共查询到20条相似文献,搜索用时 28 毫秒
1.
A correlation-based central difference image correction (CDIC) method and application in a four-roll mill flow PIV measurement 总被引:1,自引:0,他引:1
An experiment is conducted in a four-roll mill to verify a novel particle image velocimetry (PIV) recording evaluation method that combines the advantages of central difference interrogation and an image correction technique. Simulations and experiments in the four-roll mill geometry demonstrate that the central difference image correction method described in this paper can not only avoid the bias error resulting from the curvature and high-velocity-gradient flow but also effectively reduce the random error resulting from particle image distortion. Two image correction schemes and two base algorithms are discussed. A four-point image correction scheme is suggested on the basis of the traditional correlation-based interrogation algorithm to enable a fast, high-accuracy evaluation of PIV recordings in complex flows. In addition, the PIV experiment accurately determines the velocity field in the four-roll mill and confirms the linear distributions of the velocity components and the roller speed. 相似文献
2.
Sara Nauri José Nogueira Antonio Lecuona Mathieu Legrand Pedro A. Rodríguez 《Experiments in fluids》2008,45(4):609-621
Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity
in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has
shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients.
A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is
able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance
is evaluated carrying out the following two different tasks:
The first task has been achieved using synthetic images. Afterwards the coherence of the results has been checked with real
images. The results show improvement of Stereo-LFC PIV in respect to Stereo-Multigrid PIV enhanced with image distortion.
The performance of Stereo-LFC when only large scales are involved shows an increase of the dynamic range of measurable vorticity.
When small scales are analysed, the magnitude of the error resulting when using Stereo-LFC is about half of the one obtained
for the Stereo-Multigrid measurements. Results with errors below 20% have been achieved for some of the cases with peak vorticities
as large as 1.8 Δt
−1 (in the absence of out-of-plane displacements), out-of-plane loss of particle pairs of 65% (with a low peak vorticity of
0.06 Δt
−1) and peak vorticities as large as 1.5 Δt
−1 with 50% particle pair loss. For the second task most of the information has been obtained using real images. It has been
found that the resolution limit is very dependent on the robustness of the algorithms against image defects and variability.
The results show a remarkable improvement when using the Stereo-LFC PIV processing, although a full quantification and characterization
would need further study because of the variety of noise sources possible in a real image. 相似文献
– | Comparison of robustness and accuracy for large and small scale flow structures. This is carried out using three techniques: the conventional Stereo PIV, the Stereo-LFC PIV and the Stereo-Multigrid PIV enhanced with image distortion. |
– | Insight on the limit of resolvable scales for the Stereo-LFC. This task is relevant because the resolution attainable by this combination is higher than what has been obtained by the rest of the herein used algorithms. |
3.
Measurement of fully-developed turbulent pipe flow with digital particle image velocimetry 总被引:6,自引:0,他引:6
J. Westerweel A. A. Draad J. G. Th. van der Hoeven J. van Oord 《Experiments in fluids》1996,20(3):165-177
A new and unique high-resolution image acquisition system for digital particle image velocimetry (DPIV) in turbulent flows is used for the measurement of fully-developed turbulent pipe flow at a Reynolds number of 5300. The flow conditions of the pipe flow match those of a direct numerical simulation (DNS) and of measurements with conventional (viz., photographic) PIV and with laser-Doppler velocimetry (LDV). This experiment allows a direct and detailed comparison of the conventional and digital implementations of the PIV method for a non-trivial unsteady flow. The results for the turbulence statistics and power spectra show that the level of accuracy for DPIV is comparable to that of conventional PIV, despite a considerable difference in the interrogation pixel resolution, i.e. 32 × 32 (DPIV) versus 256 × 256 (PIV). This result is in agreement with an earlier analytical prediction for the measurement accuracy. One of the advantages of DPIV over conventional PIV is that the interrogation of the DPIV images takes only a fraction of the time needed for the interrogation of the PIV photographs. 相似文献
4.
