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Flow Mechanism And Hydrodynamic Noise Of3D Rigid Cavity

Posted on:2013-04-19Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y WangFull Text:PDF
GTID:1262330392469746Subject:Mechanical engineering
Abstract/Summary:PDF Full Text Request
The flow past3D rigid cavity is a common structure on the surface of theunderwater vehicle. The hydrodynamic noise generated by the structure has attractedconsiderable attention in recent years. From the perspective of ocean exploration, thehydrodynamic noise of the cavity not only affects the accuracy of the acousticequipment on the underwater vehicle, but also increases the noise level and reducesthe safety of the underwater vehicle. From the perspective of the marine environment,hydrodynamic noise of the cavity causes ocean pollution and brings harm to themarine life.However, water is usually a highly incompressible fluid medium. It is verydifficult to predict cavity hydrodynamic noise in the water because of the small Machnumber, the low sound conversion rate, and the strong coupling between sound andstructure. Therefore, the research of cavity hydrodynamic noise is very significant. Itprovides theoretical guidance for design of the underwater vehicle.In this thesis, the hydrodynamic noise induced by the cavity is investigated byLES-Lighthill equivalent sources method. The feasibility and reliability of this hybridmethod is verified by the tunnel test. Based on this method, the mechanism of cavityunsteady flow and the characteristics of hydrodynamic noise are described in detail.The characteristics of hydrodynamic noise are analyzed for cavities with differentstructural parameters. The characteristics of hydrodynamic noise with uncertainenvironmental parameters are also investigated. The results provide not only atheoretical basis for the prediction of cavity hydrodynamic noise, but also a theoreticalreference for the structural design of the underwater vehicle with low noise. The maincontributions of the thesis are summarized as follows:1. LES-Lighthill equivalent sources method is extended to analyze thehydrodynamic noise of3D rigid cavity according to computational aeroacousticstheories and methods. Feasibility and reliability of LES-Lighthill equivalent sourcesmethod for the flow and sound fields computation and the hydrodynamic noiseprediction of3D rigid cavity are verified by the tunnel test.2. Based on LES-Lighthill equivalent sources method, the unsteady flowmechanism of3D square cavity is analyzed in terms of the vortex structure and distribution, the correlation between fluctuating pressure and time, and the correlationbetween fluctuating pressure and frequency. The hydrodynamic noise and the radiatedmechanism of3D cavity are investigated from the sound source intensity, thenear-field sound pressure intensity, the far-field radiated noise, and the propagationdirectivity.3. Investigation is performed on the relationship between the hydrodynamicnoise and the flow time. It is found that the hydrodynamic noise of the cavity is anormal distribution of the flow time. Analysis on the flow structure and the soundfield characteristics of3D cavity illustrates the relationship between thehydrodynamic noise and the vortex movement. The relationship between sound andvortex of3D rigid cavity is also established.4. Based on the mechanism of the cavity hydrodynamic noise and therelationship between vortex and sound, the hydrodynamic noise is discussed forcavities with different sizes and shapes. The result provides theoretical support fordesign of the underwater vehicle with low drag and low noise.5. The influence of the environmental parameters, including water velocity,sound velocity, and density, on the hydrodynamic noise is analyzed. The theoreticalmodel of the hydrodynamic noise is established as a function of a singleenvironmental parameter by the least square method. Considering the coupling andnonlinear characteristics of the hydrodynamic noise with various environmentalparameters, a BP neural network model is built for the hydrodynamic noise powerspectrum density. These models provide a feasible and reliable method for theprediction of cavity hydrodynamic noise in the design of underwater vehicles.
Keywords/Search Tags:3D cavity structure, LES-Lighthill equivalent sources method, Flowmechanism, Hydrodynamic noise, Relationship between vortex and sound
PDF Full Text Request
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