Resonant Characteristics Of The Shell Structure And Feature Extraction

Due to the large-scale shells navigating underwater, a variety of radiation noise would be produced, which can be segregate into the following three categories:structural noise, propeller noise and current flow noise. Not only the mobility but also the performance of concealment will be limited by the three types of noise, among which the structural noise is the most impactive. In order to improve the concealment property of marine large-scale shells, it is of great necessity to investigate the sources of noise line spectrum.In this thesis, the excited resonant vibration line spectra were mainly researched. The resonant signal generated by shell vibration was extracted and converted into time slices. Then the property of phase difference changing with time at different frequency resolution condition was analyzed, which characters the shell vibration and sound radiation characteristics and is named the time shift feature. That serves as a significant approach to distinguish the sources of resonance vibration line spectra. The main contents of this thesis are as follow:Firstly, numerical analysis was done for both the spherical shell and the cylindrical shell with a hemispherical cap at both sides. Then the Vibration modes and the resonant responses were analyzed with ANSYS through which the resonant frequencies were confirmed.Secondly, the complex modulation spectrum analysis (Zoom FFT) and the all phase FFT (apFFT) spectral analysis technique are analyzed in this thesis. Zoom FFT is an effective method of zooming spectra for narrow-band signals. As for the all phase FFT, it shows great advantage in suppressing the leakage of spectrum. Further more, accurate phase estimation can be achieved without any frequency correction, which benefits from the behavior of phase invariance. In addition, for the multi-frequency signals ruined by noise, high-precision estimations of frequency, phase and amplitude will also be obtained through the apFFT algorithm.Then the feasibility of both algorithms was demonstrated through the computer simulation.Finally, the underwater experiments were carried out. The cylindrical shell with a hemispherical cap at both sides was excited by both the single frequency and the broadband signal sources. Then the resonant frequency spectra were zoomed with the Zoom FFT transformation. Meanwhile, the original resonant signals were analyzed with the all phase FFT algorithm to obtain the phase information. Consequently, the feature that the phase difference changes with time at different frequency resolution condition could be extracted. Hence, the above signal processing provides a new promising method to identify the resonant line spectra of shell structures.