气泡幕后向散射光信号特性的蒙特卡罗方法研究

利用蒙特卡罗方法对水介质中的气泡幕的后向光散射回波信号进行了系统仿真。通过对计算结果与实验结果的比较,证明了蒙特卡罗方法的有效性。根据仿真结果,结合理论分析的方法,对气泡幕位置、厚度和接收器视场角等参数对回波信号的影响进行了分析,结果表明:回波信号出现时间与气泡幕位置存在一一对应的关系;在气泡幕的衰减系数ρσt不变的情况下,存在一个有效气泡幕厚度,当大于该厚度时,气泡幕的后向光散射回波信号基本不变;在系统各项参数不变的情况下,适当增加接收器的视场角,可有效地提高回波信号的信噪比。     

气泡幕回声信号的起伏相关

实验研究了电解气泡幕的回声信号起伏特性，在两种电解电压的条件下，测量了几个频率的信号起伏相关时间，并用声学反演法测量了气泡幕中气泡的体积分数、气泡大小分布以及气泡幕的厚度。     

激光在水中气泡微粒的后向散射特性

从激光能量、目标位置、视场大小及气泡厚度等方面对气泡后向散射特性进行研究.采用实验方法研究脉冲激光气泡后向散射特性.实验结果表明,增大激光能量在一定程度上可以有效地改善气泡探测的效果,而气泡发生器位置和视场的合理选择使得气泡探测更容易些,气泡区域厚度的变化对气泡后向散射并没有明显的影响.该实验结论与蒙特卡洛数值模拟的结果一致.     

次级源和误差传感器布放对带弹性障板的充液直管声振复合有源控制的影响*

针对充液管路系统噪声有源控制问题,研究了次级源和误差传感器布放对带弹性障板的充液直管管路系统有源消声与有源消振复合控制效果的影响。基于声固耦合方法建立了带弹性障板的充液直管管路系统的有限元模型,在声激励下对比了次级声源布放对系统有源消声性能的影响,并在组合激励下分析了次级力源、次级声源和误差传感器布放对系统复合有源控制的影响。结果表明,非对称分布的次级声源容易激起管壁振动,进而带动障板振动,导致有源消声效果不佳;采用对称分布的次级声源可使低频段的降噪量提高10 dB以上。复合有源控制可进一步提升全频段的控制效果。通过增加振动误差传感器数量,可使绝大多数频点的降噪量提高1~20 dB不等。此外,在管壁上布放的两圈次级力源的间距小于管壁振动波长的1/4,且都不位于管壁振动节点附近时控制效果更好。     

气泡浓度对海洋激光雷达后向散射特性的影响

对不同浓度气泡幕水下激光雷达的后向散射特性进行了理论分析和实验研究。实验结果表明：由于水中气泡群后向散射的原因,在水下激光雷达整体后向散射信号的回波曲线上,除了水体后向散射导致的信号峰以外,还出现因气泡散射而导致的散射峰,该散射峰叠加在水体的后向散射信号峰上,其位置与水中气泡幕的位置直接相关,而其幅值则受气泡幕的浓度影响。随着气泡幕浓度的增大,其后向散射峰幅值相应增大,这一点与理论分析结果一致,而回波峰值的变化趋势则主要受水体质量的影响。     

次级声源优化布放的局部空间有源降噪

针对区域有源降噪问题,为获得更优降噪效果,根据实际次级通路传递函数,提出次级声源优化布放的有源控制系统并详细比较了两种次级声源优化布放算法与次级声源均匀布放的实际降噪效果。应用的第一种次级声源优化算法是l₂范数约束的约束匹配追踪算法,第二种次级声源优化算法是l₁范数约束的稀疏正则化方法。在全消声室中利用扬声器线阵进行多通道有源降噪实验研究,实验结果表明,在200～1000 Hz,次级声源优化布放的控制系统的平均降噪量比次级声源均匀布放的控制系统的平均降噪量多5 dB左右;在1100～1900 Hz,次级声源优化布放的控制系统的平均降噪量比次级声源均匀布放的控制系统的平均降噪量多11～13 dB左右,次级声源优化布放的控制系统的降噪量分布更加均匀且次级声源输出能量更小。此外,两种优化算法中,稀疏正则化方法的降噪效果更佳。     

封闭空间自适应有源噪声控制误差传声器的最优布放

对于封闭空间有源噪声控制,本文研究了在次级声源布放确定的情况下,如何选择误差传声器的数目及位置以取得最大降噪量的问题.理论分析及仿真结果表明:误差传声器布放准则与次级声源布放准则类似.同时,本文提出误差信号内插法,以减少误差传声器数目,提高降噪量.     

