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1.
The ultrasonic velocity profile measurement of flow structure in the near field of a square free jet
Coherent structures in the near field of a three-dimensional jet have been investigated. Experiments were carried out for
a free jet issuing from a square nozzle using a water channel. Instantaneous velocity profiles were obtained in the axial
and radial directions by using an ultrasonic velocity profile (UVP) monitor. Axial variations of dominant time-scales of vortex
structures were examined from one-dimensional wavelet spectra. Wavenumber-frequency spectra were calculated by two-dimensional
Fourier transform along the axial direction in a mixing layer, and it was found that a convective velocity of flow structures
was nearly constant independently of their scales in space and time. Coherent structures in the axial direction were investigated
in terms of proper orthogonal decomposition (POD). Eigenfunctions are similar to a sinusoidal wave, and reconstructed velocity
fields by the lower-order and higher-order POD modes demonstrate large-scale and smaller-scale coherent structures, respectively.
Received: 8 May 2000/Accepted: 23 January 2001 Published online: 29 November 2001 相似文献
2.
The spatial-temporal evolution of coherent structures(CS) is significant for turbulence control and drag reduction. Among the CS, low and high speed streak structures show typical burst phenomena. The analysis was based on a time series of three-dimensional and three-component(3D-3C) velocity fields of the flat plate turbulent boundary layer(TBL) measured by a Tomographic and Time-resolved PIV(Tomo TRPIV) system. Using multi-resolution wavelet transform and conditional sampling method, we extracted the intrinsic topologies and found that the streak structures appear in bar-like patterns. Furthermore, we seized locations and velocity information of transient CS, and then calculated the propagation velocity of CS based on spatial-temporal cross-correlation scanning. This laid a foundation for further studies on relevant dynamics properties. 相似文献
3.
Using experimental data from Particle Image Velocimetry (PIV) measurements, coherent structures of a transitional spatially developing boundary layer are determined. The coherent structures are determined utilizing the Proper Orthogonal Decomposition (POD), which is based on an expansion of the flow field variables into a set of eigenfunctions or modes. For having constant and reproducible flow field conditions, the flow is artificially excited by means of periodic velocity fluctuations. The used excitation device allows the generation of different transition scenarios, where this paper focuses on the case of thefundamental transition. Phase locked excitation signals allow the recording of instantaneous velocity fields of the flow field at certain instants of time. It can be shown that PIV is a suitable technique to provide experimental data for POD. The results of the POD show that already a small number of modes cover most of the kinetic energy of the flow. 相似文献
4.
R. Perrin M. Braza E. Cid S. Cazin F. Moradei A. Barthet A. Sevrain Y. Hoarau 《Flow, Turbulence and Combustion》2006,77(1-4):185-204
The main objective of the present experimental study is to analyse the turbulence properties in unsteady flows around bluff body wakes and to provide a database for improvement and validation of turbulence models, concerning the present class of non-equilibrium flows. The flow around a circular cylinder with a low aspect ratio () and a high blockage coefficient () is investigated. This confined environment is used in order to allow direct comparisons with realisable 3D Navier–Stokes computations avoiding ‘infinite’ conditions. The flow is investigated in the critical regime at Reynolds number 140,000. A cartography of the velocity fields in the near wake of the cylinder is obtained by PIV and Stereoscopic PIV techniques. Statistical means and phase-averaged quantities are determined. Furthermore, POD analysis is performed on the data set in order to extract coherent structures of the flow and to compare the results with those obtained by the conditional sampling technique. The Reynolds stresses, the strain-rate and vorticity fields as well as the turbulence production terms are determined. 相似文献
5.
PIV measurements of a microchannel flow 总被引:24,自引:0,他引:24
A particle image velocimetry (PIV) system has been developed to measure velocity fields with order 1-μm spatial resolution.
The technique uses 200 nm diameter flow-tracing particles, a pulsed Nd:YAG laser, an inverted epi-fluorescent microscope,
and a cooled interline-transfer CCD camera to record high-resolution particle-image fields. The spatial resolution of the
PIV technique is limited primarily by the diffraction-limited resolution of the recording optics. The accuracy of the PIV
system was demonstrated by measuring the known flow field in a 30 μm×300 μm (nominal dimension) microchannel. The resulting
velocity fields have a spatial resolution, defined by the size of the first window of the interrogation spot and out of plane
resolution of 13.6 μm× 0.9 μm×1.8 μm, in the streamwise, wall-normal, and out of plane directions, respectively. By overlapping
the interrogation spots by 50% to satisfy the Nyquist sampling criterion, a velocity-vector spacing of 450 nm in the wall-normal
direction is achieved. These measurements are accurate to within 2% full-scale resolution, and are the highest spatially resolved
PIV measurements published to date.
