Laser Doppler Anemometer Data Research Articles

Dead time effects in laser Doppler anemometry measurements

We present velocity power spectra computed by the so-called direct method from burst type laser Doppler anemometer (LDA) data, both measured in a turbulent round jet and generated in a computer. Using today's powerful computers we have been able to study more properties of the computed spectra than was previously possible, and we noted some unexpected features of the spectra that we now attribute to the unavoidable influence of a finite measurement volume (MV). The most prominent effect, which initially triggered these studies, was the appearance of damped oscillations in the higher frequency range, starting around the cut-off frequency due to the size of the MV. Using computer generated data mimicking the LDA data, these effects have previously been shown to appear due to the effect of dead time, i.e., the finite time during which the system is not able to acquire new measurements. These dead times can be traced back to the fact that the burst-mode LDA cannot measure more than one signal burst at a time. Since the dead time is approximately equal to the residence time for a particle traversing a measurement volume, we are dealing with widely varying dead times, which, however, are assumed to be measured for each data point. In addition, the detector and processor used in the current study introduce a certain amount of fixed processing and data transfer times, which further contribute to the distortion of the computed spectrum. However, we show excellent agreement between a measured spectrum and our modeled LDA data thereby confirming the validity of our model for the LDA burst processor.

Final report on EURAMET project No. 827: LDA-based intercomparison of anemometers

A regional air speed comparison between six EURAMET laboratories used an ultrasonic anemometer and a laser Doppler anemometer (LDA) at air speeds between 0.2 m s−1 and 40 m s−1. Based on periodic calibrations at the pilot laboratory (PTB), the uncertainty contributed to the comparison by the LDA was 0.2% and the uncertainty contributed by the ultrasonic anemometer depended on the air speed and varied between 2.95% and 0.11%. The degrees of equivalence between the participants were notably better for the laser Doppler anemometer data than for the ultrasonic anemometer data. For example, using the 95% chi-squared consistency check, 16 of the original 59 participant data points were removed as discrepant from the ultrasonic data set, but only one point was discrepant in the LDA data. A possible explanation is that the LDA is non-intrusive and therefore does not alter the velocity field in the wind tunnel whereas the ultrasonic device does produce blockage effects. The results of this comparison (and prior EURAMET and CIPM comparisons) indicate a need for more attention to blockage effects during air speed calibrations and their effect on air speed uncertainty statements.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by EURAMET, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Practical Aspects of the Spectral Analysis of Irregularly Sampled Data With Time-Series Models

Several algorithms for the spectral analysis of irregularly sampled random processes can estimate the spectral density for a low frequency range. A new time-series method extended that frequency range with a factor of thousand or more. The new algorithm has two requirements to give useful results. First, at least ten closest pairs of neighboring irregular observations should have a distance less than the minimum resampling distance for the chosen discrete-time frequency range. Second, a low-order time-series model should be appropriate to describe the global character of the data. The consequences and importance of this second demand are studied for irregular turbulence observations with narrow spectral details. Models of low orders are estimated from equidistant hot-wire observations and from irregularly sampled laser Doppler anemometer (LDA) data, which are obtained from the same turbulence process. The irregular data are resampled with the nearest neighbor method, both with and without slotting. Apart from the usual bias contributions of resampling irregular data, LDA data can give an additional spectral bias if the instantaneous sampling rate is correlated to the actual magnitude of the turbulent velocity. Making histograms of the amplitudes and the interarrival times provides useful information about irregularly sampled data.

Turbulence Modulation Induced by Bubble Swarm in Oscillating-Grid Turbulence

In the present study, liquid-phase turbulence modulation induced by a bubble swarm ascending in arbitrary turbulence was experimentally investigated. Liquid-phase homogeneous isotropic turbulence was formed using an oscillating grid in a cylindrical acrylic vessel of 149 mm in inner diameter. A bubble swarm consisting of 19 bubbles of 2.8 mm in equivalent diameter was examined; the bubble size and generating time were completely controlled using a bubble generating device through audio speakers. This bubble generating device was able to repeatedly control the bubble swarm arbitrarily and precisely. The bubble swarm was generated at a frequency of 4 Hz. The liquid phase motion was measured via two LDA (Laser Doppler Anemometer) probes. The turbulence intensity, spatial correlation and integral scale were calculated from LDA data obtained by the two spatially-separate-point measurement. When the bubble swarm was added, the turbulence intensity dramatically changed. The original isotropic turbulence was modulated to the anisotropic turbulence by the mutual interaction between the bubble swarm and the ambient isotropic turbulence. The integral scales were calculated from the spatial correlation function. The increase in turbulence intensity and the decrease in integral scale were observed by injecting the bubble swarm in oscillating-grid turbulence.

