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1.
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
2.
J. Sousa 《Experiments in fluids》2002,33(6):854-862
The mean turbulent flow structure around a cube mounted on the surface of an open-surface water channel was studied using a two-dimensional implementation of digital particle image velocimetry (DPIV). The out-of-plane velocity component was obtained by the use of the concept of continuity applied to two-dimensional velocity fields recorded in parallel planes. Various methods were used for the identification and localization of large-scale vortical structures in the three-dimensional flow around the surface-mounted obstacle. The results show the feasibility of its application to three-dimensional PIV data and the superior performance of recent identification techniques (namely swirling strength and normalized angular momentum), over the classical vorticity-based criterion. 相似文献
3.
This paper presents a reconstruction of the three-dimensional velocity field of a turbulent vortex ring by means of Taylor’s
hypothesis. Stereoscopic PIV is used to acquire three velocity component information on a plane. The accuracy of the Taylor’s
hypothesis for this particular flow pattern is first discussed, and the three-dimensional velocity and vorticity information
are then presented. This study also introduces an azimuthally averaging method in order to give a mean structure in cylindrical
coordinates from a single realization and from which turbulent stresses and production can be estimated. The azimuthally averaged
quantities are then compared with the ensemble-averaged results from the previous planar (two-dimensional and stereoscopic)
PIV experiments. 相似文献
4.
Cinematographic stereoscopic PIV measurements were performed in the far field of an axisymmetric co-flowing turbulent round
jet (Re
T ≈ 150, where Re
T is the Reynolds number based on Taylor micro scale) to resolve small and intermediate scales of turbulence. The time-resolved
three-component PIV measurements were performed in a plane normal to the axis of the jet and the data were converted to quasi-instantaneous
three-dimensional (volumetric) data by using Taylor’s hypothesis. The availability of the quasi-three-dimensional data enabled
the computation of all nine components of the velocity gradient tensor over a volume. The use of Taylor’s hypothesis was validated
by performing a separate set of time-resolved two component “side-view” PIV measurements in a plane along the jet axis. Probability
density distributions of the velocity gradients computed using Taylor’s hypothesis show good agreement with those computed
directly with the spatially resolved data. The overall spatial structure of the gradients computed directly exhibits excellent
similarity with that computed using Taylor’s hypothesis. The accuracy of the velocity gradients computed from the pseudo-volume
was assessed by computing the divergence error in the flow field. The root mean square (rms) of the divergence error relative
to the magnitude of the velocity gradient tensor was found to be 0.25, which is consistent with results based on other gradient
measurement techniques. The velocity gradients, vorticity components and mean dissipation in the self-similar far field of
the jet were found to satisfy the axisymmetric isotropy conditions. The divergence error present in the data is attributed
to the intrinsic uncertainty associated with performing stereoscopic PIV measurements and not to the use of Taylor’s hypothesis.
The divergence error in the data is found to affect areas of low gradient values and manifests as nonphysical values for quantities
like the normalized eigenvalues of the strain-rate tensor. However, the high gradients are less affected by the divergence
error and so it can be inferred that structural features of regions of intense vorticity and dissipation will be faithfully
rendered. 相似文献
5.
Stereoscopic particle image velocimetry 总被引:25,自引:19,他引:6
A. K. Prasad 《Experiments in fluids》2000,29(2):103-116
Stereoscopic particle image velocimetry (PIV) employs two cameras to record simultaneous but distinct off-axis views of the
same region of interest (illuminated plane within a flow seeded with tracer particles). Sufficient information is contained
in the two views to extract the out-of-plane motion of particles, and also to eliminate perspective error which can contaminate
the in-plane measurement. This review discusses the principle of stereoscopic PIV, the different stereoscopic configurations
that have been used, the relative error in the out-of-plane to the in-plane measurement, and the relative merits of calibration-based
methods for reconstructing the three-dimensional displacement vector in comparison to geometric reconstruction. It appears
that the current trend amongst practitioners of stereoscopic PIV is to use digital cameras to record the two views in the
angular displacement configuration while incorporating the Scheimpflug condition. The use of calibration methods has also
gained prominence over geometric reconstruction.
