Numerical study of glare spot phase Doppler anemometry

The phase Doppler anemometry has (PDA) been developed to measure simultaneously the velocity and the size of droplets. When the concentration of particles is high, tightly focused beams must be used, as in the dual burst PDA. The latter permits an access to the refractive index of the particle, but the effect of wave front curvature of the incident beams becomes evident. In this paper, we introduce a glare spot phase Doppler anemometry which uses two large beams. The images of the particle formed by the reflected and refracted light, known as glare spots, are separated in space. When a particle passes through the probe volume, the two parts in a signal obtained by a detector in forward direction are then separated in time. If two detectors are used the phase differences and the intensity ratios between two signals, the distance between the reflected and refracted spots can be obtained. These measured values provide information about the particle diameter and its refractive index, as well as its two velocity components. This paper is devoted to the numerical study of such a configuration with two theoretical models: geometrical optics and rigorous electromagnetism solution.     

Some effects of optical alignment errors in a differential laser Doppler velocimeter

A formulation has been developed to treat the effects of alignment errors of the optical system used in a differential laser Doppler velocimeter. It is then applied to analyse the properties of output beat signals obtained by the velocimeter. The effects of optical alignment errors on the spectral width and signal-to-noise ratio of the output beat signals and on the probing volume are investigated in some detail. The spectral width is not affected very much by the deviation of the angle between two beams incident on the probing volume, but depends mainly on the number of interference fringes produced in that volume. The signal-to-noise ratio is very sensitive to the displacement of two incident beams on the probing volume and is also affected by the intensity and beam width differences of these two beams. The probing volume is evaluated in the case of two incident beams having different beam widths.     

Coherent Scattering by Multiple Particles in Phase Doppler Interferometry

A theoretical model has been developed for studying the response of the phase Doppler interferometer when multiple particles are simultaneously present within the measurement probe volume. The developed model incorporates the geometrical optics theory for describing the coherent interaction between the scattered light signals of multiple particles, eachhaving different size, velocity, trajectory, and arrival time. The resulting Doppler signal is processed by a theoretical signal processor which can simulate the performance characteristic of different signal processing schemes that are widely used in phase Doppler interferometry, namely, zero-crossing counter, covariance, autocorrelation and DFT parocessors. The application of the developed model for studying the coherent scattering by two particles has been specifically addressed in this paper. It has been shown that a DFT processor can be used to simultaneously measure the size and velocity of the two particles in most instances. However, for more than two particles, the signal processing scheme becomes more complex because of a quadratic increase in the beat frequency components.     

光纤非接触加速度计

介绍一种新型光纤接触加速度。入射激光经长短不同的两光纤，在两个不同的时间对物体同一点的速度取样，由物体返回的光束经偏振光实现长短光路互易成近于等光程干涉，利用外差技术测量多普勒频差得到物的加速度值。     

Trajectory Ambiguities in Phase Doppler Systems: Study of a near forward and a near-backward geometry

Generalized Lorenz-Mie theory for the scattering of arbitrarily shaped beams by spherical particles has been applied to two standard phase Doppler layouts, employing receiving units at 30° and 150° off-axis locations. It is shown that the particle trajectory effects may lead to inaccurate size measurements for the near-forward receiver and may make the near-backward measurements totally misleading when a large particle size range (1:40) needs to be covered. Only limited improvements can be achieved by using two phase-shift signals from a single receiving unit for discrimination. The errors associated with the trajectories are also detrimental to the concentration measurements based on the existing criteria. However, an extended optical system employing two identical receiving units, located symmetrically about the plane of the laser beams, provides a robust solution to the trajectory ambiguity. It can be used to measure correctly the particle size and the particle location in the measuring volume. The difficulties associated with estimating the effective size of the measuring volume as a function of the particle diameter (in order to determine the true size distribution and the particle number density) may also be resolved by employing an extended system. Hence, despite a higher cost, this arrangement is attractive, at least for obtaining some benchmark simultaneous measurements of sizes and velocities in two-phase particulate flows.     