Turbulence Modulation Induced by Bubble-Liquid Mutual Interaction in an Oscillating-Grid Turbulence

Turbulence modulation induced by bubble swarms was experimentally investigated. Homogeneous isotropic turbulence is formed by using an oscillating grid in a cylindrical acrylic vessel. The bubble size and generation frequency are completely controlled using a bubble generator. A bubble swarm is generated at a frequency of 4 Hz and consists of 19 bubbles of 2.8 mm in equivalent diameter. The liquid phase velocities were measured via two LDA (Laser Doppler Anemometer). Turbulence intensity, spatial correlation and integral scale were evaluated from the LDA data obtained by the two spatially-separate-point measurement. The turbulence intensity dramatically changed by adding bubble swarms. The original isotropic turbulence was modulated to the anisotropic turbulence by the mutual interaction between bubble swarms and ambient isotropic turbulence. The turbulence modulation was also confirmed by the integral scale.

ICONE15-10170 Turbulence Modulation Induced by Bubble Swarm in Oscillating-grid Turbulence

In the present study, liquid-phase turbulence modulation induced by a bubble swarm ascending in arbitrary turbulence was experimentally investigated. Liquid-phase homogeneous isotropic turbulence was formed using an oscillating grid in a cylindrical acrylic vessel of 149 mm in inner diameter. A bubble swarm consisting of 19 bubbles of 2.8 mm in equivalent diameter was examined; the bubble size and launching time were completely controlled using a bubble launching device through audio speakers. This bubble launching device was able to repeatedly control the bubble swarm arbitrarily and precisely. The bubble swarm was launched at a frequency of 4 Hz. The liquid phase motion was measured via two LDA (Laser Doppler Anemometer) probes. The turbulence intensity, spatial correlation and integral scale were calculated from LDA data obtained by the two spatially-separate-point measurement. When the bubble swarm was added, the turbulence intensity dramatically changed. The original isotropic turbulence was modulated to the anisotropic turbulence by the mutual interference between the bubble swarm and ambient isotropic turbulence. The integral scales were calculated from the spatial correlation function. The effects of the bubble swarm on the integral scales showed the tendencies similar to those on turbulence intensity.

Estimation of cross-power spectra using sample-and-hold reconstruction of laser Doppler anemometry data

Measurement of turbulent spatial and temporal length scales is an increasingly important aspect of analysis in fluid mechanics. This information is derived either from the cross correlation or the cross spectrum between two measurements. The use of a laser Doppler anemometer (LDA) further complicates the determination of these properties because of the random data acquisition associated with LDA measurements. The calculation of the cross spectrum obtained for LDA data with equispaced data from a conventional instrument such as a hot wire anemometer (HWA) and for two LDA systems is considered. For each case, sample-and-hold reconstruction of the LDA data is used. The errors in cross spectrum estimates together with coherence function calculations (i.e. frequency domain cross covariance) are derived and procedures for correcting the cross spectrum and, hence, the phase and the coherence, are proposed. The results from numerical simulations with prescribed coherence and phase are shown to produce good results up to and beyond the equivalent Nyquist frequency based on the mean LDA data rate. Finally, the procedures are applied to two point LDA measurements in a jet and the results, using the proposed procedures, are shown to compare favourably with those obtained from the correlation slotting method.

LDA and PIV Analyses of In-Cylinder Steady Flow and Verifications of Numerical Simulation Results with Partial Cells Method

Characteristics of the flow inside an engine cylinder model have been evaluated by a particle image velocimetry (PIV) and a laser Doppler anemometer (LDA). Verifications of the results by Partial Cells in Cartesian coordinate method numerical simulation have been also made by the experiment results. The model engine head that has two intake valves and transparent cylinder are used in the steady state flow condition. The tests are conducted by single- and dual- valve open condition with 8 mm valve lift in-1470 Pa suction gauge pressure. The comparisons between the PIV and the LDA experiments show almost the same tendencies for mean velocities and turbulence intensities in the whole experimental regien qualitatively and also quantitatively. The differences in the valve open condition are clearly observed in the integral length and time scales that are estimated from the PIV and LDA data respectively. It is shown that the PIV results are useful for verification of numerical results, which agree reasonably with those of experiments.

