共查询到10条相似文献,搜索用时 125 毫秒
1.
The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase the spatial resolution of the velocity measurement to a maximum of 1.8 mm×3.1 mm. Velocity validation tests in fully developed laminar pipe flow showed good agreement with both optical PIV measurements and the expected parabolic profile. A dynamic range of 1 to 60 cm/s has been obtained to date. 相似文献
2.
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
3.
Holger Nobach 《Experiments in fluids》2011,50(4):919-927
Individual variations of intensity of tracer particles, e.g., due to out-of-plane displacements between exposures, strongly
limit the achievable accuracy of correlation-based PIV processing. The RMS error originated by this effect correlates with
the spatial resolution that can be achieved with the processing algorithm making especially high-resolution algorithms like
iterative image deformation affected by this error. Both aspects are shown, the gain of resolution by iterative image deformation
and the loss of accuracy due to individual variations of particle intensities. 相似文献
4.
The utility of particle image velocimetry (PIV) for measurement of velocity fields in many fluid flows is well established. This has created interest in overcoming difficulties with the technique when applied to increasingly larger fields of view where there exists a significant range of velocities and spatial velocity gradients are large. In this regard, a major deficiency with standard cross-correlation PIV is the inherent link between the density of vectors in the measurement field and the maximum measurable displacement. Several schemes exist to reduce this link. These iterative hierarchical/multiresolution schemes are strongly dependent on vector validation routines to remove spurious vectors. Here the design of a new framework for vector validation is described. This framework is general enough for use with both regular and irregularly spaced vector fields to make it applicable to particle image velocimetry (PIV), particle tracking velocimetry (PTV), and hybrid methods. It is based on the determination of a smoothed displacement field that robustly characterizes the measured field such that invalid vectors are attenuated more than valid vectors. In this particular study a thin-plate spline model is incorporated within an iterative regularized weighted least-squares routine to find a smoothed version of the displacement field that maintains pertinent velocity gradient information. The utility of the methodology is demonstrated for a complex flow profile containing four vortices where the valid displacement ranges from ?1/4 to +1/4 of the area of interest (AOI) dimension. Results indicate that this validation strategy can discriminate between valid and invalid vectors remarkably well over a range of parameter settings. In the example presented a flow field with significant velocity gradients and having a high number of invalid vectors (25%) is accurately validated. 相似文献
5.
T. Astarita 《Experiments in fluids》2007,43(6):859-872
The use of a weighting window (WW) in the evaluation of the cross-correlation coefficient and in the iterative procedure of
image deformation method for particle image velocimetry (PIV) applications can be used to both stabilise the process and to
increase the spatial resolution. The choice of the WW is a parameter that influences the complete PIV algorithm. Aim of this
paper is to examine the influence of this aspect on both the accuracy and spatial resolution of the PIV algorithm. Results
show an overall accordance between the theoretical approach and the simulation both with synthetic and real images. The choice
of the combination of WW influences significantly the spatial resolution and accuracy of the PIV algorithm.
相似文献
T. AstaritaEmail: |
6.
H. Huang 《Experiments in fluids》1998,24(4):364-372
In this paper digital processing techniques for PIV (Partical Image Velocimetry) using double-exposed particle images have
been studied. It has been found that a pattern matching technique is significantly superior to the traditional autocorrelation
method in the case that a large particle displacement between the double exposures is present on the image. In PIV using double-exposed
images, the image shifting technique is usually used to solve the directional ambiguity problem. The performance of PIV using
autocorrelation technique is dependent on the flow speed and the amount of image shift applied. This dependence, for example,
causes a difficulty of autocorrelation in flows close to a solid boundary. The present study shows that a pattern matching
technique eliminates such a difficulty. At the same signal-to-noise ratio, the pattern matching techndique has a better spatial
resolution than that of autocorrelation. In concert with the pattern matching technique, PID (Particle Image Distortion) can
be applied to double-exposed images, further improving the reliability and accuracy of velocity estimates of PIV in the presence
of large velocity gradients. Generally speaking, PIP-matching and PID extend the validity of PIV using double-exposed images.
The total processing time required by the PIV using the pattern matching technique and one PID iteration is of the same order
as that required by the PIV using autocorrelation.
Received: 7 July 1995 / Accepted: 11 September 1997 相似文献
7.
Benjamin H. Timmins Brandon W. Wilson Barton L. Smith Pavlos P. Vlachos 《Experiments in fluids》2012,53(4):1133-1147
The uncertainty of any measurement is the interval in which one believes the actual error lies. Particle image velocimetry (PIV) measurement error depends on the PIV algorithm used, a wide range of user inputs, flow characteristics, and the experimental setup. Since these factors vary in time and space, they lead to nonuniform error throughout the flow field. As such, a universal PIV uncertainty estimate is not adequate and can be misleading. This is of particular interest when PIV data are used for comparison with computational or experimental data. A method to estimate the uncertainty from sources detectable in the raw images and due to the PIV calculation of each individual velocity measurement is presented. The relationship between four error sources and their contribution to PIV error is first determined. The sources, or parameters, considered are particle image diameter, particle density, particle displacement, and velocity gradient, although this choice in parameters is arbitrary and may not be complete. This information provides a four-dimensional “uncertainty surface” specific to the PIV algorithm used. After PIV processing, our code “measures" the value of each of these parameters and estimates the velocity uncertainty due to the PIV algorithm for each vector in the flow field. The reliability of our methodology is validated using known flow fields so the actual error can be determined. Our analysis shows that, for most flows, the uncertainty distribution obtained using this method fits the confidence interval. An experiment is used to show that systematic uncertainties are accurately computed for a jet flow. The method is general and can be adapted to any PIV analysis, provided that the relevant error sources can be identified for a given experiment and the appropriate parameters can be quantified from the images obtained. 相似文献
8.
Digital particle image velocimetry 总被引:51,自引:13,他引:51
Digital particle image velocimetry (DPIV) is the digital counterpart of conventional laser speckle velocitmetry (LSV) and particle image velocimetry (PIV) techniques. In this novel, two-dimensional technique, digitally recorded video images are analyzed computationally, removing both the photographic and opto-mechanical processing steps inherent to PIV and LSV. The directional ambiguity generally associated with PIV and LSV is resolved by implementing local spatial cross-correlations between two sequential single-exposed particle images. The images are recorded at video rate (30 Hz or slower) which currently limits the application of the technique to low speed flows until digital, high resolution video systems with higher framing rates become more economically feasible. Sequential imaging makes it possible to study unsteady phenomena like the temporal evolution of a vortex ring described in this paper. The spatial velocity measurements are compared with data obtained by direct measurement of the separation of individual particle pairs. Recovered velocity data are used to compute the spatial and temporal vorticity distribution and the circulation of the vortex ring. 相似文献
9.
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
10.
Two iterative PIV image processing methods are introduced, which utilize displacement and deformation of the interrogation areas to maximize the correlation. The velocity gradients used for the window deformation are iteratively estimated directly from the images and no velocity values are required from neighbouring interrogation areas, as with numerical differentiation. The improved accuracy and resolution of the velocity gradient estimation compared to numerical differentiation is shown using synthetic images. The performance in a real application is shown using experimental reference images. 相似文献