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1.
Niels G. Deen Paul Willems Martin van Sint Annaland J. A. M. Kuipers Rob G. H. Lammertink Antoine J. B. Kemperman Matthias Wessling Walter G. J. van der Meer 《Experiments in fluids》2010,49(2):525-530
A novel image pre-processing scheme for PIV of single- and two-phase flows over reflecting objects which does not require
the use of additional hardware is discussed. The approach for single-phase flow consists of image normalization and intensity
stretching followed by background subtraction. For two-phase flow, an additional masking step is added after the background
subtraction. The effectiveness of the pre-processing scheme is shown for two examples: PIV of single-phase flow in spacer-filled
channels and two-phase flow in these channels. The pre-processing scheme increased the displacement peak detectability significantly
and produced high quality vector fields, without the use of additional hardware. 相似文献
2.
PIV for granular flows 总被引:4,自引:0,他引:4
Particle image velocimetry (PIV) has been adapted for use in measuring particle displacement and velocity fields in granular
flows. “Seeding” is achieved by using light and dark particles. The granular flow adjacent to a clear bounding wall is illuminated
with a strobe, and the recorded images are analyzed using standard PIV techniques. The application is demonstrated by measuring
convection rolls in a granular bed undergoing vertical oscillations. The PIV measured displacement is consistent with displacement
of a marked layer of particles.
Received: 29 January 1998/Accepted: 8 April 1999 相似文献
3.
A rotating mirror is widely used to generate the velocity shift that can resolve the directional ambiguities of PIV (particle
image velocimetry) measurements. The rotating mirror system inevitably creates the normal displacement of the resulting PIV
images and causes systematic image errors. Two corrective methods are proposed to eliminate or reduce the image biasing in
PIV system. The use of two linearly traversing mirrors, instead of a single rotating mirror, shows that image biasing can
be eliminated and the velocity shift well generated. As a second option, two co-rotating mirrors, instead of one, can reduce
the image biasing with a maximum velocity shift available. Detailed imaging kinematics of the two suggestive methods are presented
to lead to designing of practical devices that improve the PIV capabilities by reducing the systematic image errors.
The authors like to acknowledge the partial financial support provided from the Turbo and Power Machinery Research Center
(TPMRC) of Seoul National University, Seoul, Korea. 相似文献
4.
A technique is described for measuring the mean velocity gradient (rate-of-displacement) tensor by using a conventional stereoscopic particle image velocimetry (SPIV) system. Planar measurement of the mean vorticity vector, rate-of-rotation and rate-of-strain tensors and the production of turbulent kinetic energy can be accomplished. Parameters of the Q criterion and negative λ2 techniques used for vortex identification can be evaluated in the mean flow field. Experimental data obtained for a circular turbulent jet issuing normal to a crossflow in a low speed wind tunnel for a jet-to-crossflow velocity ratio of 3.3 are presented to show the applicability of the proposed technique. The results reveal the presence of a secondary counter-rotating vortex pair (SCVP) which is located within the jet core and has a sense of rotation opposite to that of the primary one (PCVP). Consistency of the measurements is verified by the agreement of data obtained in two perpendicular planes. Accuracy of the data is discussed and algebraic relations for some measurement uncertainties are presented. 相似文献
5.
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
6.
An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. Local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f+=0.44, 0.66 and 0.88) with a fixed forcing amplitude (A+=0.6) were employed at Re θ =690. The effect of three different forcing angles (α=60°, 90° and l20°) was investigated under a fixed forcing frequency (f+=0.088). The PIV results showed that the wall-region velocity decreases on imposition of the local forcing. Inspection of the phase-averaged velocity profiles revealed that spanwise large-scale vortices are generated downstream of the slot and persist farther downstream. The highest reduction in skin friction was achieved at the highest forcing frequency (f+=0.088) and a forcing angle of α=120°. The spatial fraction of the vortices was examined to analyze the skin friction reduction. 相似文献
7.
