Naval and aerodynamic designers are concerned about the sound radiated from vibrating complex structures in heavy or light fluid medium excited by broadband random forces, that is, mechanical forces or turbulent boundary layer (TBL) pressure fluctuations. They wish to minimize significant noise radiation both outward to the medium and inward to the structure inside. Hence a method for computing the vibroacoustic response of structures acted by broadband random forces is of critical value to the designer by permitting him to select appropriate structural modifications (e.g. damping treatments) or to apply appropriate mechanical forces to structures to suppress structural vibration and the associated radiation of sound. Accurate methods of computation for the vibroacoustic response of the key components of complex structures such as plates and cylinders excited by random forces will provide a useful foundation extensible to more complex structures, e.g. airplane fuselage-wing systems, missiles, ribbed sonar domes, and ship-hull systems, etc.
This thesis presents theoretical formulations of acoustic radiation from rectangular plates with a few ideal edge conditions in heavy and light fluid media excited by random forces, that is, arbitrary mechanical forces or turbulent boundary layer pressure fluctuations, in the presence of a uniform subsonic flow. This analysis is based on in vacuo modal expansion of both the plate vibration and the pressure fields. The plate vibration and the pressure fields described in modal domain are transformed into wavenumber-frequency domain. Numerical evaluation of the acoustic power is extremely time-consuming for broadband excitations. In order to obtain the approximate solutions, it is assumed that the broadband forcing functions have white spectra and remain unaltered due to the plate vibration and that the modal coupling is weak as long as structural damping is light. Under these assumptions, the approximate solutions for the surface acoustic intensity and the acoustic power are obtained with and without considering the modal coupling effects. The results obtained by using modal expansion method are also compared to the ones by using Rayleigh integral in a stationary fluid medium.
The evaluations of the modal coupling impedance, which describes the interaction of a structural mode with acoustic medium, are based on the use of modal expansion for the vibrational field and the acoustic pressure. Alternative approach is suggested to compute the modal coupling impedance more efficiently in a moving fluid medium, by using averaged Green function in wavenumber domain which can handle the square root singularity. In particular, the modified modal coupling reactance of the heavy fluid-loaded plates at low frequencies for low flow speed is obtained for the evaluation of the vibration velocity of the fluid-loaded plates. The behavior of the modal velocity is studied in terms of flow speed and heavy fluid loading by using several approximate methods. After eliminating the error caused by Gibbs phenomena essentially occuring at low frequencies for finite Mach number, good approximate solutions for the modal velocity of fluid-loaded plates in a moving fluid medium are obtained.
The broadband acoustic response is computed by either neglecting or including the modal coupling effects to see the effect of flow speed and configuration of applied mechanical forces. Those are shown that moving fluid medium has a great effect on the characteristics of acoustic radiation irrespective of edge conditions and there exists an optimal forces' spacing distance which lead to a relative minimum acoustic power radiation from plates excited by broadband random mechanical forces. The related topic of modal coupling on the acoustic power radiation from finite plates driven by random or harmonic mechanical forces is also studied. For a plate with edge conditions which allow a rigid body mode, it is shown that the modal coupling can not be neglected, in particular, at low frequencies because the coupling terms between elastic mode and rigid body mode have some effects on the broadband acoustic power radiation.
The coupling of a structural mode and the TBL pressure fluctuations is studied by using Chase model for the wavenumber-frequency spectrum. By using the approximate solutions, the effects of moving fluid and turbulence on acoustic power radiation are studied. The optimum input forces are designed theoretically to minimize the acoustic power radiation from a plate due to turbulent excitation. Obtained expression for the approximate acoustic power is used for determining the control force spectral density. By considering the type (point or area force) and number of the input forces, the optimum input forces to minimize the acoustic power radiation from a plate due to turbulent excitation are designed theoretically and so the performance of the power reduction is investigated.
구조설계자는 랜덤가진력에 의한 구조물의 진동과 음향방사의 해석 및 저감설계 기술분야에 많은 연구를 수행하였다. 구조물 내부와 외부로 방사되는 소음을 줄이기 위해 구조변경 및 능동제어기술을 적용할 수 있으나, 적절한 해석기법과 물리적 현상의 정확한 이해가 없으면 합리적 구조설계가 힘들게된다.
일반적으로 복잡한 기계구조물의 주요소인 보, 평판 및 원통셸과 같은 단순구조물의 진동과 음향해석은 보다 복잡한 구조물의 해석 및 설계에 직접 응용될 수 있기 때문에 단순구조물의 진동과 음향특성의 해석 및 물리현상의 규명은 매우 중요하다.
본 연구의 주목적은 유동이 있는 공기 혹은 수중에서 랜덤기계가진 및 난류압력에 의해 가진되는 유한평판의 진동과 음향방사, 그리고 모드연성에 대한 해석방법을 개발하는 것으로, 근사해를 구하기 위한 방법제시와 아울러 물리적 현상을 연구하였다. 광대역가진시 유한평판의 음향해석을 위해 모드해석과 파수영역의 해석기법을 이용하였으며, 유동과 모드연성이 음향파워에 미치는 영향을 평판의 경계조건에 따라 심층 분석하였다.
첫째, 평판과 음향매질과의 연성을 나타내는 연성임피던스의 평가는 그린함수(Green function)의 특이점과 유동에 의한 질량부가효과에 의해 큰 오차가 발생하므로 이를 제거할 수 있는 효과적인 방법을 제시하였다. 또한 이 방법을 이용하여 접수구조물의 진동해석에 응용하였다.
둘째, 광대역 기계력가진에 의한 음향파워를 모드연성효과를 고려하여 구하였으며, 유동의 속도와 가진력의 형상이 음향특성에 미치는 영향을 평판의 경계조건에 따라 규명하였다. 또한 조화가진에 의한 모드연성효과의 물리적 현상을 가진모드에 따라 규명하였다.
셋째, 유동에 의해 발생되는 난류압력에 의한 방사음향파워를 구하였으며, 이의 감소를 위한 제어가진력 설계방안을 제시하였다.
결론적으로 제안된 해석방법 및 물리적 현상규명은 공기 중 혹은 수중 구조물의 진동과 음향방사해석 및 이의 저감설계에 응용될 수 있을 것이다.
