共振式消声器声学性能分析

在保持连接管结构形状及尺寸不变、共振腔容积不变的条件下，分析了单节共振式消声器腔体结构对其声学特性的影响。结果表明；共振式消声器腔体深宽比对其共振频率有较大的影响，截面宽高比有一定的影响，但相对较小。当腔体形状接近于立方体时，消声器共振频率最大，此时用集中参数模型估算消声器共振频率与实际值偏差最小，腔体深宽比减小或增大，共振频率均向低频方向移动，当腔体深度大于波长的10％～16％时，用一维轴向模型估算消声器共振频率与实际值吻合较好；随着截面宽高比的增加，消声器共振频率向低频方向移动，截面形状愈扁，共振频率变化幅度愈大。     

An experimental investigation of an acoustically excited laminar premixed flame

A two-dimensional laminar premixed flame is stabilized over a burner in a confined duct and is subjected to external acoustic forcing from the downstream end. The equivalence ratio of the flame is 0.7. The flame is stabilized in the central slot of a three-slotted burner. The strength of the shear layer of the cold reactive mixture through the central slot is controlled by the flow rate of cold nitrogen gas through the side slots. The frequency range of acoustic excitation is 400-1200 Hz, and the amplitude levels are such that the acoustic velocity is less than the mean flow velocity of the reactants. Time-averaged chemiluminescence images of the perturbed flame front display time-mean changes as compared to the unperturbed flame shape at certain excitation frequencies. Prominent changes to the flame front are in the form of stretching or shrinkage, asymmetric development of its shape, increased/preferential lift-off of one or both of the stabilization points of the flame, and nearly random three-dimensional fluctuations over large time scales under some conditions. The oscillations of the shear layer and the response of the confined jet of the hot products to the acoustic forcing, such as asymmetric flow development and jet spreading, are found to be responsible for the observed mean changes in the flame shape. A distinct low-frequency component (∼60-90 Hz) relative to the excitation frequency is observed in the fluctuations of the chemiluminescent intensity in the flame under most conditions. It is observed that fluctuations in the flame area predominantly contribute to the origin of the low-frequency component. This is primarily due to the rollup of vortices and the generation of enthalpy waves at the burner lip. Both of these processes are excited at the externally imposed acoustic time scale, but convect/propagate downstream at the flow time scale, which is much larger.     

新型阻尼结构叶片振动特性试验研究

干摩擦阻尼结构被广泛应用于透平叶片中来降低其振动应力。文章设计并搭建了干摩擦阻尼块结构叶片振动特性测试试验台,通过测量一新型阻尼结构叶片的振动响应值,获得了不同激振力及模拟离心力转速下叶片频率响应曲线及模态阻尼比。试验结果表明：当激振力较小时（4N、5N和6N）,叶片的共振频率随模拟离心力转速的上升而不断增加,而模态阻尼比则先增加再减小,在40％模拟离心力转速工况下模态阻尼比最大,叶片减振效果最好;当激振力较大时（12N、18N和24N）,叶片振动更加剧烈,使得阻尼块接触刚度降低,叶片的共振频率接近自由叶片固有频率,而模态阻尼比随着模拟离心力转速的上升不断降低。     

Prescheduled adaptive control scheme for resonance cancellation of a distributed solar collector field

Distributed solar collector fields possess low frequency resonance characteristics. Consequently, whenever fast response control is attempted using conventional controllers, be they fixed or adaptive, oscillatory performance results. Accordingly, slow control has to be accepted in order to provide adequate damping. In this paper an adaptive control scheme is proposed that takes into account resonance characteristics. This scheme enables the speed of response to be increased without sacrificing stability margins and damping.     

Nonlinear combustion instability analysis based on the flame describing function applied to turbulent premixed swirling flames

Instability analysis of swirling flames is of importance in the design of advanced combustor concepts for aircraft propulsion and powerplant for electricity production. Thermoacoustic instabilities are analyzed here by making use of a nonlinear representation of flame dynamics based on a describing function. In this framework, the flame response is determined as a function of frequency and amplitude of perturbations impinging on the combustion region. This model is adapted to the case of confined swirling flames comprising an upstream manifold, an injection unit equipped with a swirler and a cylindrical flame tube. The flame describing function is experimentally determined and is combined with an acoustic transfer matrix representation of the system to provide growth rates and oscillation frequencies as a function of perturbation amplitude. These data can be used to determine regions of instability, frequency shifts with respect to the acoustic eigenfrequencies and they also yield amplitude levels when self-sustained oscillations of the system have reached a limit cycle. This equilibrium is obtained when the amplitude dependent growth rate equals the damping rate in the system. This requires an independent determination of this last quantity which is here based on measurements of the combustor resonance response curve, together with numerical estimates of the flame contribution to the system response. The geometrical parameters of the upstream manifold and flame tube are varied and the corresponding operating regimes are compared with those predicted with the FDF framework. The present demonstration of the FDF framework in a generic configuration indicates that this can be used in more general situations of technological interest.     