Received: 15 April 1999/Accepted: 1 February 2000 相似文献
6.
M. Hecklau R. van Rennings V. Zander W. Nitsche A. Huppertz M. Swoboda 《Experiments in fluids》2011,50(4):799-811
AFC (Active Flow Control) experiments have been performed by means of steady and pulsed blowing out of the sidewalls as well
as out of the blade’s suction surface in a highly loaded compressor cascade. PIV (Particle Image Velocimetry) was used to
evaluate the fully three-dimensional internal flow field and the impact of AFC methods. The aim was to observe the secondary
flow structures and flow instabilities by PIV, to tune the AFC device operation parameters. This paper summarizes the different
PIV measurements performed at the stator cascade to give an overview of the dominant flow features in the passage flow field
and to obtain a detailed view of control mechanisms. In addition, a new vortex detection method is presented, based on a 2D-wavelet
which is applicable in two-dimensional velocity data fields. 相似文献
7.
Construction of three-dimensional images of flow structure, based on the quantitative velocity field, is assessed for cases where experimental data are obtained using particle tracking technique. The experimental data are in the form of contiguous planes of particle images. These contiguous data planes are assumed to correspond to successive spatial realizations in steady flow, or to phase-referenced realizations in an unsteady flow.Given the particle images on contiguous planes, the in-plane velocity fields are determined. Then, the out-of-plane velocity field is obtained using a spectral interpolation method. Application of this method allows, in principle, construction of the three-dimensional vorticity field and the streamline patterns.A critical assessment is made of the uncertainties arising from the in-plane interpolation of the velocity field obtained from particle tracking and the evaluation of the out-of-plane velocity component. The consequences of such uncertainties on the reconstructed vorticity distributions and streamline patterns are addressed for two basic types of vortex flows: a columnar vortex, for which the streamlines are not closed and are spatially periodic in the streamwise direction; and for a spherical (Hill's) vortex exhibiting closed streamline patterns, and no spatial periodicity. 相似文献
8.
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
9.
The method to estimate the dynamic load of a flapping wing by the integration of pressure on the wing’s surface is discussed.
The flapping wing was modeled as a plate flapping sinusoidally in hovering condition. The flow around the flapping plate was
measured using stereo PIV on multiple measurement planes along the out-of-plane direction. The phase-averaged velocity field
of each measurement plane was calculated so that three-dimensional analyses could be applied. The phase-averaged pressure
field was obtained from the integration of the three-dimensional Poisson equation for pressure using the available information
acquired from stereo PIV measurements. The pressure field is visualized on the measurement planes. In this study, the estimated
load was the torque of the axis of rotation. This torque was compared with the result from strain gauge measurements. The
torque estimation, although only on a partial surface of the plate, is within reasonable agreement with the measured torque.
The integration of the Poisson equation based on stereo PIV measurements and estimations of the torque shows that an increase
in the torque at the start of a flapping stroke is caused by the stagnation on the surface of the plate from the flow that
is induced by the leading-edge vortices. 相似文献
10.
Three-component velocity field measurements of propeller wake using a stereoscopic PIV technique 总被引:1,自引:0,他引:1
A stereoscopic PIV (Particle Image Velocimetry) technique was used to measure the three-dimensional flow structure of the turbulent wake behind a marine propeller with five blades. The out-of-plane velocity component was determined using two CCD cameras with an angular displacement configuration. Four hundred instantaneous velocity fields were measured for each of four different blade phases, and ensemble averaged in order to find the spatial evolution of the propeller wake in the region from the trailing edge up to one propeller diameter (D) downstream. The influence of propeller loading conditions on the wake structure was also investigated by measuring the velocity fields at three advance ratios (J=0.59, 0.72 and 0.88). The phase-averaged velocity fields revealed that a viscous wake formed by the boundary layers developed along the blade surfaces. Tip vortices were generated periodically and the slipstream contracted in the near-wake region. The out-of-plane velocity component and strain rate had large values at the locations of the tip and trailing vortices. As the flow moved downstream, the turbulence intensity, the strength of the tip vortices, and the magnitude of the out-of-plane velocity component at trailing vortices all decreased due to effects such as viscous dissipation, turbulence diffusion, and blade-to-blade interaction. 相似文献
11.
