Acoustic Characteristics of Underwater Explosion about Metal Grid of Detonating Cords

JIA Hu,WANG Hai-bo,SHEN Zhao-wu

(1.School of Civil Engineering and Architecture,Nanyang Normal University,Nanyang 473061,China;2.School of Civil Engineering and Architecture,Anhui University of Science and Technology,Huainan 232001,China; 3.Department of Modern Mechanics,University of Science and Technology of China,Hefei 230027,China)

Acoustic Characteristics of Underwater Explosion about Metal Grid of Detonating Cords

JIA Hu1,WANG Hai-bo2,SHEN Zhao-wu3

(1.School of Civil Engineering and Architecture,Nanyang Normal University,Nanyang 473061,China;2.School of Civil Engineering and Architecture,Anhui University of Science and Technology,Huainan 232001,China; 3.Department of Modern Mechanics,University of Science and Technology of China,Hefei 230027,China)

To obtain a continuous sound source of underwater explosion,metal grid of detonating cords was designed.The pressure tests and the bubble pulsation experiments were conducted to investigate sound pressure level properties,sound duration and reverberation effect of underwater explosion about metal grid of detonating cords.The spectrum characteristics of underwater explosion signal were analyzed with Hilbert-Huang transform.The results showed that underwater explosion about metal grid of detonating cords could produce more than one pulse wave signal continuously,and form stable continuous wave.Subsequently,the produced bubble pulsation and reverberation effect could significantly improve the duration of underwater explosion.Detonating grid structure,had a strong sound power,and the highest sound pressure level of underwater explosion could reach 249 dB.Even after blasting 60 ms,the sound pressure level of the detonating cords was still above the 200 dB.Frequency range of the underwater explosion sound about metal grid of detonating cords was wide and became the highest when the energy of the low frequency band was below 100 kHz.The frequency components of the pulse signal were rich,and had the characteristics of low frequency and instant,and most of them were below 100 kHz,especially 50 kHz.The pulse shock signal had multiple energy peaks,and the energy value was more concentrated.The fluctuation energy was basically concentrated on the frequency range of 100 kHz,especially,the energy of the low frequency of below 50 kHz was the largest.Moreover,for the frequency above 100 kHz,their energy components were very small. The number and duration of the pulse shock wave could be controlled by the arrangement and the length of metal grid of detonating cords.The time interval between the pulse shock wave was adjustable,and the device was stable and easy to control.

explosion mechanics;underwater explosion;acoustic characteristics; sound pressure level

0 Introduction

Underwater explosion of energetic material can produce no directivity pulse,which has thecharacteristics of strong sound power,wide frequency,high coersion rate and long distance. Energetic material can be used as the explosion source and has applications in scientific exploration,acoustic navigation and acoustic interference,etc.In the United States,energetic materials,such as tetranitro-aniline,were exploded underwater to measure sound pressure level and other characteristics.Weston[1]had studied on the acoustic characteristics of underwater explosion about different kinds of explosive charge.Pan[2]found that explosive exploded underwater had the characteristics of high power and wide frequency.Wu[3]had studied the acoustic characteristics of three explosives underwater,including TNT,RS211 and RS3-4.

At present,the energetic materials,used as the acoustic sources of underwater explosion, are generally characterized by low controllability,poor repeatability,and short duration.Metal detonating cords has the characteristics of the strong lateral restraint,high density charge core, uniform charge,low line charge density,and stable detonation velocity.

Jia[4-5]found that metal detonating cords,exploding underwater,could generate the pulse shock wave of high sound pressure level,strong sound power,long duration and significant reverberation effect.In order to make metal detonating cords play a role in the underwater acoustic countermeasure,metal grid of detonating cords was designed to product consistent and multiple pulse wave signal,form the approximately stationary continuous wave,and improve duration of underwater explosion significantly.

1 Experiments of metal grid of detonating cords

The underwater explosion experiments,as shown in Fig.1,were carried out in the experimental tower of high 5 m,diameter 5 m,and the water depth was 4.4 m.Test system was composed by long tourmaline underwater shock pressure sensor(ICP,the voltage sensitivity of the pressure sensor is 23.43 mV/MPa,power supply of constant current(482A22)and Tektronix oscilloscope(DP07054),as shown in Fig.2.

