共查询到20条相似文献,搜索用时 31 毫秒
1.
Andreas Fouras David Lo Jacono Chuong Vinh Nguyen Kerry Hourigan 《Experiments in fluids》2009,47(4-5):569-577
A method is proposed that allows three-dimensional (3D) two-component measurements to be made by means of particle image velocimetry (PIV) in any volume illuminated over a finite thickness. The method is based on decomposing the cross-correlation function into various contributions at different depths. Because the technique is based on 3D decomposition of the correlation function and not reconstruction of particle images, there is no limit to particle seeding density as experienced by 3D particle tracking algorithms such as defocusing PIV and tomographic PIV. Correlations from different depths are differentiated by the variation in point spread function of the lens used to image the measurement volume over that range of depths. A number of examples are demonstrated by use of synthetic images which simulate micro-PIV (μPIV) experiments. These examples vary from the trivial case of Couette flow (linear variation of one velocity component over depth) to a general case where both velocity components vary by different complex functions over the depth. A final validation—the measurement of a parabolic velocity profile over the depth of a microchannel flow—is presented. The same method could also be applied using a thick light sheet in macro-scale PIV and in a stereo configuration for 3D three-component PIV. 相似文献
2.
A kilohertz frame rate cinemagraphic PIV system for laboratory-scale turbulent and unsteady flows 总被引:2,自引:0,他引:2
A kilohertz frame rate cinemagraphic particle image velocimetry (PIV) system has been developed for acquiring time-resolved
image sequences of laboratory-scale gas and liquid-phase turbulent flows. Up to 8000 instantaneous PIV images per second are
obtained, with sequence lengths exceeding 4000 images. The two-frame cross-correlation method employed precludes directional
ambiguity and has a higher signal-to-noise ratio than single-frame autocorrelation or cross-correlation methods, facilitating
acquisition of long uninterrupted sequences of valid PIV images. Low and high velocities can be measured simultaneously with
similar accuracy by adaptively cross-correlating images with the appropriate time delay. Seed particle illumination is provided
by two frequency-doubled Nd:YAG lasers producing Q-switched pulses at the camera frame rate. PIV images are acquired using
a 16 mm high-speed rotating prism camera. Frame-to-frame registration is accomplished by imaging two pairs of crossed lines
onto each frame and aligning the digitized image sequence to these markers using image processing algorithms. No flow disturbance
is created by the markers because only their image is projected to the PIV imaging plane, with the physical projection device
residing outside the flow field. The frame-to-frame alignment uncertainty contributes 2% to the overall velocity measurement
uncertainty, which is otherwise comparable to similar film-based PIV methods.
Received: 11 July 2000 / Accepted: 21 June 2001 Published online: 29 November 2001 相似文献
3.
μPIV is a widely accepted tool for making accurate measurements in microscale flows. The particles that are used to seed the
flow, due to their small size, undergo Brownian motion which adds a random noise component to the measurements. Brownian motion
introduces an undesirable error in the velocity measurements, but also contains valuable temperature information. A PIV algorithm
which detects both the location and broadening of the correlation peak can measure velocity as well as temperature simultaneously
using the same set of images. The approach presented in this work eliminates the use of the calibration constant used in the
literature (Hohreiter et al. in Meas Sci Technol 13(7):1072–1078, 2002), making the method system-independent, and reducing the uncertainty involved in the technique. The temperature in a stationary
fluid was experimentally measured using this technique and compared to that obtained using the particle tracking thermometry
method and a novel method, low image density PIV. The method of cross-correlation PIV was modified to measure the temperature
of a moving fluid. A standard epi-fluorescence μPIV system was used for all the measurements. The experiments were conducted
using spherical fluorescent polystyrene-latex particles suspended in water. Temperatures ranging from 20 to 80°C were measured.
This method allows simultaneous non-intrusive temperature and velocity measurements in integrated cooling systems and lab-on-a-chip
devices. 相似文献
4.
A common source of error in particle image velocimetry (PIV) is the presence of bright spots within the images. These bright
spots are characterized by grayscale intensities much greater than the mean intensity of the image and are typically generated
by intense scattering from seed particles. The displacement of bright spots can dominate the cross-correlation calculation
within an interrogation window, and may thereby bias the resulting velocity vector. An efficient and easy-to-implement image-enhancement
procedure is described to improve PIV results when bright spots are present. The procedure, called Intensity Capping, imposes
a user-specified upper limit to the grayscale intensity of the images. The displacement calculation then better represents
the displacement of all particles in an interrogation window and the bias due to bright spots is reduced. Four PIV codes and a large set of experimental
and simulated images were used to evaluate the performance of Intensity Capping. The results indicate that Intensity Capping
can significantly increase the number of valid vectors from experimental image pairs and reduce displacement error in the
analysis of simulated images. A comparison with other PIV image-enhancement techniques shows that Intensity Capping offers
competitive performance, low computational cost, ease of implementation, and minimal modification to the images. 相似文献
5.
