Effects of burst detection and analysis upon turbulence measurements for laser Doppler anemometry

This study shows that for turbulence measurements made using laser Doppler anemometry, errors associated with the frequency analyzer's resolution can be present. Since all analyzers are based upon some digital clock, the minimum clock cycle is proportional to the finest frequency resolution (f_r) possible. It was found that if the frequency histogram spaned less than 16f_r for ±3σ, then the calculated values for all of the Reynolds stress tensor components will be in error. This was observed for a counter system and a correlator system. The correlator system interpolated to less than one clock cycle, but the interpolation did not seem to help in the elimination of the errors.     

Derivation and comparison of fundamental uncertainty limits for laser-two-focus velocimetry,laser Doppler anemometry and Doppler global velocimetry

Laser-2-focus velocimetry (L2F), laser Doppler anemometry (LDA) and Doppler global velocimetry (DGV) are common measurement techniques for flow analysis, but a fundamental comparison of their minimum achievable measurement uncertainties is still missing. In order to reveal the measurement principle with the lowest uncertainty, the Cramér-Rao lower bounds (CRLB) are derived analytically regarding two inevitable fundamental disturbances: photon shot noise and thermal detector noise. The CRLB results are compared with each other assuming equal temporal resolutions. For both noise sources, a relative uncertainty limit results for L2F and LDA, and an absolute uncertainty limit for DGV. Hence, DGV seems to be appropriate for investigating high-speed flows. However, the threshold velocities strongly depend on the possible viewing angles. The CRLBs are calculated and compared with each other for typical conditions in turbomachinery as an example.     

Measurement of bubble and droplet diameters by the Doppler anemometry

Interaction of a probing laser beam with a liquid droplet is analyzed theoretically. It is shown that the amplitude of the Doppler signal obtained from a moving droplet depends on its position with respect to the beam waist. The possibility of determining the radii of droplets and bubbles moving in the flow from the spatial positions of the extreme points of the Doppler signal amplitude is justified theoretically and experimentally. The radius of glass spheres is measured (1.75 and 5 mm). The relative error of measurements is 1.1 and 8%. The radius of bubbles upfloating in a viscous liquid is measured. It is shown that the proposed method based on the Doppler anemometry offers prospects in studying two-phase flows.     

Evaluation of methods for assessing spectra from laser Doppler velocimetry signals

A critical evaluation is made of the spectral bias which occurs in the use of a laser Doppler velocimeter (LDV). In order to accommodate the highly turbulent random sampled LDV data, a doubly stochastic Poisson process is generated. Four well-known processing algorithms, i.e., the direct transform (RG), the transit-time weighting method (TW), the sample-and-hold method (SH), and the modified Shannon sampling technique (SR), are evaluated and compared with the theoretical spectra for various sampling and flow conditions. Assessment of these algorithms is made for varying data densities (0.05 d.d. 5.0) and turbulence levels (t.i. = 30% and 100%). Furthermore, the effects of the Reynolds stress coefficients and of the transversal standard deviations on the spectral bias are examined.     

基于双光束-双散射的激光多普勒水流速测量

激光多普勒水流速仪是一种非接触、高精度的测量仪器,但由于技术和集成度等原因还仅限于研究所和高校使用。文中介绍了一种双光束-双散射法测量流体速度,利用此种光路测量法可以提高集成度、促进激光技术的普及;还可以解决常规测量法难以解决的问题,例如风洞的风速测量等。通过实验验证了这种方法的可行性。     

A robust algorithm for the reconstruction of helical filaments using single-particle methods

Some of the earliest methods for three-dimensional reconstruction from electron microscopic images were developed for helical objects. Single-particle methods have been used with great success for the three-dimensional reconstruction of macromolecular assemblies that have no internal symmetry or closed point group symmetries. An approach is presented for the application of single-particle methods to helical filaments that surmounts many of the difficulties of helical image analysis, including indexing, unbending and the need to find long helically symmetric filament segments. It is shown using both human Rad51 and E. coli RecA nucleoprotein filaments that this approach converges without user intervention to a stable solution, and that it has the potential to overcome many of the problems associated with image analysis of disordered helical polymers. The method can be applied transparently to structures where Bessel overlap would greatly complicate helical analysis. In addition, the procedure allows for the ab initio determination of helical symmetry, when no prior knowledge exists.     

