Sound radiation from rectangular baffled and unbaffled plates

The radiation efficiency of a flat rectangular plate is often used as a basis on which to represent the sound radiation from more complex plate-like structures. The solution for a plate set in a rigid baffle is well known, including the radiation efficiency for multi-modal response of the plate. In this case the assumption is usually made that each mode within a given frequency band has equal modal vibration energy. This paper explores a number of limitations of this simple result. First, the extent to which the radiation efficiency for a particular forcing point deviates from the modal-average result is investigated. Second, the difference is shown between the results for a baffled plate and an unbaffled plate. For a multi-modal response, an empirical formula is also presented which allows the radiation efficiency to be estimated for the unbaffled case. Finally, the effect of different boundary conditions on both baffled and unbaffled results is demonstrated by comparing the results for guided boundaries with those for simply supported boundaries.     

Sound radiation from point-excited structures: Comparison of plate and sphere

Acoustic radiation from a structure can be expressed in terms of “modal radiation” and “modal coefficients”. This paper investigates the contributions of these two modal properties to radiation excited by a point force. Sound radiation from two basic structures is considered: a baffled rectangular plate and a closed spherical shell. The plate behaviour is familiar, and governed by the relation between the natural frequency of a mode and its coincidence frequency. For a closed spherical shell, there are either zero or two “critical frequencies”, depending on the radius and thickness. When there are two the shell radiates well both above and below the two frequencies, and poorly in the frequency range between them.     

Study on measurement of dispersive characteristics of higher order mode Lamb waves

The dispersive characteristics of higher order mode Lamb waves (HOMLW) excited by interdigital transducers (IDT) are measured and analyzed, which are necessary for designing micro-sensor in ultrahigh frequency (UHF). A measurement system is set up, in which dispersive characteristics of HOMLW are obtained by the method of transform between frequency and time domains. The characteristics of amplitude-frequency and phase-frequency of Lamb wave are auto-measured by the system. By IFFT, the pulse response of the IDT device was obtained. Different modes were separated in time domain and dispersive curve of each mode is calculated by FFT. The best mode is chosen to design the micro-sensor in UHF. The phase velocity of HOMLW is greater than the surface wave (SAW) velocity and an oscillator in higher frequency can be made, so the absolute sensitivity of micro-sensor can be increased. In this paper, the dispersive characteristics of HOMLW excited by an IDT in a 127.86 degrees rotated Y-cut, X propagating lithium niobate plate is analyzed. An oscillator using a(13) mode is made, the phase velocity of which is measured about 19,652 m/s when h/lambda=0.94 (h=plate thickness, lambda=wavelength).     

The effects of distributed masses on acoustic radiation behavior of plates

The acoustic radiation behavior of a plate with a distributed mass loading is studied. A set of in vacuo normal modes or fluid-loaded undamped normal modes are used for modal analysis of the acoustic radiation from a plate in air or in water. Modal radiation efficiency, modal volume displacement, modal input energy and sound power level are computed to show the effects of size and location of the mass loading on the acoustic radiation of the plate. It is observed that the acoustic radiation behavior of a mode in both cases will have relatively larger changes if the mass loading is placed on an antinode of the mode shape or the mass loading is more concentrated. The acoustic radiation behavior of a mode and the radiated power of the plate in water have less change than those in air with the same mass loading due to the added mass of the water, especially for the first few modes.     

Nonlinear capillary vibration of a charged bubble in an ideal dielectric liquid

The problem of nonlinear radial pulsations and surface vibrations of a charged bubble placed in an ideal incompressible dielectric liquid is asymptotically solved up to the second order of smallness by the method of many scales. It is shown that, in the case of nonlinear vibrations, resonance energy exchange may take place not only between surface modes but also between the radial mode and a surface mode. A new type of instability (other than Rayleigh instability against the self-charge), instability against the excess vapor pressure in the bubble, is discovered. The new type of instability shows up as energy transfer from the centrosymmetric pulsation mode to all initially excited surface vibration modes simultaneously.     