A comparative study of the MQD method and several correlation-based PIV evaluation algorithms 总被引:8,自引:0,他引:8
The Minimum Quadratic Difference (MQD) method is compared with methods conventionally used for the evaluation of PIV recordings,
i.e. correlation-based evaluation with fixed interrogation windows (auto- or cross-correlation) and correlation-based tracking.
The comparison is performed by studying the evaluation accuracy achieved when applying these methods to pairs of synthetic
PIV recordings for which the true displacements are known. The influence of the magnitude of the particle image displacement,
evaluation window size, density of particle image distribution, and particle image size on the accuracy are investigated.
In all these cases the best results in terms of a statistical error are obtained with the MQD method. The superiority of the
MQD method can be explained with its potential of accounting for non-uniformities in the particle image distribution and a
non-uniform illumination. It is also shown that the conventional correlation-based methods may produce principal errors that
are non-existent for the MQD method. The evaluation speed achievable for the MQD method by making use of the FFT is comparable
to that common for the generally used auto- or cross-correlation algorithm. Finally, a quantitative explanation is given for
the often observed phenomenon that PIV velocity results tend to be smaller than the true values.
Received: 15 May 1998/Accepted: 24 April 1999 相似文献
6.
A method to extract whole-field spatio-temporal correlations by combining global and single-point measurement techniques of different time resolutions is proposed. For fluid mechanics applications, the emphasis is on the combination of low repetition rate particle image velocimetry (PIV) results with experimental data obtained at largely higher sampling frequencies. The experimental feasibility of the procedure is established from results obtained in the wake of a cylinder, using PIV and constant temperature hot wire anemometry (CTA). The method is then applied to examine the shear layer in the core of a round subsonic jet using PIV and laser Doppler velocimetry (LDV). The accuracy of the cross-correlation functions is compared to the auto- and cross-correlation functions obtained from series of LDV and CTA measurements. 相似文献
7.
A novel multi-frame particle image velocimetry (PIV) method, able to evaluate a fluid trajectory by means of an ensemble-averaged cross-correlation, is introduced. The method integrates the advantages of the state-of-art time-resolved PIV (TR-PIV) methods to further enhance both robustness and dynamic range. The fluid trajectory follows a polynomial model with a prescribed order. A set of polynomial coefficients, which maximizes the ensemble-averaged cross-correlation value across the frames, is regarded as the most appropriate solution. To achieve a convergence of the trajectory in terms of polynomial coefficients, an ensemble-averaged cross-correlation map is constructed by sampling cross-correlation values near the predictor trajectory with respect to an imposed change of each polynomial coefficient. A relation between the given change and corresponding cross-correlation maps, which could be calculated from the ordinary cross-correlation, is derived. A disagreement between computational domain and corresponding physical domain is compensated by introducing the Jacobian matrix based on the image deformation scheme in accordance with the trajectory. An increased cost of the convergence calculation, associated with the nonlinearity of the fluid trajectory, is moderated by means of a V-cycle iteration. To validate enhancements of the present method, quantitative comparisons with the state-of-arts TR-PIV methods, e.g., the adaptive temporal interval, the multi-frame pyramid correlation and the fluid trajectory correlation, were carried out by using synthetically generated particle image sequences. The performances of the tested methods are discussed in algorithmic terms. A high-rate TR-PIV experiment of a flow over an airfoil demonstrates the effectiveness of the present method. It is shown that the present method is capable of reducing random errors in both velocity and material acceleration while suppressing spurious temporal fluctuations due to measurement noise. 相似文献
8.
