共查询到20条相似文献,搜索用时 31 毫秒
1.
Tomographic particle image velocimetry 总被引:8,自引:0,他引:8
This paper describes the principles of a novel 3D PIV system based on the illumination, recording and reconstruction of tracer particles within a 3D measurement volume. The technique makes use of several simultaneous views of the illuminated particles and their 3D reconstruction as a light intensity distribution by means of optical tomography. The technique is therefore referred to as tomographic particle image velocimetry (tomographic-PIV). The reconstruction is performed with the MART algorithm, yielding a 3D array of light intensity discretized over voxels. The reconstructed tomogram pair is then analyzed by means of 3D cross-correlation with an iterative multigrid volume deformation technique, returning the three-component velocity vector distribution over the measurement volume. The principles and details of the tomographic algorithm are discussed and a parametric study is carried out by means of a computer-simulated tomographic-PIV procedure. The study focuses on the accuracy of the light intensity field reconstruction process. The simulation also identifies the most important parameters governing the experimental method and the tomographic algorithm parameters, showing their effect on the reconstruction accuracy. A computer simulated experiment of a 3D particle motion field describing a vortex ring demonstrates the capability and potential of the proposed system with four cameras. The capability of the technique in real experimental conditions is assessed with the measurement of the turbulent flow in the near wake of a circular cylinder at Reynolds 2,700. 相似文献
2.
Matthias Kühn Klaus Ehrenfried Johannes Bosbach Claus Wagner 《Experiments in fluids》2011,50(4):929-948
To measure large-scale flow structures in air, a tomographic particle image velocimetry (tomographic PIV) system for measurement
volumes of the order of one cubic metre is developed, which employs helium-filled soap bubbles (HFSBs) as tracer particles.
The technique has several specific characteristics compared to most conventional tomographic PIV systems, which are usually
applied to small measurement volumes. One of them is spot lights on the HFSB tracers, which slightly change their position,
when the direction of observation is altered. Further issues are the large particle to voxel ratio and the short focal length
of the used camera lenses, which result in a noticeable variation of the magnification factor in volume depth direction. Taking
the specific characteristics of the HFSBs into account, the feasibility of our large-scale tomographic PIV system is demonstrated
by showing that the calibration errors can be reduced down to 0.1 pixels as required. Further, an accurate and fast implementation
of the multiplicative algebraic reconstruction technique, which calculates the weighting coefficients when needed instead
of storing them, is discussed. The tomographic PIV system is applied to measure forced convection in a convection cell at
a Reynolds number of 530 based on the inlet channel height and the mean inlet velocity. The size of the measurement volume
and the interrogation volumes amount to 750 mm × 450 mm × 165 mm and 48 mm × 48 mm × 24 mm, respectively. Validation of the
tomographic PIV technique employing HFSBs is further provided by comparing profiles of the mean velocity and of the root mean
square velocity fluctuations to respective planar PIV data. 相似文献
3.
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
4.
Volume self-calibration for 3D particle image velocimetry 总被引:4,自引:2,他引:2
B. Wieneke 《Experiments in fluids》2008,45(4):549-556
Planar self-calibration methods have become standard for stereo PIV to correct misalignments between laser light sheet and
calibration plane. Computing cross-correlation between images from camera 1 and 2 taken at the same time, non-zero disparity
vectors indicate rotational and translational misalignments relative to the coordinate system defined by a calibration plate.
This approach works well for thin light sheets but fails for extended volumes recorded in 3D-PTV or tomographic PIV experiments.
Here it is primarily necessary to correct calibration errors leading to triangulation errors in 3D-PTV or in degraded tomographic
volume reconstruction. Tomographic PIV requires calibration accuracies of a fraction of a pixel throughout the complete volume,
which is difficult to achieve experimentally. A new volumetric self-calibration technique has been developed based on the
computation of the 3D position of matching particles by triangulation as in 3D-PTV. The residual triangulation error (‘disparity’)
is then used to correct the mapping functions for all cameras. A statistical clustering method suitable for dense particle
images has been implemented to find correct disparity map peaks from true particle matches. Disparity maps from multiple recordings
are summed for better statistics. This self-calibration scheme has been validated using several tomographic PIV experiments
improving the vector quality significantly. The relevance for other 3D velocimetry methods is discussed. 相似文献
5.
