Comparison of PIV parameters effects on displacement identification and cross correlation

An algorithm is presented to determine displacements thanks to the identification method. Its main properties are described: no link with the particle size, measurement of the velocity distribution. Determination of effects of PIV parameters on displacement identification is made. Parameters used are noise, bias, velocity distribution. Therefore, we can define a validity domain of PIV parameters for identification and compare it with the domain of cross correlation. The identification validity range is based on 70% of isolated particles, on a displacement norm and on displacement gradients corresponding to less than half the size of the interrogation cell and to 10% of the average velocity. The comparison with cross correlation domains indicates that the cross correlation is more robust. However, the identification method is interesting because of the possibility of displacement distribution measurement. We use it to measure the decreasing of the turbulence intensity for a grid-generated turbulence.     

Evaluation of the 3D-PIV standard images (PIV-STD project)

Particle Imaging Velocimetry (PIV) offers many advantages for studies of fluid flows. Lots of PIV techniques have been developed and applied to various flow fields. However, there are no standard tools for evaluation of the PIV system effectiveness and accuracy. To popularize the PIV for practical use, the PIV system should have some standard.     

Recording of PIV images on photothermoplastic plates

In this paper we explore the possibility of utilizing commercially available photothermoplastic plates as recording support in Particle Image Velocimetry (PIV), to allow real time analysis of recorded data. An experimental apparatus has been developed which allows holographic recording and reconstruction of simulated PIV images. We show that double exposed photothermoplastic plates can successfully be analysed provided that a suitable recording/reconstruction scheme is used.     

Evaluation of the integrals of target/seabed scattering using the method of complex images

In this paper the integrals which arise in target/seabed scattering problems are examined. These are the integrals which express the propagation of a spherical harmonic term in a fluid above or below a half-space and the integrals which compute the conversion coefficients of outgoing spherical harmonics into incident spherical harmonics after a seabed reflection. A very efficient and accurate method of computing these integrals, using the method of complex images, is derived. Numerical comparisons with the exact integral expressions show the accuracy of this approach.     

An analysis of noise in PIV images

Particle Image Velocimetry can provide detailed velocity field information that has unparalleled value to both experimentalists and computational fluid dynamicists. As with any measurement technique, it is important to assess the error associated with the measurements. The PIV measurement chain contains several stages where error is introduced. The dynamics of the seed particles determine how well they represent the local fluid velocity. Acquiring the image, whether photographically or electronically, will introduce aberrations, distortion, diffraction, and positioning uncertainties. The image analysis procedure to determine particle displacements is the final step in the measurement chain. This paper considers the error in this step of the measurement chain. Synthetic images were generated with controlled levels of the following quantities: background mean illumination, background illumination standard deviation, mean particle diameter, number of paired particles, the ratio of unpaired particles to paired particles, displacement magnitude, and displacement direction. The analysis method tested is a generic autocorrelation technique using a two-dimensional forward FFT, PSD calculation, inverse FFT, and a peak detection algorithm based on a decreasing threshold search with subsequent sub-pixel interpolation. The error analysis principles demonstrated, however, could easily be applied to other algorithms including those implementing cross-correlation techniques. The error in the analysis technique is characterised by four quantities: the bias in the displacement magnitude, the precision index of the displacement magnitude, the bias in the displacement direction, and the precision index of the displacement direction. Each independent variable was varied over a specified range and the behaviour of the four dependent variables observed. The results showed a clear ability for this error assessment technique to illustrate the reliable operating range for each parameter. The variation in error with parameter level tended to be similar in all cases. There was a significant portion of the range where the error was very low. Then, at some critical value, the analysis technique broke down and the error became quite high.     

Unified Green's function retrieval by cross correlation

It has been shown by many authors that the cross correlation of two recordings of a diffuse wave field at different receivers yields the Green's function between these receivers. Recently the theory has been extended for situations where time-reversal invariance does not hold (e.g., in attenuating media) and where source-receiver reciprocity breaks down (in moving fluids). Here we present a unified theory for Green's function retrieval that captures all these situations and, because of the unified form, readily extends to more complex situations, such as electrokinetic Green's function retrieval in poroelastic or piezoelectric media. The unified theory has a wide range of applications in "remote sensing without a source."     

Structural damage detection using cross correlation functions of vibration response

Structural damage detection methods based on vibration responses are appealing for a variety of reasons such as their potential to observe damage from sensors placed remote from an unknown damage site. Of particular interest to the authors is online damage detection in which changes in the structure can be flagged up in an automated fashion by permanently installed transducers. In a previous paper by the authors, the inner product vector (IPV) was proposed as a damage detection algorithm which uses cross correlation functions between response measurements. Implicitly assumed in the formulation is that the response quantity is that of displacement resulting from white noise excitation. In this paper, the IPV technique is first reviewed and then generalised to address velocity and acceleration response to band pass white noise excitation. It is shown that the IPV is a weighted summation of the mode shapes, and the effect of some particular measurement noise on the IPV can be adaptively eliminated in the calculation of IPV. Then, the damage detection method based on changes in the IPV is proposed. Finally, damage detection experiments of shear frame structure, honeycomb sandwich composite beam and aircraft stiffened panel are presented to illustrate the feasibility and effectiveness of the proposed method.     

