共查询到20条相似文献,搜索用时 114 毫秒
1.
K. Prestridge P. M. Rightley P. Vorobieff R. F. Benjamin N. A. Kurnit 《Experiments in fluids》2000,29(4):339-346
We describe a highly-detailed experimental characterization of the Richtmyer-Meshkov instability (the impulsively driven
Rayleigh-Taylor instability) (Meshkov 1969; Richtmyer 1960). In our experiment, a vertical curtain of heavy gas (SF6) flows into the test section of an air-filled, horizontal shock tube. The instability evolves after a Mach 1.2 shock passes
through the curtain. For visualization, we pre-mix the SF6 with a small (∼10−5) volume fraction of sub-micron-sized glycol/water droplets. A horizontal section of the flow is illuminated by a light sheet
produced by a combination of a customized, burst-mode Nd:YAG laser and a commercial pulsed laser. Three CCD cameras are employed
in visualization. The “dynamic imaging camera” images the entire test section, but does not detect the individual droplets.
It produces a sequence of instantaneous images of local droplet concentration, which in the post-shock flow is proportional
to density. The gas curtain is convected out of the test section about 1 ms after the shock passes through the curtain. A
second camera images the initial conditions with high resolution, since the initial conditions vary from test to test. The
third camera, “PIV camera,” has a spatial resolution sufficient to detect the individual droplets in the light sheet. Images
from this camera are interrogated using Particle Image Velocimetry (PIV) to recover instantaneous snapshots of the velocity
field in a small (19 × 14 mm) field of view. The fidelity of the flow-seeding technique for density-field acquisition and
the reliability of the PIV technique are both quantified in this paper. In combination with wide-field density data, PIV measurements
give us additional physical insight into the evolution of the Richtmyer-Meshkov instability in a problem which serves as an
excellent test case for general transition-to-turbulence studies.
Received: 26 June 1999/Accepted: 29 October 1999 相似文献
2.
Simultaneous two-phase PIV by two-parameter phase discrimination 总被引:6,自引:0,他引:6
A flexible and robust phase discrimination algorithm for two-phase PIV employs second-order intensity gradients to identify
objects. Then, the objects are sorted into solids and tracers according to parametric combinations of size and brightness.
Solids velocities are computed by tracking, gas velocities by cross-correlation. Tests in a fully-developed turbulent channel
flow of air showed that the two phases do not contaminate or bias each other's velocity statistics. Error magnitude and valid
data yield were quantified with artificial images for three particle sizes (25, 33, and 63 μm), two interrogation area sizes
(32 and 64 pixels), and volumetric solids loads from 0.0022% to 0.014%. At the channel centerline, the gas valid data yield
was above 98% and the RMS error in gas velocity was less than 0.1 pixels for all variations of these parameters. The solid-to-tracer
signal ratio was found to be the major parameter affecting the magnitude of the RMS error.
Received: 20 September 2000/Accepted: 2 July 2001 Published online: 29 November 2001 相似文献
3.
The effect of a discrete window offset on the accuracy of cross-correlation analysis of digital PIV recordings 总被引:9,自引:15,他引:9
This paper describes how the accuracy for estimating the location of the displacement-correlation peak in (digital) particle
image velocimetry (PIV) can be optimized by the use of a window offset equal to the integer-pixel displacement. The method
works for both cross-correlation analysis of single-exposure image pairs and multiple-exposure images. The effect is predicted
by an analytical model for the statistical properties of estimators for the displacement, and it is observed in the analysis
of synthetic PIV images of isotropic turbulence, and in actual measurements of grid-generated turbulence and of fully-developed
turbulent pipe flow.
Received: 29 April 1996/Accepted: 29 October 1996 相似文献
4.
