矢量拖线阵水听器流噪声响应特性

针对传统拖线阵流噪声理论的局限性, 建立了完善的矢量拖线阵流噪声理论分析方法, 可全面准确地揭示矢量拖线阵流噪声响应特性. 基于细长圆柱的湍流边界层压力起伏Carpenter模型, 采用波数-频率谱分析方法对矢量拖线阵流噪声响应特性进行了理论研究, 导出了圆柱形矢量水听器流噪声响应的声压和振速自功率谱及其互功率谱的解析表达式, 定量分析了流噪声响应功率谱与拖曳速度、水听器尺寸、套管尺寸和材料等参数之间变化规律; 另外, 还讨论了圆柱形矢量水听器偏离护套轴线时矢量拖线阵流噪声响应, 导出了流噪声响应的声压、径向和轴向振速自功率谱及其互功率谱的解析表达式, 数值计算结果表明: 轴线偏移距离对声压和轴向振速的高频噪声的影响要大于对低频噪声的影响, 而对径向振速的全频段噪声都有明显影响, 且对振速分量影响要远大于对声压影响.     

多管PDE爆轰噪声传播过程物理特性实验研究

为研究多管脉冲爆轰发动机爆轰噪声传播过程及声波物理特性,对单管至四管脉冲爆轰发动机爆轰噪声物理特性开展实验研究,获得了多管脉冲爆轰发动机爆轰噪声的波形、声压衰减规律、辐射特性、持续时间和频谱特性等物理参数。结果表明:在管口区域,爆轰噪声峰值衰减较快,在远离管口区域,衰减速率逐渐放缓。随着爆轰管数量的增加,爆轰峰值噪声在各方位角上的声压级逐渐增大。在远场区域内爆轰噪声指向性图呈“M”形。随着爆轰管数量的增加,爆轰噪声的A/B持续时间均增加。爆轰噪声是能量主要集中在中低频段的宽频噪声,其频谱峰值和峰值所在频点随爆轰管数量的增加而改变。      

Symbolic regression modeling of noise generation at porous airfoils

Based on data sets from previous experimental studies, the tool of symbolic regression is applied to find empirical models that describe the noise generation at porous airfoils. Both the self noise from the interaction of a turbulent boundary layer with the trailing edge of an porous airfoil and the noise generated at the leading edge due to turbulent inflow are considered. Following a dimensional analysis, models are built for trailing edge noise and leading edge noise in terms of four and six dimensionless quantities, respectively. Models of different accuracy and complexity are proposed and discussed. For the trailing edge noise case, a general dependency of the sound power on the fifth power of the flow velocity was found and the frequency spectrum is controlled by the flow resistivity of the porous material. Leading edge noise power is proportional to the square of the turbulence intensity and shows a dependency on the fifth to sixth power of the flow velocity, while the spectrum is governed by the flow resistivity and the integral length scale of the incoming turbulence.     

Dependence of NaI(Tl) detector intrinsic efficiency on source-detector distance,energy and off-axis distance: Their implications for radioactivity measurements

In this work the dependence of intrinsic efficiency of a NaI(Tl) detector of radius 3.82 cm and height 7.62 cm on source-detector distance (d), source-off-axis distance (d ₀) and γ-photon energy have been investigated using analytical and Monte Carlo methods. The results showed that, for a given off-axis distance, there exists a value of the ratio of source-detector distance (d) to detector radius (R) where intrinsic efficiency is minimum. This d/R value at which minimum efficiency occurs approaches zero as off-axis distance increases and it is almost constant with increase in energy. In the region where d/R < 0.01, a criteria given by Jehouani et al [1] for good photon detection, intrinsic efficiency decreases with increasing off-axis distance. The implications of the results for radioactivity measurement and radiation protection are discussed. Chacteristics of intrinsic efficiency in the regions d/R < 0.01 and d/R > 10 are also compared.      