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
12.
Stereoscopic particle image velocimetry (SPIV) is applied to measure the instantaneous three component velocity field of pipe
flow over the full circular cross-section of the pipe. The light sheet is oriented perpendicular to the main flow direction,
and therefore the flow structures are advected through the measurement plane by the mean flow. Applying Taylor’s hypothesis,
the 3D flow field is reconstructed from the sequence of recorded vector fields. The large out-of-plane motion in this configuration
puts a strong constraint on the recorded particle displacements, which limits the measurement accuracy. The light sheet thickness
becomes an important parameter that determines the balance between the spatial resolution and signal to noise ratio. It is
further demonstrated that so-called registration errors, which result from a small misalignment between the laser light sheet
and the calibration target, easily become the predominant error in SPIV measurements. Measurements in laminar and turbulent
pipe flow are compared to well established direct numerical simulations, and the accuracy of the instantaneous velocity vectors
is found to be better than 1% of the mean axial velocity. This is sufficient to resolve the secondary flow patterns in transitional
pipe flow, which are an order of magnitude smaller than the mean flow. 相似文献
13.
Hemodynamic forces within the human carotid artery are well known to play a key role in the initiation and progression of
vascular diseases such as atherosclerosis. The degree and extent of the disease largely depends on the prevailing three-dimensional
flow structure and wall shear stress (WSS) distribution. This work presents tomographic PIV (Tomo-PIV) measurements of the
flow structure and WSS in a physiologically accurate model of the human carotid artery bifurcation. The vascular geometry
is reconstructed from patient-specific data and reproduced in a transparent flow phantom to demonstrate the feasibility of
Tomo-PIV in a complex three-dimensional geometry. Tomographic reconstruction is performed with the multiplicative line-of-sight
(MLOS) estimation and simultaneous multiplicative algebraic reconstruction (SMART) technique. The implemented methodology
is validated by comparing the results with Stereo-PIV measurements in the same facility. Using a steady flow assumption, the
measurement error and RMS uncertainty are directly inferred from the measured velocity field. It is shown that the measurement
uncertainty increases for increasing light sheet thickness and increasing velocity gradients, which are largest near the vessel
walls. For a typical volume depth of 6 mm (or 256 pixel), the analysis indicates that the velocity derived from 3D cross-correlation
can be measured within ±2% of the maximum velocity (or ±0.2 pixel) near the center of the vessel and within ±5% (±0.6 pixel)
near the vessel wall. The technique is then applied to acquire 3D-3C velocity field data at multiple axial locations within
the carotid artery model, which are combined to yield the flow field and WSS in a volume of approximately 26 mm × 27 mm × 60 mm.
Shear stress is computed from the velocity gradient tensor and a method for inferring the WSS distribution on the vessel wall
is presented. The results indicate the presence of a complex and three-dimensional flow structure, with regions of flow separation
and strong velocity gradients. The WSS distribution is markedly asymmetric confirming a complex swirling flow structure within
the vessel. A comparison of the measured WSS with Stereo-PIV data returns an acceptable agreement with some differences in
stress magnitude. 相似文献
14.
Application of PIV in a Mach 7 double-ramp flow 总被引:2,自引:0,他引:2
The flow over a two-dimensional double compression ramp configuration is investigated by means of schlieren visualization, quantitative infrared thermography and particle image velocimetry (PIV) in a short-duration facility producing a free-stream flow at Mach 7. The study focuses upon the accuracy assessment of PIV in the hypersonic flow regime including flow facility effects such as repeatability of test conditions. The solid tracer particles are characterized by means of electron microscopy as well as by measuring the dynamic response across a planar oblique shock wave with PIV. The experiments display a strong variation in the light scattering intensity of the seeded flow over the flow field, due to the large flow compressibility. The mean velocity spatial distribution allows to clearly identify the shock pattern and the main features of the flow downstream of the shocks. However, the spatial resolution is insufficient to determine the wall flow properties. Furthermore the velocity data obtained with the PIV technique allow the determination of the spatial distribution of the Mach number under the hypothesis of adiabatic flow. The double ramp configuration with a variable second compression angle exhibits shock–shock interactions of Edney type VI or V for the lowest and highest ramp angle, respectively. A single heat transfer peak is detected with infrared thermography on the second ramp in case of a type VI interaction while for the type V shock interaction a double heat transfer peak is found. Shock wave angles measured with PIV are in good agreement with theory and the overall flow topology is consistent with schlieren visualization. Also in this respect the results are in agreement with compressible flow theory. 相似文献
15.