Tomographic PIV measurements in a turbulent lifted jet flame 总被引:1,自引:0,他引:1
Measurements of instantaneous volumetric flow fields are required for an improved understanding of turbulent flames. In non-reacting flows, tomographic particle image velocimetry (TPIV) is an established method for three-dimensional (3D) flow measurements. In flames, the reconstruction of the particles location becomes challenging due to a locally varying index of refraction causing beam-steering. This work presents TPIV measurements within a turbulent lifted non-premixed methane jet flame. Solid seeding particles were used to provide the 3D flow field in the vicinity of the flame base, including unburned and burned regions. Four cameras were arranged in a horizontal plane around the jet flame. Following an iterative volumetric self-calibration procedure, the remaining disparity caused by the flame was less than 0.2 pixels. Comparisons with conventional two-component PIV in terms of mean and rms values provided additional confidence in the TPIV measurements. 相似文献
6.
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
7.
Holographic recording overcomes the limits in 2-D particle image velocimetry (PIV) to cover a 3-D flow field volume. Interrogation
by focusing on single planes in a reconstructed particle field is disturbed by noise from out-of-focus particles. A numerical
simulation models image reconstruction and shows how validation rates depend on aperture and volume depth. An experimental
model environment of scattering particles in moveable plastic slices gives support to the numerical results. Simulations and
tests are carried out for interrogation by autocorrelation and crosscorrelation techniques and furnish guidelines for system
design.
Received: 27 December 1996 / Accepted: 14 August 1997 相似文献
8.
The motion-tracking-enhanced MART (MTE-MART; Novara et al. in Meas Sci Technol 21:035401, 2010) has demonstrated the potential to increase the accuracy of tomographic PIV by the combined use of a short sequence of non-simultaneous recordings. A clear bottleneck of the MTE-MART technique has been its computational cost. For large datasets comprising time-resolved sequences, MTE-MART becomes unaffordable and has been barely applied even for the analysis of densely seeded tomographic PIV datasets. A novel implementation is proposed for tomographic PIV image sequences, which strongly reduces the computational burden of MTE-MART, possibly below that of regular MART. The method is a sequential algorithm that produces a time-marching estimation of the object intensity field based on an enhanced guess, which is built upon the object reconstructed at the previous time instant. As the method becomes effective after a number of snapshots (typically 5–10), the sequential MTE-MART (SMTE) is most suited for time-resolved sequences. The computational cost reduction due to SMTE simply stems from the fewer MART iterations required for each time instant. Moreover, the method yields superior reconstruction quality and higher velocity field measurement precision when compared with both MART and MTE-MART. The working principle is assessed in terms of computational effort, reconstruction quality and velocity field accuracy with both synthetic time-resolved tomographic images of a turbulent boundary layer and two experimental databases documented in the literature. The first is the time-resolved data of flow past an airfoil trailing edge used in the study of Novara and Scarano (Exp Fluids 52:1027–1041, 2012); the second is a swirling jet in a water flow. In both cases, the effective elimination of ghost particles is demonstrated in number and intensity within a short temporal transient of 5–10 frames, depending on the seeding density. The increased value of the velocity space–time correlation coefficient demonstrates the increased velocity field accuracy of SMTE compared with MART. 相似文献
9.
