Its optical and signal-analysing systems and some experimental results of flow velocity

A laser Doppler microscope, consisting of optical and signal-analysing systems, has been developed. Flow velocity can be measured with an extremely high spatial resolution over an area of up to 10μm in diameter. The optical system consists of a microscope having two types of penetrative and reflective systems. On the basis of the characteristic analysis of Doppler beat signals from the laser Doppler microscope, the signal-analysing system, which uses a tracking filter together with a digital-hold element and a time-to-pulse height converter, has been found to be the most suitable for analysing beat signals. Some experimental results obtained using the laser Dopller microscope for measuring flow velocities, both in a small square cell in a venule, and in a capillary over the web of a frog's foot are presented.     

Laser Doppler velocimeter for remote measurement of polluted water and aerosols discharges

A continuous wave Doppler velocimeter has been developed for remote estimation of the volume of polluted water and gaseous discharges using an actively stabilized ring dye laser. Doppler shifted backscattered signals induced by fluids in motion using heterodyne detection technique has been applied for measurement of Doppler shifts. Polluted water and gaseous discharge flow rates have been measured remotely for simulation purposes. The system is capable of measuring Doppler shifts as low as 0.25 MHz corresponding to 0.1 m/s velocities of the fluids.     

Studies of the dynamic behaviour of charged dust particles in high electric fields using laser Doppler velocimetry

As a preliminary study for the collection of dust particles in an electrostatic precipitator, laser Doppler velocimetry has been used to measure the velocity of small dust particles charged in the high electric field of a model electrostatic precipitator. A differential optical system is adopted for the velocimeter. The period-measuring system is used to analyse Doppler beat signals and to obtain velocity data from them. By means of the laser Doppler velocimeter, the dynamic behaviour of the velocity and direction of moving dust particles is fully revealed as a function of the applied electric voltage in the entire collecting space of the model electrostatic precipitator, and is shown in vectorial flow diagrams of particle velocity.     

激光多普勒测速实验系统的信号处理

本文详细报导了运用集成锁相环路在激光多普勒测速实验教学系统中实现多普勒信号频率跟踪处理的设计原理和具体实施方案。该处理方法将集成度较高的 L M5 6 5锁相环应用于系统 ,使系统具有电路简单、可靠、可实时测量和结果直观、易操作等特点     

双光束激光多普勒测速系统

激光多普勒测速以其高精度、使用方便而在速度测试领域得到广泛的应用. 本文介绍了激光多普勒测速和双光束测速系统的基本原理,并对流体速度进行了测量,得出了相应的实验结果.     

激光场的线宽对双光子电磁感应光透明及共振吸收增强的影响

讨论了四能级系统中激光场的线宽及多普勒效应对双光子电磁感应光透明及共振吸收增强的影响，并给出了该系统中消除多普勒效应的最佳实验方案.     

Laser Doppler imaging through tissues phantoms by using self-mixing interferometry with a laser diode

Laser Doppler imaging has been widely used for the evaluation of cutaneous blood flow. We report on how the self-mixing interferometry configuration with a laser diode is explored for what is believed to be the first time to generate flow maps. The experiment was carried out by sensing the laser intensity power spectrum at each pixel as the laser was scanned over a model that mimics the properties of skin and circulating blood.     

Pulsatile flow measurement in hydraulic systems using a laser velocimeter

Servo valves find their major application in high performance hydraulic control systems where the accurate control of position, velocity and load is essential. Because of this, it is important to determine the dynamic characteristics of the servo valve (i.e. flow rate as a function of input command signal) more precisely.Previous techniques for this evaluation depended on monitoring spool position using linear transducers, since flow measurement techniques capable of following fast transients and oscillating (or pulsating) flows were not easily applied. The introduction of the laser Doppler anemometer changes this. Steady state flow measurement, using the dual beam scatter system, is now common but since it needs a spectrum analyser to measure the Doppler frequency it is not suitable for unsteady flow. Since hydraulic systems are relatively particle free, the Doppler signals are dreceived intermittently. Because of this, a signal processing system has been developed which measures the period of one or more oscillations in the frequency burst caused by a scattering particle passing through the control volume.Using this technique steady and unsteady laminar flow profiles have been measured in order to assess its effectiveness in the performance testing of hydraulic servo systems.     

