Laser Doppler Anemometry Signal Research Articles

Experimental Investigation on Velocity Fluctuation in a Vaned Diffuser Centrifugal Pump Measured by Laser Doppler Anemometry

Turbulent flow, mainly originating from the rotor-stator interaction (RSI), is closely associated with the normal and safe operation of the centrifugal pump. In the current research, to clarify turbulent flow in the centrifugal pump with a vaned diffuser, the non-intrusive LDA (Laser Doppler Anemometry) system is applied to measure velocity pulsation signals at different regions when the pump operates at various flow rates. Time and frequency domain analysis methods are combined to investigate the velocity signals, and the velocity distribution around the volute tongue region is reconstructed from twenty measuring points. Results show that the velocity spectrum is characterized by the discrete components at the blade passing frequency and its higher harmonics, and it is caused by the RSI between the impeller and the diffuser. For the points in the volute spiral and diffusion sections, due to the significantly reduced RSI effect, the velocity spectrum shows an evident difference from comparison with the points between the impeller and diffuser, and the blade passing frequency is not always the dominant frequency. The comparison of velocity amplitudes and RMS* (root mean square of velocity) values at different points proves that the measuring position and flow rate affect velocity pulsations. As observed from velocity distribution reconstructed by LDA signals, high velocity regions are developed downstream of the diffuser channel for all the measured flow rates.

Spectral analysis of round jet instabilities at low Reynolds number

For the study of round jet flow at low Reynolds number, a fine analysis of instabilities is necessary. In this paper, we present a frequency analysis of temporal and spatial velocity variations in a round jet with a Reynolds number equal to 830. The data were obtained from Laser Doppler Anemometry (LDA) measurements for various downstream positions. Attention is focused on the characteristics of the instantaneous signal of the longitudinal velocity component u (t ) in order to characterize the transition from laminar state to turbulent state. The spectral analysis of the LDA signals of the axial component of the exit velocities highlights the presence of a single peak of energy around a characteristic frequency f 0 . This frequency corresponds to the amplified mode of the instability. The jet develops a sinuous mode which is the most unstable anti-symmetric mode. The results obtained in this study concerning the sinuous mode frequency, agree well with those obtained by laser tomography technique and images analysis.

Estimation of turbulence power spectra for bubbly flows from Laser Doppler Anemometry signals

The accuracy of the estimation of turbulence power spectra from Laser Doppler Anemometry (LDA) signals in bubbly flows is studied. Special attention is paid to the influence of the gaps in the signal created by the bubbles. The performance of reconstruction and slotting techniques for the estimation of the power spectrum is determined by application to synthetic bubbly flow LDA signals, and checked by application to real data. Estimation of spectra of bubbly flow signals with reconstruction techniques is found to give poor results. In general, the application of reconstruction techniques to LDA signals gives bias in the spectrum due to the addition of noise and low-pass filtering. The nature of the filtering and noise for bubbly flow signals differs from that for single phase flows. For bubbly flow signals, the spectrum is reliable up to a cut-off frequency which is lower than the cut-off frequency for single-phase flow signals with similar data rates. In addition, slopes close to - 5 3 may appear for signals which in reality have flat spectra. For single phase flow, it is possible to correct for these artefacts. The work shows that it is not possible to create similar correction techniques for bubbly flow signals. The application of the slotting technique for estimation of power spectra of bubbly flow signals leads to much better results. Estimation of the spectrum beyond the mean data rate is well possible. The performance of several improvements of the slotting technique is discussed.

Calibration of a micromachined particle velocity microphone in a standing wave tube using a LDA photon-correlation technique

In this paper, a new method of calibrating an acoustic particle velocity sensor using laser Doppler anemometry (LDA) is discussed. The results were compared and were in good agreement with the results obtained by conventional methods, where the sensitivity of the microflown is obtained with the use of a reference microphone and a standing wave tube. The LDA signal generated by the acoustic particle motion was analysed using the photon-correlation method, where the signal is considered to consist of a series of discrete photon events. The photon-correlation system is used to measure particle velocity amplitude next to the microflown particle velocity sensor in a standing wave. Measurements are performed for frequencies between 250 Hz and 4 kHz and velocities between 5 mm s−1 and 25 mm s−1 (root-mean-square (rms) values) which are equivalent to sound fields of 100 and 114 dB SPL in free field. From the output voltage of the probe microflown and the LDA-derived particle velocity in a standing wave, the sensitivity of the microflown is obtained. The two different calibration methods are in good agreement showing a discrepancy of 1 dB for the frequency range of 250 Hz–4 kHz.

Description of flow regime transitions in bubble columns via laser Doppler anemometry signals processing

This work aims at studying the non-linear dynamics and the flow regime transitions in bubble column reactors. For this purpose, various signal processing techniques e.g. frequency analysis, fractal analysis and deterministic chaos analysis have been applied to laser Doppler velocimetry signals. The system considered is a two-dimensional reactor allowing LDV measurements at higher void fractions than in three-dimensional systems. Each signal processing technique presents a specific capacity to describe a regime transition or a feature of the flow structure. Use of these various techniques have highlighted the occurrence of two states in the transition regime and yielded detailed information on the physical mechanisms responsible for these transitions.