This paper investigates the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination. Modern solid-state LED devices can provide averaged radiant power in excess of 10 W and by operating the LED with short high current pulses theoretical pulse energies up to several tens of mJ can be achieved. In the present work, a custom-built drive circuit is used to drive a Luminus PT-120 high-power LED at pulsed currents of up to 150 A and 1 μs duration. Volumetric illumination is achieved by directly projecting the LED into the flow to produce a measurement volume of ≈3–4 times the size of the LED die. The feasibility of the volumetric LED illumination is assessed by performing tomographic PIV of homogenous, grid-generated turbulence. Two types of LEDs are investigated, and the results are compared with measurements of the same flow using pulsed Nd:YAG laser illumination and DNS data of homogeneous isotropic turbulence. The quality of the results is similar for both investigated LEDs with no significant difference between the LED and Nd:YAG illumination. Compared with the DNS, some differences are observed in the power spectra and the probability distributions of the fluctuating velocity and velocity gradients. These differences are attributed to the limited spatial resolution of the experiments and noise introduced during the tomographic reconstruction (i.e. ghost particles). The uncertainty in the velocity measurements associated with the LED illumination is estimated to approximately 0.2–0.3 pixel for both LEDs, which compares favourably with similar tomographic PIV measurements of turbulent flows. In conclusion, the proposed high-power, pulsed LED volume illumination provides accurate and reliable tomographic PIV measurements in water and presents a promising technique for flow diagnostics and velocimetry. 相似文献
10.
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
11.
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
12.
A hybrid technique is presented that combines scanning PIV with tomographic reconstruction to make spatially and temporally resolved measurements of the fine-scale motions in turbulent flows. The technique uses one or two high-speed cameras to record particle images as a laser sheet is rapidly traversed across a measurement volume. This is combined with a fast method for tomographic reconstruction of the particle field for use in conjunction with PIV cross-correlation. The method was tested numerically using DNS data and with experiments in a large mixing tank that produces axisymmetric homogeneous turbulence at \(R_\lambda \simeq 219\) . A parametric investigation identifies the important parameters for a scanning PIV set-up and provides guidance to the interested experimentalist in achieving the best accuracy. Optimal sheet spacings and thicknesses are reported, and it was found that accurate results could be obtained at quite low scanning speeds. The two-camera method is the most robust to noise, permitting accurate measurements of the velocity gradients and direct determination of the dissipation rate. 相似文献
13.
Two-dimensional particle image velocimetry (PIV) is used to obtain a set of parallel vector maps in spanwise direction over
the delta wing configuration ELAC. The out-of-plane velocity component is then constructed by application of continuity equation.
This yields the whole three-dimensional separated flow field over the leeward side of the model. The spatial resolution of
the measurements enables a detailed examination of the three-dimensional flow structure. The growth and the helical structure
of primary vortex as well as smaller flow structures caused by secondary separation can be observed. Accuracy of the constructed
velocity component is estimated with help of a numerically obtained three-dimensional dataset of the flow field around this
configuration. The reconstruction procedure was applied to this data set taking the experimental uncertainty and the grid
spacing of the PIV measurements into consideration. A comparison of reconstructed out-of-plane component and data of the numerical
solution of Navier–Stokes equations results in a promising low error. A statistical analysis of different procedures allows
interpretation of reconstruction capabilities.
Received: 15 April 1998 / Accepted: 15 September 1998 相似文献
14.
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
15.
16.
Particle image velocimetry (PIV) was used to measure instantaneous and average particle velocity fields near the stagnation zone of a particle-laden impinging air jet. The results were compared with Lagrangian particle tracking measurements. Ensemble averages from the two methods agree well except in regions where particles have different histories, and a specific trajectory is dominant but not exclusive. The PIV autocorrelation method loses information regarding non-dominant particle trajectories. Thus, although instantaneous PIV measurements yield the dominant particle velocities correctly, the averaged measurements are biased in some regions.This work was supported by the Electric Power Research Institute under Contract RP 8034-01. We thank the 3M Corporation for their generous materials support. 相似文献
17.
