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1.
Flow field investigation in rotating rib-roughened channel by means of particle image velocimetry 总被引:2,自引:0,他引:2
The turbulent velocity field over the rib-roughened wall of an orthogonally rotating channel is investigated by means of two-dimensional
particle image velocimetry (PIV). The flow direction is outward, with a bulk Reynolds number of 1.5 × 104 and a rotation number ranging from 0.3 to 0.38. The measurements are obtained along the wall-normal/streamwise plane at mid-span.
The PIV system rotates with the channel, allowing to measure directly the relative flow velocity with high spatial resolution.
Coriolis forces affect the stability of the boundary layer and free shear layer. Due to the different levels of shear layer
entrainment, the reattachment point is moved downstream (upstream) under stabilizing (destabilizing) rotation, with respect
to the stationary case. Further increase in rotation number pushes further the reattachment point in stabilizing rotation,
but does not change the recirculation length in destabilizing rotation. Turbulent activity is inhibited along the leading
wall, both in the boundary layer and in the separated shear layer; the opposite is true along the trailing wall. Coriolis
forces affect indirectly the production of turbulent kinetic energy via the Reynolds shear stresses and the mean shear. Two-point
correlation is used to characterize the coherent motion of the separated shear layer. Destabilizing rotation is found to promote
large-scale coherent motions and accordingly leads to larger integral length scales; on the other hand, the spanwise vortices
created in the separating shear layer downstream of the rib are less organized and tend to be disrupted by the three-dimensional
turbulence promoted by the rotation. The latter observation is consistent with the distributions of span-wise vortices detected
in instantaneous flow realizations. 相似文献
2.
Experimental investigation of the flow in the vaneless diffuser of a centrifugal pump by particle image displacement velocimetry 总被引:7,自引:0,他引:7
This work presents the application of particle image displacement velocimetry to the measurement of fluid velocities in a centrifugal pump diffuser. Measurements are taken at different operating points and allow to define the variation of radial and tangential velocity components along a pitch. They are further processed to determine the relative velocity and vorticity fields. Results are also compared with laser Doppler measurements taken in the same facility. 相似文献
3.
The accuracy of the particle image velocimetry technique was investigated using synthetic images having known characteristics. Algorithms were developed to extract two-dimensional velocity information by tracking particles between successive frames of a movie automatically without operator assistance. This allowed to parametrically investigate the influence of the various parameters (image contrast, image noise, particle density, distribution of sizes of particles and particle displacement between frames) on the accuracy of the technique. It was found that as long as the images have a good contrast, particle locations can be determined with sub-pixel accuracy and particle velocities can be determined within a few percent. 相似文献
4.
Inertial waves generated by a small oscillating disk in a rotating water filled cylinder are observed by means of a corotating
particle image velocimetry system. The wave takes place in a stationary conical wavepacket, whose angle aperture depends on
the oscillation frequency. Direct visualisation of the velocity and vorticity fields in a plane normal to the rotation axis
are presented. The characteristic wavelength is found to be approximately equal to the disk diameter. The classical dispersion
relation for plane waves is verified from the radial location of the wavepacket, and from the ellipticity of the projected
velocity diagram.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
相似文献
Frédéric MoisyEmail: |
5.
M. P. Wernet 《Experiments in fluids》2000,28(2):97-115
Digital Particle Imaging Velocimetry (DPIV) is a powerful measurement technique, which can be used as an alternative or complementary
approach to Laser Doppler Velocimetry (LDV) in a wide range of research applications. The instantaneous planar velocity measurements
obtained with PIV make it an attractive technique for use in the study of the complex flow fields encountered in turbomachinery.
The ability to acquire multiple measurement points of comparable accuracy to LDV results in reduced runtime and enables the
study of both transient and steady state flow phenomena. Many of the same issues encountered in the application of LDV to
rotating machinery apply in the application of PIV. Techniques for optical access, light sheet delivery, CCD camera technology
and particulate seeding are discussed. Results from the successful application of the PIV technique to both the blade passage
region of a transonic axial compressor and the diffuser region of a high speed centrifugal compressor are presented. Both
instantaneous and time-averaged flow fields were obtained. The 95% confidence intervals for the velocity estimates were also
determined.
Received: 16 November 1998/Accepted: 10 April 1999 相似文献
6.
