Numerical And Experimental Study On The Acoustic Characteristics Of Underwater Gas Jet And Bubble Flow

Underwater exhaust,which is defined as the process of gas exhausting into the water through an underwater nozzle,orifice or slit.The process is divided into two kinds of flow patterns,such as jet flow and bubble flow according to the different of the gas flow velocity.Whatever,the complex gas-liquid two phase flow is usually accompanied by a distinct noise/acoustic signals radiations.Recently,the investigation of underwater exhaust noise/acoustic characteristics has become a hot spot of research interest in many fields.For example,in the field of underwater equipment,the exhaust gas generated by the thermal propulsion plant is discharged into the surrounding ocean environment.Noise generated by this process will not only cause the discomfort of the staff and polluting the ocean environment,but also interfere to the function of vehicle-mounted sonar and communication systems.Even make the equipment more exposed to the non-friendly sonars detection,resulting in life-threatening.For the process industry field,the phenomenon of underwater bubble flow is common in the typical process equipment such as bubbling reactors,aerations.In terms of obtaining flow field information,passive acoustic emission measurement has shown huge advantages and potential.The acoustic characterization of two phase flow pattern has gradually become one of the hottest topics for scientific researchers and application engineers.Therefore,it has important guiding significance and wide application value to study the acoustic characteristics and the mechanism of underwater exhaust process,whether from the perspective of noise suppression or acoustic applications.In this paper,underwater exhaust process is investigated by the means of numerical simulations,and accompanied with theoretical analysis and mechanism experimental studies.The sound mechanism of two kinds of flow pattern is comprehensively investigated through detailed synchronized analysis of the evolution of complex two phase flow field of underwater exhaust and the time-domain characteristics of radiation acoustic signals.The influence of typical factors on flow behavior and sound radiation characteristics is also carried out.The main content of this paper is as follow:First of all,a numerical prediction model of underwater exhaust noise/acoustic is established based on the Lighthill's acoustic analogy theory.The numerical simulation can be used for synchronized acquisition of the flow behavior and the radiation sound information of underwater exhaust.And this paper presents signal processing methods such as filtering and short time Fourier spectrum analysis,for separating and extracting the numerical prediction sound characteristics.A three-dimensional numerical study of supersonic gas jets submerged in still water is presented.The influence of stagnation pressure of gas inflow and ambient pressure outside the nozzle are considered for both over-and under-expanded jets.A multiphase-compressible-flow formulation and the large eddy simulation methodology are resolved to examine the flow structure in this interaction and characterize the observed flow patterns.The Lighthill's acoustic analogy theory is implemented for predicting the far-field acoustic radiation characteristics.Numerical results on far-field sound pressure level are validated by available experimental results,and the difference is below 3.6dB.Result shows that the formation of shock waves and its destruction due to the jet swing elevate pressure and sound pressure fluctuations in the flow field.The detachment of bubbles from an underwater nozzle inevitably generates distinct acoustic signals.Numerical simulation models are applied to predict the process of bubble detachment from an underwater nozzle.The volume of fluid(VOF)model in conjunction with continuum surface force(CSF)model is applied to numerically investigate the single bubble formation process in the bubble columns.A combination of large eddy simulation(LES)model and Ffowcs Williams-Hawkings(FW-H)equation is successfully applied to predict bubble acoustic pressures.The adaptive filtering techniques of signal processing are utilized in bubble acoustic analyses.In particular,transient spectrum based on time-frequency analyses can not only precisely count the bubbling period but also effectively measure the bubble sizes.The numerical results for bubble size are in good agreement with the theoretical data(less than 2.5%deviation).The effects of the location of FW-H integral surface and mesh size on simulated results are discussed in this paper.Numerical simulation models are proposed to investigate the effect of different nozzle wall configurations on bubble formation and acoustic characteristics.The main results of this research show that the structure of the nozzle wall plays a guiding role in vortex motion around bubbles.The bubble size also increases with the increase in exit-lip thickness,whereas the bubbling rate decreases.Experimental system and the test schema which are suitable for the study of bubble flow behaviors and acoustic characteristics of underwater exhaust are also setup in this paper.This system provides a reliable schema of the signal-video synchronized acquisition with high precision.By combining the study results of numerical simulation,volumetric bubble oscillation is found to be excited by an axial water jet up into the bubble,immediately following the bubble detachment.To sum up,the results of this study lead to the proposition that numerical simulation method could be applied to well understanding of the physics of underwater exhaust sound.The sound generation mechanism of two typical underwater exhausting process can provide theoretical and technical support for the control of noise and the active use of acoustic signals.