管道模态波入射下HQ管降噪机理研究

随着涡扇发动机风扇直径增大转速降低,低频噪声控制问题引发格外关注。HQ管作为一种抗性降噪结构,在低频段拥有良好的降噪效果。本文首先建立了带HQ管的管道风扇噪声数值模型,并对声传播特性进行了细致的仿真分析,获得了与前期实验数据对比一致性良好的预测结果。最后通过不同HQ管排布螺旋角下降噪效果和声模态分解结果的对比分析,对HQ管降噪机理进行了探讨,发现HQ管对模态波降噪的主要模式是模态的反射与散射,并且不同于将入射模态散射为截止模态的传统认知,将入射模态散射为不占优的可传播模态可以大幅削弱占优模态的幅值,同样具有较为明显的降噪效果。     

空气锤进排气噪声控制用消声器的试验研究

分析了空气锤进排气噪声的特性,全面深入地试验和分析了,不同基本滤波单元及其组合的消声器实际应用在Ｃ４１－１５０型空气锤时对排气噪声的降低功能,并取得了显著的降噪效果。     

子空间降噪在舰船噪声混沌信号处理中的应用

结合混沌信号本身的特性采用子空间投影理论,研究含噪Lorenz模型信号和水下舰船噪声信号的降噪问题。讨论了延迟时间和嵌入维对降噪效果的影响,重点研究了为保持混沌信号的局部特性如何选择参与自相关函数计算的信号长度。采用分段自相关函数多次迭代的方法,保持了混沌的局部特性。仿真结果表明,降噪效果较为理想。     

The use of an air bubble curtain to reduce the received sound levels for harbor porpoises (Phocoena phocoena)

In December 2005 construction work was started to replace a harbor wall in Kerteminde harbor, Denmark. A total of 175 wooden piles were piled into the ground at the waters edge over a period of 3 months. During the same period three harbor porpoises were housed in a marine mammal facility on the opposite side of the harbor. All animals showed strong avoidance reactions after the start of the piling activities. As a measure to reduce the sound exposure for the animals an air bubble curtain was constructed and operated in a direct path between the piling site and the opening of the animals' semi-natural pool. The sound attenuation effect achieved with this system was determined by quantitative comparison of pile driving impulses simultaneously measured in front of and behind the active air bubble curtain. Mean levels of sound attenuation over a sequence of 95 consecutive pile strikes were 14 dB (standard deviation (s.d.) 3.4 dB) for peak to peak values and 13 dB (s.d. 2.5 dB) for SEL values. As soon as the air bubble curtain was installed and operated, no further avoidance reactions of the animals to the piling activities were apparent.     

颗粒帘换热器中气粒两相换热特性实验研究

根据某1 t/h燃煤工业锅炉空气预热器的热力参数,设计并搭建了一套颗粒帘空气预热器模拟实验系统,研究了不同进气温度Tg0(150~300℃)、进气速度Vg0(0.9~1.5m/s)、颗粒帘进口厚度b0 (60~180 mm)、颗粒粒径dp(150~212μm)以及不同颗粒质量流量ms (550~2150 g/s)工况条件下热空气与进口温度tp0=20℃的硅砂颗粒帘间的换热特性。结果表明:影响颗粒帘换热器中气粒两相换热特性因素的重要性次序为进气温度、进气速度、颗粒质量流量、颗粒粒径、颗粒帘进口厚度;换热端差最低可至4.5℃,最大可达87℃;颗粒帘及颗粒帘出口气流的温度沿颗粒下落方向在前期上升迅速(186~475℃/m)而后期上升比较缓慢(60~108℃/m),并且在0~0.5 m和0.5~1.0 m的高度范围可分别用线性和对数方程来描述。     

微泡对高强度聚焦超声声压场影响的仿真研究*

微泡对高强度聚焦超声(HIFU)治疗焦域具有增效作用,而HIFU治疗中不同声学条件下微泡对HIFU形成声压场的影响尚不清楚。本文基于气液混合声波传播方程、Keller气泡运动方程、时域有限差分(FDTD)法和龙格-库塔(RK)法数值仿真研究输入声压、激励频率、气泡初始空隙率和气泡初始半径对HIFU形成声压场的影响。研究结果表明,随着输入声压的增大,焦点处声压升高但焦点处最大声压与输入声压的比值减小,焦点位置几乎不变;随着激励频率和气泡初始半径的增大,焦点处声压升高且焦点位置向远离换能器方向移动;随着气泡初始空隙率的增大,焦点处声压降低且焦点位置向换能器方向移动。     