Near field measurement of the complex radiation impedance presented to a vibrating plate in a reverberant room containing a rotating diffuser

The radiation impedance presented by the field of a reverberant room to a rectangular plate mounted in a large baffle and vibrating in its fundamental simply supported edge mode is experimentally investigated. It is shown that a large diffuser rotating at 30 rpm causes large variations in both the real and imaginary parts of the radiation impedance but the average values agree very well with values obtained in the free field of an anechoic room. The real part of the radiation impedance is in good agreement with theoretical prediction. The experimental method makes use of a time average hologram of the mode stored in computer memory, a single point acceleration measurement on the plate, and a surface pressure scan. The method allows investigation of side band frequencies generated by the plate in the presence of the rotating diffuser. It is shown that these side band frequencies contain negligible energy and that they are the result of the cyclic amplitude modulation of the power radiated by the plate.     

The sound power of an individual mode of a clamped-free annular plate

The main aim of this study is an analysis of sound radiation of some clamped-free and free-clamped annular plates. The plates are embedded in some infinite perfectly rigid baffles and vibrate with the free-field condition satisfied. The Kirchhoff-Love linear theory of a perfectly elastic plate is used to solve the plates’ equations of motion. The sound pressure at the plates’ surfaces is expressed by its Hankel transform. The closed-path integral technique known from the literature and the stationary-phase method are used to find the standardized active and reactive sound power of an individual mode in the form of some high-frequency asymptotic formulae useful for some highly efficient engineering computations. Their non-oscillating and oscillating parts have been separated. It is assumed that the vibration and sound radiation are axisymmetric and time harmonic. Low fluid loading is also assumed. The complex sound power of an individual mode forms the basis for computing the total sound power of some excited and damped vibrations in the fluid. The total sound power is not discussed in this study.     

Structural topology optimization on sound radiation at resonance frequencies in thermal environments

Thermal and acoustic environments pose severe challenges to the structural design of hypersonic vehicles.One of them is to find optimal design that exhibits ideal acoustic characteristics in a frequency band,which is discussed in this paper through topology optimization aiming at resonance sound radiation in thermal environments.The sound radiation at resonance frequencies is the main component of response,minimization on which is likely to provide a satisfactory design.A bi-material plate subjected to uniform temperature rise and excited by harmonic loading is studied here.Thermal stress is first evaluated and considered as prestress in the following dynamic analysis;radiated sound power is then calculated through Rayleigh integral.Sensitivity analysis is carried out through adjoint method considering the complicated relationship between stress-induced geometric stiffness and design variables.As the resonance frequency is constantly changing during the optimization,its sensitivity should be considered.It is also noticed that mode switching may occur,so mode tracking technique is employed in this work.Some numerical examples are finally discussed.     

Sound radiation from a water-filled pipe,radiation into light fluid

This paper is concerned with the sound radiation from a water-filled exhaust pipe. The pipe opening and a plate attached to it form a vibrating surface for this radiation. Fluid-structural coupling between the pipe and enclosed fluid is included in the system modeling, but light fluid assumption is used for sound radiation into the space above the vibrating surface. In this paper, a numerical study on the n = 0 mode in the pipe shows that the wave types associated with this mode have different characteristics in two regions of the nondimensional frequency omega. In the first region of 0相似文献   

流体加载下加肋板结构的声辐射特性研究

本文采用有限元和边界元方法对加肋板结构的声辐射进行了计算分析,研究分析了加肋板结构的板厚、板面积、板边长比、肋骨惯性矩和间距、边界条件以及板材和流体介质等对结构辐射声功率的影响,得到了一些有意义的结论,从而为揭示加板结构声辐射规律,降低结构的声辐射提供了一定的依据。     

Reduction of radiated exterior noise from the flexible vibrating plate of a rectangular enclosure using multi-channel active control

This study attempted to control the radiated exterior noise from a rectangular enclosure in which an internal plate vibrates by acoustic excitation and noise is thus radiated from that plate. Multi-channel active control was applied to reduce the vibration and external radiation of this enclosed plate. A piezoelectric ceramic was used as a distributed actuator for multiple mode control of the vibration and radiated noise in the acoustically excited plate. To maximize the effective control, an approach was proposed for attachment the piezoelectric actuator in the optimal location. The plate and internal acoustic space in the enclosure are coupled with each other. This will change dominant frequency characteristics of the plate and, thus, those of the externally radiated noise. Active noise control was accomplished using an accelerometer attached to the plate and a microphone placed adjacent to that plate as an error sensor under acoustic excitation of sine wave and white noise. It was found that the control of radiated external radiation noise requires a microphone as an error sensor, a sound pressure sensor due to vibration of the plate, differences in the dominant frequency of externally radiated noise, and complex vibration modes of the plate.     