Fig.1 Experimental tower of underwater explosion

Fig.2 Metal detonating cord

The experimental sample was cord metal detonation cord(MDC),as shown in Fig.2,the diameter is 1.5 mm,the charge of cyclotrimethylene trinitramine(RDX)was 1.5 g/m.The MDC mesh was arranged in a square steel ring(length of the side was 1 m)and wound in the upperand lower sides,the horizontal distance between the grid was 5 cm,as shown in Fig.4.The MDC mesh was detonated through one end of a detonator.At the same time,in order to prevent the influence of detonator detonation on the flow field in water, the detonator was placed in a specially processed antiknock device which could effectively weaken the effect.

During the underwater explosion tests,the grid center of metal detonating cords and the sensors were placed below water surface 2.9 m.The line between the pressure sensor and the MDC mesh center was perpendicular to the square steel ring. The distance between the sensors and grid center of metal detonating cords is 1.0 m,the signal sampling frequency is 10 MHz, and the curve of pressure test about underwater explosion is shown in Fig.5.

From Fig.5,it could be seen that metal grid of detonating cords exploded the explosion along the winding direction,and the test system received 19 explosive shock waves of metal detonating cords.The number equals to that of metal detonating cord grid structure.Each metal detonating cord generates a shock wave,it can be concluded that metal grid of detonating cords is a shock wave generat by device of underwater explosion.

1-metal grid of detonating cord, 2-ICP sensor,3-water vessel of underwater explosion,4-initiator,5-power supply of constant current,6-oscilloscope,7-computerFig.3 Experimental device of underwater explosion

Fig.4 Metal grid of detonating cords

Fig.5 Pressure-time curve of underwater explosion(r=1.0 m)

It can be seen from Fig.5 that each value of the peak pressure varied with the continuous detonation of the MDC,although the length of each vertical NDC was equal.The main reason for this might be divided into two aspects.On the one hand,it was mainly related to the signal propagation distance in the water.On the other hand,when the MDC exploded under water,the shock wave produced by the explosion was separated from the detonation product; subsequently,high temperature and high pressure detonation gas could promote the surrounding water body outward movement and so as to form the bubble pulsation;while the MDC grid was arranged in the horizontal distance of approximately 5 cm which was very close to the bub-ble radius,as a result,the burst of the bubble formed by the explosion of the front MDC just reached the position of the subsequent MDC;thus,the free surface of cavitation was formed, which leading to the decrease of the explosion pressure of adjacent MDC.

2 The sound pressure level of underwater explosion

When metal detonating cords explode underwater,the relation between effective sound pressure and acoustic pressure amplitude can be expressed by:

where pm=pressure amplitude(MPa),pr,expressed as pr=1 μPa,is reference sound pressure, and the sound pressure level of underwater explosion(SPL,Unit:dB)can be expressed by:

According to the Eq.(2),the SPL of metal grid of detonating cords is shown in Fig.6.

Fig.6 gives the sound pressure level time domain diagram of the MDC mesh underwater explosion.It can be seen that the MDC underwater explosion sound pressure could reach more than 200 dB,up to 249 dB,and the sound pressure level of the MDC was still around 200 dB after 60ms.This was shown that the underwater explosion of MDC mesh had strong acoustic power and long duration,and could fully achieve the effect of underwater interference.

Fig.6 Sound pressure level of underwater pulse wave

3 Duration and reverberation effect of underwater explosion

Fig.5 and Fig.6 show that metal grid of detonating cords not only produce continuous shock wave,but also produce bubble pulsation and reverberation effect.The peak value of bubble pulsation is small,but the pulse width is larger than that of the shock wave.In order to further explain the phenomenon,a high speed camera(2 000 PS-1)was used to capture the bubble pulsation when two metal detonating cords were blasting,as shown in Fig.7.