用声响应法测量直喷式柴油机燃烧压力振荡共振频率的研究

本文根据声波动理论推导出声压波动方程，证明了在密闭空间中声压波动具有与柴油机燃烧室内气体压力振荡相类似的规律性。实验还表明，通过声响应法测量的声压波动共振频率与燃烧压力振荡共振频率是基本相等的。     

Flow Induced Acoustic Resonance in In-line Tube Banks

In the present paper the attention is focused on the relation between vortex shedding phenomena andacoustic resonance which occurred in the two-dimensional model of boiler.There were tube bankswith in-line arrangement for small tube pitch ratio.We measured the sound pressure level,the phasedelay of acoustic pressures,the spectrum of velocity fluctuation and the gap velocity.As a result, wefound two peak frequencies of sound pressure level with different Strouhal numbers S_t,mainly about0.26 and 0.52.The noise of S_t=0.26 was the resonance of transverse mode and S_t=0.52 waslongitudinal mode.The vortex shedding of S_t=0.15 was generated inside the tube banks withoutacoustic resonance. As gap velocity increased, we observed that the peak level of spectrum was weakand broad-banded. The onset velocity of the acoustic resonance of longitudinal mode was lower thanthat of transverse mode.     

Acoustic time-of-flight imaging of polymer electrolyte membrane water electrolysers to probe internal structure and flow characteristics

Acoustic time-of-flight (AToF) imaging has been demonstrated as a low-cost, rapid, non-destructive, operando tool to characterize processes in the flow channels and liquid-gas diffusion layer (LGDL) of polymer electrolyte membrane water electrolysers (PEMWEs). An array of 64 piezoelectric sensors was used, with all sensors emitting input pulses and detecting the acoustic wave reflected by the sample (pulse-echo mode). The shape and intensity of this reflected waveform depends on the ratio of reflection and transmission at phase interfaces and is strongly affected by resonant scattering of acoustic waves by gas bubbles. This AToF imaging technique was deployed to produce reflection intensity maps of the anode flow-field and LGDL; by measuring the AToF response for current densities ranging from 0.00 A cm^?2 to 2.00 A cm^?2, a close correlation was found between the acoustic attenuation in the flow-field and the production and removal of oxygen gas through the flow channels. Furthermore, a close link between the AToF response and water thickness in the LGDL was demonstrated, as supported by literature data. The application of the AToF technique has been established as a novel way of investigating PEMWE operation and as an alternative to more complex imaging techniques such as neutron imaging.     

Oscillatory velocity response of premixed flat flames stabilized in axial acoustic fields

This article describes a combined theoretical-experimental investigation of the processes that drive axial instabilities in solid propellant rocket motors. In this study, the solid propellant flame has been simulated by a premixed flat flame that has been stabilized on the porous side-wall of a duct. The driving processes have been investigated by studying the interaction of the premixed flame with an axial acoustic field. Using experimentally determined acoustic pressures, burner surface admittances, and steady-state flame temperature distributions as input data, the developed model was used to determine the characteristics of the velocity field in the flame region under a variety of test conditions. The predicted velocity field was then compared with LDV velocity measurements to check the validity of the model and determine the flame driving. These studies reveal that the investigated flame responds to the presence of an axial acoustic field by producing an oscillatory velocity component, ν′, normal to the duct wall, that can drive or damp the acoustic field. Comparison of the measured data with the model predictions reveal satisfactory agreement. These studies also showed that driving and damping of the acoustic field occur simultaneously in different regions of the flame. The net effect of the flame upon the acoustic field depends upon the relative magnitudes of these opposing tendencies. It is also shown that the flame driving depends upon the acoustic admittance of the side-wall surface and the frequency of the acoustic field.     