The two-dimensional space–time turbulence statistics of free shear jet flows in the form of the two-point velocity correlation tensor are important for aeroacoustic noise source modelling based on the acoustic analogy approach. This paper presents a direct application of the Point-Referenced Global Correlation (PRGC) technique to measure the components of this correlation tensor for a sector of the flow field in two jet configurations. The PRGC approach combines single point and global measurement techniques and enables two-point space–time correlations over a region of the flow to be obtained. The technique is applied to a single stream jet and a co-axial coplanar jet at a Mach number of 0.24 using commercial Laser Doppler velocimetry (LDV) and low-speed particle image velocimetry (PIV) systems. Results for the one-dimensional correlations are shown to compare well with two-point measurements. The results for the two-dimensional space–time correlations are presented and the characteristics for both configurations discussed. 相似文献
8.
The present analysis shows that the EVM can not reflect the turbulence physics in non-inertial frame. The effects of Coriolis
force on turbulence is embodied naturally in the Reynolds-stress transport equation. It is observed that the existing second-moment
closure model with appropriate near-wall treatment can adequately predict flows in rotating channel and in axially rotating
pipe for moderate rotation rate.
The project supported by the National Natural Science Foundation of China, State Education Commission and Tsinghua University 相似文献
9.
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
10.
Particle image velocimetry (PIV) was used to measure instantaneous and average particle velocity fields near the stagnation zone of a particle-laden impinging air jet. The results were compared with Lagrangian particle tracking measurements. Ensemble averages from the two methods agree well except in regions where particles have different histories, and a specific trajectory is dominant but not exclusive. The PIV autocorrelation method loses information regarding non-dominant particle trajectories. Thus, although instantaneous PIV measurements yield the dominant particle velocities correctly, the averaged measurements are biased in some regions.This work was supported by the Electric Power Research Institute under Contract RP 8034-01. We thank the 3M Corporation for their generous materials support. 相似文献
11.
We describe a non-intrusive PIV system developed for performing high-resolution measurements in a field of view of 2 m×3 m,
as required on the LEGI-Coriolis 13 m diameter rotating platform. Particle preparation, laser illumination, photographic digitization,
and cross-correlation analysis techniques are explained. Some results on the wake behind a cylinder illustrate the possibilities
of this PIV system.
Received: 29 October 1996/Accepted: 5 April 1997 相似文献
12.
The problems related with the development of secondary structures in time-dependent rotating flows are considered. The experimental results on the generation of these structures (inhomogeneities placed in series in a gas) which occur outside (inside) a cylindrical tube, when it starts (ends) suddenly to rotate, are presented. The qualitative explanation of these results is given on the basis of the ideas on the development of time-dependent viscous boundary layers, the condition of the stability of axisymmetric rotating flows obtained in the inviscid approximation, and the condition for the Görtler vortex generation. To obtain a more complete idea of the effect of secondary structure generation in time-dependent rotating flows two flows are numerically modeled on the basis of the complete Navier-Stokes equations. The first problem is that of rotating gas (air) flow in a round cylindrical tube of finite length with closed ends when it starts or ends suddenly to rotate. The second situation concerns with the generation of inside or outside (with respect to the tube surface) time-dependent vortex structures, when a tube with one closed end starts or ends suddenly to rotate; a gas flows into the tube from an external finite volume through the central region of its second open end and flows out of the tube into the volume along the periphery of this section. The flow parameters are analyzed both qualitatively and quantitatively. 相似文献
13.
14.
M. V. Kalashnik 《Fluid Dynamics》2007,42(3):376-388
The stability of rotating horizontal-shear flows is investigated within the framework of the linear approximation. The shear flow perturbations are divided into three classes (symmetric and two- and three-dimensional) and sufficient conditions of stability are obtained for each class. The perturbation dynamics in a flow with constant horizontal shear are described and the algebraic instability of the flow with respect to three-dimensional perturbations is detected. It is shown that the symmetric perturbations may be localized (trapped) inside the shear layer. The problem of finding the growth rates and frequencies of the trapped waves is reduced to a quantum-mechanical Schrödinger equation. Exact solutions are obtained for a “triangular” jet and hyperbolic shear. 相似文献
15.
重点研究高速离心压气机叶轮与机匣间的间隙流动及其温度分布。研究将离心压气机简化为高速转动圆盘,搭建了相关实验平台,并开展了相应的数值模拟研究。通过改变转动圆盘的转速和轴向进入的冷却流的流量,研究了转速和流量对于间隙内温度和速度分布的影响。结果显示,转速是影响温度变化的最主要因素,转速越大,温度越高;同等幅度的流量变化对温度的影响则较小。研究发现,在实验和模拟对应的大雷诺数条件下,无量纲的速度分布基本不受到圆盘转速、冷却流量和温度场的影响。 相似文献
16.
