3I3Ld.~3 NOI.LVIOVèJ — ——-.•,--- ,• - -~ -—-,.- — - •~~—.-— _ - - 1. Tech Memo No. 457 Bolt Beranek and Newman Inc. 0 11 H Q N ~~ (Li U ~ -~ 0 I-. 10 Sn — ooo ~~ w0 —“3 O O O I- )(~~ I- U)LiJ F- W _ _ _ _ _ _ - ~~~~~ (Aip/!$d OOS)_3~flSS3~d _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~~~~~~ ! ~~~~~~~~~~~~ ~~~~ (w (4D 39S~Od T/[/,,9p) (6P) 2 13A31 3OèIflOS ADd3NJ AON3I3I~d3 NOIIVIQV èI f .1 ~~~~~~~~~~~~ - — ~~~~~~~~—- -— - ----- -~~~~~~~ - -~~~~~~ -- -—- - ~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Tech Memo No. 457 Bolt Beranek and Newman Inc. 00 ‘1 i 2 I — —~~~~~ I-, IV~~~~ ~ EE 0 r ______________ 0 ~~~ tO — . 0 ‘a’ W — __J — V . . E ,_. Lu I— 0 LU It) ~ -J ~ 0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ — _ N 0 ~~ - w lal ‘-‘N ” _ Q__ ~ •0 ~ # ,‘~ ~ OZ 0 LU ~~~~ .. -...LLi ~~~~ 0 00~~0LUNW~~~ 0 000i- cr l-Nw 0 00 x~ X0_J j w ~- W Z ct I e~ 4 0 S LU p.. I >- ~~ (A!p, $dOOS)3~j fl SS~~~d ~~ I 1’ 1-~~ L1; (wUD 3aS~Od7l(/,’9p) (8P) 13A31 3O~flOS ADdJN3 ADN3IDIJJ3 NOI1~ IGV~ 17 ~~ - —— Tech Memo No. 457 Bolt Beranek and Newman Inc. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 8 I I I 0 - 1 i I I ...~~~~.. — — - - p. - - ID CS - - Sn i-a - . ~~~~~~~~ - - ~~~~~~ N - 0 • I 1 -N ~~~~ = —j----~~ - A ’? I j - ~~~~~~~~~~~~~ • 1 4 V ~~~~~~~ . I I H 2 2 ‘ 2 2 . N N N I I (WI 4O 3e$~Dd7l(//9p) (8P) 13A31 33W)OS A983N3 A3N313W.13 NOIiVIQV~ [1 L 18 [1 — —— ----- --4 — — ~~~ ---— — - -~ _-——--.— ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ — - - ~~~~~~~~~~~~~~~ --- ---- - -~- Tech Memo No. 457 Bolt Beranek and Ne~~~an Inc . - The difference in source levels due to the difference in air—gun volume (400—cu in. vs 2000—cu in.) is about 7 dB , approxima tely that seen at the lower frequencies . The analysis of the lower chamber pressure wavefo rms suggests that the shuttle is staying open 20 to 25 msec. At the lowe r fre quenci es (shallow depths) , the time to bubble collapse is suffici ently long to allow the shuttle to close before the bubble collapse forces air back into the gun chamber. At the higher frequenc ies (deep depths), the t ime to bubble collapse is much less than 20 to 25 msec; thus , the bubble collapse is cushioned by outflowing air and eventually repressurizes the gun ’s chamber. The lower—chamb er pressure waveforms shown in Fig. 8 for the standard conf iguration with a 2000—cu in. chamber demonstrate this effect. The slope of 30 ps i/msec is assoc iated wi th the electrical—d ischarge time constant of the voltag e amplifier used with the pressure transd ucer. Pressure increases having slopes grea ter than 30 psi/msec are interpreted as bubble—collapse repressur ization of the air chamber. No repressurization is observed for the 200—ft (15—Hz) and 400—ft (25—Hz) conditions , since the bubble period is longer than the shuttle closure time of about 25 msec . At 800 ft (63 Hz) , some repressur ization is evident , wi th the resultant drop in source level . In this case, the bubble period is slightly shorter than the closure time. At 1600 ft (90 Hz), the repressurization is very evident , as indicated by the slope of 130 psi/msec after the maximum pressure discharge. This effect is greatest in the large—volum e configuration , where a significant amount of air is still discharg ing at the time of bubble collapse. For the small—volume configuration , the air discharge is essentially complete at the bubble—period timing and there is no outgoing air to cushion the bubble and red uce source levels. I I! • ~~~~~~~~~~~~~~~~~~ _ _ ~~~~~ • - - . .. - - - - - . - _ • -- ~~~~~~~~~~~~~~~—-~~~-~~~--— - --~~~~~~ - ~~~-- — ~~ -- .