In vitro and in silico (IVIS) flow characterization in an idealized human airway geometry using laser Doppler anemometry and computational fluid dynamics techniques

The air flow in idealized human airway geometry was studied using computational and experimental methods. A computational fluid dynamics model developed to determine the air flow characteristics in airways was validated by comparison of the experimental velocity profiles obtained with laser Doppler anemometric measurements with numerical data. A good correlation was found between the values obtained with the two methods. Both the measurements and the calculations showed the flow to be laminar in the trachea region of the airway model, but it is affected by the airway geometry in subsequent airways.     

Comparative analysis of the performance of laser Doppler systems using maximum likelihood and phase increment methods

This paper presents a comparative analysis of the errors of two alternative methods of estimating the central frequency of signals of laser Doppler systems, one of which is based on the maximum likelihood criterion and the other on the so-called pulse-pair technique. Using computer simulation, the standard deviations of the Doppler signal frequency from its true values are determined for both methods and plots of the ratios of these deviations as a measure of the accuracy gain of one of them are constructed. The results can be used by developers of appropriate systems to choose an optimal algorithm of signal processing based on a compromise between the accuracy and speed of the systems as well as the labor intensity of calculations.     

Mode shape reconstruction of an impulse excited structure using continuous scanning laser Doppler vibrometer and empirical mode decomposition

For vibration testing, discrete types of scanning laser Doppler vibrometer (SLDV) have been developed and have proven to be very useful. For complex structures, however, SLDV takes considerable time to scan the surface of structures and require large amounts of data storage. To overcome these problems, a continuous scan was introduced as an alternative. In this continuous method, the Chebyshev demodulation (or polynomial) technique and the Hilbert transform approach have been used for mode shape reconstruction with harmonic excitation. As an alternative, in this paper, the Hilbert-Huang transform approach is applied to impact excitation cases in terms of a numerical approach, where the vibration of the tested structure is modeled using impulse response functions. In order to verify this technique, a clamped-clamped beam was chosen as the test rig in the numerical simulation and real experiment. This paper shows that with additional innovative steps of using ideal bandpass filters and nodal point determination in the postprocessing, the Hilbert-Huang transformation can be used to create a better mode shape reconstruction even in the impact excitation case.     

Velocity correction of the Janus configuration laser Doppler velocimeter

The error of laser Doppler velocimeter (LDV) based on Janus configuration is analyzed as the vehicle inclines. Analysis shows that the error of the Janus configuration LDV depends on the vertical velocity to horizontal velocity ratio (VHR) only. But the measurement accuracy of the Janus configuration LDV will be not high enough to meet the requirement of vehicle self-contained navigation system, if the VHR of the vehicle is overlarge in powerful rough ground. The error of the Janus configuration LDV is corrected through an inclinometer whose precision of angle needed is only 2%. Simulation shows that the error of the Janus configuration LDV after correction can be limited below 0.1% even as the VHR reaches 0.3 and pitch angle is 15°.     

Evolution of methods of assessing the accuracy of metal-cutting machines

The evolution of concepts of accuracy and attainable machining accuracy are considered. The physical aspects of accuracy are discussed. The changes in the level of precision since 1800 are outlined, with predictions of what is attainable in the future. The energy barrier to accuracy is analyzed for various cutting technologies.     

Quasi-optimal processing of laser Doppler vibrometer signals

A method of quasi-optimal processing of laser Doppler vibrometer (LDV) signals in the case of monoharmonic vibrations is considered. Under certain assumptions, the problem of quasi-optimal estimation of the information parameter of the LDV signal, related through a known constant multiplier to the relative amplitude of vibrations, is solved. A structure of the device for optimal estimation is proposed. Conditions of optimality of the method are given. Statistical characteristics of estimates of the relative vibration amplitude are investigated by a numerical modeling method. The error of these estimates is compared with the Rao-Kramer bounds.     

提高经济型数控车床车削几何精度的加工方法

文章提出了几种提高数控车削几何精度的加工方法，对于生产实践具有指导意义。     

Calibration of laser Doppler vibrometer and laser interferometers in high-frequency regions using electro-optical modulator

We proposed a new primary calibration method of a laser Doppler vibrometer (LDV) using the optical modulated excitation of an electro-optical modulator and validated this approach by comparing mechanical modulated excitation with homodyne laser interferometer at the National Metrology Institute of Japan (NMIJ). The velocity sensitivity of the LDV was evaluated with the uncertainty budget at a high-frequency range up to 1 MHz, which is indicated as the upper limit of 50 kHz in ISO (International Organization for Standardization) 16063–41. As the results, we confirmed that velocity sensitivities between optical and mechanical modulated excitations are flat within their uncertainty from 100 Hz up to 1 MHz. Moreover, in order to evaluate the accurate phase shift of the accelerometer up to 20 kHz using optical modulated excitation, we also revealed the reliability of two time shifts between homodyne and heterodyne laser interferometers.     

