Dropwise chains as the elements of water fog spatial structure

Video images of water fog drops were acquired under standard atmospheric conditions with weak turbulence of the environment. Pair correlation functions of the drops were performed and traces of the spatial arrangement of the drops inside the fog determined. The fog structure carriers are the drop chains with a fixed interdroplet distance. The possible influence of the drop chains on the shear viscosity and fog surface tension has been analysed.     

双声源位置和强度识别的三维声强方法

目前在远场识别声源空间位置和强度缺乏行之有效的方法。针对此问题,提出采用四传声器进行三维声强测量,从而构建出声强、声源坐标和声功率的非车线性方程组,求解方程得出声源空间坐标和强度的方法。以3个三维声强探头对两个同频率单极子声源的识别为例,分别利用数值仿真和半消声室内的实验进行方法验证,并对声源的识别空间分辨率做了测试,得出角度识别最大误差为3.83°,为真实值的8.5%,距离识别最大误差0.1 m,为真实距离的10%。结果表明采用该方法空间坐标和声功率识别均具有很高的准确度,双声源的空间位置分辨力也优于远场声全息方法。      

用于土壤水分测量的磁共振射频线圈宽频匹配方法

准确检测土壤水分对于改善农业用水管理至关重要.为了实现土壤水分的准确测量,本文设计了一种微型inside-out核磁共振（NMR）传感器（测量区域位于传感器周围）.其中磁体由三个轴向磁化的圆柱磁体组成,射频线圈由四个相同的弧面螺旋线圈串联组成.通过调整圆柱体之间的半径和高度,获得了一个圆周对称恒定梯度场.并利用宽频带匹配方法,增加射频线圈的激励区域,从而提高整体信噪比.将本文设计的传感器用于测量含水量不同的小颗粒沙土和大颗粒石英砂土壤模型样品的CPMG（Carr-Purcell-Meiboom-Gill）信号,通过逆拉普拉斯变换获得了T₂分布谱,并将T₂谱峰面积与称重法获得的含水量进行了相关性分析,探讨了该传感器用于测量土壤水分的可行性.     

不同质量含水量的土壤反射率光谱模拟模型

土壤含水量的时空分布与变化情况对土壤温度变化、陆地—大气间热量平衡以及陆面大气环流产生显著的影响,因此,对大范围内土壤含水量进行实时动态监测,获得某段时间内土壤含水量的连续变化情况具有重要的意义。研究目的是借助高光谱遥感手段,通过构建不同质量含水量的土壤反射率光谱模拟模型,深入了解土壤质量含水量与土壤反射率光谱之间的关系,为监测土壤含水量提供有效手段。利用ASD Field Spectral FR野外光谱仪和加水称重法获得北京市8个采样点的土壤样品不同质量含水量下的土壤反射率光谱实测数据,利用其中2个土壤样品不同质量含水量下的光谱数据构建含水土壤反射率光谱模拟模型,并利用未参与建模的另外6个土壤样品数据对该模型的模拟效果进行了检验。通过数据验证发现,当土壤质量含水量小于田间持水量时,该模型的模拟精度较高;而且对于不同的土壤样品,模型的模拟效果都比较好。最后又利用北京大学校园内三个采样点的实地测量光谱数据对模型进行了验证,光谱的模拟值与实测值之间的均方根误差最小可达0.005 8。因此该模型可实现对质量含水量小于田间持水量的不同类型土壤的反射率光谱进行较高精度的模拟。     

