流场刺激下海洋生物发光的光学特性及数理模型

生物受激发光探测是一种感知水中目标的新技术、新手段。选取多边膝沟藻、夜光虫和月芽形梨甲藻3种典型海洋受激发光生物体为对象,采用光谱仪、光子计数器、库埃特流场旋转圆筒等装置搭建了生物受激发光实验平台,研究了不同刺激下生物体受激发光的光学特性,并在此基础上结合概率论和数理统计理论,建立了流场刺激下的生物发光的数理模型。结果表明:发光藻的发光具有较好的单色谱特性,峰值波长集中在472 nm左右,谱线的半波宽度为30～40 nm。发光藻的发光随着机体的疲劳而变弱,但发光能力随着机体的休息会慢慢恢复。同种刺激条件下,3种藻发光强度的强弱依次为月芽形梨甲藻、夜光虫、多边膝沟藻。     

超微弱光子图像的相关检测及计算机模拟

在超微弱发光的研究中（例如生物发光）,由于发光强度极弱,由像增强器得到的光子图像由于样本（光子）数量太少和受系统暗噪声的影响使其信噪比极低,提出了一种基于统计学的光子图像相关积分方法和基于相关处理的光子图像处理方法,用累积光子来提高图像信噪比并得到相应灰度图像并对该方法进行了计算机模拟,同时展示了光子图像和传统灰度图像的信噪比关系,是进行微弱生物发光信号检测的有效方法。     

微泡对高强度聚焦超声焦域影响的研究

微泡对高强度聚焦超声(HIFU)治疗具有增效作用,而HIFU治疗中不同声学条件下微泡对HIFU治疗焦域的影响尚不清楚。本文基于声传播方程、Yang-Church气泡运动方程、生物热传导方程、时域有限差分法(FDTD)、龙格-库塔(RK)法数值仿真研究输入功率、激励频率和气泡初始半径对HIFU在含气泡体模中形成焦域的影响,并利用含Sono Vue造影剂的仿组织体模研究进行实验验证。结果表明,增大输入功率、气泡初始半径和升高激励频率均可增大焦域,随着输入功率的增大,焦域形状可能发生变化,而随着激励频率升高和气泡初始半径的增大,焦域会向远离换能器的方向移动。     

基于受激发光实验的生物光尾流特性

 基于生物受激发光与流场机械刺激间的相关性,构建了库埃特流场刺激下的生物受激发光实验平台,分析了影响生物发光的水动力因素剪应力。实验结果显示：引起多边膝沟藻受激发光的剪应力阈值为0.1 N/m²;当流场中的剪应力大于发光阈值后,生物发光强度随着剪应力的增大而增强。采用FLUENT软件模拟计算了不同航速下某潜艇尾流场中的剪应力,计算结果表明：剪应力随着尾流长度的增加而减小,在近场区域剪应力出现了类似正弦波振荡的减小,且航速越大减幅越大,减速越快;当潜艇航速大于4.1 m/s航行时, 2 000 m处尾流的剪应力仍然大于引起生物发光的剪应力阈值。     

微秒脉冲激励的大气压氦等离子体射流放电特性北大核心CSCD

研究了不同电极结构以及放电参数对微秒脉冲激励的氦等离子体射流放电特性的影响。实验中采用不同管内径、不同电极形状、不同重复频率等参数,通过采集放电阶段的电流电压图、发光图像以及发射光谱等,对等离子体射流的电学特性和光学特性进行诊断。实验结果表明,随着管内直径的增大,氦等离子体射流的长度减小;管内径为8mm时,等离子体射流的击穿电压与放电电流最小,同时,其发射光谱中第二正带系N2,N+2和O等高能活性粒子的强度最高;管内径为5mm的等离子体射流的放电电流、功率及消耗的能量最大;在相同实验条件下,针尖电极结构中的放电电流、消耗的功率还有发射光谱强度都较大;随着重复频率的增加,氦等离子体射流的长度会增加,但击穿电压减小。     

光子图像统计处理

讨论了一种对有限光子图像累加的统计处理方法。在超微弱发光的研究中（例如细胞的超微弱光），由于发光强度较弱，需要用像增强器对超微弱发光图像进行增强得到可视光子图像，超微弱发光的光子图像不可避免地受到像增强系统暗噪声及背景噪声的影响，使光子图像湮没在噪声中。为从原始图像中检验出信号，常将采集到的有限光子图像进行累加以增加样本数，根据信号光子和噪声光子的不同统计分布，运用似然比检验方法得到有效判断累加图像中某像素是否含有信号光子，由此剔除图像中的噪声光子，得到信噪比改善的光子图像。     