Analysis of the Self-Protection Characteristics of a 1.5T Bitter-Like HTS Magnet Operated at 65K

We present a conceptual configuration of a high-temperature superconducting(HTS)magnet made from REBCO(Re=Rare Earth,B=Barium,C=Copper,O=Oxide)annular plates,called a Bitter-like HTS magnet,which can operate in persistent current mode without joint resistance and can be excited by a flux pump and without current leads and a persistent power supply.An REBCO annular magnet which can generate 1.5 T corresponding to the operating current density 80%of critical current density of the magnet at an operating temperature of65 K is conceptually designed.Then the thermal stability of the magnet is numerically simulated by Comsol software.Whein a piece of RBCO annular plate quenches,the maximum released energy is its stored energy because each REBCO annular plate in the Bitter-like magnet is in parallel.To calculate the stored energy in the REBCO annular plate,the inductance of every annular plate,including self-inductance and mutual inductance,is calculated.Compared with the minimum quench energy(MQE)and stored energy in one REBCO annular plate,the stored energy in one REBCO annular plate is always smaller than the MQE,and the REBCO annular plate will not be damaged even though the stored energy in the REBCO annular plate is fully released,which indicates that this 1.5 T Bitter-like magnet has the property of self-protection.     

全维高精度势能面上F+CH3OH反应的产物振动态分布

甲醇与氟原子之间的抽氢反应可以生成HF和CH₃O、CH₂OH自由基等产物. 该反应在环境化学、燃烧化学、辐射化学和星际化学中都非常重要. 基于之前构建的全维高精度势能面,本文采用准经典轨线方法研究了该典型反应的动力学. 特别是使用正则模式分析方法确定了多原子产物CH₃O和CH₂OH的振动态分布. 研究发现,当反应物处于振转基态时,CH₃O和CH₂OH主要分布在基态. 当反应物CH₃OH的OH伸缩模式激发为第一激发态时,产物CH₂OH的OH伸缩模式、扭转模式、H₂CO 面外弯曲模式及其组合会被有效激发. 在两条通道中,可用能量大部分都流入HF的振动能和产物的平动能,而自由基产物CH₃O或CH₂OH只得到非常少的能量,与实验结果一致,这也表明了自由基的旁观者性质.     

Nonlinear resonance interaction between the oscillation modes of a spherical liquid layer on the surface of a melting hailstone

Evolutionary equations are derived and solved that describe the time dependence of the oscillation mode amplitudes on the surface of a charged conducting liquid layer resting on a solid core. It is assumed that the layer experiences a multimode initial deformation. The equations are solved asymptotically in the second order of smallness in the small dimensionless amplitude of capillary oscillations on the surface of the layer. Mechanisms behind internal nonlinear resonance interaction between the modes of the liquid layer oscillations and behind energy transfer between the modes both in degenerate and in secondary combination resonances are investigated. It is found that in the degenerate resonance interaction between oscillation modes, the energy may be transferred not only from lower to higher modes but also vice versa if the higher mode is excited at the zero time. This conclusion is valid not only for a liquid layer on the surface of a solid core but also for a drop.     

The Beam-Wave Interaction in a Ka-Band Relativistic Coupled-Cavity TWT

By the method of the eigen mode expansion, a full three-dimensional (3-D) model has been developed that can be used to investigate the beam-wave interaction in a high-power, Ka-band relativistic coupled-cavity traveling-wave tube (TWT). In the tube studied by us, a sever in the interaction section is used to restrain the oscillation of RF electromagnetic fields between the input and output end. In this case, the asymmetric hybrid HTM₁₁ mode has little impact on the main interaction process ^[1,2], so the RF electromagnetic fields mainly interacting with the modulated electron beam belong to the symmetric mode TM₀₁. The presented model includes three-dimensional RF fields, three-dimensional electron motions, and improved three-dimensional space-charge fields including dc and ac space-charge fields. Moreover, this model can also calculate backward radiation excited by modulated electron beam and the direct effect of the transverse electron motion on the energy exchange. Our calculation results show that the space-charge field has evident effect on the interaction process, the transverse electron motion has some, and the backward radiation has little.     