空心高斯涡旋光束的远场特性

基于角谱法和稳相法,推导出了空心高斯涡旋光束的TE波和TM波在自由空间远场传输和能流密度的解析表达式,研究了其相位奇点和能流密度分布。结果表明,空心高斯涡旋光束的远场特性主要跟控制参数有关。改变光束中的涡旋离轴量,光涡旋和能流密度黑核会发生移动。圆刃型位错线的半径和能流密度暗环位置跟束腰宽度有关,而能流密度的对称性主要受涡旋离轴量的影响。     

矢量水听器拖曳阵的信号模拟器设计

本文中所设计的基于矢量水听器的拖线阵声纳模拟器,仿真了拖线阵的拖船干扰时域信号和空间传播特性,以及矢量水听器偶极子指向性。对于拖船干扰的仿真用Ecs模型来设定宽带功率谱,以AR模型来拟合指定功率谱,利用海底、海面对声场的一次反射作用仿真了拖船干扰的空间多途信道。利用插值滤波器的方法按照设定的时间精度仿真了宽带信号的阵元间精确延时。处理该模拟器所产生阵元时空采样仿真信号而得到的波束形成结果验证了该模拟器的有效性。     

伴流场中对转桨空化初生的判定与辐射噪声预报和校验

采用空化多相流瞬态模拟和边界元数值声学计算相结合的混合方法,预报了全附体假尾后对转桨在初生空化状态下的线谱和宽带谱噪声,分析了初生空化状态下对转桨噪声谱级相对于无空化状态的增量,提出了同时从流场与声场角度判定对转桨空化初生的四个充分条件。预报值与空泡水筒噪声测量值进行了比较。多相流瞬态模拟包括非定常雷诺时均模拟、尺度适应模拟和分离涡模拟三种方法。计算结果表明,在大范围进速系数范围内预报对转桨敞水性能曲线与测量值吻合很好。尺度适应模拟(SAS)和分离涡模拟(DES)在捕捉对转桨空化脉动压力时精度相当,均能满足非定常负载噪声预报的精度要求;雷诺时均模拟(URAN S)仅对于低频负载噪声来说基本适用。空泡体积脉动诱导线谱噪声在800 Hz处的预报误差小于4 dB;在800 Hz^3 kHz频段内,预报得到1/3oct中心频率处谱级的平均误差小于1.5 dB,总声级预报误差小于2.4 dB。空化初生的充分条件为:空化面积与桨盘面面积的比值小于2%、积分力和叶梢截面压力系数分布较无空化状态基本不变且中高频段噪声谱级增加8~10 dB。较好地解决了伴流场中对转桨空化初生判定和初生空化状态下辐射噪声预报的两个技术难题,可直接服务于高速、低噪声鱼雷的设计研发。      

Anisotropic packing and one-dimensional fluctuations of C60 molecules in carbon nanotubes

The confinement of a C60 molecule encapsulated in a cylindrical nanotube depends on the tube radius. In small tubes with radius RT approximately < 7 A, a fivefold axis of the molecule coincides with the tube axis. The interaction between C60 molecules in the nanotube is then described by a O2-rotor model on a 1D liquid chain with coupling between orientational and displacive correlations. This coupling leads to chain contraction. The structure factor of the 1D liquid is derived. In tubes with a larger radius the molecular centers of mass are displaced off the tube axis. The distinction of two groups of peapods with on- and off-axis molecules suggests an explanation of the apparent splitting of Ag modes of C60 in nanotubes measured by resonant Raman scattering.     

矢量拖线阵不同形状水听器流噪声响应特性

拖线阵声呐流噪声大小与水听器形状密切相关,为了揭示矢量拖线阵流噪声响应特性,研究了不同形状矢量水听器滤波特性及其噪声抑制特点。基于波数-频率谱分析方法,导出了圆柱形、球形、胶囊形矢量水听器的波数响应函数以及噪声功率谱解析表达式;并根据湍流边界层压力起伏Carpenter模型数值计算了矢量水听器波数响应函数、流噪声响应特性与水听器形状和几何尺寸之间的变化规律,得到了不同形状矢量水听器抑制流噪声特点。研究结果表明,细长胶囊形矢量水听器具有较好的流噪声抑制性能。      

Far-field spectral behaviour and spectral switching of super-Gaussian pulses

We have presented the far-field spectral behaviour of a super-Gaussian pulse diffracted through a rectangular aperture. It has been found that the on-axis modified power spectrum is always blue shifted, whose value increases with the increase in the value of the super-Gaussian parameter (m). For off-axis case, at some critical angle of diffraction, the diffraction induced far-field power spectrum is split into two lines of equal height. The far-field power spectrum is blue shifted when the diffraction angle is greater than the critical angle, whereas it is red shifted when the diffraction angle is less than the critical angle, i.e., spectral switching occurs. The critical angle for switching significantly increases with the increase in the super-Gaussian parameter (m). An important finding is that the switching frequency, the blue and red shift frequencies can be tuned by varying m.     