On the choice of the number of samples in laser Doppler anemometry signal processing

The minimum number of samples that must be taken from a sinusoidal signal affected by white Gaussian noise, in order to find its frequency with a predetermined maximum error, is derived. This analysis is of interest in evaluating the performance of velocity-measurement sys- tems based on the Doppler effect. Specifically, in laser Doppler an- emometry (LDA) it is usual to receive bursts with a poor signal-to-noise ratio, yet high accuracy is required for the measurement. In recent years special attention has been paid to the problem of monitoring the tempo- ral evolution of turbulent flows. In this kind of situation averaging or fil- tering the data sequences cannot be allowed: in a rapidly changing en- vironment each one of the measurements should rather be performed within a maximum permissible error and the bursts strongly affected by noise removed. The method for velocity extraction that will be considered here is the spectral analysis through the squared discrete Fourier trans- form, or periodogram, of the received bursts. This paper has two parts. In the first an approximate expression for the error committed in LDA is derived and discussed. In the second a mathematical formalism for the exact calculation of the error as a function of the signal-to-noise ratio is obtained, and some universal curves for the expected error are provided. The results presented here appear to represent a fundamental limitation on the accuracy of LDA measurements, yet, to our knowledge, they have not been reported in the literature so far. © 2001 Society of Photo-Optical

Review of LDA and PIV applied to the measurement of sound and acoustic streaming

A review of laser Doppler anemometry (LDA) and particle image velocimetry (PIV) with their application to the measurement of sound is presented. The fundamental principles behind LDA and PIV are discussed and extended to the application of sound measurement. Special attention is paid to analysis of LDA signals including the Hilbert transform, which enables amplitude information to be obtained about various frequency components of a signal and wavelet analysis, which allows non-stationary signals to be accurately analysed. The influence of the refractive index variations in a medium due to a sound wave on the laser beams of an LDA signal is discussed. Attention is also paid to acoustic streaming which arises due to high-intensity sound, and PIV results are presented to demonstrate the effect.

Laser Doppler anemometry applied to airborne acoustics: Influence of the signal processing parameters

Laser Doppler anemometry (LDA) is widely used for fluid mechanics but may also provide measurements of the acoustic particle velocity. The validation of its use in acoustics, which is being achieved, requires knowledge of the appropriate data processing parameters. In order to understand their effects, an experiment is conducted in which LDA measures structural velocities. In this experiment, the velocity of a shaker tip measured by LDA is compared with the velocity measured by a laser vibrometer. The LDA signal is a sinusoidal frequency modulation whose instantaneous frequency is proportional to the measured velocity. LDA-measured velocities are estimated using either spectrum analysis or time-frequency analysis. These comparisons point out three major results. Even if the velocity is small, which corresponds to a narrow bandwidth of the measured signal, oversampling should be preferred. Time windows should be chosen as short as possible despite the poor estimation of the instantaneous frequency. These first two results lead to a precise temporal description of the measured velocity but a rough one in the amplitude domain. However, the a priori knowledge of the excitation signal frequency enables the precise estimation of the velocity amplitude by means of a least-square method.

Influence of the acousto-optic effect on laser Doppler anemometry signals

When laser Doppler anemometry (LDA) is used to measure sound fields in water it is important to take into account the refractive index variations in the water due to the sound wave. These have the effect of creating a phase difference between the two laser beams in the LDA setup so that when they intersect they create moving fringes. In some situations this acousto-optic effect can dominate over the movement of the particles due to the sound wave, thereby influencing the Doppler signal. This article determines in which situations the acousto-optic effect can be ignored and in which situations it has a dominant effect. Theoretical expressions are derived for the magnitude of the acousto-optic effect on the LDA signal in terms of the distance of the laser beam propagation and acoustic wave number. The results show that varying the value of the wave number, the distance the laser beams have traveled in the direction perpendicular to the optical axis, or the angle of the sound wave, has an effect on the amplitude, yamp, of the fringe movement. For low wave number values, an angle of 0, ±π from the optic axis (y direction) corresponds to the situation in which the laser beams have been affected to the same degree by refractive index changes in the water and thus the path difference is zero and the value of yamp is a minimum. A maximum value of yamp is obtained for an angle of ±π/2 from the y direction for low wave number values. It is shown that the ratio of the acousto-optic effect to the amplitude of the particle movement due to the sound wave varies as the square of both the acoustic wave number and the distance of the laser beam propagation for low wave number values. An equation for the apparent motion of the particles in the fringes is determined and is used to show that the acousto-optic effect dominates as the value of the wave number and the distance of propagation of the beams increases. Also, for very low wave numbers and short distances, the acousto-optic effect is negligible. This is because at low wave numbers, corresponding to low sound frequencies, the refractive index gradients, and hence the phase changes along the lengths of the laser beams, are small.