Second-order accurate particle image velocimetry 总被引:1,自引:0,他引:1
An adaptive, second-order accurate particle image velocimetry (PIV) technique is presented. The technique uses two singly
exposed images that are interrogated using a modified cross-correlation algorithm. Consequently, any of the equipment commonly
available for conventional PIV (such as dual head Nd: YAG lasers, interline transfer CCD cameras, etc.) can be used with this
more accurate algorithm. At the heart of the algorithm is a central difference approximation to the flow velocity (accurate
to order Δt
2) versus the forward difference approximation (accurate to order Δt) common in PIV. An adaptive interrogation region-shifting algorithm is used to implement the central difference approximation.
Adaptive shifting algorithms have been gaining popularity in recent years because they allow the spatial resolution of the
PIV technique to be maximized. Adaptive shifting algorithms also have the virtue of helping to eliminate velocity bias errors.
The second- order accuracy resulting from the central difference approximation can be obtained with relatively little additional
computational effort compared to that required for a standard first-order accurate forward difference approximation.
The adaptive central difference interrogation (CDI) algorithm has two main advantages over adaptive forward difference interrogation
(FDI) algorithms: it is more accurate, especially at large time delays between camera exposures; and it provides a temporally
symmetric view of the flow. By comparing measurements of flow around a single red blood cell made using both algorithms, the
CDI technique is shown to perform better than conventional FDI-PIV interrogation algorithms near flow boundaries. Cylindrical
Taylor–Couette flow images, both experimental and simulated, are used to demonstrate that the CDI algorithm is significantly
more accurate than conventional PIV algorithms, especially as the time delay between exposures is increased. The results of
the interrogations are shown to agree quite well with analytical predictions and confirm that the CDI algorithm is indeed
second-order accurate while the conventional FDI algorithm is only first-order accurate.
Received: 15 June 2000/Accepted: 2 February 2001 相似文献
5.
A new particle tracking algorithm based on deterministic annealing and alternative distance measures
We describe a new particle tracking algorithm for the interrogation of double frame single exposure data, which is obtained
with particle image velocimetry. The new procedure is based on an algorithm which has recently been proposed by Gold et al.
(Gold et al., 1998) for solving point matching problems in statistical pattern recognition. For a given interrogation window,
the algorithm simultaneously extracts: (i) the correct correspondences between particles in both frames and (ii) an estimate
of the local flow-field parameters. Contrary to previous methods, the algorithm determines not only the local velocity, but
other local components of the flow field, for example rotation and shear. This makes the new interrogation method superior
to standard methods in particular in regions with high velocity gradients (e.g. vortices or shear flows). We perform benchmarks
with three standard particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods: cross-correlation, nearest
neighbour search, and image relaxation. We show that the new algorithm requires less particles per interrogation window than
cross-correlation and allows for much higher particle densities than the other PTV methods. Consequently, one may obtain the
velocity field at high spatial resolution even in regions of very fast flows. Finally, we find that the new algorithm is more
robust against out-of-plane noise than previously proposed methods.
Received: 1 March 1999 / Accepted: 29 July 1999 相似文献
6.
This paper describes an experimental development for temporal and spatial reconstruction of continuously varying flow fields
by means of digital cinematographic particle image velocimetry (PIV). The system uses a copper-vapor laser illumination synchronized
with a high-speed camera, and continuously samples at 250 fps to measure transient and non-periodic turbulent flows with relatively
low frequencies, i.e., the surf zone turbulence produced by depth-limited wave break in a long laboratory flume. The use of
the developed PIV system comprehensively records the temporal development of both phase-averaged and instantaneous turbulent
vortex flows descended from the breaking waves to the bottom. Also, the measured power spectra show harmonic frequencies,
ranging from the orbital frequency of 0.5 Hz up to the order of 5 Hz, and the well-known −5/3 dependence upon the turbulence
fluctuation frequencies thereafter.
Received: 2 December 1999/Accepted: 6 September 2000 相似文献
7.