A combination correlation-based interrogation and tracking algorithm for digital PIV evaluation 总被引:4,自引:0,他引:4
A combination of the correlation-based interrogation algorithm and the correlation-based tracking algorithm is proposed for digital PIV evaluation. A zero-padding interrogation algorithm is adopted in which the interrogation windows differ in size and in which the number of pixels in the side length of the smaller window is not restricted to a power of 2. This greatly improves the algorithm's accuracy and measurement range. The correlation-based tracking algorithm is employed when the already-measured vector can serve as a good predictor of the next vector to be measured. In this case, only a very small searching scope is required and computation can be fast. Computational intensity analysis shows that, using the same-sized sampled window, the correlation-based tracking algorithm is more efficient than the conventional correlation-based interrogation algorithm if the searching scope is less than 4. Compared with some of the other correlation-based algorithms, the proposed combination method is faster, is more accurate, has a larger measurable range, and can utilize a sampled window of any size. 相似文献
5.
A digital mask technique for reducing the bias error of the correlation-based PIV interrogation algorithm 总被引:1,自引:0,他引:1
In this paper the bias phenomenon in the evaluation of PIV recordings by using the correlation-based interrogation algorithm
is discussed, and a digital mask technique, that can effectively reduce the bias error, is introduced. The correlation-based
interrogation algorithm, when masked with a Gaussian window function, can achieve a higher evaluation accuracy not only for
PIV recordings of flows with small velocity gradients, but also for that of flows with large gradients.
Received: 14 October 1998/Accepted: 20 July 1999 相似文献
6.
PIV measurements of a microchannel flow 总被引:24,自引:0,他引:24
A particle image velocimetry (PIV) system has been developed to measure velocity fields with order 1-μm spatial resolution.
The technique uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope,
and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution of the
PIV technique is limited primarily by the diffraction-limited resolution of the recording optics. The accuracy of the PIV
system was demonstrated by measuring the known flow field in a 30 μm×300 μm (nominal dimension) microchannel. The resulting
velocity fields have a spatial resolution, defined by the size of the first window of the interrogation spot and out of plane
resolution of 13.6 μm× 0.9 μm×1.8 μm, in the streamwise, wall-normal, and out of plane directions, respectively. By overlapping
the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal
direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved
PIV measurements published to date.
Received: 29 October 1998/Accepted: 10 March 1999 相似文献
7.
Experimental dual plane particle image velocimetry (PIV) data are assessed using direct numerical simulation (DNS) data of a similar flow with the aim of studying the effect of averaging within the interrogation window. The primary reason for the use of dual plane PIV is that the entire velocity gradient tensor and hence the full vorticity vector can be obtained. One limitation of PIV is the limit on dynamic range, while DNS is typically limited by the Reynolds number of the flow. In this study, the DNS data are resolved more finely than the PIV data, and an averaging scheme is implemented on the DNS data of similar Reynolds number to compare the effects of averaging inherent to the present PIV technique. The effects of averaging on the RMS values of the velocity and vorticity are analyzed in order to estimate the percentage of turbulence intensity and enstrophy captured for a given PIV resolution in turbulent boundary layers. The focus is also to identify vortex core angle distributions, for which the two-dimensional and three-dimensional swirl strengths are used. The studies are performed in the logarithmic region of a turbulent boundary layer at z
+ = 110 from the wall. The dual plane PIV data are measured in a zero pressure gradient flow over a flat plate at Re
τ = 1,160, while the DNS data are extracted from a channel flow at Re
τ = 934. Representative plots at various wall-normal locations for the RMS values of velocity and vorticity indicate the attenuation of the variance with increasing filter size. Further, the effect of averaging on the vortex core angle statistics is negligible when compared with the raw DNS data. These results indicate that the present PIV technique is an accurate and reliable method for the purposes of statistical analysis and identification of vortex structures. 相似文献
8.
A new stereoscopic PIV system to measure the three velocity components is developed and applied to grid turbulence flows.