Methods of processing laser Doppler anemometry signals to extract sound field information

For several years laser Doppler anemometry (LDA) has been used to measure mean flow rates in gaseous and liquid flows. It is only recently that LDA has been developed as a technique for recording instantaneous particle velocities, enabling nonintrusive measurements to be made in sound fields. The paper describes a typical experimental setup and discusses various methods of digitally processing the LDA photodetector signals to extract the required velocity information. Particular emphasis is placed on the zero counting and Hilbert transformation techniques. The problem of signal dropout is also addressed. LDA measurements of acoustic particle velocity amplitudes are compared with acoustic velocity amplitudes deduced from probe microphone and impedance measurements. It is found that LDA measurements could be made not only in monofrequency sound fields but also in more complex sound fields containing a fundamental component and additional harmonics. Theoretical and practical limitations of the techniques are investigated and used to assess the viability of applying them to studies in the bores and mouthpieces of brass instruments.

LASER DOPPLER ANEMOMETRY MEASUREMENTS OF BUBBLE RISE VELOCITY AND DEPARTURE FREQUENCY

Previous experimental investigations have employed photographic techniques to quantify the rise velocity and departure frequency of bubbles departing from a boiling surface, a method that requires a significant time investment to provide accurate time-averaged results. The current study focuses on applying laser Doppler anemometry (LDA) techniques to quantify these parameters. Initially, the LDA system is employed to measure the rise velocity and departure frequency of air bubbles in FC-72, R-113, and water. These air bubble tests verify the ability of the LDA measurement technique to quantify the bubble parameters. The LDA technique is then used to quantify the bubble parameters from a wire during isolated nucleate boiling conditions in saturated FC-72. Highly time-dependent departure frequencies (7 to 242 Hz) are measured in the isolated nucleate boiling regime. LDA signals representing departure frequencies in the fully developed nucleate boiling regime are discussed.

The feasibility of fast single-bit sampling for multi-point laser Doppler anemometry

Consideration is given to a number of aspects concerning the design and implementation of a simultaneous multi-point laser Doppler anemometry (LDA) system. The difficulties of data collection and processing for many channels is seen to be the principle limiting factor to system development, and a practical route making use of highspeed components for fast LDA signal sampling and storage, followed by fast Fourier transform (FFT) signal analysis in software is explored. A demonstration system is constructed consisting of ten optical channels arranged in a linear array, a single LDA sampling card and a controlling microcomputer, and the arrangement is applied to a closed water circuit. Received signals are compared with software-generated equivalents and the effect of various signal-to-noise ratios on the accuracy of a reconstructed velocity distribution is examined. The overall configuration is shown to provide a modular approach enabling a piecewise increase in the number of processing channels according to requirements.

Spectral estimation and analysis of LDA data in pulsatile flow through heart valves

The Laser Doppler Anemometry (LDA) measurement technique presents an inherent difficulty when spectral analysis is applied to it. The random nature of the LDA signal prohibits sampling at regular, equi-spaced, time instants. Irregular sampling presents additional variability of the spectral estimator. In order to reduce this variability, spectral analysis of LDA data is performed according to the method of direct Fourier transform of short blocks of data, as suggested by Gaster and Roberts. The LDA data is measured in a flow field distal to prosthetic heart valves with varying degrees of stenosis. The spectral estimates of velocity data sampled during the rapid closure stage of the valve are achieved with excellent frequency resolution. Important and useful information about dominant frequency peaks and preferred modes which exist in the flow, otherwise smeared or concealed in the spectral contents, are then derived from the spectral information. These modes are quantitatively analysed in light of vortex formation and related flow mechanisms. Comparative studies of normal and stenosed valves show that the preferred modes are governed by the valve geometry and dynamic behavior and are correlated to the severity of the stenosis.

Distortion of LDA fringe pattern by tracer particles

For precise flow velocity measurements laser Doppler anemometry (LDA) is wide-spread in use in the laboratories of industry and universitarian research institutions. The LDA method has the advantage of being not intrusive and able to discriminate between forward and reverse velocities. So far, laser Doppler anemometry is believed to be one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not constant within an individual burst. This can induce an additional scatter in the signal frequency and in the determination of the flow velocity. Until now, the reason for the period variations has not been investigated in detail although the problem was observed before. This paper describes experimental investigations which show that the particle passage through the laser beams shortly before the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern. Thus, the signal period length from an individual particle, passing the center of the measuring volume at the same time, varies according to the distortion of the fringe spacing.

Processor operated correlator with applications to laser Doppler signals

A 64-channel correlator is designed with application to the processing of laser Doppler anemometry signals in the range 200 Hz to 250 kHz. The correlator is processor operated to enable the consecutive sampling of 448 correlation functions at a rate up to 500 Hz. Software is described to identify a Doppler frequency from each correlation and the system is especially designed for transient flow signals. Doppler frequencies are determined with an accuracy of about 0.1%.