中红外光谱的HFC水分含量和粘度测量

针对水-乙二醇抗燃液压液（HFC）目前常用的水分含量和粘度测量方法存在操作复杂,耗时长,测量成本较高等问题,研究了将中红外光谱分析法应用于HFC水分含量和粘度测量。以85个实际在用油样为样本集,采用传统的电量法和毛细管法分别测量水分含量和运动粘度;使用衰减全反射液体池（ATR）采集各样本的中红外光谱并进行光谱校正、背景扣除、Savitzky-Golay(SG)平滑处理和基线校正等预处理。水分含量测量模型构建过程中,利用学生化残差-杠杆值法寻找强影响点,判定为异常样本并加以剔除;利用马氏距离SPXY法划分建模样本集和验证样本集。根据Beer-Lambert定律,水分含量和红外光谱吸光度成正比,采用线性方法来构建水分含量和吸光度红外光谱之间的关系模型。同时为提高模型的鲁棒性和泛化能力需要消除光谱中的无用信息、冗余信息和噪声,因此采用后向间隔偏最小二乘法（BiPLS）进行水分含量测量特征谱区优选,并建立线性校正分析模型。结果表明剩余3 526~3 354,1 790~1 618,3 005~2 660和1 096~924 cm-1波段时获得最小的交叉验证均方根误差（RMSECV）。特征波段优选结果可以理解为,根据3 526~3 354 cm-1波段预测HFC体系中所有OH的量,减去根据3 005~2 660和1 096~924 cm-1波段换算得到的乙二醇中的OH,进而计算得到水分含量,为进一步提高精度,将水中OH变角振动吸收峰对应的1 790~1 618 cm-1波段作为水分含量测量的辅助波段。利用独立验证样本进行模型检验,结果表明所建立的线性模型相关系数（r）为0.989 5,预测均方根误差（RMSEP）为0.405 2,满足HFC中水分含量测量精度需求。粘度测量模型构建过程中,利用马氏距离法寻找离群样本,判定为异常样本并加以剔除,利用马氏距离SPXY法进行建模样本集和验证样本集划分。粘度是一个与多重因素有复杂关系的理化指标,和红外光谱之间表现为非线性关系。基于主成分分析结合BP神经网络法（PCA-BPNN）构建非线性粘度校正分析模型,提取累积贡献率为95.12%的前10个主成分作为输入,测得的粘度值为输出,创建单隐层BP神经网络并进行训练,训练结果相关系数r为0.996 8。利用独立验证样本进行模型检验,结果表明所建立的非线性模型r为0.984 3,RMSEP为0.615 1,满足HFC粘度测量精度需求,且优于采用BiPLS线性模型获得的结果。确定了可用于HFC中水分含量测量的四个中红外波段,可为窄带红外光谱分析法或其他类似研究提供依据。研究结果表明中红外光谱分析法结合BiPLS和PCA-BPNN分析法可分别应用于HFC水分含量和粘度测量,精度符合监测需求,为HFC中分含量和粘度测量提供了一种新的测试方法,相比于传统测量方法具有快速、无损、低成本的特点。     

Spectral-energy characteristics of sound fields scattered by fine-structured inhomogeneities of sound velocity into the geometrical shadow zone

In analyzing experimental results obtained with explosive sources in the tropical zone of the Indian Ocean, a good agreement was obtained for spectral-energy characteristics of signals observed in the first geometrical shadow zone with computer calculations of the sound field scattered by fine structure inhomogeneities of the fractal type. From the comparison of the results of calculating the frequency characteristics of sound fields in the shadow zone by the wave code and by the method combining ray acoustics with the wave theory of sound scattering, it was found that both methods are appropriate for describing the real processes of scattering and propagation of sound in the ocean with fine-structured stratification and that these methods can be used for solving inverse problems.     

基于高光谱技术的土壤水分无损检测

利用高光谱成像仪(光谱范围400～1 000 nm)对土壤含水率进行了无损检测。比较了208个土样不同天数下土壤含水率与光谱变化、不同质量含水量光谱的差异;对比分析了不同光谱预处理方法、不同方法提取特征波长、采用多元线性回归(multiple linear regression,MLR)、主成分回归(principal component regression,PCR)与偏最小二乘回归(partial least squares regression,PLSR)建模,优选出最佳模型。结果表明：光谱曲线的反射率随着土壤含水率的增加而减小。当超过田间持水率时,光谱曲线的反射率会随着土壤含水率的增加而增大。对比分析了不同预处理方法,近红外波段优选出单位向量归一化预处理方法。采用无信息变量消除法(UVE)、竞争自适应加权采样(CARS)、β系数法、连续投影算法(SPA)方法提取特征波长为49,30,5和7。为了减少数据冗余,对UVE与CARS提取的特征波长进一步采用SPA方法进行特征提取,UVE+SPA,CARS+SPA提取特征波长数分别为5和8个。在此基础上,利用MLR,PCR和PLSR方法对400～1 000 nm范围的特征波长建立模型,对比分析不同建模效果,优选出β系数提取的特征波长的MLR模型。最优的特征波长为411,440,622,713和790 nm,最优模型的预测相关系数R_p=0.979,预测均方根误差RMSEP为0.763。因此,今后可采用不同波段对土壤含水率进行定量分析。     