A new stereoscopic approach based on telecentric lenses is introduced. The method offers in-focus imaging at high viewing angles (highly tilted object planes) with diminishing systematic image distortion. In single-view particle image velocimetry (PIV) applications telecentric lenses can be used to eliminate projection errors. Normal and oblique viewing are tested using a commercial telecentric lens with particle image velocimetry. 相似文献
7.
Results are presented from an experimental investigation into the interaction of a planar shock wave with a vortex ring. A
free-falling spherical soap bubble is traversed by the incident shock wave and develops into a vortex ring as a result of
baroclinically deposited vorticity (?r×?p 1 0{\nabla\rho\times\nabla p \neq 0}). The vortex ring translates with a velocity relative to the particle velocity behind the shock wave due to circulation.
After the shock wave reflects from the tube end wall, it traverses the vortex ring (this process is called “reshock”) and
deposits additional vorticity. Planar Mie scattering is used to visualize the atomized soap film at high frame rates (up to
10,000 fps). Particle image velocimetry (PIV) was performed for an argon bubble in nitrogen accelerated by a M = 1.35 shock wave. Circulation was determined from the PIV velocity field and found to agree well with Kelvin’s vortex ring
model. 相似文献
8.
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
9.
10.
The present paper presents time-resolved volumetric Particle Tracking Velocimetry measurements in a water towing tank on a
SD7003 airfoil, performed at a Reynolds number of 60,000 and a 4° angle of attack. The SD7003 airfoil was chosen because of
its long mid-chord and stable laminar separation bubble (LSB), occurring on the suction side of the airfoil at low Reynolds
numbers. The present study focuses on the temporal resolution of unsteady large-scale vortex structures emitted from the LSB.
In contrast to other studies, where only the observation of the flow in the transition region was examined, the entire flow
from the leading edge to the far wake of the airfoil was investigated here. 相似文献
11.
An optical method of altimetric imaging velocimetry (AIV) for measuring the slope of the surface elevation in the rotating
fluid with free surface is described. This method allows one to obtain the major dynamical fields in the fluid layer including
velocity, vorticity and surface elevation. When used in combination with the Optical Thickness method the AIV can be used
to render the full dynamical characteristics of a two-layer flow. Both methods allow one to achieve very high spatial resolution
by rendering a velocity vector in each pixel of the image. An example of the two-layer source-driven flow on a γ-plane (also
called polar β-plane) is offered to demonstrate the application of these methods. This “β-plume’ is a gyre-like response to
a point source of fluid, including intense jets, eddies and Rossby waves. 相似文献
12.
An explicit solution of two-dimensional Gaussian regression for the estimation of particle displacement from the correlation function in particle image velocimetry (PIV) or particle position from the images in particle tracking velocimetry (PTV) with sub-pixel accuracy is introduced. The accuracy and the ability of the methods to avoid pixel locking due to non-axially orientated, elliptically shaped particle images or correlation peaks are investigated using simulated and experimentally obtained images. 相似文献
13.
An experimental study of a turbulent backward-facing step (BFS) was undertaken to investigate the vortex structures behind the step. Attention was given to the secondary vortex because of its poor representation in literature and its potential for evaluating computational turbulence models. A 2D, cross-correlation particle image velocimeter (PIV) was developed, which allowed measurement of the highly turbulent, reversing step flow. Global, high resolution data was obtained for the cross-sectional plane of the BFS and for several other planes parallel to it. Measurement planes across the step revealed the 3D nature of the secondary vortex and an unexpected flow structure was identified. The secondary vortex was found to traverse across the flow, from the cross-sectional plane towards the step edge–sidewall corner.List of symbols AR aspect ratio - d particle displacement (m) - d error in particle displacement (m) - D expansion channel height (mm) - D0 inlet channel height (mm) - ER expansion ratio - H step height (mm) - N number of samples - ReH Reynolds number based on step height - Sp(x,y) centre coordinates of primary vortex (mm) - Ss(x,y) centre coordinates of secondary vortex (mm) - t laser pulse separation time (s) - t error in pulse separation time (s) - U horizontal velocity (m/s) -
̄ mean horizontal velocity (m/s) -
horizontal velocity variance (m2/s2) -
inlet centreline mean velocity (m/s) -
inlet centreline velocity variance (m2/s2) - V vertical velocity (m/s) - VM velocity magnitude (m/s) - VM error in velocity magnitude (m/s) - W step width (mm) - x length dimension (mm) - y height dimension (mm) - z width dimension (mm) - Xr shear layer reattachment point (mm) - Xr reattachment point for infinite step width (mm) - Xs secondary vortex separation point (mm) - Ys secondary vortex reattachment point (mm) - U velocity error (m/s) -
mean velocity error estimate (m/s) -
velocity variance error estimate (m2/s2) - bot bottom inlet boundary layer thickness (mm) - top top inlet boundary layer thickness (mm) - 99 0.99
boundary layer thickness (mm) 相似文献
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17.
