共查询到20条相似文献,搜索用时 46 毫秒
1.
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
2.
A new orthogonal-plane cinema-stereoscopic particle image velocimetry (OPCS-PIV) diagnostic has been used to measure the dynamics
of three-dimensional turbulence–flame interactions. The diagnostic employed two orthogonal PIV planes, with one aligned perpendicular
and one aligned parallel to the streamwise flow direction. In the plane normal to the flow, temporally resolved slices of
the nine-component velocity gradient tensor were determined using Taylor’s hypothesis. Volumetric reconstruction of the 3D
turbulence was performed using these slices. The PIV plane parallel to the streamwise flow direction was then used to measure
the evolution of the turbulence; the path and strength of 3D turbulent structures as they interacted with the flame were determined
from their image in this second plane. Structures of both vorticity and strain-rate magnitude were extracted from the flow.
The geometry of these structures agreed well with predictions from direct numerical simulations. The interaction of turbulent
structures with the flame also was observed. In three dimensions, these interactions had complex geometries that could not
be reflected in either planar measurements or simple flame–vortex configurations. 相似文献
3.
Simultaneous velocity measurements at two points have been carried out with a two-component laser Doppler velocimeter in
order to characterise the turbulence along the impeller stream of a Rushton turbine in a water tank. In addition to mean values
of the velocity and its fluctuations, space–time correlations have been studied to enable the determination of the convection
velocity as well as of the scales of the turbulent structures and to investigate the validity of Taylor’s hypothesis. A correction
to the relationship between space and time correlations is proposed.
Received: 22 March 1996/Accepted: 23 April 1997 相似文献
4.
The effect of variation of the integral length scale on the accuracy of Taylor’s hypothesis and the assumption of local isotropy
when a mean strain is applied to a nearly homogeneous and isotropic, heated turbulent flow is investigated. The mean strain
is produced by placing one of two cylinders downstream of the heated wire mesh. Results show, the mean strain rate where the
directly measured variance of the streamwise spatial gradient differs by less than ten percent from the corresponding value
computed using Taylor’s hypothesis and temporal derivative decreases with increasing the ratio of the integral length scale
to the cylinder diameter.
Received: 13 November 1997 / Accepted: 23 April 1998 相似文献
5.
A numerical implementation of the advection equation is proposed to increase the temporal resolution of PIV time series. The
method is based on the principle that velocity fluctuations are transported passively, similar to Taylor’s hypothesis of frozen turbulence. In the present work, the advection model is extended to unsteady three-dimensional flows. The main objective of the method
is that of lowering the requirement on the PIV repetition rate from the Eulerian frequency toward the Lagrangian one. The
local trajectory of the fluid parcel is obtained by forward projection of the instantaneous velocity at the preceding time
instant and backward projection from the subsequent time step. The trajectories are approximated by the instantaneous streamlines,
which yields accurate results when the amplitude of velocity fluctuations is small with respect to the convective motion.
The verification is performed with two experiments conducted at temporal resolutions significantly higher than that dictated
by Nyquist criterion. The flow past the trailing edge of a NACA0012 airfoil closely approximates frozen turbulence, where the largest ratio between the Lagrangian and Eulerian temporal scales is expected. An order of magnitude reduction
of the needed acquisition frequency is demonstrated by the velocity spectra of super-sampled series. The application to three-dimensional
data is made with time-resolved tomographic PIV measurements of a transitional jet. Here, the 3D advection equation is implemented
to estimate the fluid trajectories. The reduction in the minimum sampling rate by the use of super-sampling in this case is
less, due to the fact that vortices occurring in the jet shear layer are not well approximated by sole advection at large
time separation. Both cases reveal that the current requirements for time-resolved PIV experiments can be revised when information is poured from space to time. An additional favorable effect is observed by the analysis in the frequency domain whereby the spectrum becomes significantly
less prone to aliasing error for the super-sampled data series. 相似文献
6.