超声换能器并联谐振频率的复合式跟踪方法研究

为了使超声换能器适应变化比较剧烈的负载,本文通过分析超声换能器在并联谐振频率附近工作时的频率特性和实际需求特点,利用变压器初级匹配方法得到了更好的频率特性,并对比了换能器空载和带负荷情况下的阻抗特性曲线,提出了复合式自动频率跟踪方法,空载时找到超声换能器最小电流的对应的频率点,加载过程中利用比例积分微分算法实现频率的快速跟踪。并对超声换能器在不同负载时的功率输出进行了实验,结果表明,复合式频率跟踪方法可以稳定地跟踪到超声换能器的并联谐振频率,能实现超声换能器的功率自调节,对提高换能器的工作效率和负荷适应能力具有实际的指导意义和应用价值。     

超微弱生物发光图像中的统计检验

应用光电阴极探测灵敏度为０．５ｃｐｓ／ｍｍ＾２的超高灵敏度的光电成像系统，获得了绿豆芽和活体昆明鼠的超微弱生物发光图像，并用统计理论研究了极弱光强条件下光子图像的信号检测和问题，文中在信号和噪声均为泊松分布的条件下，分析了从光子噪声中检验是否有信号的判据以及影响到检验的５个因素对检验结论的影响，以此判据成功地检验到实验获得的昆明鼠发光光子图像中的信号。     

小样本光子图像的统计处理

讨论了一种对小样本光子图像的统计处理方法。在超微弱发光的研究中（例如细胞的超微弱荧光）,由于发光强度极弱,需要用像增强器对超微弱发光图像进行增强得到可视图像,超微弱发光图像不可避免地受到像增强系统暗噪声及背景噪声的影响,使光子图像湮没在噪声中。为从原始图像中检验出信号,根据信号光子和噪声光子的不同统计分布,运用信号检测与的方法判断光子是否属于信号光子,并得到一简明的判据,由此判据剔除图像中的噪声光子,得到信噪比改善的光子图像。并用此方法处理了人掌的超微弱发光光子图像。     

超微弱发光图像的统计处理方法

应用超高灵敏度的光电成像系统，获得了活体昆明鼠的超微弱生物发光图像，并用概率统计理论研究了光子图像中的光子频数分布，得到了一种去除噪声的原则。文中首次提出了用Ｘ＾２准则拟合实际图像中的信号区和背景区分布，并根据得到的统计估计值对光子图像进行处理的方法，该法对实验获得的昆明鼠的超微弱生物发光有较好的处理效果，提高了图像的信噪比，得到了好的观察效果。     

Acoustic vibration of the amphibian eardrum studied by white noise analysis and holographic interferometry

The motion of the amphibian eardrum under free-field acoustic stimulation was investigated using time-averaged holography. We show that the amplitude is linearly related to sound pressure up to +/- 1000 nm. The frequency response of the eardrum shows broad resonance characteristics with a main peak between 1200-2200 Hz. The velocity of the tympanic membrane's motion at its resonance frequency matches the acoustic velocity of air particles. The resonance characteristics of the eardrum are also revealed by white noise stimulation. The power spectrum obtained by Fourier transformation of the autocorrelation of the response to noise resembles closely that obtained by holography.     

Underwater radiated noise from modern commercial ships

Underwater radiated noise measurements for seven types of modern commercial ships during normal operating conditions are presented. Calibrated acoustic data (<1000 Hz) from an autonomous seafloor-mounted acoustic recorder were combined with ship passage information from the Automatic Identification System. This approach allowed for detailed measurements (i.e., source level, sound exposure level, and transmission range) on ships of opportunity. A key result was different acoustic levels and spectral shapes observed from different ship-types. A 54 kGT container ship had the highest broadband source level at 188 dB re 1 μPa@1m; a 26 kGT chemical tanker had the lowest at 177 dB re 1 μPa@1m. Bulk carriers had higher source levels near 100 Hz, while container ship and tanker noise was predominantly below 40 Hz. Simple models to predict source levels of modern merchant ships as a group from particular ship characteristics (e.g., length, gross tonnage, and speed) were not possible given individual ship-type differences. Furthermore, ship noise was observed to radiate asymmetrically. Stern aspect noise levels are 5 to 10 dB higher than bow aspect noise levels. Collectively, these results emphasize the importance of including modern ship-types in quantifying shipping noise for predictive models of global, regional, and local marine environments.     

Experimental Estimation of the Emission Directivity of a Moving Surface Vessel in Shallow Water

The possibility of constructing an acoustic model of a surface ship’s noise emission in the far field using monopole-type emitters uniformly distributed along the hull is investigated. Experimental data obtained in shallow water are used to calculate the characteristics of equivalent monopole emission sources that form a total sound field similar to the sound field from a moving surface ship. The powers of each monopole and the cross-correlations between them are calculated. For the selected discrete components and linear model of an extended source, the directivity patterns are constructed, reduced to the free space. In the experiments and calculations, technical tools and algorithms were used that ensure high-precision positioning of the vessel with respect to the receiving elements of the array. An equivalent model of the waveguide transfer function in the operations area was preliminarily obtained by acoustic waveguide calibration using specially developed equipment, experimental techniques, and processing algorithms. This made it possible to use adequate seafloor models and the waveguide transfer function when calculating the equivalent sound field and directivity pattern. Good agreement is shown between the calculated and experimental data, both of the directivity pattern and field distribution along the transit characteristics. Practical recommendations are given for developing methods to measure the noise fields of surface vessels.

