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1.
 A non-biased estimator of power spectral density (PSD) is introduced for data obtained from a zeroth order interpolated laser Doppler anemometer (LDA) data set. The systematic error, sometimes referred to as the “particle-rate filter” effect, is removed using an FIR filter parameterized using the mean particle rate. Independent from this, a procedure for estimating the measurement system noise is introduced and applied to the estimated spectra. The spectral estimation is performed in the domain of the autocorrelation function and assumes no further process parameters. The new technique is illustrated using simulated and measured data, in the latter case with direct comparison to simultaneously acquired hot-wire data. Received: 9 June 1997/Accepted: 14 October 1997  相似文献   

2.
Spectral estimation of irregularly sampled velocity data issued from Laser Doppler Anemometry measurements is considered in this paper. A new method is proposed based on linear interpolation followed by a deconvolution procedure. In this method, the analytic expression of the autocorrelation function of the interpolated data is expressed as a linear function of the autocorrelation function of the data to be estimated. For the analysis of both simulated and experimental data, the results of the proposed method is compared with the one of the reference methods in LDA: refinement of autocorrelation function of sample-and-hold interpolated signal method given by Nobach et al. (Exp Fluids 24:499–509, 1998), refinement of power spectral density of sample-and-hold interpolated signal method given by Simon and Fitzpatrick (Exp Fluids 37:272–280, 2004) and fuzzy slotting technique with local normalization and weighting algorithm given by Nobach (Exp Fluids 32:337–345, 2002). Based on these results, it is concluded that the performances of the proposed method are better than the one of the other methods, especially for what concerns bias and variance.  相似文献   

3.
An unequally spaced fractal interpolation method is developed to reconstruct the low data rate laser Doppler velocimetry (LDV) signal for better spectral results. The reconstructed result not only augments the data points to improve the statistics but also retains the turbulent fractal dimension of the flow. In other words, the interpolated data are flow turbulence-correlated data which are physically reasonable. This method is demonstrated on the LDV spectral measurements in the shear layer of a jet flow and the results are compared against the hot-wire spectral data. When the data rate of the LDV signal is low, the fractal interpolation method can suitably restore the major spectral peaks while the conventional sample-and-hold fails. Effects of the data rate, data record length, sampling rate and the fractal dimension on the accuracy of the spectral reconstruction are also discussed. A version of this paper was presented at the 4th International Symposium on Transport Phenomena in Heat and Mass Transfer, July 14–18, 1991, Sydney, Australia  相似文献   

4.
Experimental results are presented of the power spectral density of the random bending strain on the surface of a beam obtained using a scanning laser vibrometer. The strain spectra were obtained by processing vibration data measured at discrete locations along the beam's length. The beam was driven by a stationary broad-band random force. The experimental setup is described along with the data analysis procedure. The results presented here indicate that the method is practical and can lead to reliable estimates.  相似文献   

5.
Many techniques have been developed in order to obtain spectral density function from randomly sampled data, such as the computation of a slotted autocovariance function. Nevertheless, one may be interested in obtaining more information from laser Doppler signals than a spectral content, using more or less complex computations that can be easily conducted with an evenly sampled signal. That is the reason why reconstructing an evenly sampled signal from the original LDV data is of interest. The ability of a wavelet-based technique to reconstruct the signal with respect to statistical properties of the original one is explored, and spectral content of the reconstructed signal is given and compared with estimated spectral density function obtained through classical slotting technique. Furthermore, LDV signals taken from a screeching jet are reconstructed in order to perform spectral and bispectral analysis, showing the ability of the technique in recovering accurate information’s with only few LDV samples.  相似文献   

6.
We consider the origin of noise and distortion in power spectral estimates of randomly sampled data, specifically velocity data measured with a burst-mode laser Doppler anemometer. The analysis guides us to new ways of reducing noise and removing spectral bias, e.g., distortions caused by modifications of the ideal Poisson sample rate caused by dead time effects and correlations between velocity and sample rate. The noise and dead time effects for finite records are shown to tend to previous results for infinite time records and ensemble averages. For finite records, we show that the measured sampling function can be used to correct the spectra for noise and dead time effects by a deconvolution process. We also describe a novel version of a power spectral estimator based on a fast slotted autocovariance algorithm.  相似文献   

7.
 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. Received: 24 April 2001 / Accepted: 8 August 2001  相似文献   

8.
Spectral analysis is a useful tool in engineering applications but many practical problems are encountered in computing power spectral densities (PSD), particularly in laser Doppler velocimetry where data are irregularly sampled. Methods to compute PSD are usely based on discrete Fourier transforms such as FFT. A non linear spectral analysis approach is tested and found to give better results, particularly with short data records. Results are also quite promising when non linear spectral analysis is combined with signal reconstruction to process irregularly sampled data.A preliminary version of this paper has been presented at LDA specialist meeting on the use of computers, May 1987, Institut Saint Louis, France  相似文献   