Accuracy Analysis of the Fabry–Perot Etalon Based Doppler Wind Lidar

A direct detection Doppler wind lidar with the dual Fabry–Perot technique has been built in Hefei, China, and the system is described in this paper. A dual, air-spaced Fabry–Perot etalon is designed as the frequency discriminator and the transmission characteristics are measured with a stabilized cw laser and a pulse laser. The experimental data are found to be in agreement with the designed parameters. The Doppler shift is measured by taking the ratio of the transmitted intensities from the dual etalon. Analysis of the accuracy of Doppler measurement is made by considering the factors of the number of iterations in converting the measured ratio to the Doppler shift, the laser intensity fluctuation and the signal-to-noise ratio. In the sufficient signal-to-noise ratio the velocity accuracy of the Doppler lidar system is estimated to be better than 0.5 m/s.     

The characterisation and application of a pulsed neodymium YAG laser DGV system to a time-varying high-speed flow

Doppler Global Velocimetry (DGV) is a whole-field measurement technique which has attracted significant interest from the fluid-flow research community since its introduction in 1991. Practical implementations of the methodology have focused on two principal laser light sources: the argon ion laser, applied to steady state or slowly varying flows; and the pulsed neodymium YAG laser for the measurement of instantaneous velocity fields. However, the emphasis in the published literature has been very much on research using the argon laser. This paper reports the application of a Q-switched, injection-seeded neodymium YAG laser to the proven Oxford DGV system, and the use of this combination in a short duration unsteady high-speed flow. The pertinent characteristics of the apparatus are described, and the impact of these on the integrity of the resulting velocity measurements is presented. Adaptations to the commercial laser system that make it suitable for application to the measurement of transient high-speed flows are described. Finally, the application of this system to a short duration unsteady flow is described. This application is based on the flow found in a new type of transdermal drug delivery device, where particles of the drug material are projected at high speed through the skin. Whole-field velocities are recorded, and values as high as 800 m/s are evident.     

Simultaneous measurements of multiple flow velocity components using frequency modulated lasers and a single molecular absorption cell

Since unsteady, complex flow phenomena play an important role, optical measurements techniques are required for flow investigations, which provide simultaneous measurements of multiple velocity components. Doppler global velocimetry has this potential. It is a flow measurement technique, where the Doppler shift of scattered light is measured by a molecular absorption cell for frequency-to-intensity conversion. However, novel Doppler global velocimeters with laser frequency modulation were only used for single component measurements yet. In order to enhance such a system for the simultaneous measurement of multiple components, a concept based on frequency division multiplexing is introduced for the first time. Besides multiple lasers, only a single molecular absorption cell and a single detector unit is required. Two-component measurements of velocity profiles from nozzle flows are presented as a proof of principle. The designed measurement system provides high measurement rates of up to 20 kHz, which is three orders of magnitude higher than for typical Doppler global velocimetry setups.     

Time-Resolved Laser Doppler Measurements in the Underexpanded Jet of a Model Gas-Blast Circuit Breaker under Arcing Conditions

A differential laser Doppler system has been used in conjunction with a self-triggered oscillographic raster display system for measuring particle concentrations and flow velocities in a model gas-blast circuit breaker. The flow velocities have been correlated with particle size to enable extrapolation to be made to determine the true plasma flow velocity. The results have been deconvoluted to take account of lateral movements of the arc column. The measurements have been taken with air as the host arcing medium and a flow through a 25-mm-diameter orifice sustained by an upstream pressure of about 7 bar (downstream pressure 1 bar). The arc was sustained by sinusoidal cuffents of peak values 3 and 8 kA and a frequency of 85 Hz. These two currents corresponded to conditions when the model circuit breaker successfully and unsuccessfully interrupted the arc-sustaining current, respectively. The results show that in the case of the 8-kA arc, the flow pattern is severely distorted from that under nonarcing conditions, suggesting that this is a contributory reason for the poorer perfonnance of the model circuit breaker at this higher current level.     

