Translated Abstract
Lightweight sandwich structures have been increasingly used in a wide range of engineering applications (e.g., automobiles, express trains, ship/submarine hulls and aircraft fuselages), and hence their vibroacoustic characteristics are of paramount importance for interior noise reduction. In the pursuit of vibration and noise reduction in civil and military applications, this dissertation deals with the vibroacoustic problems of lightweight sandwich structures immersed in either static or convected fluid. Specifically, the structural wave and sound wave propagation, the dynamic responses and vibroacoustic performances of these structures are systematically investigated by incorporating theoretical modeling, experimental measurment and numerical simulation. An integrated optimal algorithm toward lightweight, high-stiffness and superior sound insulation capability is subsequently proposed, based on which the optimal design of prototype sandwich structures is performed. The contents and contributions of the dissertation are summarized as follows: On the basis of sound velocity potentials, analytical models are separately developed for the transmission of sound across simply supported and clamped double-panel partitions. The proposed models are capable of describing more accurately the air cavity coupling than the commonly used method of rigid cavity modal function, thus more applicable for vibroacoustic studies. It is found that the overall vibroacoustic behavior of a double-panel partition can be significantly changed by altering the thickness of the enclosed air cavity, without changing other geometrical dimensions of the structure. The peaks and dips in the STL (sound transmission loss) versus frequency curves shift to low frequencies as the air cavity thickness is increased, resulting in enhanced STL values. Within the low frequency range, the STL value of a finite structure is higher than that of the correspodning infinite structure, whereas at high frequencies the infinite structure provides an upper bound estimate of STL for finite configurations. Increasing the panel thickness can effectively enhance the sound insulation capability of double-panel structures. Moreover, whilst the elevation angle of the incident sound affects significantly the sound insulation capability of finite double-panel structures, the influence of incident azimuth angle is negligible. Experimental measurements are performed to validate the applicability and feasibility of the proposed analytical models, with good overall agreement achieved. Obtained results suggest that the natural frequencies of a fully clamped double-panel partition are higher than those of a fully simply supported one. At low frequencies, the STL values show noticeable discrepancies between the two boundary conditions (simply suppored versus clamped), while at high frequencies the discrepancies are dependent upon the incident elevation angle. Whilst the vibration mode shapes of a simply supported panel can be approximated as those of its clamped counterpart only in the case of normal sound incidence, dramatic distinctions exist between them in the case of oblique sound incidence. An analytical approach is formulated to account for the effects of mean flow on sound transmission across a simply supported rectangular aeroelastic panel. The application of the convected wave equation and the displacement continuity condition at the fluid-panel interfaces ensures the exact handling of the complex aeroelastic coupling between panel vibration and fluid disturbances. Unlike previous researches, the model is capable of dealing with the general case that the fluids on both sides of the panel are convecting, which lays theoretical basis for further investigation of the issue. The results show that the influence of the incident side mean flow upon sound penetration is significantly different from that of the transmitted side mean flow. The contour plot of refraction angle versus incident angle for the case when the mean flow is on the transmitted side is just a reverse of that when the mean flow is on the incident side. The aerodynamic damping effects on the transmission of sound are well captured by plotting the STL as a function of frequency for varying Mach numbers. However, as the Mach number is increased, the coincidence dip frequency increases when the flow is on the incident side but remains unchanged when the flow is on the radiating side. In the most general case when the fluids on both sides of the panel are convecting, the refraction angular relations are significantly different from those when the fluid on one side of the panel is moving and that on the other side is at rest. An anlytical study is carried out for the transmission of external jet-noise through a double-leaf skin plate of aircraft cabin fuselage in the presence of external mean flow. An aero-acoustic-elastic theoretical model is developed by applying the structural vibration theorem, the convected wave equation, the Navier-Stokes equation and the displacement continuity condition at the fluid-plate interface. Four distinct acoustic phenomena (i.e., mass-air-mass resonance, standing-wave attenuation, standing-wave resonance, and coincidence resonance) for a flat double-leaf plate as well as the ring frequency resonance for a curved double-leaf plate are identified. Independent of the proposed theoretical model, simple closed-form formulae for the natural frequencies associated with the above phenomena are derived using physical principles. Excellent agreement between the model predictions and closed-form formulae is achieved. It is demonstrated that: (i). In the case of sound incidence along the downstream direction, as the Mach number is increased, the STL values increase over a broad frequency range and the natural frequencies for the associated acoustic phenomena (except for coincidence resonance) are shifted considerably to lower frequencies due to the added-mass effects of the mean flow. The exception of the coincidence resonance is attributed to its