9.
Techniques for deriving the auto or power spectrum (PSD) of turbulence from laser Doppler anemometry (LDA) measurements are reviewed briefly. The low pass filter and step noise errors associated with the sample-and-hold process are considered and a discrete version of the low pass filter for the resampled signal is derived. This is then used to develop a procedure by which the PSD estimates obtained from sample and hold measurements can be corrected. The application of the procedures is examined using simulated data and the results show that the frequency range of the analysis can be extended beyond the Nyquist frequency based on the mean sample rate. The results are shown to be comparable to those obtained using the method of Nobach et al. (1998) but the new procedures are more straightforward to implement. The technique is then used to determine the PSD of real LDA data and the results are compared with those from a hot wire anemometer.  相似文献   

10.
 The evaluation of turbulence power spectra from velocity data acquired by LDA is considered. An analysis of the effect of the validation procedure, implemented in many data processors, on the power spectrum obtained by the sample and hold method is reported and compared to experimental results. The existence of a new term in the measured power spectrum step noise is predicted and experimentally verified and a procedure for eliminating spectral bias and estimating the Taylor time scale is suggested and applied. Received: 13 May 1997/Accepted: 21 October 1997  相似文献   

11.
重力场匹配导航的重力图生成技术   总被引:2,自引:2,他引:2  
通过对当前用于海洋重力场格网插值的四种常用算法(距离倒数加权法、Kriging法、径向基函数法和改进的二次曲面Shepard方法)进行分析,以相对规则、不同密度的两组数据作为基准数据进行了插值比较。实验结果表明,基于改进的Shepard插值算法相对于其它三种算法具有速度快、精度高的优点,比较符合当前海洋重力数据获取的现状,是进行高精度重力图生成的有效方法。  相似文献   

12.
大跨度桥梁空间脉动风场的计算机模拟   总被引:2,自引:0,他引:2  
针对现有Deodatis方法模拟大跨度桥梁空间脉动风场中存在的计算量问题,通过对谱分解矩阵引入插值近似,减少谱分解的次数,从而提高该谐波合成法的计算效率,并节省内存花费,实现了对三维空间脉动风场的有效模拟。改进方法模拟的脉动风速样本仍保持各态历经性,且逐渐收敛到目标功率谱。用改进的Deodatis方法模拟了润扬长江悬索桥桥面主梁上作用的纵向脉动风速。结果表明,该改进措施对Deodatis方法的应用效果非常明显,改进的Deodatis方法模拟脉动风速样本的相关函数与目标相关函数均吻合良好。尽管改进后的Deo-datis方法对谱分解矩阵采用了插值近似,但模拟的随机风速样本仍具有很好的精度。  相似文献   

13.
Scalar concentration measurements in liquid-phase flows with pulsed lasers   总被引:1,自引:0,他引:1  
The suitability of pulsed lasers for laser-induced-fluorescence (LIF) measurements of the local concentration of scalars in liquid-phase flows is investigated. Experiments were performed to measure the fluorescence intensity of aqueous solutions of rhodamine-6G chloride excited by a Q-switched, frequency-doubled, Nd:YAG laser. The fluorescence intensity is found to be linear with dye concentration, but not with illumination power density. The fluorescence intensity saturates at laser power densities easily exceeded by short-pulse-duration lasers. A procedure for calibrating and normalizing the raw image data is discussed which relies only on weak absorption and the linearity of fluorescence intensity with concentration. This procedure enables quantitative concentration measurement with pulsed-laser-induced fluorescence.  相似文献   

14.
基于连续正弦速度扫描方式的激光多普勒测振技术   总被引:2,自引:0,他引:2  
陈强  臧朝平 《实验力学》2015,30(5):613-620
本文针对逐点扫描激光多普勒非接触测振技术只能进行离散空间域振动测试的缺陷,研究了基于连续正弦速度扫描方式的激光多普勒测振新技术,实现了连续空间域的振动测试。并以简支梁为测试对象,进行了实验验证。实验结果表明:应用连续扫描激光多普勒测振技术与扫描激光多普勒测振技术获取的模态振型匹配一致,而且前者在测试时间、测试数据量及空间分辨率上都优于后者。  相似文献   

15.
An experimental analysis of the precessing vortex core (PVC) instability in a free swirling jet of air at ambient pressure and temperature is performed by means of laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Two parametric studies are considered, varying the swirl parameter and the Reynolds number. The range of parameters considered allowed to study conditions of strong precession as well as the inception and settlement of the instability. Mean velocity and standard deviation profiles, power spectral density functions and probability density functions for the axial and tangential velocity components are presented. Average as well as instantaneous PIV maps are considered in the characterization of the flowfield structure and detection of the instantaneous position of the vortex center. Joint analysis of velocity PDFs and power spectra shows that the PVC contribution to the global statistics of the velocity field can be properly separated from the contribution of the true flow turbulence, giving additional insight to the physics of the precession phenomenon. The results obtained in the explored range of conditions indicate that the true turbulence intensity is not dependent on the swirl parameter. An erratum to this article can be found at  相似文献   