From Fig.7,it can be seen that the two shock waves encounter after the two detonating cords blast,and the radius of two cylindrical bubbles,generated by two blasting detonating cords,is increasing,and it results in the compression of the surrounding water medium and the rapid increasing density.Under the action of explosion shock wave and bubble expansion,small bubbles can be clearly observed around the two detonating cords,and such small bubbles arepersistent during the whole expansion process of the first bubble pulsation.In Fig.7,the first bubble pulsation period is 27.5 ms,and the initial expansion is cylindrical.With the increase of radius,the outer contour is no longer the standard cylindrical.Two adjacent bubbles begin to fuse in 62 ms,and form a bubble curtain in 79.5 ms.Such curtain can be seen clearly in 323 ms,and a lot of small bubbles are still in the area in 1 130 ms.Such bubbles can enhance the reverberation effect of underwater explosion significantly.Bubble pulse generated by metal grid of detonating cords blasting can last a long time,and the subsequent generating small bubbles can also exist longer.Bubbles can produce a strong reflection and scattering effect, which significantly enhance underwater explosion reverberation effect and improve underwater explosion sound duration.

Fig.7 Pictures of bubble pulses of two metal detonating cords

4 Hilbert-Huang transform analysis of metal grid of detonating cords

In order to further investigate the composition and characteristics of the underwater explo-sive pulse signal,the data were analyzed by Hilbert-Huang transform(HHT).About the shock wave and the bubble pulsation,generated by the underwater explosion of metal grid of detonating cords,the frequency components are complex,the distribution frequency band is wide. The sampling frequency is higher,and thus the signal can be accurately captured and meet the demands of the signal analysis[6].To analyze easily,the pulse signal from-0.000 167 7 s to 0.003 001 9 s was analyzed by HHT,as shown in Fig.8.

Fig.8 Pressure time history curve of metal grid of detonating cords about underwater explosion(r=1.0 m)

4.1 EMD decomposition of pulse signal

In order to further study the characteristics of the pulse signal propagation of underwater blasting about metal grid of detonating cords,the pulse signal measured by experiments(Fig.8)was analyzed by EMD decomposition.The reconstructed signal after the EMD decomposition and the relative error distribution between the measured signal and the reconstructed signal are shown in Fig.9.It can be concluded that the EMD decomposition of the original signal can achieve a good effect of information reconstruction.

IMF components and the marginal spectrum,as shown in Fig.10,were obtained by EMD decomposition.

The EMD signal decomposition is shown in Fig.10(left),and they are arranged in the order from high frequency to low frequency[7-8].High frequency component of underwater explosive pulse signal of metal grid of detonating cords(c1-c5),has the characteristics of high frequency and fast decay,and each IMF component has multiple peaks,which are consistent with multiple impact peaks generated by the underwater explosion of metal grid of detonating cords. From c1 to c5,it is dominant frequency band of the original signal,and contain most of the energy of the signal.Intermediate frequency component of signal(c6-c10)shows the characteristics of repeated fluctuations.Low frequency components of signal(c11-c13)show the characteristics of less frequency components and slow decay.Margin of signal decomposition(c14) characterizes signal itself weak trend or falling instrument.

Fig.9 Reconstructed signal after EMD decomposition and relative error distribution

Fig.10 EMD decomposition and the marginal spectrum of underwater blasting signal about metal grid of detonating cords

4.2 Hilbert transform of pulse signal

The marginal spectrum of the Hilbert transform is shown on the right side of Fig.10. The frequency components of the signal show the law from high to low,and the high frequency bandwidth is large while the low frequency bandwidth is narrow.The frequency of the IMF components and the numerical value of the main instantaneous frequency can be clearly seen in the marginal spectrum.The frequency components of the signal are rich,and most of the signal is below 100 kHz,especially below 50 kHz.The energy of the band above 100 kHz is very small,and it is consistent with the spectrum of the original signal shown in Fig.11.It shows that the Hilbert can meet the signal analysis requirements of the underwater blasting of the metal grid of detonating cords.The frequency is mainly located in the low frequency range.The metal grid of detonating cords can be considered as a series of discrete point sources of continuous explosion.The long time of detonation and the mutual interference of the pulse wave cause the wide frequency distribution of the continuous pulse signal.

Hilbert energy spectrum(Fig.12),of the underwater blasting signal of the metal grid of detonating cords,was characterized by the accumulation of the energy of each frequency in the whole time.From the figure,it can be clearly seen that the signal energy changes with frequency.The signal frequency components,when the metal grid of detonating cords underwater blasting,are more abundant,and the energy is mainly concentrated in the frequency band of 0-50 kHz.Hilbert energy spectrum,when the metal grid of detonating cords blast underwater, reflects the frequency domain distribution of the signal energy,and it can be seen that the underwater explosion signal has low frequency and instantaneous characteristics.