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.     

Measurement of absolute flow velocity vector using dual-angle, delay-encoded Doppler optical coherence tomography

Single-beam laser Doppler measurements of flow velocity are only sensitive to the velocity component parallel to the optical axis. We describe a simple modification to a standard Doppler optical coherence tomography (OCT) system using a single sample beam that provides velocity information from multiple angles within the beam. By introducing a glass plate midway into the OCT beam path, the sample beam is divided into several components, each with a different group delay and each providing a separate interferogram with its own effective Doppler angle. By combining the Doppler shift measured in each of these component interferograms, the flow velocity vector is fully determined.     

Localized velocity and turbulence measurement in turbulent swirling flows using laser Doppler anemometry

Turbulent swirling water flow in a pipe has complex velocity distributions with special measurement problems tor experimental work. Some of these problems are described in the introduction to the paper along with comments on the basic characteristics of turbulent swirling flow.The form of laser Doppler anemometer used is briefly noted, with some special optical features developed to suit this class of flow. Experience during some two years of comprehensive experimentation is then outlined, including comment on the data processing system.Results from experiments with tangentially injected water flow in 50 mm bore pipes up to 60 diameters long are reported, with local measurements of mean velocity and turbulence intensity. The variation of mean velocity components and turbulence intensity with radius and distance along the pipe are discussed with a few comparisons of results obtained from other experimental techniques, e.g. pitot-static probe and hot films.For the particular problem examined here it would have been difficult, if not impossible, to obtain the measurements using a technique other than laser anemometry.     

Real-time laser Doppler and phase Doppler signal processors

The laser Doppler anemometry/phase Doppler anemometry (LDA/PDA) spectrum processor with a maximum likelihood criterion has been developed in recent years. A review of such a processor, including the algorithm, architecture structure and performance, is given in this paper. The optimization of LDA/PDA processors and the advantages of the developed computer-based system are described.     

Description of a Doppler rayleigh LIDAR for measuring winds in the middle atmosphere

The possibility to measure winds in the middle atmosphere with a Doppler LIDAR was demonstrated in 1989. It has been used since then to study the wave-mean flow interaction, in association with the Rayleigh LIDAR providing density and temperature and their fluctuations. The Doppler LIDAR relies on Rayleigh scattering from air molecules and was originally designed to cover the height range 25–60 km, a region where radars cannot operate. The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of two narrow band-passes of a single dual Fabry-Perot interferometer tuned to either side of the emitted laser line. Its extension to lower altitudes where Mie scattering is present is under study.     

Laser Doppler vibrometry based on self-mixing effect

In this paper, a simple, compact and low cost laser Doppler vibrometer is presented. It is simply composed of a laser diode (9 mm in diameter, 2 g of weight, emitting at 825 nm) and by a two-lens optical system designed to focus the laser radiation on the vibrating target. This sensor can measure target velocity. The working principle is based on the self-mixing effect that occurs in a semiconductor laser diode when the emitted radiation is back reflected toward the cavity and then re-introduced inside. The target velocity is calculated measuring the frequency of the peak of the spectrum of the signal generated by a photodiode mounted on the back facet of the laser diode. In this article, the design and the realisation of the vibrometer is reported. The self-mixing vibrometer has been calibrated in the range 0–300 mm/s, using a rotating disk covered with white paper. The effect of the angle of measurement and target distance are reported. Finally, the sensor has been compared with a commercial laser Doppler vibrometer using a vibrating surface as a target.     

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.     

时频分析在激光多普勒信号处理中的应用

建立了用来检测微机电系统谐振器面内运动的激光差动多普勒系统,测量了谐振器的振动速度。在信号处理过程中,首先使用电路系统去除高频载波成分,然后基于Matlab平台,选取时频分析中的Wigner-Ville分布对采集进入计算机的多普勒信号进行分析。提取了其中的速度信息,并与用计数法处理的结果进行了比较。结果表明具有较好的效果。