--- ,----- --- -—- -• - - -- •- — ._ ---.— .- -- - - - - Tec h Memo No. 457 Bolt Beranek and Newman Inc. ~~~~~~ — 0 0 0 0 o I_I— _.~~~~~~~~ w ____ I f ii : : .>- - • E • E • Ic) itS) — Lu Lu • • — S — 0 I.- 0 LU I- I- ‘ ‘ ‘ I I I ’ ( I I I ’ ’ ’ (!$d) 3W1SS38d ~i3BI1VH3 L_ -~ ~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~- --—.~~~~~~~~~~ - ~~~~~~~~~~~~~~~~~~ — -----•— .- - - --~~~~~~~~ -~~~~~~~~ --~ -~~~~~~~~~~~~~~~ - -.. -- - •~~~-- Tech Memo No. 457 Bolt Beranek and Newman Inc . A spec ial shuttle hav ing a larger diame ter shaft than that of the standard shuttle was tested with the 800C air gun . Theoretical analysis of the air—gun parameters (see Appendix B) showed that the larger shaft provided a larger restoring force on the down motion , thereby decreasing the closure time (as indicated in Fig. B.3) The effect of this larger shuttle on energy source level is seen in Fig. 9. At low frequencies , the effect is minimized , since bubble—pulse cushioning is not an issue. As the depth is increased , the effec t of a faster shuttle is seen in larger source levels. The pressure waveform seen in Fig. 9 for the 1000—ft condition using the faster shuttle shows no evidence of repressurization. For this condition , the source level Is greater by about 7 dB than for the slow—shu ttle case . 21 ,- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ • .- ——~~~~~~~ .1~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ - - ~~ - ~ —-—.----- - -~~~~~~~~~ - Tec h Memo No. 457 Bolt Beranek and Newman Inc. _ _ _ J 000 z ~ 00 0 4 — LU • 0001— 4 LU -J • I o • I l LU N I I I c I-. >. ~~ (A !P/!SdOOS)3èIflSS3èId 0 — / — Ø Q CS - / / p - ~~~~~~~~~~~ ~~~ — I .. LU - i4/ ~~~~~~~~~~~~~~~~ ~~~ • l f - * g2 (wt 4D 3as~od?lI/,iep) (ep) 13A31 3DèiflOS A983N3 A3N3I3tdd3 NOIJ.VIOV~ - Tech Memo No. 457 Bolt Beranek and Newman Inc . 5. SUMMARY AND CONCLUSIONS 1. The maximum ene rgy source level for the 800C at 80 Hz is about 216 dB re Pa 2 sec , for the fast—shuttle 2000—cu in.—chamber configuration at 4000 psi. Similarly, the max imum source level at 80 Hz for the smaller l500C air gun is about 210 dB re Pa2 sec a t 4000 psi. 2. The lower—chamber pressure wavefo rms indicate that the shuttle is not closing fast eno ugh relative to the bubble period , resulting in reduced source levels. Evidence indicates that It is more important to close the shuttle rapidly than to attempt to utilize the remainin g air (typically 10 to 20%). 3. The characteristics of air discharge for the various confi gurations tested did not show port or throat size to be restrictive. About 80 to 90% of the initial charge pressure was discharged in less than 10 msec independent of configuration. 4. These air guns have the potential of hi gher radiation efficiency, which could be achieved by: Increasing the shuttle speed to prevent the interference of the bubble collapse with the outgoing air discharge , and Eliminating water transport by the upgoirsg shuttle. I I- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~ ..-- . -.• •.-•— - - ~~~~~~ - ~~~~~~~~~ --- • - Tech Memo No. 457 Bolt Beranek and Newman Inc . References 1. J.E. Barger and P.W. Smith , Jr., “Descr iption of a Long—Range - Impulsive Source Surveillance System (U),” BBN Tech Memo No. - 358, 30 June 1977 (CONFIDENTIAL) . . 2. J.E. Barger and C.M. Gogos , “Results of an Air—Gun Test at Key - Wes t, December 1977 ,” BBN Tech Memo NO. 423, January 1978. 3. J.E. Barger , “Test Plan: Air Gun Tests (April) , “ BBN Tech Memo No. 442, April 1978. - - .: ~ . 