Measurement of the gradient field of a turbulent free surface 总被引:1,自引:1,他引:0
We study the free surface above a turbulent channel flow. We describe a laser scanning technique that can be used to measure the space–time turbulent surface gradient field along a line. A harmonically swiveling laser beam is focused on the surface and its angle of refraction is measured using a position sensing device. The registered signals can be converted easily to the desired gradient field, and spectra and correlations can be measured. Examples of measured spectra and correlation functions of the surface above a turbulent channel flow (Reynolds number R
λ ≈ 250) demonstrate the viability of the technique. We further assess the validity of Taylor’s frozen turbulence hypothesis that implies that time-dependent signals measured along a line that is oriented perpendicularly to the mean channel velocity can be interpreted as 2D measurements of the surface slope. While Taylor’s hypothesis works for a turbulent velocity field, it does not work for its free surface.
相似文献
Willem van de WaterEmail: |
7.
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
8.
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
9.
Measurements have been made using Laser Doppler Anemometry (LDA) in a fully developed turbulent channel flow with the aim
of determining second-order and third-order temporal and spatial structure functions of the longitudinal velocity fluctuation.
A reliable determination of these moments requires the data to be corrected for the effect of noise. Correction procedures
are outlined, based on the behaviour of temporal or spatial correlation functions in the limit of small time intervals or
small separations. No a priori assumptions about the nature of the noise are made so that the procedure should be quite general.
The corrected LDA data indicate that, especially for spatial separations, the effect of noise can be felt even within the
inertial range. The corrected structure functions should allow an unambiguous assessment to be made of Taylor’s hypothesis
and of the extended self-similarity (ESS) method; examples are given in each case. Temporal structure functions obtained by
hot wire anemometry (HWA) are much less affected by noise than the LDA data.
Received: 8 April 1998/Accepted: 26 October 1998 相似文献
10.
11.
Particle image velocimetry (PIV) has been used in order to measure the three mean components and turbulence intensities of
the velocity vector in a swirling decaying flow induced by a tangential inlet in an annulus. This kind of flow motion is found
to be very complex, exhibiting three-dimensional and non-axisymmetric characteristics coupled with a free decay of the swirling
intensity along the flow path, thereby making it difficult to study. A method allowing the measurement of the three components
of the velocity flow-field with a standard PIV system with two-dimensional acquisitions, is presented. The evolution of each
velocity component between the inlet and the outlet of the annulus is obtained. Furthermore, the PIV technique is extended
to the measurement of turbulent characteristics such as turbulent intensities and dimensionless turbulent energy. The main
characteristics of the swirling flow are discussed and the swirl number is estimated as a function of the axial distance from
the tangential inlet.
Received: 6 July 1998/Accepted: 20 March 1999 相似文献
12.
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
13.
Yannis Hardalupas Srikrishna Sahu Alex M. K. P. Taylor Konstantinos Zarogoulidis 《Experiments in fluids》2010,49(2):417-434
A new approach for simultaneous planar measurement of droplet velocity and size with gas phase velocities is reported, which
combines the out-of-focus imaging technique ‘Interferometric Laser Imaging Droplet Sizing’ (ILIDS) for planar simultaneous
droplet size and velocity measurements with the in-focus technique ‘Particle Image Velocimetry’ (PIV) for gas velocity measurements
in the vicinity of individual droplets. Discrimination between the gas phase seeding and the droplets is achieved in the PIV
images by removing the glare points of focused droplet images, using the droplet position obtained through ILIDS processing.
Combination of the two optical arrangements can result in a discrepancy in the location of the centre of a droplet, when imaging
through ILIDS and PIV techniques, of up to about 1 mm, which may lead to erroneous identification of the glare points from
droplets on the PIV images. The magnitude of the discrepancy is a function of position of the droplet’s image on the CCD array
and the degree of defocus, but almost independent of droplet size. Specifically, it varies approximately linearly across the
image along the direction corresponding to the direction of propagation of the laser sheet for a given defocus setting in
ILIDS. The experimental finding is supported by a theoretical analysis, which was based on geometrical optics for a simple
optical configuration that replicates the essential features of the optical system. The discrepancy in the location was measured
using a monodisperse droplet generator, and this was subtracted from the droplet centres identified in the ILIDS images of
a polydisperse spray without ‘seeding’ particles. This reduced the discrepancy between PIV and ILIDS droplet centres from
about 1 mm to about 0.1 mm and hence increased the probability of finding the corresponding fringe patterns on the ILIDS image
and glare points on the PIV image. In conclusion, it is shown that the proposed combined method can discriminate between droplets
and ‘seeding’ particles and is capable of two-phase measurements in polydisperse sprays. 相似文献
14.