This system uses two CCD cameras coupled with an accurate cross-correlation calculation method. An experimental test (based
upon three-dimensional displacements) has been carried out to demonstrate the capability of this process to locate the maximum
of correlation, and to detect accurately the 3D displacements. Experiments in a well-established turbulent flow have validated
the method for quantitative measurements and a comparison with LDV results showed a good agreement in terms of mean and fluctuating velocities. Combined PIV and stereoscopic PIV measurements on a turbulent flow revealed the need to the stereoscopic systems to measure accurate 2D
velocity fields. It has been shown that an error of up to 10% in the velocity fluctuation measured by conventional PIV could
be attained due to 3D effects in highly turbulent cases. Finally, the digital cross-correlation technique adapted to the determination
of small displacements seems to be the most suitable technique for stereoscopic PIV.
Received: 22 July 1997/Accepted: 27 January 1998 相似文献
9.
We describe a technique that allows an arbitrary size of the interrogation window when using the traditional FFT algorithm
in analysing PIV recordings by either cross- or auto-correlation methods. The length and width of the effective interrogation
window are no longer required to be composed of a number of pixels making a power of 2 (16, 32, 64 etc). This gives a higher
flexibility in selecting the appropriate window size.
Received: 28 January 1997/Accepted: 11 August 1997 相似文献
10.
The possibility of using different times between laser pulses (Δt) in a PIV (Particle Image Velocimetry) measurement of the same real flow field for error assessment has already been proposed
by the authors in a recent paper Nogueira et al. (Meas Sci Technol 20, 2009). It is a simple procedure that is available with the usual PIV setup. In that work, peak locking was considered basically
as a bias error. Later measurements indicated that, using appropriate processing algorithms, this error is not the main peak-locking
effect. Scenarios with the rms (root mean square) error due to peak locking as the most relevant contribution are more common
than initially expected and require a differentiated approach. This issue is relevant due to the impact of the rms error in
the evaluation of flow quantities like turbulent kinetic energy. The first part of this work is centred on showing that peak-locking
error in PIV is not always a measurement bias towards the closest pixel integer displacement. Insight in the subject indicates
that this is the case only for algorithm-induced peak locking. The peak locking coming out of image acquisition limitations
(i.e. resolution) is not ‘a priory’ biased. It is a random error with a peculiar probability density function. Discussion
on the subject is offered, and a particular approach to use a simple multiple Δt strategy to asses this error is proposed. The results reveal that in real images where amplitude of the peak-locking bias
error is assessed to be as small as 0.02 pixels, rms errors can be in the order of 0.1 pixels. As PIV approaches maturity,
providing a quantitative confidence interval by estimating measurement error seems essential. The method developed is robust
enough to quantify these values in the presence of turbulence with rms up to ~0.6 pixels. This proposal constitutes a relevant
step forward from the traditional histogram-based considerations that only reveal whether strong peak-locking error is present
or not, without any information on its magnitude or whether its origin is bias or rms. 相似文献
11.
A correlation-based continuous window-shift technique to reduce the peak-locking effect in digital PIV image evaluation 总被引:3,自引:0,他引:3
In this paper the peak-locking phenomenon is investigated in the evaluation of digital PIV recordings by using a correlation-based
interrogation algorithm with a discrete window shift and a correlation-based tracking algorithm. Statistical analyses indicate
that nonuniformly distributed bias errors are the main cause of the peak-locking effect, and the amplitude variation of the
random error is also an important source of the peak locking. Simulations and experimental examples demonstrate that very
strong peak-locking effects exist for the correlation-based interrogation algorithm with discrete window shift in the cases
of large particle images, small interrogation windows, and very small particle images. Very strong peak-locking effects are
also observed for the correlation-based tracking algorithm when the particle images are overexposed, binarized, or very small.
These strong peak-locking effects can be avoided without loss of evaluation accuracy by using a continuous window-shift technique
in combination with the correlation-based interrogation algorithm.
Received: 2 July 2001 / Accepted: 28 November 2001 相似文献
12.
In PIV, the optimal time separation (t) between successive laser pulses is influenced by a number of parameters. In the present paper, only two kinds of error affecting the choice of t are studied: (i) random error arising from noise during recording of the flow seeded with tracer particles and subsequent interrogation of the particle images, and (ii) acceleration error arising from approximation of the local Eulerian velocity based on small (but non-zero) particle displacements. These two kinds of error place conflicting requirements on t. A model to optimize t with respect to these errors is described, and the model is confirmed by the results of a Monte Carlo simulation. This model for optimal t is extended to various acceleration distributions. An estimate for the spatial resolution of the velocity field resulting from cross-correlation PIV is proposed.We wish to thank the University of Delaware and the Department of Mechanical Engineering for providing a graduate fellowship to support this work. 相似文献
13.