A particle timing laser velocity meter

A fluid velocity meter is described; this operates by measuring the time taken for individual dust particles carried in the flow to travel between two points in space. A laser is used to produce two light beams parallel to one another and approximately normal to the flow direction. These beams are focused to two point or line foci in the region of interest: these foci are separated by a distance of the order of 1 mm in the flow direction. A particle crossing both foci in succession gives two scattered light pulses, and the time interval between these is measured by an oscilloscope or digital chronometer. The particles are natural dust particles occurring in the laboratory air or tap water. As in Doppler and fringe anemometers, the advantage of the laser is its spatial coherence: it enables extreme intensity to be obtained in small, well-defined regions. The instrument might be described as a fringe anemometer in which all the light is concentrated into the two end fringes, the others being eliminated.     

Measurements of the velocity gradient using a laser Doppler phenomenon

This paper reports on a method for measuring the velocity gradient using a laser Doppler phenomenon. The velocity gradient is determined from the velocity difference between two different points of the probing object and is actually obtained from the optical heterodyning of two differently Doppler-shifted scattered light fields from two points of the object having different velocities. The properties of output beat signals are theoretically investigated from their spectral broadening including the effect of the detecting aperture. The preliminary experiment was performed by using a rotating glass disk whose velocity is different from its center to the outside. The experimental results shows the usefulness of the present method for measurements of the velocity gradient.     

非球形椭球粒子参数变化对光偏振特性的影响

针对自然界中多数沙尘、烟煤粒子的非球形问题, 在球形粒子偏振特性的基础上, 进一步研究非球形椭球粒子的折射率、有效半径、粒子形状等参数变化对光偏振特性的影响, 采用基于T矩阵的非球形粒子仿真方法, 模拟非偏振光经椭球粒子传输后光的偏振特性及其与球形粒子间的差异, 并以实际沙尘、海洋、烟煤三种气溶胶粒子为例说明结果的正确性. 结果表明: 当折射率实部越小, 虚部越大时, 球形粒子与非球形粒子的偏振差异越不明显; 当粒子有效半径增加时, 球形粒子偏振度的变化比非球形粒子更为明显, 且最大值分别出现在散射角为150°和120°的位置; 当粒子形状不同时, 不同形状椭球及球形粒子的差异在散射角小于60° 时并不明显, 且当椭球粒子纵横比互为倒数时, 两种粒子的偏振特性近似相同. 通过以上分析可知, 在光传输过程中, 椭球粒子多数情况下无法被近似为球形粒子进行计算.     

Absorption and Scattering of Light by Highly Concentrated Two‐phase Flows

The spray cone emerging during an extended metal atomization process (called spray forming) has been investigated in order to quantify the influence of highly concentrated multiphase flows on phase‐Doppler‐anemometry (PDA) measurements. Using this non‐intrusive, optical measurement technique not only the local particle size and velocity distributions of the spray can be obtained but also additional information about the mass flux in the multiphase flow. Since standard phase‐Doppler systems can be easily applied to low concentrated particle systems (spherical particles with smooth surfaces and an optical transparent continuous phase taken for granted) the application of this measurement technique to highly concentrated multiphase flows is more complex. Both the laser light propagating from the PDA device to the probe volume and the scattered one going backward to the PDA receiving system are disturbed by passing the highly concentrated multiphase flow. The resulting significant loss in signal quality especially concerns the measurement of the smaller particles of the spray because of their reduced silhouette (in comparison with the bigger ones). Thus, the detection of the smallest particles becomes partially impossible leading to measurement of a distorted diameter distribution of the entire particle collective. In this study the distortions of the measured distributions dependent on the particle number concentration as well as on the path length of the laser light are discussed.     

Low-Concentration Photon Correlation Spectroscopy

Photon correlation spectroscopy (PCS) is a technique to measure rapidly particle size in the sub-micrometre region. The use of PCS is, however, limited by concentration. The upper limit is due to multiple scattering of the incident light and the lower limit is determined by the fact that fluctuations of the number of particles in the measuring zone have a significant influence on the apparent diffusion coefficient. In this paper a signal processing method is described which differentiates this influence. With this system the lower limit is no longer limited to about 100 particles in the measuring volume corresponding to a concentration of 10⁹ particles/cm³. The limitation is now the intensity of the scattered light, which becomes too weak at a concentration of about 50 particles/cm³. As a consequence of this work, a revision to the basic theory of PCS may be necessary. Moreover, the new processing method also permits the measurement of the particle concentration in the sample.     