农田土壤水分含量的激光诱导荧光光谱表征

通过实验探讨了植物的激光诱导叶绿素荧光光谱与土壤水分间的关系。实验利用Y-形光纤探头获取了植物在450 nm LED光源照射下所发出的叶绿素荧光光谱,同时利用TDR传感器测量土壤湿度。实验以水稻为研究对象,研究了在持续水分胁迫和间歇水分胁迫下,叶绿素荧光光谱743 nm附近波峰的变化,结果发现,波峰强度与土壤水分含量具有相关性。最后,利用Lorentzian方程,将持续水分胁迫下的土壤含水量与叶绿素荧光强度进行建模,发现所建的模型具有较高的决定系数,说明该方法可以应用于农业生产中对土壤水分的测量。     

深海近距离声场频率-距离干涉结构反演海底声学参数

地声反演一般存在多解性的问题,不易有效分离沉积层声速与厚度,本文提出了一种当声源与接收水听器分别位于海面与海底附近时匹配声场干涉周期反演深海沉积层声速与厚度的方法。该声场周期结构由直达波与海底沉积层反射回波干涉形成,其干涉周期受沉积层声速和厚度影响,通过匹配实测与理论计算的声场干涉周期可有效反演沉积层声速和厚度。开展了深海实验验证,利用水听器接收到的近距离船舶噪声的声场干涉条纹,反演得到的实验海域海底沉积层声速与该海域海底表层采样平均值差小于20 m/s。数值仿真与实验结果表明该方法可以有效反演沉积层的声速与厚度。      

低频声音在水雾中衰减的测量

一.引言 低频声音在霧气中的衰减的测量可以在充霧的混响室中行之,但是这种测定方法只限制在一些分立的低频率——即相当于混响室的低简正振动方式。为了使这种测量能够在较广的连续频程中进行,作者採取了阻抗管的驻波分析方     

微观结构对膜换湿能力影响研究

对多孔膜换湿过程进行了理论分析,得到了透湿量和湿阻的计算式.同时对三种膜包括PVDF,PES和纤维素膜进行了湿阻的测量实验,通过对实验数据的回归处理,得到了多孔膜的结构参数.结果表明,膜厚、孔径分布、孔隙率及曲折因子对膜的换湿特性都有较大影响,在平均孔径相同的情况下,其它因素仍然导致了不同膜换湿能力的较大差距。在所获得的结构参数基础上,通过数值模拟的方法研究了平均孔径对膜换湿能力的影响。结果表明湿阻随平均孔径的增大而减小,减小趋势逐渐平缓最后趋向一定值。     

Isotopic composition of atmospheric moisture from pan water evaporation measurements

A continuous and reliable time series data of the stable isotopic composition of atmospheric moisture is an important requirement for the wider applicability of isotope mass balance methods in atmospheric and water balance studies. This requires routine sampling of atmospheric moisture by an appropriate technique and analysis of moisture for its isotopic composition. We have, therefore, used a much simpler method based on an isotope mass balance approach to derive the isotopic composition of atmospheric moisture using a class-A drying evaporation pan. We have carried out the study by collecting water samples from a class-A drying evaporation pan and also by collecting atmospheric moisture using the cryogenic trap method at the National Institute of Hydrology, Roorkee, India, during a pre-monsoon period. We compared the isotopic composition of atmospheric moisture obtained by using the class-A drying evaporation pan method with the cryogenic trap method. The results obtained from the evaporation pan water compare well with the cryogenic based method. Thus, the study establishes a cost-effective means of maintaining time series data of the isotopic composition of atmospheric moisture at meteorological observatories. The conclusions drawn in the present study are based on experiments conducted at Roorkee, India, and may be examined at other regions for its general applicability.     