The paper discusses bias errors introduced in Tomographic-PIV velocity measurements by the coherent motion of ghost particles
under some circumstances. It occurs when a ghost particle is formed from the same set of actual particles in both reconstructed
volumes used in the cross-correlation analysis. The displacement of the resulting ghost particle pair is approximately the
average displacement of the set of associated actual particles. The effect is further quantified in a theoretical analysis
and in numerical simulations and illustrated in an actual experiment. It is shown that the bias error does not significantly
affect the measured flow topology as deduced in an evaluation of the local velocity gradients. Instead, it leads to a systematic
underestimation of the measured particle displacement gradient magnitude. This phenomenon is alleviated when the difference
between particles displacement along the volume depth is increased beyond a particle image diameter, or when the reconstruction
quality is increased or when the accuracy of the tomographic reconstruction is improved. Furthermore, guidelines to detect
and avoid such bias errors are proposed. 相似文献
18.
《Experimental Thermal and Fluid Science》2009,33(1):123-131
The motion of gas within an air-filled rigid-walled square channel subjected to acoustic standing waves is experimentally investigated. The synchronized particle image velocimetry (PIV) technique has been used to measure the acoustic velocity fields at different phases over the excitation signal period. The acoustic velocity measurements have been conducted for two different acoustic intensities in the quasi-nonlinear range (in which the nonlinear effects can be neglected in comparison with the dissipation effects), and one acoustic intensity in the finite-amplitude nonlinear range (in which both the nonlinear term and the dissipative term play a role in the wave equation). The experimental velocity fields for the quasi-nonlinear cases are compared with the analytical results obtained from the time-harmonic solution of the wave equation. Good agreement between the experimental and analytical velocity fields proves the ability of the synchronized PIV technique to accurately measure both temporal and spatial variations of the acoustic velocity fields. The verified technique is then used to measure the acoustic velocity fields of the finite-amplitude nonlinear case at different phases. 相似文献
19.
The technical basis and system set-up of a dual-plane stereoscopic particle image velocimetry (PIV) system, which can obtain
the flow velocity (all three components) fields at two spatially separated planes simultaneously, is summarized. The simultaneous
measurements were achieved by using two sets of double-pulsed Nd:Yag lasers with additional optics to illuminate the objective
fluid flow with two orthogonally linearly polarized laser sheets at two spatially separated planes, as proposed by Kaehler
and Kompenhans in 1999. The light scattered by the tracer particles illuminated by laser sheets with orthogonal linear polarization
were separated by using polarizing beam-splitter cubes, then recorded by high-resolution CCD cameras. A three-dimensional
in-situ calibration procedure was used to determine the relationships between the 2-D image planes and three-dimensional object
fields for both position mapping and velocity three-component reconstruction. Unlike conventional two-component PIV systems
or single-plane stereoscopic PIV systems, which can only get one-component of vorticity vectors, the present dual-plane stereoscopic
PIV system can provide all the three components of the vorticity vectors and various auto-correlation and cross-correlation
coefficients of flow variables instantaneously and simultaneously. The present dual-plane stereoscopic PIV system was applied
to measure an air jet mixing flow exhausted from a lobed nozzle. Various vortex structures in the lobed jet mixing flow were
revealed quantitatively and instantaneously. In order to evaluate the measurement accuracy of the present dual-plane stereoscopic
PIV system, the measurement results were compared with the simultaneous measurement results of a laser Doppler velocimetry
(LDV) system. It was found that both the instantaneous data and ensemble-averaged values of the stereoscopic PIV measurement
results and the LDV measurement results agree well. For the ensemble-averaged values of the out-of-plane velocity component
at comparison points, the differences between the stereoscopic PIV and LDV measurement results were found to be less than
2%.
Received: 18 April 2000/Accepted: 2 February 2001 相似文献
20.
Tomographic particle image velocimetry (Tomo-PIV) is a promising new PIV technique. However, its high computational costs
often make time-resolved measurements impractical. In this paper, a new preprocessing method is proposed to estimate the initial
volume intensity distribution. This relatively inexpensive “first guess” procedure significantly reduces the computational
costs, accelerates solution convergence, and can be used directly to obtain results up to 35 times faster than an iterative
reconstruction algorithm (with only a slight accuracy penalty). Reconstruction accuracy is also assessed by examining the
errors in recovering velocity fields from artificial data (rather than errors in the particle reconstructions themselves). 相似文献