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1.
Stereo particle image velocimetry (PIV) has been employed to study a vortex generated via tangential injection of water in a 2.25 inch (57 mm) diameter pipe for Reynolds numbers ranging from 1,118 to 63,367. Methods of decreasing pipe-induced optical distortion and the PIV calibration technique are addressed. The mean velocity field analyses have shown spatial similarity and revealed four distinct flow regions starting from the central axis of rotation to the pipe wall in the vortex flows. Turbulence statistical data and vortex core location data suggest that velocity fluctuations are due to the axis of the in-line vortex distorting in the shape of a spiral. 相似文献
2.
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
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4.
T. Mizukaki 《Shock Waves》2010,20(6):531-537
In this article, we attempt to validate flow visualization using the high-speed background-oriented schlieren (HiBOS) method,
which is the BOS technique combined with a high-speed video camera as the recording device in the experiment. The method has
been applied to shock-induced flow near the open end of a shock tube. Three incident shock Mach numbers were examined so that
the BOS measurements could be compared with results given in the literature of particle-image velocimetry (PIV) measurements.
Using the HiBOS technique, we were able to clearly view developing, compressible vortex rings and diffracted shock waves discharged
from the open end of the shock tube. From the BOS images, we extracted the history of the propagation velocity, the diameter
of the vortex ring, and the diameter of the vortex core, all of which agree with the corresponding PIV values reported in
the literature. 相似文献
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6.
A generalized processing technique in digital particle image velocimetry with direct estimation of velocity gradients 总被引:1,自引:0,他引:1
S. Mayer 《Experiments in fluids》2002,33(3):443-457
A technique is proposed for the processing of digital particle image velocimetry (PIV) images, in one single step providing direct estimates of fluid velocity, out-of-plane vorticity and in-plane shear rate tensor. The method is based on a generalization of the standard PIV cross-correlation technique and substitutes the usual discrete cross-correlation of image pairs with a correlation of interpolated two-dimensional image intensity functions, being subject to affine transformations. The correlation is implemented by using collocation points, on which image intensity values are interpolated. The resulting six-dimensional correlation function is maximized using a general purpose optimization algorithm. The use of the method is demonstrated by application to different types of synthetically generated image pairs constructed with known particle displacement functions. The resulting errors are assessed and compared with those of a representative standard PIV method as well as with those of the present technique using no differential quantities in the search of the peak location. The examples demonstrate that significant improvements in accuracy can be obtained for flow fields with regions containing strong velocity gradients. 相似文献
7.
An extension of two color particle image velocimetry (PIV) is described where the color images are recorded onto a single
high-resolution (3060×2036 pixel) color CCD sensor. Unlike mono-color CCD sensors, this system not only eliminates the processing
time and the subsequent digitization time of film-based PIV but also resolves the directional ambiguity of the velocity vector
without using conventional image-shifting techniques. For comparing the spatial resolutions of film and CCD data, a calibration
experiment is conducted by recording the speckle pattern onto 35 mm color film and using a CCD sensor under identical conditions.
This technique has been successfully implemented for simulated turbine film-cooling flows in order to obtain a more detailed
characterization of the coolant-injection phenomenon and its interaction with freestream disturbances.
Received: 20 November 1996/Accepted: 29 January 1998 相似文献
8.
PIV error correction 总被引:15,自引:9,他引:6
D. P. Hart 《Experiments in fluids》2000,29(1):13-22
A non-post-interrogation method of reducing subpixel errors and eliminating spurious vectors from particle image velocimetry
(PIV) results is presented. Unlike methods that rely on the accuracy or similarity of neighboring vectors, errors are eliminated
before correlation information is discarded using available spatial and/or temporal data. Anomalies are removed from the data
set through direct element-by-element comparison of the correlation tables calculated from adjacent regions. The result is
a processing technique that yields a symmetric correlation profile representing the velocity at the boundary of the combined
regions. This correlation based correction (CBC) technique greatly improves subpixel accuracy, and is highly robust to out-of-boundary
particle motion, particle overlap, unmatched particle correlations, and electronic and optical imaging noise.
Received: 22 June 1999/Accepted: 21 September 1999 相似文献
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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.
Results are presented from an experimental investigation into the interaction of a planar shock wave with a vortex ring. A
free-falling spherical soap bubble is traversed by the incident shock wave and develops into a vortex ring as a result of
baroclinically deposited vorticity (?r×?p 1 0{\nabla\rho\times\nabla p \neq 0}). The vortex ring translates with a velocity relative to the particle velocity behind the shock wave due to circulation.
After the shock wave reflects from the tube end wall, it traverses the vortex ring (this process is called “reshock”) and
deposits additional vorticity. Planar Mie scattering is used to visualize the atomized soap film at high frame rates (up to
10,000 fps). Particle image velocimetry (PIV) was performed for an argon bubble in nitrogen accelerated by a M = 1.35 shock wave. Circulation was determined from the PIV velocity field and found to agree well with Kelvin’s vortex ring
model. 相似文献
12.