A new sub-pixel correlation peak locating algorithm for PIV analysis is introduced. The method is theoretically consistent with the method of continuously shifting interrogation sub-windows by fractional displacements, which has proven to be an effective way to reduce the bias error associated with integer pixel aliasing, or peak-locking. However the proposed algorithm performs continuous window shifting in the spatial frequency domain using the shift property of the Fourier transform, thus it is equivalent to interpolating the original digital image with the Fourier transform reconstruction. Synthetic and real PIV images are used to test the new algorithms performance relative to that of traditional (non-iterative) peak-finding methods and other peak-locking reduction algorithms, such as the continuous window shifting technique. The resultant bias error of the proposed algorithm is smaller (by an order of magnitude in some cases), and importantly, the periodic nature of the bias error, the characteristic signature of peak-locking, is eliminated as long as the discrete particle images have been sampled at a rate greater than the Nyquist sampling frequency. Moreover, this new algorithm is shown to be computationally efficient and it converges faster than the competing algorithms. 相似文献
14.
The accuracy of turbulent kinetic energy (TKE) dissipation rate measured by PIV is studied. The critical issue for PIV-based
dissipation measurements is the strong dependency on the spatial resolution, Δx, as reported by Saarenrinne and Piirto (Exp Fluids Suppl:S300–S307, 2000). When the PIV spacing is larger than the Kolmogorov scale, η, the dissipation is underestimated because the small scale
fluctuations are filtered. For the case of Δx smaller than the Kolmogorov scale, the error rapidly increases due to noise. We introduce a correction method to eliminate
the dominant error for the small Δx case. The correction method is validated by using a novel PIV benchmark, random Oseen vortices synthetic image test (ROST),
in which quasi-turbulence is generated by randomly superposing multiple Oseen vortices. The error of the measured dissipation
can be more than 1,000% of the analytical dissipation for the small Δx case, while the dissipation rate is underestimated for the large Δx case. Though the correction method does not correct the underestimate due to the low resolution, the dissipation was accurately
obtained within a few percent of the true value by using the correction method for the optimal resolution of η/10 < Δx < η/2. 相似文献
15.
J. de Jong L. Cao S. H. Woodward J. P. L. C. Salazar L. R. Collins H. Meng 《Experiments in fluids》2009,46(3):499-515
Measuring the turbulent kinetic energy dissipation rate in an enclosed turbulence chamber that produces zero-mean flow is
an experimental challenge. Traditional single-point dissipation rate measurement techniques are not applicable to flows with
zero-mean velocity. Particle image velocimetry (PIV) affords calculation of the spatial derivative as well as the use of multi-point
statistics to determine the dissipation rate. However, there is no consensus in the literature as to the best method to obtain
dissipation rates from PIV measurements in such flows. We apply PIV in an enclosed zero-mean turbulent flow chamber and investigate
five methods for dissipation rate estimation. We examine the influence of the PIV interrogation cell size on the performance
of different dissipation rate estimation methods and evaluate correction factors that account for errors related to measurement
uncertainty, finite spatial resolution, and low Reynolds number effects. We find the Re
λ
corrected, second-order, longitudinal velocity structure function method to be the most robust method to estimate the dissipation
rate in our zero-mean, gaseous flow system. 相似文献
16.
J. D. Luff T. Drouillard A. M. Rompage M. A. Linne J. R. Hertzberg 《Experiments in fluids》1999,26(1-2):36-54
We have recently used Particle Image Velocimetry (PIV) to study the dynamics of vortex propagation in reacting and non-reacting
flows. In order to do so, it became necessary to assess the uncertainty in PIV-based vorticity data. A computer simulation
was developed to investigate how uncertainty propagates throughout the post-processing, numerical data smoothing, and vorticity
calculating algorithms commonly used in the analysis of PIV data. Results indicate that the average uncertainty in vorticity
per interrogation cell (normalized to the average vorticity, and then surface averaged), for a simple vortex, can be reduced
to approximately ±4% with appropriate measures. This value was obtained using PIV autocorrelation software, a local regression
technique combined with a Gaussian-smoothing filter. Our best experimental results (these areas with no lost or spurious vectors)
are consistent with Stoke’s theorem.