1 . • i i i ii 214 •. T _ _ _ _ _ - - . •. - - Tech Memo No. 457 Bolt Beranek and Newman Inc . APPENDIX A. CALCULATION OF RADIATION EFFIC IENCY The radiation (acoustical) efficiencyof anairgun isdefined to be the fraction of available energy that is radiated in the fundamental frequency band . We take the available energy to be the work done on the ocean by the adiabatic expansion of the gas to the max imum bubble radius . The analysis assumes the initial gas to be in a spherical conta iner hav ing volume V1, ra di us r 1, and pressure P1. The maximum bubble volume is V2, where the radius is r 2, and the pressure is P2. The work W done on the ocean is P0V2, where the ambient pressure is P0. The adiabatic work of expansion of an ideal gas is equal to (P1V1—P 2V2) (y_1) 1 , where is the ratio of specific heats. Since all of the work done on the ocean is done by the gas , these two quantities of work are equal. Together with the relationship for adiabatic expansion of an ideal gas , P2/P1=(V2/V 1)~~’, the equality of the two work quantities give 1 = (y—1) (P0/P1 )x+x~~~ ’ ~’~ , (A.1) where the volume ratio V2/V1 is equal to x . Equation A.1 has been solved for spherical air bubbles initially pressurized to 4000 psi and hav ing initial volumes of both 300 cu in. and 2000 cu in. The maximum bubble radius r 2 in cm and ava ilable work W in MJ are plotted on Fig. A .1 for bubbles at initial depths of from 100 to 1600 ft. The quantities are plotted as functions of the bubble fundamental frequency in Hz. The bubble period T is estimated as twice the collapse time of a hollow spherical void , as or iginally calculated by Rayle igh . A-i -• .—____________ — - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ -.- — - - Tech Memo No. 457 Bolt Berane k and Newman Inc . ~~~~~~~~ (J) sniav~ 3189fl8 8 o o o o — 0 (0 ‘ ~~~ N 0 : I I i1~~I = ~I~~ I 4~-’(P)H.Ld3Q NOISNVdX3 ‘1 CS — ~~. ~. — o _ _ 1 /1 _ _ I I 0 I 0~~~ — _ — 4 ~ — — I—. LU — — — 10 — — ~~~ 0 - — = I— N La. - . ~~~ — I i . 0 -I . Lu .E -I o - U, o 0 o N _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ O~~ ° •1 _ I- .~~~ ‘I.C .— — s-a o o 4 I I 0 Q 0 N H f ry4 ’(M)A 9~3N3 3LLYIIfl3Nd ii — — A— 2 - H ___ -----—— - - • - - — - —~—----——— ~ ——-—--- - - Tech Memo No. 457 Bolt Beranek and Newman Inc. T = 1.83 r 2 (p/P 0)112. The acoustical energy radiated in the fundamental frequency band is calculated from the pressure waveform p (t) , measured at a radius and through a bandpass filter centered on the fundamental frequency. E (4 r 02/p c) f p2(t)dt. The acous ti cal eff ic iency,~~, of the air gun is the fraction of available energy that is radiated : I E/W, where the adiabatic work of expansion for an ideal gas is W = ________ y—l • • ~ 0~ - - - - ~ - Tech Memo No. 457 Bolt Beranek and Newman Inc. APPENDIX B. DEPENDENCE OF AIR DISCHARGE ON A IR GUN PARAMETERS The equation for the fractional pressure remaining in an air gun t seconds after firing may be written: (At 3+lY 7’’4 0<t< v p( t ) = (3~~ ARt 2—ARt3+3(1—R)BAt—2AB 312 (1—R/2)+l] 7~~ 4 B<t<t P1 C where ~‘ s ~ and A = 1 L W K p1 = initial pressure S 1 V1 = initial volume = effective port width = effective port leng th 2PLMS Ms = mass of shuttle B = P S 5L = effective area of lower 1 L shuttle face SI~ = effective area of upper shuttle face = U K = cons tant SL t~ = port closure time = B (1+1//if) The depen dence of these equa tions on A , B, and R an d , hence , upon the 7 air—gun parameters , has been plotted in Figs. B.1, 8.2, and B .3. Figure B.]