Modeling of offshore pile driving noise using a semi-analytical variational formulation

Underwater noise radiated from offshore pile driving got much attention in recent years due to its threat to the marine environment. This study develops a three-dimensional semi-analytical method, in which the pile is modeled as an elastic thin cylindrical shell, to predict vibration and underwater acoustic radiation caused by hammer impact. The cylindrical shell, subject to the Reissner–Naghdi’s thin shell theory, is decomposed uniformly into shell segments whose motion is governed by a variational equation. The sound pressures in both exterior and interior fluid fields are expanded as analytical functions in frequency domain. The soil is modeled as uncoupled springs and dashpots distributed in three directions. The sound propagation characteristics are investigated based on the dispersion curves. The case study of a model subject to a non-axisymmetric force demonstrates that the radiated sound pressure has dependence on circumferential angle. The case study including an anvil shows that the presence of the anvil tends to lower the frequencies and the amplitudes of the peaks of sound pressure spectrum. A comparison to the measured data shows that the model is capable of predicting the pile driving noise quantitatively. This mechanical model can be used to predict underwater noise of piling and explore potential noise reduction measures to protect marine animals.     

Enhanced bandgap in annular photonic-crystal silicon-on-insulator asymmetric slabs

Photonic band structures of annular photonic-crystal (APC) silicon-on-insulator (SOI) asymmetric slabs with finite thickness were investigated by the three-dimensional plane-wave expansion method. The results show that for a broad range of air-volume filling factors, APC slabs can exhibit a significantly larger bandgap than conventional circular-hole photonic-crystal (PC) slabs. Bandgap enhancements over conventional air hole PC SOI slabs as large as twofold are predicted for low air-volume filling factors below 15%. This desirable behavior suggests a potential for APC SOI slabs to serve as the basis of various optical cavities, waveguides, and mirrors.     

考虑噪声源深度分布的海洋环境噪声模型及地声参数反演

考虑到海洋环境噪声源深度分布不集中,建立了噪声源随深度分布的海洋环境噪声模型,分析了源深度对噪声场垂向特征的影响并从简正波角度予以解释,发现海底声阻抗和声源深度都显著影响由海洋环境噪声获得的等效海底反射损失大掠射角部分,进而将该模型用于地声参数反演.两段实测噪声数据200—525 Hz频段的反演结果表明:基于海洋环境噪声的地声参数反演最优值与声传播的反演结果相近;源平均深度最优值随频率增加有变小的趋势,说明随频率增加环境噪声主要贡献源逐渐由航船转为风浪;当海况大于3级时,400 Hz以上频段噪声源深度平均值很小,与Monahan气泡理论的描述一致.     

Acoustic measurements of the sound-speed profile in the bubbly wake formed by a small motor boat

In situ measurements of the bubble field within wakes generated by a small motorboat show that the bubble field, shortly after the initial turbulent generation period, consists mainly of bubbles with radii between 20 and 200 microm. The subsequent dispersion of the wake field can be described using a model that includes bubble buoyancy and dissolution only, and the air volume fraction within the wakes decay exponentially with an e-folding time of between 40 and 60 s. Simultaneous measurements of sound propagating through the bubbly wake exhibit spectral banding due to waveguide propagation. Inversions using the inverse-square theory developed by Buckingham [Philos. Trans. R. Soc. London, Ser. A 335, 513-555 (1991)] show that this acoustic inversion technique provide a viable means of estimating the low-frequency sound-speed profile in an upward refractive bubble layer when dispersion can be neglected.     

Suspended water droplets as a means of noise reduction

The propagation of acoustic waves and energy absorption in a medium of air containing suspended water droplets have been investigated experimentally. It has been found that an increase in atomisation pressure or mass flow ratio reduces the size of droplets, thus yielding higher noise reduction. It should be possible to develop an appropriate system, for reducing the noise of hammers or other impact machinery, where the creation of a temporary air-water skirt around the noise source can be synchronised with the blow rate, i.e. the timing of the noise generation.