     

Suppression of acoustic noise in the measurements of wall pressure fluctuations

It is shown that, in hydrodynamic noise measurements in the presence of acoustic noise acting upon the pressure fluctuation receiver, spatial filtering methods should provide the best results. Active methods are developed for suppressing the acoustic noise that affects a miniature receiver in the course of turbulent pressure fluctuation measurements. The methods are based on complicating the structure of the measuring transducer by introducing an extra compensating sensing element whose characteristics are identical with those of the main sensing element. The spatial filtering of small-scale turbulent pressure fluctuations by a finite-size electroacoustic transducer is used as the basis for the development of noise-compensated measuring systems, as well as methods of measuring the turbulent pressure fluctuations by receivers with noise compensation. A numerical study of the wave-number filtering of acoustic noise in wall pressure measurements by a noise-compensated receiver is performed.     

Acoustic ship signature measurements by cross-correlation method

Cross-correlation methods were applied for the estimation of the power spectral density and modulation spectrum of underwater noise generated by moving vessels. The cross-correlation of the signal from two hydrophones allows the separation of vessel acoustic signatures in a busy estuary. Experimental data recorded in the Hudson River are used for demonstration that cross-correlation method measured the same ship noise and ship noise modulation spectra as conventional methods. The cross-correlation method was then applied for the separation of the acoustic signatures of two ships present simultaneously. Presented methods can be useful for ship traffic monitoring and small ship classification, even in noisy harbor environments.     

Analysis on spatial difference of ocean ambient noise spectrum in the northern South China Sea

To study the spatial difference of ocean ambient noise in offshore China and reveal its generation mechanism, the difference between the noise level(NL) of two stations is compared based on long-term observation data collected in the same season. Considering the influence of underwater acoustic channels and the characteristics of noise sources, the transmission loss level(TL) from the port ship noise sources to the measuring station is analyzed and it shows a strong correlation between the transmission loss difference(TLD) and noise level difference(NLD) at the band of 50-500 Hz. At the same time, the analysis of ship noise source level(SL) presents a difference about 20 dB between the two ports, corresponding to the variation of the ship density before and during the fishing moratorium. The results prove that the ambient noise is closely related to ship noise sources at this frequency band, and the noise difference between the two stations results from the transmission loss and the ship noise sources.     

界面条件下线型超声相控阵声场特性研究

开展了界面条件下线型超声相控阵声场特性的研究.将带有楔块的超声相控阵问题合理简化为液固界面的情况进行讨论.根据射线声学理论,计算了单阵元在液固界面存在时的辐射声场,进而推导了聚焦法则,得到了超声线型阵在液固界面存在时的声场、位移场表达式.对安装在楔块上的相控阵换能器的辐射声场进行了仿真,并讨论了聚焦对换能器轴向和横向声场的影响,结果表明利用聚焦能提高分辨率和灵敏度,但聚焦区域之外声束性能更差,在实际检测中要合理利用聚焦. 关键词： 超声相控阵 界面 声场 聚焦     

南海北部海洋环境噪声谱级空间差异性分析

为研究中国近海海洋环境噪声的空间差异性,揭示其形成原因,以同一海域同一季节两个测量站位的长期观测数据为研究对象,对比两个站位噪声谱级的差异.结合海洋信道和噪声源特性的影响,分析港口强噪声源到噪声采集站位的传播损失,发现在50~500 Hz频段传播损失差值和噪声谱级差值呈现较强相关性;对港口航船噪声源级的分析发现,该频段内源级相差20 dB左右,与休渔期前后航船密度差异相对应。试验分析和研究结果表明,南海北部海域50~500 Hz频段内海洋环境噪声与航船噪声源密切相关,两站位噪声空间差异由海区传播条件差异与航船噪声源级差异共同引起。      

浅海舰船噪声声强流的垂直方向性

浅海环境中,确定性声源的多途声信号干涉使得接收点处声强流的方向发生改变,不再与声源位置处的声强流方向一致。只测量声场的标量声强时,无法得到接收点处声强流的垂直方向性,而基于简正波矢量场建模和仿真,可获得理想条件下宽带点声源激发声场声强流的垂直方向性。本文采用单矢量水听器进行海上实验,获得了海洋环境噪声和干扰条件下舰船噪声声强流的垂直方向性。仿真和实验结果表明:远场条件下,浅海干涉现象引起接收点处声强流的方向(极角)随频率和距离变化,其时间-频率分布呈现与LOFAR谱干涉条纹相似的条纹,声强流的极角值主要分布在70?～110?范围内。