Local time estimation for the slotted correlation function of randomly sampled LDA data

The task of autocorrelation and power spectral density estimation from velocity data sampled irregularly in time by a laser-Doppler anemometer (LDA) is addressed in this article. A new method based on the slotting technique was found to be a very reliable estimator. This article describes specific improvements of the slotting technique, the model-based variance estimation and the spectral transform leading to more accurate estimates of the autocorrelation function and the power spectral density. Furthermore, the new method yields more information especially at short time lags of the autocorrelation function, which can be used to derive improved estimates of the Taylor time scale.

A high-resolution laser Doppler anemometer: design, qualification, and uncertainty

A new high-resolution laser Doppler anemometer (LDA) has been developed with a working distance of 350 mm, allowing operation in lab-scale wind tunnels. The measurement volume size is 35 μm in diameter by 60 μm in length, allowing resolution of the smallest turbulence scales even at fairly high Reynolds numbers. The controversial question of velocity and validation bias in LDA data is resolved with an experimental method for measuring and removing those effects. Uncertainty estimates are also derived for all the mean and Reynolds stress measurements.

Pulsed hot-wire measurements in two- and three-dimensional wall jets

Pulsed Hot-Wire Anemometry (PHWA) measurements are performed in well defined two- and three-dimensional turbulent wall jets. For the two-dimensional wall jet the objective is to study reported differences between conventional Hot-Wire Anemometry (HWA) and Laser Doppler Anemometry (LDA) results. In the three dimensional wall jet, new improved data are provided, employing a measuring technique suitable for highly turbulent flows. This, since only hot-wire results previously have been published for this flow. The pulsed wire results show good agreement with existing Laser Doppler anemometer data in the two-dimensional wall jet, both reporting significantly higher turbulence levels in the outer region of the flow than hot-wires do. The hot-wire anemometer errors generally increase with increasing local turbulence intensity and since the three-dimensional wall jet has a higher turbulence level than its two-dimensional equivalent, the new pulsed hot-wire results improve the information available for the turbulence field in this flow significantly.

Phase-locked eduction of vortex shedding in flow past an inclined flat plate

Flow past an inclined flat plate at an angle of attack of 30° and a Reynolds number of 30 000 is investigated experimentally. The velocity field in the wake is measured with a laser doppler anemometer (LDA) in the region from one plate breadth downstream to three and a half-plate breadths downstream. Controlled forcing is applied to the wake by vibrating the plate in the across-wind direction at a frequency in the middle of the lock-in range. The forcing serves to enhance the regularity and two-dimensionality of vortex shedding from the plate. It also facilitates phase-locked averaging of the LDA data. The LDA bursts are sorted according to their arrival instants relative to a particular phase of the vortex shedding cycles. The phase-averaged velocity results reveal large-scale vortical structures in the wake. Dynamical properties of these structures such as coherent vorticity and Reynolds stress production are discussed. The wake is found to be strongly asymmetric. The flow dynamics in the wake are dominated by a train of counterclockwise vortices shed from the trailing edge of the plate. The development, shedding and subsequent convection of these vortices are studied by following the consecutive phases of the shedding cycle.

Numerical solutions for spin-up from rest in a cylinder

A set of three-dimensional flow-field data for the region around a cylinder impulsively spun-up from rest was derived with a numerical model based on the Navier-Stokes equations. Laser-Doppler anemometer data in the azimuthal direction was employed to test the model predictions, and data was developed for a flowfield with Ekman numbers from 9.18/1,000,000 to 9.18/10,000. The contributions of inviscid and viscous terms were determined as functions of radius and time. It was found that immediately after start-up viscous diffusion is the dominant factor, which is replaced by nonlinear radial advection. The Coriolis force dominates in the later stages of spin-up. The inward radial flow is a maximum near the front, where the vertical velocity is small, but features strong radial gradients, as it does at the edge of the Ekman layer.

Spectral analysis of signals from a laser Doppler anemometer operating in the burst mode

Recently developed methods of spectral analysis for randomly sampled signals are applied to data acquired by a laser Doppler anemometer (LDA) operating in the burst mode, where the velocity of individual particles is computed. The measurements, which were obtained in a seeded air jet, support the adoption of a Poisson model for the distribution of the particle arrival times. Spectral estimates formed from the LDA data are found to agree well with corresponding estimates obtained from a hot wire probe, and have a variability which conforms with theoretical predictions. The advantages and limitations of a simple method of reducing the variability of the spectral estimates is discussed.