Prediction of environmental noise from fast electric trains

A model is presented for the environmental noise of fast electric trains on continuous welded rails, based on a consideration of the possible modes of vibration of the wheels. Evidence is presented to support the idea that the wheels are the dominant sources of noise and radiate as resonant dipoles. The model is calibrated by the results of measurements and methods of predicting train noise level and noise energy (per unit area) are developed. The noise intensity is found to increase as the fourth power of the speed. Energy (per unit area) at an observer increases as the third power of the speed and is proportional to train length divided by the distance of the observer from the track. The relevance of the model as a basis for the calculation of train noise in terms of some noise indices is then discussed and it is shown that it can be used to predict the parameters required by some indices which may be relevant to the subjective effects of environmental train noise.     

Resistance and Reactance of Monopole Fields Induced by a Test Charge Drifting Off-Axis in a Cold and Collisional Cylindrical Plasma

We study the interaction of a uniform, cold and collisional plasma with a test charged particle moving off-axis at a constant speed down a cylindrical tube with a resistive thick metallic wall. Upon matching the electromagnetic field components at all interfaces, the induced monopole electromagnetic fields in the plasma are obtained in the frequency domain. An expression for the plasma electric resistance and reactance is derived and analyzed numerically for some representative parameters. Near the plasma resonant frequency, the plasma resistance evolves with frequency like a parallel RLC resonator with peak resistance at the plasma frequency pe, while the plasma reactance can be capacitive or inductive in nature depending on the frequency under consideration.     

Cavity-enhanced absorption spectroscopy for shocktubes: Design and optimization

Cavity-enhanced absorption spectroscopy (CEAS) has generated much interest in shocktube kinetics studies because of its recent success in achieving improved sensitivity and high time resolution with robust optical alignment. While recent progress demonstrated experimental schemes including off-axis scanned-wavelength approach and on-axis ps-pulsed laser approach, that both successfully suppressed the laser-cavity coupling noise, this paper develops a theoretical model to predict the CEAS sensor performance that can be used as a design tool applicable to more generalized cases. The method models the optical field in the cavity based on the decentered Gaussian beam model, from which the cavity transmission spectrum and the laser-cavity coupling noise can be numerically calculated. The simulation results predict sensor performance for different cavity configurations and laser characteristics, including various degrees of laser-cavity mode-matching, laser linewidths, scanning rates, and cavity filling conditions. Simulation with example wavelengths in the ultraviolet, near-infrared, and mid-infrared showed increasing mode-matched beam waist size for increasing wavelengths. An off-axis alignment scheme was found to be capable of suppressing the coupling noise by two orders-of-magnitude at a moderate laser linewidth of 1?GHz. Coupling noise level on the order of 1e-5 for scanned-wavelength off-axis alignment case with a narrowband mid-infrared laser was obtained by model calculation and agreed with experimental results within acceptable uncertainty range. The developed method can serve to guide future design and optimization of CEAS system in shocktube studies.     

Effect of wall scattering on SNR in off-axis differential-type laser Doppler velocimetry

Though various properties and applications of laser Doppler velocimetry have been extensively studied in the past decade, there is little discussion on the effect of light scattering from the surface of a cell on Doppler beat signals or on methods of reducing it. In this paper, the effect of light scattering from the surface of the cell is treated as a background noise and is studied theoretically and experimentally on the detecting process of Doppler beat signals in off-axis differential-type laser Doppler velocimetry. Laser Doppler velocimetry of an off-axis type is verified to be effective for measurement of the flow velocity in the vicinity of a scattering wall. The effect of the light scattered from the wall surface on the signal-to-noise ratio (SNR) of Doppler beat signals is discussed in detail. The minimum distance, which is close to the wall and at which good Doppler beat signals can be obtained, is defined and determined quantitatively. This minimum distance is found to be strongly affected by the off-axis angle of the detecting optical system.     

Effective swimming strategies in low Reynolds number flows

The optimal strategy for a microscopic swimmer to migrate across a linear shear flow is discussed. The two cases, in which the swimmer is located at large distance, and in the proximity of a solid wall, are taken into account. It is shown that migration can be achieved by means of a combination of sailing through the flow and swimming, where the swimming strokes are induced by the external flow without need of internal energy sources or external drives. The structural dynamics required for the swimmer to move in the desired direction is discussed and two simple models, based respectively on the presence of an elastic structure, and on an orientation dependent friction, to control the deformations induced by the external flow, are analyzed. In all cases, the deformation sequence is a generalization of the tank-treading motion regimes observed in vesicles in shear flows. Analytic expressions for the migration velocity as a function of the deformation pattern and amplitude are provided. The effects of thermal fluctuations on propulsion have been discussed and the possibility that noise be exploited to overcome the limitations imposed on the microswimmer by the scallop theorem have been discussed.     