激光多普勒测速中散射粒子的大小对信噪比的影响

采用双光束干涉条纹模型,用光散射理论分析了激光多普勒测速中球形散射粒子的大小对散射光信噪比的影响。用数值解法求得了信噪比随散射粒子半径变化的曲线。仅从穿过测控区时光信号的信噪比这一方面考虑,结论是:粒子半径的取值不应大于干涉条纹宽度的0.35倍。     

Measuring the polarization of coherent light beams

We consider the conditions of interference of totally polarized light beams of arbitrary polarization that allow one to measure their vector characteristics by means of optical heterodyning. It is shown that the intensity of a light beam resulted from the superposition of two plane totally polarized waves with arbitrary polarizations represents a sum of two interference patterns, each of them being the result of superposition of conamed components of an orthogonal expansion of the electric field of the total light wave. The possibility of electronic registration of information in real time is demonstrated.     

A Fiber Optic Non- Contact Accelerometer

A novel non-contact fiber optic accelerometer is presented. Two laser beams that have passed through two different lengths of feed fibers are launched to the same moving target. The frequency of each beam is Doppler shifted by movement of the target. The frequency shift of the laser beam corresponds to the velocity of the target at the time of launch. The difference in launching time due to the difference in feed fiber lengths samples the velocities of the target at times spaced by the differential arrival times of the laser beams. Two return passes of the reflected light are also made different so that the light passed through the shorter feed fiber is steered to the longer return fiber by means of a combination of A/4 plate and polarization beam splitter (PBS). This arrangement of equalizing the total paths relieved a stringent requirement of the coherency of the laser. Two reflected light beams are mixed and the beat frequency corresponds to the difference in the Doppler shift frequency at two different times of arrival of the launcher. An accelerometer based upon this principle was constructed and its performance was examined using a sinusoidally vibrating hydrophone as a target. Features of such an accelerometer are reduction in the coherency requirements of the light source and simpler amplification of the signal using heterodyne detection.On leave of absence from Department of Optical Engineering, Zhejiang University, HangZhou, China 310027     

Measurements on Particle Size Distribution and Concentration by Transmission Fluctuation Spectrometry with Temporal Correlation

Transmission fluctuation spectrometry with temporal correlation (TFS‐TC) is a new method for particle analysis. When a narrow light beam irradiates on a particle dispersion flow, the variation of the number of particles in the small measuring zone will cause the transmitted light to fluctuate, which includes the complete information on both particle size distribution (PSD) and particle concentration. The method may be used for real‐time, inline/online applications due to its simplicity of measuring principle and experimental setup. Until recently, the theory has been limited to low particle concentrations. In this work, an experimental study of the TFS‐TC measurement is presented for a very wide range of the particle concentrations. By introducing an empirical correction including the high concentration effects and considering the effect from rheological conditions in the inversion algorithm, the particle size distribution and particle concentration are reconstructed, resulting in the coverage of a broad range of particle size and concentration.     

Light Scattering by Homogeneous Axisymmetric Particles for PDA Calculations to Measure Both Axes of Spheroidal Particles

Mathematical tools are provided for the computation of the scattered field produced by a non-spherical particle moving through the measurement volume of a phase Doppler anemometer. The Doppler signal is modeled by using the rigorous extended boundary condition method and an approximate ray theory. The region of applicability of ray theory is discussed. In particular, a pure refraction model is considered for large particles. It is shown that under some circumstances the phase Doppler technique may be used for sizing of spheroidal particles.     