Bradley A. Ochs Dan Fries Devesh Ranjan Suresh Menon 《Flow, Turbulence and Combustion》2018,101(3):927-951
It is unclear whether turbulent flame speed scalings established in low speed regimes are applicable to supersonic flames. To investigate this question, the canonical flame kernel is investigated in a scramjet-like channel having a one degree wall divergence. The growth, shape and internal kernel dynamics are investigated. Results are presented for three Mach numbers, four equivalence ratios, and three turbulence generators. Schlieren photography provides flame images for growth rate statistics and particle image velocimetry (PIV) provides turbulence statistics and investigation of internal kernel dynamics. Supersonic flame kernels are self-propagating and respond to the equivalence ratio in a fashion that is similar to low speed flames. However, supersonic flame kernels have features that are not present in subsonic flame kernels. Baroclinicity, resulting from pressure-density misalignment, creates a reacting vortex ring structure. Further, the mean kernel shape has a Mach number dependence and the vortex ring enhances the turbulent flame speed through entrainment of reactants and augmented flame surface growth. Hence, the previously established (low speed) flame speed scalings are inappropriate for supersonic flame kernels. Drawing motivation from vortex ring literature, the ring propagation velocity is used as the characteristic velocity and a new flame speed scaling is proposed. 相似文献
13.
《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
14.
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
15.
Wake vortex flow experiments are performed in a water tank where a 1:48 scaled model of a large transport aircraft A340-300 is towed at the speed of 3 and 5 ms-1 with values of the angle of attack !={2°, 4°, 8°}. Particle image velocimetry (PIV) measurements are performed in a plane perpendicular to the towing direction describing the streamwise component of the wake vorticity. The instantaneous field of view (I-FOV) is traversed vertically with an underwater moving-camera device tracking the vortex core during the downward motion. An adaptive resolution (AR) image-processing technique is introduced that enhances the PIV interrogation in terms of spatial resolution and accuracy. The main objectives of the investigation are to demonstrate the applicability of PIV diagnostics in wake vortex research with towing-tank facilities. The specific implementation of the traversing field-of-view (T-FOV) technique and the AR image processing are driven by the need to characterize the vortex wake global properties as well as the vortex decay phenomenon in the mid- and far-field. Relevant aerodynamic information is obtained in the mid-field where the time evolution of the vortex structure (core radius and tangential velocity) and of the overall vortex wake (vortex trajectory, descent velocity, circulation) are discussed. 相似文献
16.
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
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18.
A new method called direct measurement of vorticity (DMV) for digital particle images is described in this paper. Unlike
previous methods for calculating the vorticity in particle image velocimetry (PIV), the vorticity is determined directly from
the average angular displacement of rotation between two matched patterns. In order to improve the stability and precision
of the angular displacement, polar coordinates are used instead of Cartesian coordinates to depict gray level patterns. The
results of a Monte Carlo simulation of an Oseen-vortex flow indicate that the accuracy of the DMV method is independent of
the spatial resolution of the velocity sampling, and the errors in the velocity field will not be propagated into the vorticity
field as occurs with some finite difference methods. Therefore, the DMV method is a good method to be used to extract the
vorticity field from velocity data that has higher levels of uncertainty.
Received: 26 November 1999/Accepted: 30 October 2000 相似文献
19.
L. Gan 《Flow, Turbulence and Combustion》2016,97(1):141-170
This article proposes a technique to estimate the cross-sectional scalar interface (outer boundary) in an inhomogeneous turbulent flow from a conditioned particle image velocimetry (PIV) experiment, which is suitable for medium to high Reynolds numbers. The scalar interface is estimated directly by using conditioned PIV particle images which have distinguishably high particle seeding density in the area of interest, whereas conventionally in water based experiments, scalar interface is often determined from planar laser induced fluorescence (PLIF) or equivalent dye images. By comparing quantities in the vicinity of this scalar interface, it also shows that in terms of separate turbulent and non-turbulent regions, this technique could also replace the function of PLIF images in water experiments, with slightly lower spatial resolution. At the same time, if velocity information is also required simultaneously then the cost of a separate camera-laser system can be saved. The effect of particle field inhomogeneity on the PIV accuracy can be well reduced to an insignificant level by an image local normalisation treatment. This article shows that the interfacial layer could be detected fairly accurately by enhancing the particle images by wavelet based thresholding methods. The degree of detection accuracy is quantified by synthetic particle image analyses, where a scalar interface can be artificially pre-defined. The proposed technique is tested in two water based experiments but is expected to be particularly useful in gas-phase based experiments or some combustion applications, where liquid-phase dye cannot be applied. 相似文献
20.
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