Received: 29 October 1998/Accepted: 10 March 1999 相似文献
6.
Probing the velocity fields of gas and liquid phase simultaneously in a two-phase flow 总被引:2,自引:0,他引:2
The feasibility of simultaneous measurements of the instantaneous velocity fields of gaseous and liquid phase is demonstrated
in a laminar, unsteady two-phase flow. Thus, the instantaneous relative velocity field can be measured in such media. This
is achieved by combining Particle Image Velocimetry (PIV) and a gas-phase velocimetry technique, which is based on laser-induced
fluorescence (LIF) from a gaseous tracer. The wavelength shift of LIF is exploited to separate it from Mie scattering from
the liquid phase. The new technique and the PIV measurement system work independently in this approach. Thus, the measurement
accuracy and precision of the new technique can be validated by comparing it to the PIV results in regions of the flow field
where the relative velocity vanishes.
Received: 18 October 1998/Accepted: 16 October 1999 相似文献
7.
Hot wire signals obtained in grid-turbulence are processed through orthogonal wavelet transform. It is shown that using wavelet
decomposition in combination with the form of scaling named Extended Self Similarity, some statistical properties of fully
developed turbulence can be extended to very low Reλ flows. Furthermore, based on the wavelet decomposition, a new technique for coherent structures identification is introduced.
We present results obtained in grid turbulence data at low and very low Reλ conditions. 相似文献
8.
Planar velocity data of the unsteady separated flow in the turbulent wake of a circular cylinder obtained by particle image
velocimetry (PIV) are analyzed in order to visualize the large-scale coherent structures associated with alternating vortex
shedding at a Reynolds number of 2,150. Two different cases are examined: unforced vortex shedding in the natural wake and
vortex lock-on incited by forced perturbations superimposed in the inflow velocity. Proper orthogonal decomposition (POD)
is employed to reconstruct the low-order wake dynamics from randomly sampled snapshots of the velocity field. The reconstructed
flow is subsequently used to determine the evolution of the finite-time Lyapunov exponent (FTLE) fields which identify the
Lagrangian coherent structures. The results demonstrate that the combination of methods employed offers a powerful visualization
tool to uncover large-scale coherent structures and to exemplify vortex dynamics in natural and forced bluff-body wakes. 相似文献
9.
A new stereoscopic PIV system to measure the three velocity components is developed and applied to grid turbulence flows.
This system uses two CCD cameras coupled with an accurate cross-correlation calculation method. An experimental test (based
upon three-dimensional displacements) has been carried out to demonstrate the capability of this process to locate the maximum
of correlation, and to detect accurately the 3D displacements. Experiments in a well-established turbulent flow have validated
the method for quantitative measurements and a comparison with LDV results showed a good agreement in terms of mean and fluctuating velocities. Combined PIV and stereoscopic PIV measurements on a turbulent flow revealed the need to the stereoscopic systems to measure accurate 2D
velocity fields. It has been shown that an error of up to 10% in the velocity fluctuation measured by conventional PIV could
be attained due to 3D effects in highly turbulent cases. Finally, the digital cross-correlation technique adapted to the determination
of small displacements seems to be the most suitable technique for stereoscopic PIV.
Received: 22 July 1997/Accepted: 27 January 1998 相似文献
10.
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
11.
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
12.
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
13.
Holographic PIV (HPIV) is the most promising candidate for the next generation full-field velocimetry that can measure high
spatial resolution instantaneous three-dimensional (3D) velocity fields. To explore the maximum performance capabilities of
HPIV including spatial resolution, off-axis holography based HPIV has become a major direction of development. A fully automated
off-axis HPIV system based on an injection-seeded dual-pulsed YAG laser and 3D data processing software has been implemented
in the laser flow diagnostics lab (LFD). In our system, 90-degree particle scattering, dual reference beams, in situ reconstruction/data
processing, and 3D velocity extraction based on a fast “concise cross correlation” (CCC) algorithm are utilized. The off-axis
HPIV system is tested for an acoustically excited air jet and the wake of a surface-mounted tab in a water channel flow, giving
instantaneous 3D velocity fields for both flows. Experimental data of instantaneously measured 3D flow structures using this
technique show great promise.
Received: 12 August 1998/Accepted: 20 October 1999 相似文献
14.