. shows the dependence on A alone. Figure B.2 shows the dependence on B , and Fig. B.3 shows the dependence on R. In addition to the fractional pressures , the fraction of available mass that is actually emitted is also shown on the same plots. Figure B.1 shows that variations in A serve to vary the rate of air discharge. High values of A discharge more rapidly than lower values and , conse quently, the fractional pressure falls more rap idly and reaches a lowe r level than with a lowe r value of A. Thus , A may be identified as the discharge—rate parameter. Note that the port closure times are unaffected by variations in A. B-i _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ... — ~~n _ _ --~~--~~-~~~~~~~~~ • • -- Tec h Memo No. 457 Bolt Beranek and Newma n Inc. SSVI*I ~3flM3 .~O NOI13V~~ 0 1 1 1 1 1 1 l i i i I I I - (I) 9 C) - LU ~~~~~~~~ ,J J C S III _ _ _ _ I I - _ _ _ - 0 - ~~~~~~~~~~~~~ Wz -! ! ‘I’ 1- - I l l l i i i I l i i i . i N 0 1 N 0 [~d/ 4)d] NA9 8W Nt ONINIVV438 38flSS38d 1VNOI13V8~ T ] L _ _ • B—2 — -- - _ -~ - - - --. — --fl— - • . — — ~~~~~ - -• . - - ~~~~ r—- - -~~~~- .~~~~~~~~ -• - -..- - H Tech Memo No. 457 Bolt Berane k and Newma n Inc. [‘vd/ 4w~] SSVI1 03JJ.M3 .~O NOI L3V8.J Z N 0 i l i t I~~~I l i l t l i lf l — I — - - u j I.- — 1~~ I 10 — ‘I ‘CJ N — — - _ _ _ _ _ _ - - _ I 2 [‘dn4 d] Nt19 8W NI ONINIVV(38 38flSS38d 1VNOII3VèI I B— 3 1~ ~ _ _ _ _ _ ________ _ _ _ • S. - —— —— - — .- ~~~ —• - .— ~~~~~~—. - ~~~~~~ - .— Tech Memo No. 457 Bolt Beranek and Newman Inc. [‘w/(lwd] SSVI~I 03U.IVda ~O NOLL3V8.J N 0 ID 1 1 1 1 1 1 1 1 1 l ii i :~~~1 IPI) I I — I I — - _ _ _ _ _ ~iN Ni/ - ~~~ o~~1 ~L _ I I > t ~J~~~~~~~~~~~~~ Q? I ~~~~~~~i i l l -j — i w ~~~~~ii i — L) I I~~ I- C) -j Cl) Mi 17 — C) - w 0 [‘d/(s dJ NflO 8W NI ONINIVP438 38flSS38d 1VNOI13Y8~ - - Ii B- 14 _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Tech Memo No. 457 Bolt Beranek and Newman Inc. B is essentially a measure of the duration of the air discharge. /~ is the time un ti l the shu ttle reaches it s max imum he igh t and ~~~~~ is the time it takes the shuttle to return. As illustrated in Fig. B.2, large values of B imply a long discharge time; consequen tly, the fractional pressure continues to fall. Note that beyond a certain point further increases in B have little effec t on the frac tional pr essure , since the curve is very fla t for large t; i.e., all the air has already been exhausted and keeping the ports open longer is of no value. R, the ra tio of effec tive shu ttle fac e areas , reflects the ratio between the upward and downward travel times of the shuttle. From Fig. B.3, we see that the main effect of R is on the duration of port opening . Unlike B, however , R has relatively little effect on the fractional pressure or the fractional emitted mass. Since R will not be too different from unity, its effec t on pr essure w ill be slight. Thus 1, we have loosely identified A wi th the rate of air discharge , B with the duration (hence , the amount) of air di scharge , and R with the duration of port opening (but not greatly affecting air discharge) . Note that one gun parameter may affect one or more of the three parameters . In particular , the group ing P1SL/Ms appears directly in A and inversely in B. Thus , for exampl e, decreasing M5, the shu ttle mass , will increase A and decrease B ; thus , the gun w ill d ischar ge a ir a t a h igher ra te for a shorter time. The net effect may be positive or negative , depend ing on the amo unt of change. Also note that while in some cases the frac tion of emi tted mass may decr ease w ith a parame ter chan ge (increasin g V 1), the total amoun t of mass em itted w ill Increase because of the greater initial mass (proportional to P1V1). I l~ - - - • - - B-S -