Benjamin Böhm Oliver Stein Andreas Kempf Andreas Dreizler 《Flow, Turbulence and Combustion》2010,85(1):73-93
Turbulent opposed jet burners are an excellent test case for combustion research and model development due to the burners’
compactness, relative simplicity, and the good optical access they provide. The flow-field in the flame region depends strongly
on the turbulence generation inside the nozzles, so that realistic flow simulations can only be achieved if the flow inside
the nozzles is represented correctly, which must be verified by comparison to suitable experimental data. This paper presents
detailed particle image velocimetry (PIV) measurements of the flow issuing from the turbulence generating plates (TGP) inside
a glass nozzle. The resulting data is analyzed in terms of first and second moments, time-series, frequency spectra and phase
averages. The measurements show how individual high velocity jets emerging from the TGP interact and recirculation zones are
formed behind the solid parts of the TGP. Vortex shedding is observed in the jet’s shear layer were high levels of turbulent
kinetic energy are generated. Time series measurements revealed periodic pulsations of the individual jets and implied a coupling
between adjacent jets. The peak frequencies were found to be a function of the Reynolds-number. 相似文献
15.
In the present work, the distribution of the local dissipation rate of turbulent kinetic energy in Taylor–Couette flow was studied with the help of the particle image velocimetry (PIV). The experimental values of dissipation rate are strongly affected by spatial resolution of PIV measurements. Therefore, a reference value of the average specific power input is needed. Such a value was achieved from an independent torque measurement. Using these values it was possible to quantify the true local values of the dissipation rate. The distribution of mixing times in the gap could thus be calculated and was found to become more homogeneous with increasing turbulence intensity. 相似文献
16.
17.
Time-resolved particle image velocimetry (PIV) measurements performed in wall parallel planes at three wall normal locations,
y
+ = 34, 108, and 278, in a zero pressure gradient turbulent boundary layer at Re
τ = 470 are used to illuminate the distribution of streamwise velocity fluctuations in a three-dimensional energy spectrum
(2D in space and 1D in time) over streamwise, spanwise, and temporal wavelengths. Two high-speed cameras placed side by side
in the streamwise direction give a 10δ × 5δ streamwise by spanwise field of view with a vector spacing of
_boxclose = z^+ 37\Updelta x^+ = \Updelta z^+ \approx 37 and a time step of
\Updelta t+=0.5\Updelta t^+=0.5. Although 3D wavenumber--frequency spectra have been calculated in acoustics studies, to the authors’ knowledge this is the
first time they has been calculated and presented for a turbulent boundary layer. The calculation and normalization of this
spectrum, its relation to 2D and 1D spectra, and the effects of the PIV algorithm on its shape are carefully analyzed and
outlined. 相似文献
18.
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
19.
20.
High-resolution two-dimensional (2D) measurements on a large plane mixing layer provide new quantitative information of its
spatial and temporal evolution to turbulence. Periodic acoustic excitation with three frequencies was used to stabilize the
fundamental instability of the mixing layer (roll-up) and its first and second subharmonics (vortex pairings). Phase-locked
velocity measurements of the time evolution in 2D space (x, y, t) reveal accurate spatially resolved primary (2D) instabilities of the mixing layer and turbulence transition. The measurements
unveil new quantitative details of the initial Kelvin–Helmholtz waves and their spatial and temporal evolution into vortex
shedding and the effect of the second subharmonic on the first vortex pairing. The second-subharmonic effect hastens alternate
first pairings of the rollers, with the result that pairing is completed at two downstream locations. The pairings that occur
closer to the knife-edge are more organized (laminar) than those occurring farther downstream (transitional). This effect
is corroborated using Taylor’s hypothesis to compute the vorticity distributions from the measured velocity field and a pseudo-spectral
simulation of the temporal evolution of the mixing layer.
Received: 26 March 1998/Accepted: 2 March 1999 相似文献