In this communication, the Digital Image Compression (DIC) – PIV system is introduced. The present system allows the measurement
of mean and RMS velocities in turbulent flow fields, using JPEG digital image compression technique for on-line recording
of thousands of images. The decompression and subsequent analysis of the images, performed by means of digital cross-correlation
technique, is carried out off-line. Errors incurred by the application of the compression method are assessed and discussed.
The effect of the compression is firstly analysed by linearly traversing (synthetic) computer-generated PIV-images at constant
velocity. Secondly, accurate LDA measurements and data from direct numerical simulation (DNS) are used as a basis for the
analysis in a low Reynolds number open water channel flow. The results show that excellent agreement between LDA and DIC–PIV
measurements for mean and RMS velocities can be achieved using a compression factor up to 12.
Received: 27 August 1996 / Accepted: 15 December 1998 相似文献
15.
Simultaneous velocity and concentration measurements have been performed in a gas-turbine combustor model. Particle image velocimetry (PIV) was used to acquire planar velocity information and to identify coherent flow structures. The Mie scattering technique, based on a slightly modified experimental setup, was used for concentration measurements in this mixing flow. The degree of mixing was assessed by examining local concentration measurements while inhomogeneously seeding the primary and secondary stream of the mixing layer. Connections between flow field and concentration distribution were highlighted using the proper orthogonal decomposition algorithm (POD). Uncertainties and systematic errors for the PIV measurements due to the suboptimal seeding are discussed using a comparison with a second test series at optimal seeding conditions. Results are presented for several flow parameters and at various lateral planes. 相似文献
16.
Experiments are carried out in the wake of a cylinder of d
c
= 10 mm diameter placed symmetrically between two parallel walls with a blockage ratio r = 1/3 and a Reynolds number varying between 75 ≤ Re ≤ 277. Particle image velocimetry is exerted to obtain the instantaneous velocity components in the cylinder wake. A snapshot
proper orthogonal decomposition (POD) is also applied to these PIV results in order to extract the dominant modes through
the implementation of an inhomogeneous filtering of these different snapshots, apart from an interpolation to estimate the
wall shear rate at the lower wall downstream the cylinder. Mass transfer circular probes are placed at the lower wall downstream
this obstacle so as to further determine the time evolution of the wall shear rate, by bringing the inverse method to bear
on the convective-diffusion equation. Comparisons between the two synchronized techniques demonstrate that electrochemical
method can give more accurate information about the coherent structures present in the flow and about the interaction of the
von Kármán vortices with the walls of the tunnel as well. The comparison between the two measurement techniques in the flow
regions concerns the spatiotemporal evolutions of the wall shear rate obtained from PIV measurements and the wall shear rate
using mass transfer probes. Discrepancy between the PIV measurements and the electrochemical ones near the wall, where the
secondary vortices P
1′ are generated at wall, are caused by a PIV bias and a limitations of the singular mass transfer probes. 相似文献
17.
We analyzed the non-Newtonian flow characteristics of blood moving in a circular tube flow using an X-ray PIV method and compared the experimental results with hemodynamic models. The X-ray PIV method was improved for measuring quantitative velocity fields of blood flows using a coherent synchrotron X-ray. Without using any contrast media, this method can visualize flow pattern of blood by enhancing the phase-contrast and interference characteristics of blood cells. The enhanced X-ray images were achieved by optimizing the sample-to-scintillator distance, the sample thickness, and hematocrit in detail. The quantitative velocity fields of blood flows inside opaque conduits were obtained by applying a two-frame PIV algorithm to the X-ray images of the blood flows. The measured velocity data show typical features of blood flow such as the yield stress and shear-thinning effects. 相似文献
18.
The current work experimentally investigates the flow characteristics in the near-wall region of the 61-pin wire-wrapped hexagon fuel bundle via the matched-index-of-refraction technique. Particle image velocimetry (PIV) measurements were taken in the region near the surfaces of the pins, wires and enclosure wall at the Reynolds number of 19,000. From the obtained PIV velocity vector fields, flow statistics such as mean velocity and root-mean-square fluctuating velocity profiles were computed. In addition, spatial-temporal cross-correlations of velocity-velocity and pressure-velocity were analyzed. A strong correlation between the wall fluctuating pressure signal and flow structures was observed. Finally, we applied the POD analysis to the vorticity snapshots obtained in the near-wall region to reveal the dominant flow structures. It was found that the large-scale structures were elongated and aligned with the mean flow direction. 相似文献
19.
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