A first-order accurate method of extending the capability of image velocimetry to interfaces is presented. In this method,
the image fields are locally extended across interfaces using fields from the other image of an image pair. During this image
parity exchange, the extension of the image fields amounts to locally reversing and reflecting the relative velocity field
across the interface. Numerous experimental examples are given to demonstrate and validate the accuracy of the method. These
are the plane Couette flow and the laminar pipe flow demonstrating straight rigid boundaries; uniform flow past a sphere and
a sphere moving in a stagnant fluid demonstrating curved rigid surfaces; and a free-surface flow and a liquid–liquid interface
flow demonstrating compliant interfaces.
Received: 3 November 1998/Accepted: 18 August 1999 相似文献
18.
G. Blois G. H. Sambrook Smith J. L. Best R. J. Hardy J. R. Lead 《Experiments in fluids》2012,53(1):51-76
This paper presents results of an experimental study investigating the mean and temporal evolution of flow within the pore space of a packed bed overlain by a free-surface flow. Data were collected by an endoscopic PIV (EPIV) technique. EPIV allows the instantaneous velocity field within the pore space to be quantified at a high spatio-temporal resolution, thus permitting investigation of the structure of turbulent subsurface flow produced by a high Reynolds number freestream flow (Re s in the range 9.8?×?103?C9.7?×?104). Evolution of coherent flow structures within the pore space is shown to be driven by jet flow, with the interaction of this jet with the pore flow generating distinct coherent flow structures. The effects of freestream water depth, Reynolds and Froude numbers are investigated. 相似文献
19.
A digital in-line holographic particle tracking velocimetry (HPTV) system was developed to measure 3D (three-dimensional)
velocity fields of turbulent flows. The digital HPTV (DHPTV) procedure consists of four steps: recording, numerical reconstruction,
particle extraction and velocity extraction. In the recording step, a digital CCD camera was used as a recording device. Holograms
contained many unwanted images or noise. To get clean holograms, digital image processing techniques were adopted. In the
velocity extraction routine, we improved the HPTV algorithm to extract 3D displacement information of tracer particles. In
general, the results obtained using HPTV were not fully acceptable due to technical limitations such as low spatial resolution,
small volume size, and low numerical aperture (NA). The problems of spatial resolution and NA are closely related with a recording
device. As one experimental parameter that can be optimized, we focused on the particle number density. Variation of the reconstruction
efficiency and recovery ratio were compared quantitatively with varying particle number density to check performance of the
developed in-line DHPTV system. The reconstruction efficiency represented the particle number distribution acquired through
the numerical reconstruction procedure. In addition the recovery ratio showed the performance of 3D PTV algorithm employed
for DHPTV measurements. The particle number density in the range of C
o = 13–17 particles/mm3 was found to be optimum for the DHPTV system tested in this study. 相似文献
20.
A series of numerical simulations were conducted to investigate the performance of two particle center estimation algorithms
for Particle Tracking Velocimetry: a simple three-point Gaussian estimator and a least-square Gaussian. The smallest position
error for images with reasonable noise levels was found to be approximately 0.03 pixels for both estimators using particles
with diameters of 4 pixels. As both estimators performed equally well, use of the simple three-point Gaussian algorithm is
recommended because it executes 100 times faster than the least-square algorithm. The maximum achievable measurement density
and accuracy for the three-point Gaussian estimator were determined with a numerical simulation of an Oseen vortex. Uncertainty
measures have been introduced to filter out unreliable displacement measurements. It was found that 4 to 5 velocity vectors
could be obtained within a 32 × 32 pixel area with an average displacement error of 0.1 pixels. This doubles the spatial resolution
of conventional cross-correlation based Particle Image Velocimetry at comparable accuracy.
Received: 26 June 1998/Accepted: 12 October 1999 相似文献