R. Camussi 《Experiments in fluids》2002,32(1):76-86
A new technique based on wavelet transform is applied to bidimensional velocity fields obtained by particle image velocimetry
(PIV) measurements, in order to extract and characterize swirling motion associated with coherent structures. The proposed
technique is based on the selectivity property of the wavelet transform and permits the detection of regions of the flow field
associated with coherent structures and their spatial localization. Furthermore, being the method based on the analysis of
the local energy content at separated scales, it is possible to extract the typical wavenumber associated with structures
and therefore the typical length-scale. The procedure is validated by the application to velocity vector fields obtained from
PIV measurements in different flow conditions and turbulence levels. Results are compared with those obtained by other more
standard procedures, and the advantages and limitations of the proposed method are then discussed.
Received: 16 October 2000 / Accepted: 18 June 2001 Published online: 29 November 2001 相似文献
8.
This paper describes the tests of accuracy and the first application of a combined planar visualization technique. Its goal is two-phase flow discrimination, i.e. simultaneous measurements of velocity of droplets and ambient gas in the case of two-phase flow mixing, at the same location and with possible conditioning by “apparent diameter” (AD) of the droplets. It combines the mature techniques of particle image velocimetry (PIV), planar Mie scattering diffusion (PMSD), planar laser-induced fluorescence (PLIF), and it necessitates two synchronized cross-correlation systems, digital image treatment and analysis. This technique was developed with the objective of better describing the mixing between liquid and gaseous phases as in the case of high-pressure spray atomization in quiescent ambient gas. The basic principle of separation is to seed the ambient gas with micrometer particles and to tag the liquid with fluorescent dye. We use digital image treatment and analysis to discriminate between the phases. We use two cross-correlation PIV systems in order to obtain the velocity field of the droplets and gas simultaneously and separately at the same location. The digital image processing for separating the phases involves geometric measurement of droplet shapes. This leads to measurement of droplet parameters close to their real diameter, which could be used for analysis of actual mixing. A synchronized system composed of two CCD cameras is used for image recording, and two Nd:YAG lasers are used for generating pulsed light sheets at times t and t + δt. Tests were performed to check for different sources of errors. The combined technique was applied to measurements in high-pressure spray flow atomizing in a quiescent ambient gas, and first results are presented. 相似文献
9.
A digital dual-camera cinematographic particle image velocimetry (CPIV) system has been developed to provide time-resolved,
high resolution flow measurements in high-Reynolds number, turbulent flows. Two high-speed CMOS cameras were optically combined
to acquire double-pulsed CPIV images at kilohertz frame rates. Bias and random errors due to camera misalignment, camera vibration,
and lens aberration were corrected or estimated. Systematic errors due to the camera misalignment were reduced to less than
2 pixels throughout the image plane using mechanical alignment, resulting in 3.1% positional uncertainty of velocity measurements.
Frame-to-frame uncertainties caused by mechanical vibration were eliminated with the aid of digital image calibration and
frame-to-frame camera registration. This dual-camera CPIV system is capable of resolving high speed, unsteady flows with high
temporal and spatial resolutions. It also allows time intervals between the two exposures down to 4 μs, enabling the measurements
of speed flows 5–10 times higher than possible with frame-straddling using similar cameras. A turbulent shallow cavity was
then chosen as the experimental object investigated by this dual-camera CPIV technique. 相似文献
10.
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
11.
Real-time image processing for particle tracking velocimetry 总被引:2,自引:1,他引:1
We present a novel high-speed particle tracking velocimetry (PTV) experimental system. Its novelty is due to the FPGA-based,
real-time image processing “on camera”. Instead of an image, the camera transfers to the computer using a network card, only
the relevant information of the identified flow tracers. Therefore, the system is ideal for the remote particle tracking systems
in research and industrial applications, while the camera can be controlled and data can be transferred over any high-bandwidth
network. We present the hardware and the open source software aspects of the PTV experiments. The tracking results of the
new experimental system has been compared to the flow visualization and particle image velocimetry measurements. The canonical
flow in the central cross section of a a cubic cavity (1:1:1 aspect ratio) in our lid-driven cavity apparatus is used for
validation purposes. The downstream secondary eddy (DSE) is the sensitive portion of this flow and its size was measured with
increasing Reynolds number (via increasing belt velocity). The size of DSE estimated from the flow visualization, PIV and
compressed PTV is shown to agree within the experimental uncertainty of the methods applied. 相似文献
12.
Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV 总被引:3,自引:0,他引:3
This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously
using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity
field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination,
each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient
fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength
shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of
each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images,
one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields
of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images
of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition.
Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters.
The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle
are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the
potential of the method. 相似文献
13.
14.
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
15.
New techniques are developed to improve the velocity flow-field measurement capability within a free-surface boundary layer
region on which progressive capillary-gravity waves are present. Due to the extremely thin but rather vortical characteristics
of the aforementioned boundary layer, conventional particle image velocimetry (PIV) methods fail to estimate velocity (and
vorticity) vectors at an acceptable detection rate. This failure is a direct consequence of optimal PIV parameters that are
difficult to achieve in practice for such flow situations. A new technique, Sub-pattern PIV, is developed. This method has
features similar to both the super-resolution PIV (Keane et al. 1995) and the particle image distortion (PID) technique (Huang
et al. 1993), but is predicated upon a very differential philosophy. Another difficulty that arises in experiments to investigate
surface boundary layer flows is that the oscillating and deforming air–water interface has a mirror-like behavior that affects
the images, and generates very noisy data. An alternative experimental setup that utilizes the Brewster angle phenomenon is
adopted and the specular effects of the free-surface are removed successfully. This Brewster angle imaging, along with the
Sub-pattern PIV technique, is used for the target application – a free-surface boundary layer investigation. It proved to
be very effective. The methodology of both techniques is discussed, and the modified PIV procedure is validated by numerical
probabilistic simulations. Application to the capillary-gravity wave boundary layer is presented in a subsequent paper.
Received: 31 July 1997/Accepted: 4 February 1998 相似文献
16.
17.
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
18.
Particle image velocimetry (PIV) processing of free surface flow images often requires the use of digital masks to overcome
the problems caused by the interface. In cases where a large number of particle images are collected it is essential that
the time-varying boundary between the two phases can be tracked automatically to produce the binary masks. The Radon transform-based
technique presented in this paper allows the automatic detection of the air–water interface in a stream of particle images
acquired from a single camera. It is applied to time-resolved PIV measurements in the liquid phase of a stratified multiphase
flow in a circular pipe. Accuracy estimations are provided using synthetic and real wave profiles. An extension to the more
complex case of an overturning wave is also discussed. 相似文献
19.
A particle image velocimetry system for microfluidics 总被引:20,自引:0,他引:20
J. G. Santiago S. T. Wereley C. D. Meinhart D. J. Beebe R. J. Adrian 《Experiments in fluids》1998,25(4):316-319
A micron-resolution particle image velocimetry (micro-PIV) system has been developed to measure instantaneous and ensemble-averaged
flow fields in micron-scale fluidic devices. The system utilizes an epifluorescent microscope, 100–300 nm diameter seed particles,
and an intensified CCD camera to record high-resolution particle-image fields. Velocity vector fields can be measured with
spatial resolutions down to 6.9×6.9×1.5 μm. The vector fields are analyzed using a double-frame cross-correlation algorithm.
In this technique, the spatial resolution and the accuracy of the velocity measurements is limited by the diffraction limit
of the recording optics, noise in the particle image field, and the interaction of the fluid with the finite-sized seed particles.
The stochastic influence of Brownian motion plays a significant role in the accuracy of instantaneous velocity measurements.
The micro-PIV technique is applied to measure velocities in a Hele–Shaw flow around a 30 μm (major diameter) elliptical cylinder,
with a bulk velocity of approximately 50 μm s-1.
Received: 26 November 1997/Accepted: 26 February 1998 相似文献
20.
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