16.
This paper proceeds from the general case of the unsymmetric linearized multi-degrees of freedom (MDOF) systems. By adopting the general complex modal theory of the state space, the response analysis for a system subjected to random excitation of the same source is carried out using as a kind of direct spectrum analysis method in frequency domain. With the input of power spectral density function given, the explicit expression of the power spectral density function matrix of the output response can be obtained. By taking Fourier inverse transform, the integrated expressions of the correlation function matrix and of the spectrum moment matrix are obtained. Comparing with the time domain method, this method enjoys the merit of visualization and avoids the procedure of transformation from the obtained response correlation function to be solved for the output spectrum utilizing Fourier transform. This paper has extended the application range of the traditional frequency domain analysis method. The mean square values and variety of statistical values can be obtained conveniently. This method and the time domain method are different in approach but equally satisfactory in their results. The project is supported by National Natural Science Foundation of China and National Education Commission Science Foundation of China  相似文献   

17.
Two techniques — a modified correlation method and a direct transform method — have been evaluated for use in making spectral estimates from randomly-sampled random data (such as turbulence data obtained with an individual realization laser Doppler velocimeter (LDV)). The effect of bad points (which usually appear randomly in actual LDV data sets) on the spectral estimates has been studied. Necessary modifications and extensions to the techniques have been determined based on studies using simulated data with known spectral characteristics. The direct transform method is found to have certain advantages over the correlation method.From the LDV measurements taken in a simulated coal-fired magnetohydrodynamic flow field, major differences in turbulence spectra obtained from the two methods are observed in the region immediately downstream of the combustor where the flow was evidently dominated by (uncorrelated) combustion instabilities. Spectra are reported for three axial positions, and comparisons with classic turbulent pipe flow data are presented.  相似文献   

18.
By using a laser profiler, the roughness of sowed and plowed surfaces was obtained. Through evaluation of the precision of fractal dimensions based on the Weierstrass–Mandelbort (W–M) function, we found the rescaled (R/S) analysis method and the structure spectral method were not suitable for the calculation of the fractal dimension on a soft terrain surface. Therefore, the fractal dimension, non-scale range and correlation coefficient for each kind of terrain were analyzed using the following fractal computational methods – (i) variate-difference method, (ii) power spectral density method, and (iii) root mean square method. The results showed that: fractal dimension of plowed terrain was large with small fluctuations, while its internal structure was complex. The power spectral density method was not robust enough to compute the fractal dimension of a soft terrain surface. The fractal dimension computed using the root mean square method was found to be more accurate for the soft terrain surface. Moreover, the correlation coefficient of linear regression when using the root mean square method was good and the range of non-scale variation was small.  相似文献   

19.
A relatively simple optical scheme using the reference-mode laser Doppler anemometry for the in situ measurement of flow properties of a dilute particle-fluid two-phase suspension having a predominant flow direction is hereby proposed. It is an extension of the established technique of optical gating for particle sizing which is fully integrated into the established technique of laser Doppler anemometry for velocity measurement. Particles that can be measured by this scheme are limited to those with sizes greater than the smaller dimension of the optical measuring volume. Inherent in the methodology is a procedure for providing information on the local particle number density and velocity distributions for each size range of the particles and the local velocity distribution of the continuous phase. The accompanying electronics and interfaces are also established for data processing and analysis in a mini computer. Validation of the scheme has been accomplished by controlled experiments using stainless steel balls and water droplets of 1 mm and greater in diameter.  相似文献   

20.
Displacements of mechanical waves superposed onto wind waves were measured with a laser displacement gauge in a wind-wave tank. The effects of wave breaking, especially the spilling breaking type, on the wave-variance spectra are investigated. In the absence of wave breaking, the quasi-equilibrium spectrum consists of an f –7/3 subrange in the capillary regime, and its spectral density increases with increasing wind speed. When intense spilling breaking occurs, the water surface is saturated with small-scale features that cause not only an increase in the spectral density but also a reduction in the slope of the spectrum at high frequencies. Velocity components under the water surface were measured with a laser Doppler velocimeter. The energy spectra of the vertical and longitudinal velocity components in breaking waves are practically identical in the frequency range near the dominant wave frequency. At higher frequencies, the spectra generally follow Kolmogorov's –5/3 law. In the intermediate frequency range, we observed a higher spectral density for the vertical velocity component than for its longitudinal counterpart. These results suggest that turbulence energy is transferred from the vertical component to the longitudinal component in breaking waves. The acceleration of the water motion becomes as large as gravitational acceleration when intense wave breaking takes place. The flow field in breaking waves is highly dissipative.  相似文献   

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