湖泊沉积物微量元素ICP-AES与XRF分析方法和相关性研究

首次以托素湖沉积物为研究对象, ICP-AES法测定沉积物微量元素Cd,Cr,Cu,Zn和Pb的含量, 不同的消解法研究和优化结果确定了一个最适合托素湖沉积物前处理的体系为： 5∶5∶5∶1∶1的HCl-HNO₃-HF-HClO₄-H₂O₂体系。同时与定性的XRF岩芯扫描法对托素湖沉积物微量元素进行分析,以及用含水量校正的方法,讨论了含水量对扫描方法的影响。数据比较表明,与背景值相对比,托素湖沉积物中的微量元素Cd和Zn含量偏高,Cr,Cu和Pb的含量均在背景值范围内,而XRF岩芯扫描主要受沉积物元素的控制,在一定程度上也受到沉积物中含水量的影响,两种方法测量结果有显著的正相关关系(r为0.673～0.925),具有很好的可比性。     

Measuring thickness change of transparent plate by electronic speckle pattern interferometry and digital image correlation

The thickness change of transparent plates was measured by electronic speckle pattern interferometry (ESPI) method and digital image correlation (DIC) method. An out-of-plane ESPI system was developed based on the Michelson interferometer, and a new thickness measurement method was designed, which is on the basis of Snell's law of refraction and DIC. The main principles and experimental procedures of these two methods were presented. The thickness change of polymethyl specimens under uniaxial tensile loading were measured by the optical techniques and compared with each other. The results reveal that the data obtained with DIC method achieve better linearity than ESPI.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

Optimized constraints for the linearized geoacoustic inverse problem

A geoacoustic inversion scheme to estimate the depth-dependent sound speed characteristics of the shallow-water waveguide is presented. The approach is based on the linearized perturbative technique developed by Rajan et al. [J. Acoust. Soc. Am. 82, 998-1017 (1987)]. This method is applied by assuming a background starting model for the environment that includes both the water column and the seabed. Typically, the water column properties are assumed to be known and held fixed in the inversion. Successful application of the perturbative inverse technique lies in handling issues of stability and uniqueness associated with solving a discrete ill-posed problem. Conventionally, such problems are regularized, a procedure which results in a smooth solution. Past applications of this inverse technique have been restricted to cases for which the water column sound speed profile was known and sound speed in the seabed could be approximated by a smooth profile. In this work, constraints that are better suited to specific aspects of the geoacoustic inverse problem are applied. These techniques expand on the original application of the perturbative inverse technique by including the water column sound speed profile in the solution and by allowing for discontinuities in the seabed sound speed profile.     

