Research On Mechanism Of Underwater Acoustic Radiation Generated By Rainfall

Rainfall is a common natural phenomenon,an indispensable link in the global water cycle,and an indispensable factor to maintain the balance of the global ecosystem and life system.The acoustic radiation generated by rainfall is one of the important components of marine environmental noise,which is the interference background of various underwater acoustic equipment and directly affects the performance of underwater acoustic system.At the same time,the underwater noise generated by rainfall contains abundant information,which can reflect some characteristics of rainfall.The study of underwater noise generated by rainfall can deepen the understanding of the physical mechanism of rainfall noise,provide mechanism support for underwater noise suppression and noise cancellation,and provide reference for improving the performance of sonar.By measuring the underwater noise generated by rainfall to invert the rainfall intensity at sea,it can provide a remote sensing means for large-scale and long-distance monitoring of rainfall at sea,and provide a large range of ocean rainfall data for global climate change research.In the study of underwater acoustic radiation characteristics,the difficulty lies in the modeling of rainfall noise source and the quantitative measurement of rainfall noise source intensity.The sound sources of rainfall noise are spatially distributed,and the characteristics of sound sources of raindrops of different sizes are different,so the distribution characteristics of rainfall noise sources cannot be accurately described.The noise field generated by rainfall is a strong coupling between the characteristics of sound source and environment.The actual measured rainfall noise contains the influence of many environmental factors.The rainfall noise generated by the same intensity of rainfall in different environments is significantly different,so the intensity of rainfall noise source cannot be directly obtained from the measurement results of sound field.This paper mainly studies the mechanism of underwater sound radiation generated by rainfall.Firstly,the model of rainfall noise source is established,and then the sound source characteristics of single raindrop,underwater sound radiation generated by artificial rainfall and sound radiation generated by actual rainfall are studied in depth.The specific research contents are as follows:1.Underwater noise source model and sound field analysis generated by rainfallIn view of the present use of the definition of sound intensity rainfall intensity of noise sources with the environmental change and the positioning precision,this paper has redefined the rainfall intensity of noise source,rainfall intensity of noise source of accurate description is given,and the method of reverberation tank measurement is first applied to rainfall measurement noise.The laboratory measurement of rainfall noise source intensity is realized.Is obtained by the weighted average method for the average volume of single raindrop radiation spectrum curve of the large uncertainty,not smooth,the problem of the missing at high frequency,this paper proposes a noise measured by artificial rainfall,the new method to calculate the energy spectrum of single raindrops,can be smooth,continuous,covering the whole frequency band energy spectrum of single raindrop.2.Underwater acoustic radiation mechanism of single raindrop impacting water surfaceIn view of the problem that the current research on single raindrop mainly focuses on qualitative research,while quantitative research is lacking,this paper quantitatively studies the characteristics of impact sound and bubble sound produced by single raindrop in spectrum,amplitude of sound pressure,energy and other aspects.Aiming at the problem that the acoustic signal radiated by bubbles in water pool is easily disturbed by reflected sound,a method is proposed to calculate the acoustic energy radiated by bubbles generated by single raindrop underwater using direct acoustic signal.By using this method,only the amplitude of the direct sound signal and the pulsation frequency of the bubble can be measured,and the underwater acoustic energy radiated by the bubble can be calculated accurately.At present,there is no quantitative method to calculate the underwater acoustic energy radiated by a single raindrop hitting the smooth water surface at its final velocity.In this paper,a model for calculating the average acoustic energy radiated by a single raindrop with any particle size is proposed.By using this model,the conversion efficiency of raindrop kinetic energy into underwater acoustic energy is multiplied by the kinetic energy of raindrop,and the average acoustic energy radiated by a single raindrop under water can be obtained.In view of the fact that there are few researches on underwater acoustic radiation after raindrops impact on water surface at a lower height,this paper studies the mechanism and characteristics of acoustic signals generated by raindrops hitting water surface at a very small height(several centimeters to more than ten centimeters),which provides an important reference for the research on the formation mechanism of bubbles in rainfall.3.Research on underwater acoustic radiation generated by rainfallThe reverberation pool and artificial rainfall generating device are designed to meet the measurement conditions,and the rainfall noise is measured by using reverberation method,and the radiated sound power of rainfall per unit area under water is obtained,which actually solves the problem that the intensity of rainfall noise source cannot be measured accurately.Generate artificial rain containing only one particle size raindrop.By measuring the acoustic energy spectrum generated by artificial rain and calculating the acoustic energy spectrum of a single raindrop,the smooth,continuous underwater acoustic power spectrum covering the full frequency band of a single raindrop is obtained in the reverberation field for the first time.The acoustic energy of single raindrop radiation calculated by sound power spectrum is consistent with that of single raindrop radiation in free field,which verifies the feasibility of the method.In order to solve the problem that the consistency of rainfall noise measured in different environments is poor and the empirical formula used to invert rainfall intensity is only applicable to specific conditions,a new empirical formula for inverting rainfall intensity based on the intensity of rainfall noise source is proposed in this paper.The inversion results of the new empirical formula are consistent with the actual rainfall data at sea,which verifies the applicability of the empirical formula.Based on the above research,the model of rainfall noise source is established to accurately describe the intensity of rainfall noise source,the measurement method of rainfall noise source intensity is proposed and the actual measurement is carried out,and the mechanism of underwater sound radiation generated by rainfall is preliminarily understood,which lays a foundation for the inversion and monitoring of rainfall intensity.