单一节理边坡模态及谐响应分析

为有效避免地震作用下边坡的共振现象,将含有控制结构面的顺层边坡简化为单一节理边坡模型,采用ANSYS软件对简化模型进行模态分析和谐响应分析。结果表明,随着节理结构面剪切刚度的增大,边坡的一阶振型由沿结构面的错动振动变为水平振动,且临界剪切刚度为0.5 GPa/m。当边坡岩石弹性模量由20GPa增加到50 GPa时,临界剪切刚度增加。地震作用下只有边坡的第一阶自振能被激发产生水平方向共振现象,结构面的存在能减小边坡的一阶自振频率,使边坡的自振频率更接近地震波的主频,更易产生共振,可通过支护方式增加边坡节理剪切刚度来控制地震作用下边坡的共振现象。     

A study of modal damping for offshore wind turbines considering soil properties and foundation types

The modal damping ratio for each mode is crucial to characterize the dynamic behavior of offshore wind turbines and widely used by simulation software in wind turbine engineering, such as Bladed and FAST. In this study, modal damping ratios of offshore wind turbines are systematically studied for different soil properties and foundation types. Firstly, the modal damping ratios and modal frequencies for the first and second modes of a gravity foundation–supported offshore wind turbine are studied. An offshore wind turbine supported by a monopile foundation is then investigated to clarify the characteristics of modal damping ratios and modal frequencies for the monopile foundation. The soil parameters are identified by means of genetic algorithm (GA). Predicted modal damping ratios and modal frequencies as well as modal shapes show good agreement with the field measurements for both foundations. Finally, a sensitivity analysis study is carried out to investigate the effects of soil properties and foundation types on modal damping ratios. For the gravity foundation–supported offshore wind turbine, soil properties affect the modal damping ratio of the second mode largely, but affect that of the first mode little, while for the monopile‐supported offshore wind turbine, soil properties affect the modal damping ratios of the first and second modes significantly. Predicted natural periods and modal damping ratios of the first mode for both foundations by a pair of simple models agree well with those by numerical models.     

非线性阻尼模型对重力坝地震响应的影响

阻尼对结构的振动反应有重要影响,目前重力坝设计阶段通常取阻尼比为常数,但不能真实反映结构的阻尼机理。为此,基于大型通用有限元软件ABAQUS,利用混凝土损伤塑性模型分析了重力坝在地震荷载作用下的损伤发展和能量耗散,提出了重力坝结构非线性阻尼模型的表达式,分别采用非线性阻尼模型和常阻尼模型计算了重力坝在三向地震波作用下的响应,并对结果进行了对比。结果表明,采用非线性阻尼模型计算的结构响应较大,因此采用常阻尼比模型进行结构地震响应分析可能产生不安全的结果。     

Low carbon ultrasonic production of alternate fuel: Operational and mechanistic concerns of the sonochemical process of hydrogen generation under various scenarios

Sonochemistry is considered as one of the cleaner pathways for hydrogen production. The present paper investigates the potential of this technique based upon mass, mass to energy and energy conversion metrics, using modelling and experimental approaches. Four scenarios are examined assuming four saturating gases, namely O₂, air, N₂ and Ar, four acoustic frequencies, i.e., 20, 210, 326 and 488 kHz, and considering common acoustic intensities then common net electric power. The study revealed that Ar is the best fitting saturating gas for the sonochemical production of hydrogen. With a common acoustic intensity of 0.48 W/cm², an optimum ratio of H₂ molar yield to acoustic energy intensity is retrieved at 210 kHz, while with a common net electric power of 87 W, the highest ratio of hydrogen yield to electric energy was observed at 20 kHz. Results were interpreted based upon emitter surface, energy conversion and distinction of calorimetric and cavitational energies.     

贫预混预蒸发燃烧室振荡燃烧特性的实验研究

针对燃用航空煤油的贫预混预蒸发模型燃烧室的振荡燃烧特性开展了实验研究。实验表明:在相同的燃烧室入口空气燃料混合物流速下,随着当量比的增加,燃烧室振荡燃烧的振荡主频从132 Hz增加到144 Hz,但燃烧室的均方根脉动压力幅值却从1 464 Pa下降到342 Pa。在当量比不变情况下,入流空气燃料混合物流速较低时,容易引发振荡燃烧现象,而当入流空气燃料混合物流速较高时,则燃烧会变得稳定。分析了整个燃烧实验装置的前4阶轴向声学模态频率,发现实验中所激励出的振荡燃烧主频和第二阶轴向声学模态频率吻合的很好。     

Mitigating subsynchronous resonance and damping power system oscillation in a series compensated wind park using a novel static synchronous series compensator control algorithm