Remarkable zeroth-order resonant transmission of microwaves through a single subwavelength metal slit

A slit in a thick metal plate that is extremely subwavelength will not transmit microwaves polarized parallel to it. It is shown here that cuts perpendicular to the slit allow parallel polarized radiation to resonantly transmit. Furthermore, a zero-order mode may be excited within the slit, the frequency of which, to first order, is independent of the plate depth. Remarkably, for this novel type of resonance, the field in the slit oscillates with a constant phase and little amplitude variation throughout the plate depth, while the resonant wavelength tends to infinity as the slit width approaches zero.     

FORCED AND FREE VIBRATIONS OF RECTANGULAR SANDWICH PLATES WITH PARAMETRIC STIFFNESS MODULATION

An active control of the resonant vibrations of a rectangular sandwich plate performed by the parametric stiffness modulation is analyzed. The controlled vibrations are those of the dominantly flexural type excited by the transverse force acting at the first resonant frequency of dominantly flexural vibrations. The stiffness modulation is performed at a comparatively high frequency identified by the resonance of a mode of the dominantly shear type. The method of direct partition of motions is used that predicts an existence of the modal interaction between these two modes of vibrations due to the parametric stiffness modulation. It is shown that such a parametric control can provide a significant shift of the first eigenfrequency of a controlled plate (the one subjected to the stiffness modulation) from its nominal value for an uncontrolled plate. Heavy fluid loading conditions are accounted for as well as the energy dissipation in the material of a plate. It is demonstrated that although heavy fluid loading reduces resonant frequencies of forced vibrations, the suggested mechanism of control remains valid in these cases. Dynamics of an elementary two-degree-of-freedom model mechanical system is considered to illustrate the mechanism of modal interaction, which is involved in the suggested way of an active control of vibrations of sandwich plates.     

Atmospheric interaction radiation from high altitude rocket exhausts

A model for calculating the infrared radiation from rocket exhaust gases at high altitudes (typically above 200 km), caused by collisions between exhaust molecular species and atmospheric species is presented. At altitudes where the atmospheric mean free path is larger than a typical rocket exhaust plume lenght scale, the evolution in space and time of the exhaust gases is described by the kinetic theory of gases. In addition, the collision frequency between exhaust and atmospheric species is sufficiently low that excited molecules have time, on the average, to loose thsis excitation energy via radiative emission before experiencing another collision. Thus, the distribution of excited molecular states is nonthermal in this model. Two examples of such radiation are presented: one for the radiation from the CO₂ (υ₃) mode and the other for the H₂O(υ₃) mode. The atmospheric collision partner for excitation of both of these exhaust species is taken to be atomic oxygen, the dominant atmospheric constituent at high altitudes.     

液体中光击穿所激发声场的方向性研究

从液体中光击穿所激发声场的柱体模型和靶盘模型出发,运用声学基础的理论原理,提出了等离子体椭球模型,对光击穿所激发声场的方向性进行了理论研究.通过MATLAB仿真得到此声场的方向特性图,进而分析和比较了能量不同、照射区域大小不同的激光束所激发声场的方向性.研究表明等离子体椭球模型更符合实验情况.     

Propagation of the radiation field excited on discontinuities of optical waveguides

Numerical and semi-analytical techniques widely used to model the complicated structure of the radiation field excited on sharp and smooth discontinuities of a dielectric slab are reviewed. Comparative analysis of the radiation field modelling by the fast Fourier transformation-beam propagation method (FFT-BPM) and the finite difference(FD)-BPM is presented. Propagation of a higher-order mode through the cut-off cross-section of smooth and sharp discontinuities is simulated numerically by the FD-BPM, the total field is analysed using the leaky mode approach.