Polarization-insensitive cross correlation using two-photon absorption in a silicon photodiode

We present experimental measurements of the polarization dependence of two-photon absorption in silicon photodiodes at 1550 nm, and we offer a simple theory that explains our observations. Based on this theory, we propose and demonstrate that it is possible to construct an optical cross-correlation system that is polarization insensitive, provided that one of the two input polarization states can be controlled.     

Segmentation of holographic images using the level set method

Image segmentation, which is to distinguish objects from background, has played an important role in holographic image processing. In this work, we propose a technique that uses the combination of rough segmentation and the level set methods for segmentation of holographic images with zero-order diffraction interference. To improve the image quality, we eliminate the interference of zero-order diffraction in digital holography using Sobel differential gradient algorithm based on digital image processing. After reconstruction, the rough segmentation including intensity transformations and Morphology methods is applied to acquire the rough contour of the reconstruction image. Finally, we adopt level set methods to smooth the object contour and improve its accuracy. Segmentation results for objects in holographic images are presented.     

Investigations of three-dimensional flow characteristics in a liquid ramjet combustor using the PIV method

Three-dimensional flow characteristics in a liquid fuel ramjet combustor were investigated using the PIV method. The combustor had two rectangular inlets that form a 90-degree angle with each other, with intake angles of 30 degrees. Three guide vanes were installed in each rectangular inlet to improve flow stability. The experiments were performed in a water tunnel test with the same Reynolds number as Mach 0.3 at the inlet. PIV software was developed to measure the characteristics of the flow field in the combustor. Accuracy of the developed PIV program was verified with a rotating disk experiment and standard data. The experimental results showed that the two main streams from the rectangular intakes collided near the plane of symmetry and generated two large longitudinal vortices, which was in agreement with three dimensional computational results. A large and complex threedimensional recirculating flow was measured behind the intakes.     

基于辛Runge-Kutta-Nystrom方法的雷达散射截面计算

从基本的差分概念和Maxwell方程出发, 引入电磁场方程的Hamilton函数.提出一种基于Runge-Kutta-Nystrom辛算法的高阶时域有限差分方法，该方法保持了系统的相空间体积不变和总能量不变，并导出了迭代公式.在此基础上计算了一种金属圆柱的雷达散射截面.计算结果表明该方法的正确性及快速、精确的特性. 关键词： 雷达散射截面 高阶算法 辛Runge-Kutta-Nystrom方法 时域有限差分     

Small-scale seismic inversion using surface waves extracted from noise cross correlation

Green's functions can be retrieved between receivers from the correlation of ambient seismic noise or with an appropriate set of randomly distributed sources. This principle is demonstrated in small-scale geophysics using noise sources generated by human steps during a 10-min walk in the alignment of a 14-m-long accelerometer line array. The time-domain correlation of the records yields two surface wave modes extracted from the Green's function between each pair of accelerometers. A frequency-wave-number Fourier analysis yields each mode contribution and their dispersion curve. These dispersion curves are then inverted to provide the one-dimensional shear velocity of the near surface.     

Whole field spatial frequency analysis of double-or multiple-exposed PIV images

Speckle Velocimetry or high-image density PIV gives a velocity vector map of a twodimensional flow field by point-by-point spatial frequency analysis of local pattern at an interrogation spot in a double- or a multiple-exposed image of laser speckle or pseudo-speckle pattern generated by pulsed laser-light-sheet illuminations of a plane in the flow densely seeded with fine particles. The whole field spatial frequency analysis of the double- or multiple-exposed PIV image gives more useful information of the flow field. Optical Fourier transform is a conventional technique not only for the local spatial frequency analysis but for the whole field analysis. Filtering of spatial frequency is one of the typical techniques for the latter which can reconstruct a velocity contour or a component velocity contour map of the flow. Fundamentally this technique is a simple and efficient analogue method to get more information in the velocity field analysis of the flow than a digital image processing, but in practical applications the measurement is restricted to simple flows because of optical noise and low spatial resolution. In order to improve the technique, the fundamental characteristics of the filtering and the noise yielded in the filtering process must be investigated. Meanwhile, wavelet analysis can also be applied to the whole field spatial frequency analysis of PIV image. In this paper the filtering technique is examined by numerical convolution integral, and the results obtained are compared with ones obtained by the wavelet analysis.     