光溅射镀膜均匀性的优化模拟

为了改善脉冲激光溅射沉积大面积薄膜的均匀性,发展了基片离轴旋转的扫描技术.根据基片离轴旋转的基本原理和等离子体羽空间余弦分布规律,建立了径向膜厚分布公式.数值模拟了各种因素对基片离轴旋转扫描沉积薄膜均匀性的影响.分析表明,优化粒子束中心与基片中心偏置距离、溅射点与基片的距离是改善基片离轴旋转扫描镀膜均匀性的主要途径.同时也考虑了电机转速、镀膜时间和激光重频的影响.通过参量优化,当均匀度要求在95％时,计算得到薄膜的最大半径超过40 mm.     

New vehicle noise emission for French traffic noise prediction

Traffic noise prediction models in France are based on vehicle noise emission values defined by the French Guide du Bruit des Transports Terrestres (Noise Guide for Ground Transport - Noise levels prediction). These emission values are suited for models addressing the noise assessments of road infrastructures and the dimensioning of acoustic protections, needing traffic noise estimations in terms of ?_Aeq over a long period of time (an hour or more).The values, obtained from measurements collected in the 70s, are updated in the publication of a new guide (Methodological Guide, Vehicle noise emissions, to be published), which addresses the road surface influence on tyre/road noise. The emission values are now expressed through the contributions of a power unit component, function of traffic speed, traffic flow type and road declivity, and of a rolling noise component, function of traffic speed and road pavement.The paper outlines the procedures followed to determine the components, gives their numerical values, and illustrates some vehicle noise emissions.     

Inreased Power Concentration and its Effect on the Discharge Parameters on the Low Pressure Hg-Rare Gas Positive Column III. Effects of an Axial Rod

After the investigation of the influence of current and tube radius on the power concentration of the Hg-rare gas discharge in the two previous parts, part III deals with the corresponding effects of additional recombination surfaces. The plasma parameters (radiant flux of the resonance lines, field strength, electron density, electron energy distribution function) are measured in a discharge tube with an axial glass rod. With increasing rod diameter and increasing distance from the tube axis radial radiation intensity distribution becomes asymmetric. The field strength increases with the radius of the rod. The dependence of the field strength and that of the radiant flux on the parameters of the discharge do not basically change. The measured UV-radiation efficiency of the positive column is higher than that of discharges without axial rod for equal tube radius. This is attributed primarily to the changed radial electron density distribution and its effect on the collision processes.     

Noise shaping by interval correlations increases information transfer

The influence of intrinsic firing interspike interval correlations on the noise spectrum and information transfer is studied. This is done through the comparison of two simple firing models, one of which is a renewal process while the other displays interval correlations. These correlations are shown to shape the spike train power spectrum and, in particular, to decrease the noise power at low frequencies. Linear response theory and numerical simulations reveal how this shaping can increase the transmission of information about a time-varying signal. Our results are relevant to the analysis of nonrenewal point processes and signal detection in physics and biology.     

EXPERIMENTAL STUDY OF NOISE PRODUCED BY STEADY FLOW THROUGH A SIMULATED VASCULAR STENOSIS

An in vitro experiment is carried out in order to study the acoustic effects of a vascular constriction (stenosis) in people and provide correlations between these effects and parameters relevant to the hydrodynamic and acoustic processes. For this purpose, we measure the sound produced when water flows through an elastic tube which is either unobstructed or contains a rigid axisymmetric constriction. The sound is measured at the outside of a large annular container filled with water and bounded at the inside by the coaxial elastic tube. The analysis of the acoustic fields shows that a stenosis has two basic acoustic effects. These are a general increase in the sound level and the production of a number of additional distinct peaks (new frequency components) in the acoustic power spectrum. The frequencies of these peaks are close to the characteristic frequencies of vortex formation in the disturbed flow region behind a stenosis and the resonance frequencies of vibration of the post-stenotic segment of the tube. Another important result is that the stenosis generated acoustic power is approximately proportional to the fourth power of the stenosis severity and the same power of the flow Reynolds number.