PIV study on a shock-induced separation in a transonic flow 总被引:1,自引:0,他引:1
A transonic interaction between a steady shock wave and a turbulent boundary layer in a Mach 1.4 channel flow is experimentally investigated by means of particle image velocimetry (PIV). In the test section, the lower wall is equipped with a contour profile shaped as a bump allowing flow separation. The transonic interaction, characterized by the existence in the outer flow of a lambda shock pattern, causes the separation of the boundary layer, and a low-speed recirculating bubble is observed downstream of the shock foot. Two-component PIV velocity measurements have been performed using an iterative gradient-based cross-correlation algorithm, providing high-speed and flexible calculations, instead of the classic multi-pass processing with FFT-based cross-correlation. The experiments are performed discussing all the hypotheses linked to the experimental set-up and the technique of investigation such as the two-dimensionality assumption of the flow, the particle response assessment, the seeding system, and the PIV correlation uncertainty. Mean velocity fields are presented for the whole interaction with particular attention for the recirculating bubble downstream of the detachment, especially in the mixing layer zone where the effects of the shear stress are most relevant. Turbulence is discussed in details, the results are compared to previous study, and new results are given for the turbulent production term and the return to isotropy mechanism. Finally, using different camera lens, a zoom in the vicinity of the wall presents mean and turbulent velocity fields for the incoming boundary layer. 相似文献
9.
Volumetric-correlation particle image velocimetry (VPIV) is a new technique that provides a 3-dimensional 2-component velocity
field from a single image plane. This single camera technique is simpler and cheaper to implement than multi-camera systems
and has the capacity to measure time-varying flows. Additionally, this technique has significant advantages over other 3D
PIV velocity measurement techniques, most notably in the capacity to measure highly seeded flows. Highly seeded flows, often
unavoidable in industrial and biological flows, offer considerable advantages due to higher information density and better
overall signal-to-noise ratio allowing for optimal spatial and temporal resolution. Here, we further develop VPIV adding the
capability to measure concentration and increasing the robustness and accuracy of the technique. Particle concentrations are
calculated using volumetric auto-correlations, and subsequently the velocities are calculated using volumetric cross-correlation
corrected for variations in particle concentration. Along with the ability to calculate the particle concentration profile,
our enhanced VPIV produces significant improvement in the accuracy of velocity measurements. Furthermore, this technique has
been demonstrated to be insensitive to out-of-plane flows. The velocity measurement accuracy of the enhanced VPIV exceeds
that of standard micro-PIV measurements, especially in near-wall regions. The 3D velocity and particle-concentration measurement
capability of VPIV are demonstrated using both synthetic and experimental results. 相似文献
10.
C. Poelma J. M. Mari N. Foin M.-X. Tang R. Krams C. G. Caro P. D. Weinberg J. Westerweel 《Experiments in fluids》2011,50(4):777-785
Ultrasound particle image velocimetry (PIV) can be used to obtain velocity fields in non-transparent geometries and/or fluids. In the current study, we use this technique to document the flow in a curved tube, using ultrasound contrast bubbles as flow tracer particles. The performance of the technique is first tested in a straight tube, with both steady laminar and pulsatile flows. Both experiments confirm that the technique is capable of reliable measurements. A number of adaptations are introduced that improve the accuracy and applicability of ultrasound PIV. Firstly, due to the method of ultrasound image acquisition, a correction is required for the estimation of velocities from tracer displacements. This correction accounts for the fact that columns in the image are recorded at slightly different instances. The second improvement uses a slice-by-slice scanning approach to obtain three-dimensional velocity data. This approach is here demonstrated in a strongly curved tube. The resulting flow profiles and wall shear stress distribution shows a distinct asymmetry. To meaningfully interpret these three-dimensional results, knowledge of the measurement thickness is required. Our third contribution is a method to determine this quantity, using the correlation peak heights. The latter method can also provide the third (out-of-plane) component if the measurement thickness is known, so that all three velocity components are available using a single probe. 相似文献
11.
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
12.
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
13.
Stereoscopic PIV on multiple color-coded light sheets and its application to axial flow in flapping robotic insect wings 总被引:1,自引:0,他引:1
Non-scanning volume flow measurement techniques such as 3D-PTV, holographic and tomographic particle image velocimetry (PIV)
permit reconstructions of all three components (3C) of velocity and vorticity vectors in a fluid volume (3D). In this study,
we present a novel 3D3C technique termed Multiple-Color-Plane Stereo Particle-Image-Velocimetry (color PIV), which allows
instantaneous measurements of 3C velocity vectors in six parallel, colored light sheets. We generated the light sheets by
passing white light of two strobes through dichroic color filters and imaged the slices by two 3CCD color cameras in Stereo-PIV
configuration. The stereo-color images were processed by custom software routines that sorted each colored fluid particle
into one of six gray-scale images according to its hue, saturation, and luminance. We used conventional Stereo PIV cross-correlation
algorithms to compute a 3D planar vector field for each light sheet and subsequently interpolated a volume flow map from the
six vector fields. As a first application, we quantified the wake and axial flow in the vortical structures of a robotic insect
(fruit fly) model wing. In contrast to previous findings, the measured data indicate strong axial flow components on the upper
wing surface, including axial flow in the leading-edge vortex core. Collectively, color PIV is robust against mechanical misalignments,
avoids laser safety issues, and computes instantaneous 3D vector fields in a fraction of the time typical for other 3D systems.