Measurement of Size,Number Concentration and Velocity of Aerosol Particles using an optical particle counter

An aerosol measurement instrument is presented which allows for the simultaneous measurement of the size distribution, number concentration and velocities of particles. A commercial optical particle counter (OPC) was modified in terms of optics and signal evaluation to provide the required measurement information. The design of this instrument allows the definition of a cubic measuring volume by purely optical means. This is achieved by an aperture/lens system which projects a sharply defined light beam into a stream of aerosol flow. Light scattered from single particles at average angles of 90° is collected by two opposite receiver units, each projecting light on to a separate photomultiplier. The intensity of the scattered light with this instrument is found to be an unambiguous function of the particle size. The total number of particles detected per unit time results in the particle flux. The particle velocity can be calculated, in principle, through the correlation of the signal length and the optical length of the measuring volume, provided that the particles have a straight trajectory through the measuring volume and the measuring volume length in the mean flow direction is well defined. The absence of sharpness in real optical projections effects a border zone of definite length, in which the illumination declines to zero. This leads, together with the low-pass filtering of the particle signals, to an increase in the length of the signal slopes, causing some difficulties in the determination of the signal length. A digital signal evaluation technique was developed that renders possible the clear differentiation between the slope and the kernel region of the signal. The latter represents the motion of particles through the completely illuminated region, which can be a more accurate parameter to define the signal length. In addition to the signal length determination, a cross-correlation technique was tested for its potential to obtain particle velocity. the instrument has two interlaced measuring volumes of nearly the same size, which are shifted for this special application in the main flow direction by 20 μm. The phase difference between the signals from the two photomultipliers, together with the optical distance, yields the particle velocity.     

双光路DOAS系统获取大气颗粒物粒谱分布方法研究

当前大气复合污染日趋严重,造成大气氧化性增强,气体向颗粒物的转化加快。大气颗粒物粒径大小及谱分布决定其在大气中的行为,以差分吸收光谱法（DOAS）为基础,结合双光路设计技术,开展实时、在线、获取近地面大气气溶胶颗粒物的粒谱分布的光谱方法研究。首先构建低噪声性能稳定的宽带氙弧灯为光源的双光路差分吸收光谱系统,基于干净天气条件下大气的能见度数据对系统进行校准,通过两个不同光路获得的光谱信号强度之比获取近地面紫外-近红外波段的大气总宽波段消光系数。基于宽波段消光系数,在去除瑞利散射以及气体吸收对消光系数的影响后,解析出气溶胶颗粒物的消光系数。基于核函数准则,利用均匀球型粒子的电磁场Mie理论来反演气溶胶物理特性,获得气溶胶粒子在该测量谱段的体积谱分布,利用体积谱与数密度谱的关系,反演出气溶胶粒子的数密度谱分布。开展利用直方图方法来表现颗粒物的粒谱分布方法研究,首先将DOAS测量波段近似等分为若干谱段,利用谱段处平均值,获取气溶胶粒谱直方分布图。最后把该系统和方法应用于外场实验,获得了气溶胶颗粒物在300～650 nm范围内的消光系数,将测量波段等分为11个谱段,反演了颗粒物的在0.1～1.25 μm粒径范围的数密度谱分布。该研究为整治我国灰霾天气,研究大气气相/粒子非均相化学反应提供科学依据。同时将推动DOAS技术的进一步发展和应用。     

On the influence of the scattering particle concentration on the probe beam coherence, signal quality, and measuring accuracy of interferometric systems

Experimental data on the influence of the scattering particle concentration in a turbid medium on the probe beam coherence, statistic characteristics of responses, and measuring accuracy of interferometric systems are reported. It is shown that even if the power of probe beams is low (on the order of 10 mW) and the concentration of scattering particles is so high that they attenuate the input power by several hundred times, light beams almost completely retain time and spatial coherences and their respective interference pattern is highly contrast. The spectral characteristics of the signal, signal-to-noise ratio, and measuring accuracy remain nearly unchanged.     

The detection of atmospheric turbulence using two ultrasonic light modulators and heterodyne signal processing

The use of double diffraction of coherent light, passing through a pair of ultrasonic light modulators (ULM) with slightly different frequencies, has been studied for optical heterodyne detection processing. A beat signal can be generated with a difference or sum sound frequency between the two ULM's when a pair of diffracted light beams of first order, produced after passage through the two ULM's are photomixed with each other. This system has the merit that it requires no optical components except the two ULM's. It is also convenient for heterodyning techniques that the alignment of the two light beams to be photomixed is not critical. As an application of this system the phase fluctuations of the beat caused by atmospheric turbulence have been processed electronically to obtain its statistical properties.