Modification of near-wall turbulence structure in a shear-driven three-dimensional turbulent boundary layer 总被引:2,自引:0,他引:2
Most high Reynolds number flows of engineering interest are three-dimensional in nature. Key features of three-dimensional
turbulent boundary layers (3DTBLs) include: non-colateral shear stress and strain rate vectors, and decreasing ratio of the
shear stresses to the turbulent kinetic energy with increasing three-dimensionality. These are indicators that the skewing
has a significant effect on the structure of turbulence. In order to further investigate the flow physics and turbulence structure
of these complex flows, an innovative method for generating a planar shear-driven 3DTBL was developed. A specialized facility
incorporating a relatively simple geometry and allowing for varying strengths of crossflow was constructed to facilitate studies
where the skewing is decoupled from the confounding effects of streamwise pressure gradient and curvature. On-line planar
particle image velocimetry (PIV) measurements and flow visualization results indicate that the experimental configuration
generates the desired complex flow, which exhibits typical characteristics associated with 3DTBLs. Furthermore, spanwise shear
results in modification of the near-wall turbulence structure. Analysis of near-wall flow visualization photographs revealed
a reduction of mean streak length with increasing spanwise shear, while streak spacing remained relatively constant. In the
most strongly sheared case, where the belt velocity is twice that of the freestream velocity, the mean streak length was reduced
by approximately 50%.
Received: 28 October 1997/Accepted: 4 February 1998 相似文献
16.
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
17.
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
18.
A PIV based technique is developed to perform flow measurements in the vicinity of the air–water interface of a submerged
confined jet. Both the interface movement and the velocity field immediately beneath it are measured simultaneously. A detailed
turbulence structure in the surface influence region is thus obtained. Flow parameters evaluated without and w.r.t. the interface
are quantified and compared against previous works obtained using the conventional Eulerian-based instrumentation which do
not account for the interface fluctuation, and checked against analytical model characterising the turbulence close to a assumed
flat air–water interface.
Received: 15 March 1998/Accepted: 19 October 1998 相似文献
19.
Digital-Particle-Image-Velocimetry (DPIV) in a scanning light-sheet: 3D starting flow around a short cylinder 总被引:2,自引:0,他引:2
Ch. Brücker 《Experiments in fluids》1995,19(4):255-263
Scanning-Particle-Image-Velocimetry Technique (SPIV), introduced by Brücker (1992) and Brücker and Althaus (1992), offers the quantitative investigation of three-dimensional vortical structures in unsteady flows. On principle, this technique combines classical Particle-Image-Velocimetry (PIV) with volume scanning using a scanning light-sheet. In our previous studies, single scans obtained from photographic frame series were evaluated to show the instantaneous vortical structure of the respective flow phenomena. Here, continuous video recordings are processed to capture also the temporal information for the study of the set-up of 3D effects in the cylinder wake. The flow is continuously sampled in depth by the scanning light-sheet and in each of the parallel planes frame-to-frame cross-correlation of the video images (DPIV) is applied to obtain the 2D velocity field. Because the scanning frequency and repetition rate is high in comparison with the characteristic time-scale of the flow, the evaluation provides a complete time-record of the 3D flow during the starting process. With use of the continuity concept as described by Robinson and Rockwell (1993), we obtained in addition the out-of-plane component of the velocity in spanwise direction. This in view, the described technique enabled the reconstruction of the three-dimensional time-dependent velocity and vorticity field. The visualization of the dynamical behaviour of these quantities as, e.g. by video, gave a good impression of the spanwise flow showing the tornado-like suction effect of the starting vortices.A version of this paper was presented at the 2. Int. Conference on Experimental Fluid Mechanics, 4–8 July 1994, Torino (Italy) 相似文献