Color PIV might thus be of value for volume measurements of highly unsteady flows. 相似文献
14.
A variant of the particle image velocimetry (PIV) technique is described for measuring velocity and density simultaneously in a turbulent Rayleigh-Taylor mixing layer. The velocity field is computed by the usual PIV technique of cross-correlating two consecutive images, and deducing particle displacements from correlation peaks of intensity fields. Different concentrations of seed particles are used in the two streams of different temperature (density) fluids, and a local measure of the density is obtained by spatially averaging over an interrogation window. Good agreement is reported between the first- and second-order statistics for density obtained from this technique and from a thermocouple. Velocity-density correlations computed by cross-correlating individual time series are presented. The errors in the density measurements are quantified and analyzed, and the issue of spatial resolution is also discussed. Our purpose for this paper is to introduce the PIV-S method and validate its accuracy against corresponding thermocouple measurements. 相似文献
15.
A simple phase separation method using vector post-processing techniques is evaluated to measure velocity fields in a bubble
plume. To provide for validation, fluorescent seeding is used, and two sets of synoptic images are obtained: mixed-phase images
containing bubbles and fluorescent particles, and fluid-phase images containing only fluorescent particles. A third dataset
is derived by applying a digital mask to remove bubbles from the mixed-phase images. All datasets are processed using cross-correlation
particle image velocimetry (PIV). The resulting vector maps for the raw, mixed-phase data contain both bubble and continuous-phase
velocity vectors. To separate the phases, a vector post-processing algorithm applies a maximum velocity threshold for the
continuous-phase velocities coupled with the vector median filter to identify remaining bubble-velocity vectors and remove
them from the mixed-phase velocity field. To validate the phase separation algorithm, the post-processed fluid-phase vectors
are compared to PIV results obtained from both the optically separated and digitally masked data. The comparison among these
methods shows that the post-processed mixed-phase data have small errors in regions near some bubbles, but for dilute environmental
flows (low void fraction and slip velocity approximately equal to the entrained fluid velocity), the algorithm predicts well
both instantaneous and time average statistical quantities. The method is reliable for flows having 10% or less of the field
of view occupied by bubbles. The resulting instantaneous data provide information on plume wandering and eddy-size distributions
within the bubble plume. By comparison among the datasets, it is shown that the patchiness of the vector-post processed and
image masked data limit the diameter of identifiable eddy structures to the average distance between bubbles in the image,
and that both datasets give identical probability density functions of eddy size. The optically filtered data have better
data coverage and predict a greater probability of larger eddies as compared to the other two datasets. 相似文献
16.
Three different particle image processing algorithms have been developed for the improvement of PIV velocity measurements
characterized by large velocity gradients. The objectives of this study are to point out the limitations of the standard processing
methods and to propose a complete algorithm to enhance the measurement accuracy. The heart of the PIV image processing is
a direct cross-correlation calculation in order to obtain complete flexibility in the choice of the size and the shape of
the interrogation window (IW). An iterative procedure is then applied for the reduction of the size of IW at each measurement
location. This procedure allows taking into account the local particle concentration in the image. The results of this first
iterative processing, applied to synthetic images, show both a significant improvement of measurement accuracy and an increase
of the spatial resolution. Finally, a super-resolution algorithm is developed to further increase the spatial resolution of
the measurement by determining the displacement of each particle. The computer time for a complete image processing is optimized
by the introduction of original data storage in Binary Space Partitions trees. It is shown that measurement errors for large
velocity gradient flows are similar to those obtained in simpler cases with uniform translation displacements. This last result
validates the ability of the developed super-resolution algorithm for the aerodynamic characterization of large velocity gradient
flows. 相似文献
17.