This paper presents a novel damping control algorithm for static synchronous series compensator (SSSC) in a series compensated wind park for mitigating subsynchronous resonance (SSR) and for damping power system oscillations. The sample test system, adapted from the IEEE first benchmark model on SSR replacing the synchronous generator, is employed aggregating wind park based self‐excited induction generator. Consequently, it investigates the SSR phenomena and the damping power system oscillation while integrating large wind park based on SEIG. The potential occurrence and mitigation of the SSR caused by induction generator effects as well as torsional interactions, in a series compensated wind park, are investigated. The auxiliary subsynchronous damping control loops for the SSSC based on a novel design procedure of non‐linear optimization are developed. The performance of the controller is tested in steady state operation and in response to system contingencies, taking into account the impact of short circuit ratios (SCRs). The simulation results are presented to demonstrate the capability of the controllers for mitigating the SSR, damping the power system oscillation and enhancing the transient stability margin in response to different SCRs. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of boundary layer trip on reduction of jet noise in over-expanded nozzle flow

A flow resonance accompanied by the emission of acoustic tones occurs in an over-expanded convergent-divergent (C-D) nozzle when operated at comparatively low pressure ratios.This phenomenon is distinguished from conventional screech tones and is referred to as “transonic tones”.In contrast to screech tones,the peak resonant frequency for transonic tones increases with pressure ratio;the peak sound pressure level exceeds 110 dB.In this study,we investigated the basic characteristics of transonic resonance and tones using a circular C-D nozzle in an anechoic room.The effects of the boundary layer trip were also evaluated using a tripping wire for the suppressing transonic resonance and tones.The results of acoustic measurements show that several predominant peaks correspond to transonic tones.However,the boundary layer trip inside the nozzle effectively eliminated these tones and suppressed the unsteadiness of the flow inside the nozzle.     

The nonlinear heat release response of stratified lean-premixed flames to acoustic velocity oscillations

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to high-amplitude acoustic forcing in a laboratory-scale stratified burner operated with CH₄ and air at atmospheric pressure. The double-swirler, double-channel annular burner was specially designed to generate high-amplitude acoustic velocity oscillations and a radial equivalence ratio gradient at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the response are not considered in the present investigation. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a constant temperature anemometer and photomultiplier tubes with narrow-band OH^∗/CH^∗ interference filters. Time-averaged and phase-synchronized CH^∗ chemiluminescence intensities were measured using an intensified CCD camera. The measurements show that flame stabilization mechanisms vary depending on equivalence ratio gradients for a constant global equivalence ratio (?_g = 0.60). Under uniformly premixed conditions, an enveloped M-shaped flame is observed. In contrast, under stratified conditions, a dihedral V-flame and a toroidal detached flame develop in the outer stream and inner stream fuel enrichment cases, respectively. The modification of the stabilization mechanism has a significant impact on the nonlinear response of stratified flames to high-amplitude acoustic forcing (u′/U ∼ 0.45 and f = 60, 160 Hz). Outer stream enrichment tends to improve the flame’s stiffness with respect to incident acoustic/vortical disturbances, whereas inner stream stratification tends to enhance the nonlinear flame dynamics, as manifested by the complex interaction between the swirl flame and large-scale coherent vortices with different length scales and shedding points. It was found that the behavior of the measured flame describing functions (FDF), which depend on radial fuel stratification, are well correlated with previous measurements of the intensity of self-excited combustion instabilities in the stratified swirl burner. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, its effect on flame location and the interaction with unsteady flow structures.     

考虑黏性效应的半潜浮式风力机动力特性研究

对半潜浮式风力机动力特性进行研究，推导考虑黏性阻尼的动力学方程及传递函数。对黏性效应的影响及其计算方法进行探讨，对比附加阻尼矩阵法、Morison单元法的优缺点，并提出考虑黏性阻尼效应水动力计算的混合法，在此基础上对半潜浮式风力机气动-水动-锚泊全耦合动力响应进行分析。结果表明：黏性效应主要影响共振周期附近的响应值，在数值分析时不可忽略；附加阻尼矩阵法在考虑水平面内运动黏性阻尼时有所不足，且无法考虑黏性效应对共振周期的影响，Morison单元法在考虑垂荡、转动黏性阻尼时有所不足，混合法是考虑黏性阻尼水动力计算的更有效方法；该半潜浮式风力机垂荡和纵摇响应主要受波浪控制，而水平面内运动受风、浪、流联合作用的影响；浮式风力机运动和加速度的最危险工况可发生在发电工况时，最大锚链张力发生在极端环境条件时。     

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.     

基于ATV的内燃机结构声辐射预测技术研究

基于声传递向量（ATV）的概念,研究探讨了ATV技术在内燃机结构辐射噪声仿真预测上的应用方法和途径。以某四缸柴油机为例,利用ATV技术,计算得到了整机结构辐射声功率、外场域点声压（级）、结构面板声学贡献、辐射效率等声学响应。在内燃机研制过程中,基于振动和声压域之间振一声关系的ATV技术,是快速进行振一声响应预测的一个有效方法。