Received: 14 August 1996/Accepted: 13 April 1998 相似文献
17.
Theory of non-isotropic spatial resolution in PIV 总被引:2,自引:0,他引:2
The spatial resolution of the PIV interrogation technique is discussed from an analytical standpoint and assessed with Monte Carlo numerical simulation of particle image motion. The PIV measurement error associated with lack of spatial resolution is modelled associating the cross-correlation operator to a moving average filter. The error associated with the "low-pass filtering" effect is investigated by adopting a second-order polynomial expression for the velocity spatial distribution. According to the present error analysis, the measurement error is proportional to the second-order spatial derivative of the velocity field and increases with the square of the window linear size. The strategy for the selection of the window size and properties (aspect ratio and orientation) so as to minimize the error is discussed. The principle is based on nonisotropic interrogation windows of elliptical shape, with a constant area and elongated in the direction of the largest curvature radius. The nonisotropic parameters are defined as eccentricity and orientation, which are based on the local eigenvalues/vectors of the Hessian tensor of the displacement spatial distribution. The technique is implemented in a recursive PIV interrogation method. The performance of nonisotropic interrogation technique is assessed by means of synthetic PIV images, which simulate three situations: first, a one-dimensional sinusoidal shear displacement, which allows comparison of the cross-correlation spatial response with the transfer function of linear filters. Second, the stream-wise exponential velocity decay is simulated, which simulates the particle tracers decelerating downstream of a shock wave and gives an example of a flow with main velocity differences aligned with the velocity direction. The results show that keeping the image density fixed, the error caused by insufficient spatial resolution can be reduced by a factor two when a preferential direction is found in the flow field. Finally, a Lamb–Oseen vortex flow is presented, which shows the complex pattern formed by the interrogation windows in a two-dimensional case. In this case, the improvement in interrogation performance is limited due to the isotropic nature of the velocity spatial fluctuation. 相似文献
18.
The features of an improved algorithm for the interrogation of (digital) particle image velocimetry (PIV) pictures are described.
The method is based on cross-correlation. It makes use of a translation of the interrogation areas. Such a displacement is
predicted and corrected by means of an iterative procedure. In addition, while iterating, the method allows a refinement of
the size of the interrogation areas. The quality of the measured vectors is controlled with data validation criteria applied
at each intermediate step of the iteration process.
A brief section explains the expected improvements in terms of dynamic range and resolution. The accuracy is assessed analysing
images with imposed displacement fields.
The improved cross-correlation algorithm has been applied to the measurement of the turbulent flow past a backward facing
step (BFS).
A systematic comparison is presented with Direct Numerical Simulation (DNS) data available on the subject.
Received: 7 October 1997/Accepted: 11 August 1998 相似文献
19.
基于BICC算法的PIV技术 总被引:12,自引:0,他引:12
对基于BICC(BinaryImageCross-Correlation)算法的PIV(ParticleImageVelocimetry)技术的原理进行了较详细的介绍.以封闭的正方形容器内的旋转流场为例,用计算机模拟检验了BICC算法的可靠性,最后利用PIV技术对阀腔内真实流场进行了测量.结果表明,BICC算法能有效地进行图像粒子的识别,利用该算法可以得到精确的PIV测量结果. 相似文献
20.
E. Momoniat 《Meccanica》2011,46(2):313-323
We compare two finite difference schemes to solve the third-order ordinary differential equation y'=y
−k
from thin film flow. The boundary conditions come from Tanner’s problem for the surface tension driven flow of a thin film.
We show that a central difference approximation to the third derivative in the model equation produces a solution curve with
oscillations. A difference scheme based on a combination of forward and backward differences produces a smooth accurate solution
curve. Both the 0-stability and von Neumann stability properties of the different finite difference schemes are analyzed.
The solution curves obtained from both approaches are presented and discussed. 相似文献