Fig.11 Signal spectrum of the metal grid of detonating cords about underwater explosion

Fig.12 Hilbert energy spectrum of the underwater blasting signal of the metal grid of detonating cords

5 Conclusions

The following conclusions are drawn by the study on the underwater acoustic characteristics when the metal grid of detonating cords blast:

(1)Underwater explosion of metal grid of detonating cords,based on the design,can produce more than one pulse wave signal continuously,and form stable continuous wave.Subsequently,the produced bubble pulsation and reverberation effect can significantly improve the duration of underwater explosion.

(2)Detonating grid structure,at the measurement place,has a strong sound power,and the highest sound pressure level of underwater explosion can reach 249 dB.Even after blasting 60 ms,the sound pressure level of the detonating cords was still above the 200 dB.

(3)For metal grid of detonating cords,frequency range of the underwater explosion sound is wide.In the range of 100 kHz,the sound power spectrum is above 200 dB,and the energy of the low frequency band of below 100 kHz is the highest.

(4)Based on the HHT analysis,the frequency components of the pulse signal are rich, and have the characteristics of low frequency and instantaneous,and most of them are below 100 kHz,especially 50 kHz.The measured signal has multiple energy peaks,and the energy value is more concentrated.The fluctuation energy is basically concentrated in the frequency range of 100 kHz,especially,the energy of the low frequency of below 50 kHz is the largest, and for the frequency above 100 kHz,their energy components are very small.

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金屬導爆索網柵結構水下爆炸聲學特性研究

賈虎1，汪海波2，沈兆武3
（1.南陽師范學院土木建筑工程學院，河南南陽473061；2.安徽理工大學土木建筑工程學院，安徽淮南232001；3.中國科學技術大學近代力學系，合肥230027）

為了獲得連續水下爆炸聲源，文章設計了金屬導爆索網柵結構，并開展了水下爆炸壓力測試和氣泡脈動實驗，研究了金屬導爆索網柵結構水下爆炸的聲壓級特性，水下爆炸的聲持續時間，對應的混響效應，利用Hilbert-Huang變換對水下爆炸信號的頻譜特性進行了分析。研究結果表明：金屬導爆索網柵結構水下爆炸可根據設計連續產生多個脈沖沖擊波信號，形成近似平穩的連續波，隨后產生的氣泡脈動和混響效應能夠明顯提高水下爆炸聲的持續時間；金屬導爆索網柵結構具有很強的聲功率，水下爆炸聲壓級最高能夠達到249 dB，爆炸60 ms后金屬導爆索的爆炸聲壓級仍在200 dB以上；金屬導爆索網柵結構水下爆炸聲頻率范圍廣，在100 kHz以下低頻段能量最高。金屬導爆索網柵結構水下爆炸脈沖信號頻率成分豐富，具有低頻和瞬時的特點，且大部分集中在100 kHz以下，尤以50 kHz以下的最為明顯；脈沖信號具有多個能量峰值，能量值較集中，波動能量基本上都集中在頻率為100 kHz范圍以內，尤以50 kHz以下的低頻能量最大，高于100 kHz以上頻段的能量的分量很小。脈沖沖擊波的個數和聲持續時間可由金屬導爆索網柵的排列方式和長度控制，脈沖沖擊波間的時間間隔可調，發生裝置穩定易控。

爆炸力學；水下爆炸；水聲特性；金屬導爆索網柵；聲壓級

TH212TH213.3

:A

賈虎（1980-），男，南陽師范學院副教授；

TH212 TH213.3

:A

10.3969/j.issn.1007-7294.2017.06.011

1007-7294（2017）06-0769-10

汪海波（1983-），男，安徽理工大學副教授；

date：2016-12-07

Supported by the National Natural Science Foundation of China(Grant No.11202109; Grant No.11304168);He’nan Educational Committee(Grant No.2011B130002); High Level Talents Project of Nanyang Normal University

Biography：JIA Hu(1980-),male,associate professor,E-mail:jiahu@mail.ustc.edu.cn;WANG Hai-bo(1983-), male,associate professor,E-mail:wanghb_aust@163.com.

沈兆武（1953-），男，中國科學技術大學教授，博士生導師。