Measurement of surface profile using digital image correlation 总被引:5,自引:0,他引:5
Presented in this paper is the development of a system for measuring surface profile using digital image correlation. All
needed equations for camera calibration and system profile measurements are shown. Equations included needed transformation
to account for subset perspective distortions. The system is capable of an accuracy of 1/20,000 of the camera-to-object distance.
Experimental results are shown for several cases, with results giving a maximum error of 0.05 mm with camera-to-object distance
of 1000 mm. 相似文献
17.
Alberto Di Sante Raf Theunissen René A. Van den Braembussche 《Experiments in fluids》2008,44(2):179-188
A new facility to measure the time evolution of 2D velocity fields in a rotating channel is presented, and the accuracy is
discussed in detail. Measurements are made by means of a time-resolved PIV system composed of a continuous laser diode, coupled
by a fiber optics cable to a laser plane optical module, and a CMOS high-speed camera. Both the PIV system and divergent channel
are fixed on a 2.5 m rotating disk. This allows a direct measurement of the relative velocity of flows with Reynolds numbers
between 3 × 103 and 3 × 104 and Rotation numbers between 0.0 and 0.52. These values correspond to the flow conditions in small radial impellers and can
be independently adjusted by a change of the relative flow velocity and RPM. It is shown that this new facility allows high
signal-to-noise ratios, and that the direct acquisition of the data in a rotating frame drastically reduces the measurement
error. The accuracy and high spatial and temporal resolution of the measurements allow a detailed analysis of boundary layer
characteristics in stationary and rotating conditions. 相似文献
18.
A novel PIV technique for measurements in multiphase flows and its application to two-phase bubbly flows 总被引:2,自引:0,他引:2
A new experimental procedure for performing simultaneous, phase-separated velocity measurements in two-phase flows is introduced. Basically, the novel particle image velocimetry (PIV) technique is a combination of the three most often used PIV techniques in multiphase flows: PIV with fluorescent tracer particles, shadowgraphy, and the digital phase separation with a masking technique. The combination of these three independent measurement techniques is achieved by shifting the background intensity of a PIV recording to a higher, but uniform gray value level. In order to combine the advantages of these multiphase-PIV methods, a new PIV set-up was developed. With this set-up the velocity distributions of the two phases are measured simultaneously with only one b/w camera. This experimental set-up is aimed at providing a means for characterizing the modification of turbulence in the liquid phase by bubbles. This phenomenon is often called "pseudo-turbulence". 相似文献
19.
20.
Andrew M. Walker Joel Scott David E. Rival Clifton R. Johnston 《Experiments in fluids》2014,55(10):1-16
Echo particle image velocimetry (Echo PIV) presents itself as an attractive in vivo flow quantification technique to traditional approaches. Promising results have been acquired; however, limited quantification and validation is available for post-stenotic flows. We focus here on the comprehensive evaluation of in vitro downstream stenotic flow quantified by Echo PIV and validated in relation to digital particle image velocimetry (DPIV). A Newtonian blood analog was circulated through a closed flow loop and quantified immediately downstream of a 50 % axisymmetric blockage at two Reynolds numbers (Re) using time-averaged Echo PIV and DPIV. Centerline velocities were in good agreement at all Re; however, Echo PIV measurements presented with elevated standard deviation (SD) at all measurements points. SD was improved using increased line density (LD); however, frame rate or field of view (FOV) is compromised. Radial velocity profiles showed close agreement with DPIV with the largest disparity in the shear layer and near-wall recirculation. Downstream recirculation zones were resolved by Echo PIV at both Re; however, magnitude and spatial coverage was reduced compared to DPIV that coincided with reduced contrast agent penetration beyond the shear layer. Our findings support the use of increased LD at a cost to FOV and highlight reduced microbubble penetration beyond the shear layer. High local SD at near-wall measurements suggests that further refinement is required before proceeding to in vivo quantification studies of wall shear stress in complex flow environments. 相似文献