南疆沙尘区骏枣叶片水分含量检测的近红外光谱预处理方法对比

南疆地区沙尘多、灰尘大,枣树叶片表面经常覆盖一定程度的粗颗粒度沙尘,为了有效去除沙尘、灰尘在枣树叶片水分光谱测量过程中产生的散射噪声和基线漂移,研究一种适用于风沙较大地区的枣树叶片水分含量的快速检测方法,以不同灌溉梯度下的枣树叶片为研究对象,通过近红外光谱仪获取120个叶片样本的1 000～1 800 nm的光谱数据,并同步测量叶片水分含量,采用归一化、移动窗口平滑、SavitZky-Golay（SG）卷积平滑、SG求导、标准正态变量校正(SNV)和多元散射校正(MSC)等方法对原始光谱进行预处理,分析对比不同方法对散射噪声的处理能力,采用偏最小二乘回归分析方法筛选了敏感波段和建立预测模型。实验结果表明,枣树叶片水分含量强吸收峰为1 443 nm,波谷为1 661 nm;归一化光谱并未消除1 000～1 400 nm波段的散射噪声;移动窗口平滑和SG卷积平滑并未改进光谱曲线,散射噪声仍然存在;SG导数光谱的光谱特征峰和特征谷明显左移,光谱曲线不够平滑,噪声明显;SNV和MSC方法具有较好的散射噪声消除能力。偏最小回归分析方法筛选特征波长的结果表明（设置筛选波长数量为5）,基于原始光谱未筛选到1 443 nm的强波峰和1 661 nm的波谷附近的波段;基于归一化光谱在1 450 nm波峰附近筛选的波长有一定的偏差,在1 661 nm波谷附近的筛选的波长明显高于1 700 nm;基于移动窗口和SG卷积平滑光谱在1 443 nm具有一定的筛选能力,但并未筛选到1 661 nm附近的波长;导数光谱并未筛选到1 443和1 661 nm波段;SNV和MSC在波峰和波谷位置附近均筛选了敏感的光谱波段,其中MSC略优于SNV方法恰好在波峰和波谷位置,共筛选了1 002, 1 383, 1 411, 1 443和1 661 nm五个特征波段,也证明了MSC方法散射噪声和基线漂移处理能力最优,提高了敏感波长的筛选能力。偏最小二乘回归模型结果表明,不同预处理方法的RMSE值均较低,SNV和MSC方法改进了模型的预测结果,R2高于0.7,其中基于MSC方法的模型具有最高的R2和最低的RMSEP和RMSEPCV,R2=0.750 4,RMSEP=0.034 3,RMSECV=0.021 5,预测结果较优。证明MSC方法对沙尘和颗粒度引入的散射噪声具有较好的去除能力,可改进波长的筛选、提高预测模型精度,为南疆沙尘区的枣树叶片水分含量的无损检测提供了有效方法。     

窄脉冲声用于大样品的吸声测量

本文利用逆滤波器原理,在空间产生了波形可控、长度在毫秒量级的窄脉冲声信号,分别采用脉冲分离法和脉冲叠加法,对一种毛毡材料和三种不同厚度的海绵材料进行了吸声系数的测量。实验证明,基于窄脉冲声信号的吸声测量结果与ISO13472-1:2002中的MLS脉冲法及阻抗管的测量结果基本吻合。采用窄脉冲进行吸声测量,可以减少样品边缘和周围环境对测量信号的干扰,提高现场测量的准确性。     

Sound field prediction in long enclosures with branches: A combined method

The propagation of sound in long enclosures with branches has been studied theoretically and experimentally, and an efficient combined method is proposed to predict the sound field in long enclosures with branches. Based on the wave-acoustics theory, the theoretical analysis of the sound field of the long enclosures with branches is performed. This paper also investigated the sound field prediction of long enclosures with branches, by using the acoustic modeling program, ODEON. The results obtained by the theoretical analysis and the numerical simulation ODEON are compared with the experimental measurements, and the characteristics of the two methods for predicting the sound field of long enclosures with branches are analyzed. Compared with the experimental results, it is found that: (1) the results predicted by the theoretical analysis fluctuate relatively large with respect to the source-receiver distance, and the sound pressure level (SPL) attenuation obtained is smaller than that measured; and (2) the results predicted by the numerical simulation is smoother, and the calculated SPL attenuation is larger than that measured. To effectively predict the sound field of long enclosures with branches, a combined numerical method is thus proposed. The effectiveness of the proposed combined method is demonstrated by the scale-model experiments.     

Experimental identification of physical parameters of fluid-filled pipes using acoustical signal processing

A method is formulated for the identification of an unknown physical parameter of a fluid-filled pipe using the measurement of sound speed in the pipe. The method uses a simple formula which provides the relationship between the sound speed and a few physical parameters of the pipe: thickness, diameter, wall material constants and fluid constants. Once the sound speed in the pipe is measured, the simple formula can be used to extract one of the unknown parameters providing the remaining ones are known.The sound speed in the pipe is measured using a 3-transducer array. In order to demonstrate the potential of the technique the results of several measurements obtained in a water-filled steel pipe are presented.The required accuracy of the measurement of sound speed and of the specification of known parameters is analysed. The accuracy depends on the unknown parameter which is to be identified. For example, if the pipe thickness is the unknown parameter, the other parameters have to be known within a very narrow margin of error. On the contrary, if the fluid properties have to be identified the needed accuracy of known parameters gets much lower.