Analysis and treatment of errors due to high velocity gradients in particle image velocimetry 总被引:1,自引:0,他引:1
This paper deals with errors occurring in two-dimensional cross-correlation particle image velocimetry (PIV) algorithms (with window shifting), when high velocity gradients are present. A first bias error is due to the difference between the Lagrangian displacement of a particle and the real velocity. This error is calculated theoretically as a function of the velocity gradients, and is shown to reach values up to 1 pixel if only one window is translated. However, it becomes negligible when both windows are shifted in a symmetric way. A second error source is linked to the image pattern deformation, which decreases the height of the correlation peaks. In order to reduce this effect, the windows are deformed according to the velocity gradients in an iterative process. The problem of finding a sufficiently reliable starting point for the iteration is solved by applying a Gaussian filter to the images for the first correlation. Tests of a PIV algorithm based on these techniques are performed, showing their efficiency, and allowing the determination of an optimum time separation between images for a given velocity field. An application of the new algorithm to experimental particle images containing concentrated vortices is shown. 相似文献
18.
Chuong V. Nguyen Thien D. Nguyen John C. Wells Akihiko Nakayama 《Experiments in fluids》2010,48(4):577-587
PIV measurements near a wall are generally difficult due to low seeding density, low velocity, high velocity gradient, and
strong reflections. Such problems are often compounded by curved boundaries, which are commonly found in many industrial and
medical applications. To systematically solve these problems, this paper presents two novel techniques for near-wall measurement,
together named Interfacial PIV, which extracts both wall-shear gradient and near-wall tangential velocity profiles at one-pixel
resolution. To deal with curved walls, image strips at a curved wall are stretched into rectangles by means of conformal transformation.
To extract the maximal spatial information on the near-wall tangential velocity field, a novel 1D correlation function is
performed on each horizontal pixel line of the transformed image template to form a “correlation stack”. This 1D correlation
function requires that the wall-normal displacement component of the particles be smaller than the particle image diameter
in order to produce a correlation signal. Within the image regions satisfying this condition, the correlation function yields
peaks that form a tangential velocity profile. To determine this profile robustly, we propose to integrate gradients of tangential
velocity outward from the wall, wherein the gradient at each wall-normal position is measured by fitting a straight line to
the correlation peaks. The capability of Interfacial PIV was validated against Particle Image Distortion using synthetic image
pairs generated from a DNS velocity field over a sinusoidal bed. Different velocity measurement schemes performed on the same
correlation stacks were also demonstrated. The results suggest that Interfacial PIV using line fitting and gradient integration
provides the best accuracy of all cases in the measurements of velocity gradient and velocity profile near wall surfaces. 相似文献
19.
A novel technique is introduced to increase the precision and robustness of time-resolved particle image velocimetry (TR-PIV) measurements. The innovative element of the technique is the linear combination of the correlation signal computed at different separation time intervals. The domain of the correlation signal resulting from different temporal separations is matched via homothetic transformation prior to the averaging of the correlation maps. The resulting ensemble-averaged correlation function features a significantly higher signal-to-noise ratio and a more precise velocity estimation due to the evaluation of a larger particle image displacement. The method relies on a local optimization of the observation time between snapshots taking into account the local out-of-plane motion, continuum deformation due to in-plane velocity gradient and acceleration errors. The performance of the pyramid correlation algorithm is assessed on a synthetically generated image sequence reproducing a three-dimensional Batchelor vortex; experiments conducted in air and water flows are used to assess the performance on time-resolved PIV image sequences. The numerical assessment demonstrates the effectiveness of the pyramid correlation technique in reducing both random and bias errors by a factor 3 and one order of magnitude, respectively. The experimental assessment yields a significant increase of signal strength indicating enhanced measurement robustness. Moreover, the amplitude of noisy fluctuations is considerably attenuated and higher precision is obtained for the evaluation of time-resolved velocity and acceleration. 相似文献
20.
Micro PIV uses volume illumination; therefore, the velocity measured at the focal plane is a weighted average of the velocities
within the measurement volume. The contribution of out-of-focus particles to the PIV correlation can generate significant
measurement errors particularly in near wall regions. We present a new application of image overlapping, which is shown to
be very effective in improving the accuracy of time-averaged velocity measurements by effectively reducing the measurement
depth. The performance of image overlapping and correlation averaging were studied using synthetic and experimental images
of micro channel flow, both with and without image pre-processing. The results show that for flows without particle clumping,
image overlapping provides the best measurement accuracy without any need for image pre-processing. For flows with particle
clumping, image overlapping combined with band-pass filtering provides the best measurement accuracy. When overlapped images
are saturated with particles due to a large number of image pairs, image overlapping measurement still does not show any visible
pixel-locking effect. Image overlapping was found to have comparable or slightly reduced pixel-locking effects compared to
correlation averaging. In addition, image overlapping utilizes significantly fewer computational resources than the other
techniques. 相似文献