Velocity resolved laser Doppler blood flow measurements in skin

A Doppler flow meter was developed, consisting of a PC and a control card for two Doppler sensors to be fixed on skin. From the measured Doppler signals a power spectrum is calculated by a software FFT. In contrast to usual laser Doppler flow measurements in skin, which provide only a mean flow, a flow spectrum is calculated with the assumption of an isotropic distribution of the directions of the velocity of the erythrocytes and irradiation vectors in the skin. Up to four partial flows (integral over a certain frequency region of the flow spectrum) are displayed on the screen simultaneously with a resolution in time < 10 ms. The frequency span can be set independently for each flow. Corresponding to low and high Doppler frequencies the flows show different behavior and provide the possibility to distinguish between the flow in the superficial dermal plexus and larger micro-capillaries.     

Water jet velocity uncertainty in laser Doppler velocimetry measurements

The present work deals with the uncertainty evaluation in water jet velocity measurements carried out by means of a laser Doppler dual-incident-beam velocimeter in reference-beam configuration developed at the WJLab (Water Jet Laboratory of Dipartimento di Meccanica of Politecnico di Milano). The applied experimental procedure makes it possible to calculate the measurement uncertainty through the determination of its various components. Once uncertainty is known, the laser Doppler system is suitable for objective and significant velocity evaluations but also for improvements allowed by the knowledge of the most effective uncertainty sources. Such a subject is typically not considered by the specific water jet literature, but is becoming more and more important due to the evolution of water jet machining towards high precision applications.     

提高CCD激光自准直测角精度的硬件方法

设计了一个CCD激光自准直测角系统。为提高系统的测量精度和实时性,对系统中的硬件参数进行研究,有效改善了采集到图像的质量。降低数据处理难度的同时,缩短了图像处理时间和提高了系统的测量精度。对系统中小孔光阑的直径,CCD的积分时间,会聚镜头的焦距值,采集卡的亮度和对比度参数分别进行分析,寻找优化值对系统进行改进。对改进前后的系统进行对比实验,实验结果表明,改进后系统的测角精度由原来的5.0″提高到了1.0″,处理一帧图像的时间缩短到了0.417s。     

Boundary effects of light scattering in laser diagnostics of two-phase flows

This study is motivated by the advent of methods for measuring particle sizes in two-phase flows based on the local interaction of light with the scattering particle. Localization is easily interpreted in terms of the ray optics of a particle, which, however, does not cover all interaction effects. This paper consider effects that extend beyond the traditional concept of the total effect of the mechanisms of geometrical optics and wave diffraction by a particle as an opaque disk, in particular, surface waves and complex rays.. The results of the mathematical modeling performed in this study clarifies the contributions of the effects to the scattering and the position of the localization points of the interaction with the particle.     

Apparatus for laser-assisted electron scattering in femtosecond intense laser fields

An apparatus for observation of laser-assisted electron scattering (LAES) in femtosecond intense laser fields was developed. The unique apparatus has three essential components, i.e., a photocathode-type ultrashort pulsed-electron gun, a toroidal-type electron energy analyzer enabling simultaneous detection of energy and angular distributions of scattered electrons with high efficiency, and a high repetition-rate data acquisition system combined with a high power 5 kHz Ti:sapphire laser system. These advantages make extremely weak femtosecond-LAES signals distinguishable from the huge elastic scattering signals. A precise method for securing a spatial overlap between three beams, that is, an atomic beam, an electron beam, and a laser beam, and synchronization between the electron and laser pulses is described. As a demonstration of this apparatus, an electron energy spectrum of the LAES signals with 1.4 × 10(12) W/cm(2), 795 nm, 50 fs laser pulses was observed, and the detection limit and further improvements of the apparatus are examined.     

He-Ne激光器纵模分析实验

为了很好地验证纵模频率间隔与激光谐振腔长度之间的关系,实验中改变光学谐振腔的长度,纵模频率间隔相应地发生改变,把某一腔长下的纵模频率通过共焦球面扫描干涉仪将激光器模式输出到示波器进行显示。通过对示波器显示的频率—时间图进行分析,可以测出一定腔长下激光纵模的频率间隔,该测量值与理论计算值基本一致。