Investigation of roughness cross correlation in a Ni/C multilayer mirror by X-ray diffuse scattering method

The possibility of applying X-ray diffuse scattering for studying roughness in multilayer X-ray mirrors, including the correlation of roughnesses of neighboring interfaces (roughness cross-correlation) is considered. It is shown that the reliability and informativeness of this method can be improved by rejecting the classical experimental schemes and using alternative schemes in which not only the intensity of diffuse scattering itself, but also its dependence on certain experimental parameters (conditions), vary. Such parameters can be the spatial coherence of incident radiation, the direction of the momentum transfer relative to the specular diffraction plane, or the X-ray wavelength. In the framework of this approach, the results of comparative measurements of diffuse scattering from a Ni/C multilayer X-ray mirror prepared by laser ablation are considered for two close values of photon energy: below (8.325 keV) and above (8.350 keV) the K absorption edge for nickel. It is shown that, in view of effective screening of deep layers in the hard photoabsorption mode, this method provides more reliable (as compared to the standard diffuse scattering method) information on the evolution of interfaces between the layers. It is found that the smoothing of roughness in the experimental sample occurs over large spatial scales such as the micrometer scale. Only large-scale defects with a size exceeding 10 µm are replicated well from layer to layer. Possible physical reasons for the observed effect are considered. It is shown that effective smoothing on the micrometer and submicrometer spatial scales is of fundamental importance for preparing multilayer X-ray mirrors with high reflectances.     

Characterization of ultrashort electron pulses by electron-laser pulse cross correlation

An all-optical method to determine the duration of ultrashort electron pulses is presented. This technique makes use of the laser pulse ponderomotive potential to effectively sample the temporal envelope of the electron pulse by sequentially scattering different sections of the pulse out of the main beam. Using laser pulse parameters that are easily accessible with modern tabletop chirped-pulse amplification laser sources, it is possible to measure the instantaneous duration of electron pulses shorter than 100 fs in the energy range that is most useful for electron diffraction studies, 10-300 keV.     

Fusion method for infrared and visible images by using non-negative sparse representation

In this paper, an interesting fusion method, named as NNSP, is developed for infrared and visible image fusion, where non-negative sparse representation is used to extract the features of source images. The characteristics of non-negative sparse representation coefficients are described according to their activity levels and sparseness levels. Multiple methods are developed to detect the salient features of the source images, which include the target and contour features in the infrared images and the texture features in the visible images. The regional consistency rule is proposed to obtain the fusion guide vector for determining the fused image automatically, where the features of the source images are seamlessly integrated into the fused image. Compared with the classical and state-of-the-art methods, our experimental results have indicated that our NNSP method has better fusion performance in both noiseless and noisy situations.     

Small target detection using bilateral filter and temporal cross product in infrared images

We introduce a spatial and temporal target detection method using spatial bilateral filter (BF) and temporal cross product (TCP) of temporal pixels in infrared (IR) image sequences. At first, the TCP is presented to extract the characteristics of temporal pixels by using temporal profile in respective spatial coordinates of pixels. The TCP represents the cross product values by the gray level distance vector of a current temporal pixel and the adjacent temporal pixel, as well as the horizontal distance vector of the current temporal pixel and a temporal pixel corresponding to potential target center. The summation of TCP values of temporal pixels in spatial coordinates makes the temporal target image (TTI), which represents the temporal target information of temporal pixels in spatial coordinates. And then the proposed BF filter is used to extract the spatial target information. In order to predict background without targets, the proposed BF filter uses standard deviations obtained by an exponential mapping of the TCP value corresponding to the coordinate of a pixel processed spatially. The spatial target image (STI) is made by subtracting the predicted image from the original image. Thus, the spatial and temporal target image (STTI) is achieved by multiplying the STI and the TTI, and then targets finally are detected in STTI. In experimental result, the receiver operating characteristics (ROC) curves were computed experimentally to compare the objective performance. From the results, the proposed algorithm shows better discrimination of target and clutters and lower false alarm rates than the existing target detection methods.     

Construction of zero cross correlation code using a type of anti-diagonal-identity-column block matrices

This paper presents a new method for constructing zero cross correlation code with the help of anti-diagonal type-identity-column block matrices. The advantages of this code are easy code construction, existence for every natural number and acceptable code lengths. The numerical comparison shows that the newly constructed code is better code length and it supports more users than other Optical Spectrum Code Division Multiple Access (OSCDMA) codes.     

Minimizing correlation effect using zero cross correlation code in spectral amplitude coding optical code division multiple access

The use of minimal multiple access interference (MAI) in code design is investigated. Applying a projection and mapping techniques, a code that has a zero cross correlation (ZCC) between users in optical code division multiple access (OCDMA) is presented in this paper. The system is based on an incoherent light source—LED, spectral amplitude coding (SAC), and direct detection techniques at the receiver. Using power spectral density (PSD) function and Gaussian approximation, we obtain the signal-to-noise ratio (SNR) and the bit-error rate (BER) to measure the code performance. Making a comparison with other existing codes, e.g., Hadamard, MFH and MDW codes, we show that our code performs better at BER 10⁻⁹ in terms of number of simultaneous users. We also demonstrate the comparison between the theoretical and simulation analyses, where the results are close to one another.