strong dependence on the refraction angle but not on the convected fluid loading. (ii). For sound incidence along the upstream direction, the corresponding frequencies increase until the Mach number is increased up to a critical value, except again for the coincidence resonance. Further increase of the Mach number beyond the critical value results in the disappearance of the mass-air-mass resonance, the standing-wave attenuation and the standing-wave resonance, but the coincidence resonance is always existent. Increasing the Mach number induces a noticeable enhancement of STL due to the total reflection effects. (iii). The combined effects of panel curvature and internal pressurization significantly affect the STL value, which are particularly noticeable in the low frequency range adjacent to the ring frequency. The transmission of sound through all-metallic sandwich panels with corrugated cores is investigated using the space-harmonic method, with focus placed upon the influence of core topology on STL. The predicted dispersion relation of the structure shows the typical pass/stop band phenomena in wave propagation, providing intrinsic physical interpretations for the appearance of various peaks and dips on the STL versus frequency curves. The following conclusions are drawn: (i). Sound incident angle exerts a significant effect on the sound insulation capability of the sanwich structure, with its best sound insulation performance achieved for normal incident sound. (ii). The core topology plays a noticeable role in the sound insulation performance of the structure. The STL peaks and dips shift to higher frequencies as the core geometry angle is increased, leading to enhanced STL values. (iii). The appearance of STL peaks and dips is attributable to the standing-wave vibration of the face panel and the coincidence resonance of the bending waves, respectively. (iv). By defining an integrated index for evaluating both the structural stiffness and sound insulation capability of the sandwich, optimal structure design can be performed toward lightweight, high-stiffness and superior sound insulation capability. The sound radiation and transmission characteristics of infinite sandwich structures reinforced by two sets of orthogonal rib-stiffeners are theoretically formulated in terms of the Fourier transform technique and space-harmonic approach, respectively. Unlike previous researches on rib-stiffened panels without considering the inertial effects of the rib-stiffeners, the vibration motion of the rib-stiffeners is accurately described by introducing their tensional forces, bending moments and torsional moments as well as the corresponding inertial terms into the governing equations of the face panels. It is established that: (i). The incident elevation angle has a significant effect on the STL of the sandwich, i.e., oblique incident sound power transmits through the structure more easily than that of normal incident sound, which may be attributed to the constructive interference between incident sound wave and structural bending waves in the former. (ii). With the inertial effects of the rib-stiffeners accounted for, the model can capture more detailed physical features, thus much helpful for accurately predicting the vibroacoustic characteristics of the structure (iii). As a key parameter describing the periodic nature of the sandwich, the rib-stiffener spacing plays a dominant role. The natural frequencies of the structure decrease as the spacing is increased and, as a result, the peaks and dips on the characteristic curves are shifted to lower frequencies. Nonetheless, the overall tendency of these curves remains unchanged. Regarding lightweight composite sandwich structures commonly used in constructing aircraft fuselages, two comprehensive theoretical models are developed separately for sound radiation and sound transmission of infinite orthogonally rib-stiffened sandwiches filled with fibrous sound absorptive material in the partitioned cavities. The process of sound penetration across the fiberglass is characterized by adopting the equivalent fluid model, in which the viscous forces between air and fibers as well as the thermal exchanges are taken into account by introducing two variables, i.e., dynamic density and dynamic bulk modulus. It is found that the fluid-structure coupling effect induces remarkable changes of the SPL/STL versus frequency curves, especially for large stiffener separations. The fiberglass-filled cavity affects sound penetration via the combined effects of fiberglass stiffness and damping (both frequency dependent), the balance of which is significantly affected by the stiffener separation. As a highlight of this research, an integrated optimal algorithm toward lightweight, high-stiffness and superior sound insulation capability is proposed, based on which the optimal design of prototype sandwich structures is carried out.In summary, drawn by the important requirement of vibration and noise reduction in civil and military applications, the vibroacoustic characteristics of typical hull structures of automobiles, express trains, ships/submarines, aircrafts and so on are systematically investigated in this dissertation by incorporating theoretical modeling, experimental measurements and numerical simulation. Apart from establishing a relatively rigorous and reliable system for the vibroacoustic characteristics of typical structures, the effects of key structural parameters are systematically quantified and the physical mechanisms underlying the various vibroacoustic phenomena explored. Moreover, an integrated algorithm for optimal structural design toward lightweight, high-stiffness and superior sound insulation capability is proposed. The present research paves a solid theoretical and experimental foundation for investigating vibroacoustic problems associated with lightweight sandwich structures and provides useful technical guidance for their practical applications in a wide range of civial and military fields.
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