Particle image velocimetry with optical flow

Quénot, Georges; Pakleza, J.; Kowalewski, T. A.

doi:10.1007/s003480050222

Cited by 172 publications

(99 citation statements)

References 14 publications

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“…For this purpose, the colour images of TLC tracers are transformed to B&W intensity images. The magnitude and direction of the velocity vectors are determined using the recently developed evaluation technique based on the Optical Flow approach [14].…”

Section: Velocity and Temperature Measurementsmentioning

confidence: 99%

Phase change problems with free convection: fixed grid numerical simulation

1999

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Abstract.A numerical and experimental study of unsteady natural convection during freezing of water is presented. The mathematical model for the numerical simulations is based on the enthalpy-porosity method in vorticity-velocity formulation, equations are discretised on a fixed grid by means of a finite volume technique. A fully implicit method has been adopted for the mass and momentum equations. Experiments are performed for water in a differentially heated cube surrounded by air. The experimental data for natural convection with freezing in the cavity are collected to create a reference for comparison with numerical results. The method of simultaneous measurement of the flow and temperature fields using liquid crystal tracers is used. It allows us to collect transient data on the interface position, and the temperature and velocity fields. In order to improve the capability of the numerical method to predict experimental results, a conjugate heat transfer problem was solved, with finite thickness and internal heat conductivity of the non-isothermal walls. These results have been compared with the simulations obtained for the idealised case of perfectly adiabatic side walls, and with our experimental findings. Results obtained for the improved numerical model shown a very good agreement with the experimental data only for pure convection and initial time of freezing process. As time passes the discrepancies between numerical predictions and the experiment became more significant, suggesting a necessity for further improvements of the physical model used for freezing water.

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Section: Velocity and Temperature Measurementsmentioning

confidence: 99%

Phase change problems with free convection: fixed grid numerical simulation

1999

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“…Several methods of performing velocimetry have been developed and optimized. The orthogonal dynamic programming (ODP) 7 method is applied here as it has good spatial resolution, accuracy, and is applicable to images of amorphous structures, such as "smoke" or "bubble" images in fluid applications, 13 or turbulent eddies in the case of plasma measurements. Other techniques have been developed and applied to particle-imaging velocimetry (PIV) such as spatial cross-correlation methods.…”

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confidence: 99%

Turbulence velocimetry of density fluctuation imaging data

McKee

Fonck

Gupta

et al. 2004

Review of Scientific Instruments

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Analysis techniques to measure the time-resolved flow field of turbulence are developed and applied to images of density fluctuations obtained with the beam emission spectroscopy diagnostic system on the DIII-D tokamak. Velocimetry applications include measurement of turbulent particle flux, zonal flows, and the Reynolds stress. The flow field of turbulent eddies exhibits quasisteady poloidal flows as well as high-frequency radial and poloidal motion associated with electrostatic potential fluctuations and strongly nonlinear multifield interactions. The orthogonal dynamic programming technique, developed for fluid-based particle and amorphous shape (smoke) flow analysis, is investigated to measure such turbulence flows. Sensitivity and accuracy are assessed and sample results discussed. The equilibrium and fluctuating velocity of turbulent eddies in a magnetized plasma is a fundamental quantity characterizing the underlying turbulent-driven density fluctuations. The advent of multipoint, high-time-resolution, density fluctuation diagnostics and their turbulence imaging capability makes it feasible to directly measure such velocities and derived quantities that include the turbulent-driven particle transport, zonal flows, Reynolds Stress, and perhaps the vorticity of the turbulent fluctuating field. Several turbulence imaging diagnostics for magnetically confined plasmas have been or are being developed, including beam emission spectroscopy (BES), 1-3 gas puff imaging, 4,5 and the microwave reflectometer imaging array. 6 Methods of image-based velocimetry have been developed and utilized extensively in fluid dynamics, and application of such techniques to plasma fluctuation imaging data can provide deeper insight into turbulence phenomenon. Here, a particular velocimetry technique, orthogonal dynamic programming (ODP) 7 is applied to beam emission spectroscopy data to ascertain the high-frequency motion of turbulent eddies, which are themselves constantly moving and morphing in the presence of the turbulent flow field. The eddy motion should itself result from fluctuations in the (radial and poloidal) ExB (electric cross magnetic field) fluctuations and therefore the underlying but unseen electrostatic fluctuations.One-dimensional velocity fluctuations in the poloidal direction have been obtained using wavelet and other timedelay-estimation methods. 8 These measurements exhibited clear signatures of zonal flows, 9,10 coherent, radially localized and poloidally extended electrostatic potential structures. Here, the fluctuating velocity measurement method is extended to two dimensions.Velocimetry techniques are applied to two-dimensional (2D) measurements of density fluctuations obtained with BES 11 at the DIII-D tokamak. 12 BES provides localized measurements of long-wavelength ͑k Ќ 1 Ͻ 1͒ density fluctuations by observing collisionally induced fluorescence of the heating neutral beams as beam atoms interact with the background plasma. Thirty two available spatial channels have been configured to obtain imaging da...

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“…A second caveat is that while optical flow and orthogonal dynamic programming techniques are gaining in popularity, 16,28 we have not formally considered them here. Use of these techniques could shed some more light on, for example, how blob velocity scales with velocity and shape.…”

Section: B Remaining Issues With the Analysis Techniques Of Experimementioning

confidence: 99%

“…Some measure speeds in one spatial dimension, others in two spatial dimensions, depending on whether the data sets are arrays of one spatial dimension versus 2D images. Some of the methods include optical flow or pattern tracking techniques which find local velocity fields which map one image to the next, 16,27,28 time-delay estimation (TDE) with either wavelets and cross correlations, 19,23 and direct Fourier analysis (FA) which finds a wavenumber-frequency spectrum and thus a phase velocity. 15,29,30 We will not attempt to treat the exhaustive list of techniques but rather focus on specific implementations (codes) whose results have been examined in detail and have been used in the literature recently for C-Mod data.…”

Section: Introductionmentioning

confidence: 99%

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

Sierchio

Cziegler²,

Terry

et al. 2016

Review of Scientific Instruments

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Recent analysis of Gas Puff Imaging (GPI) data from Alcator C-Mod found blob velocities with a modified tracking time delay estimation (TDE). These results disagree with velocity analysis performed using direct Fourier methods. In this paper, the two analysis methods are compared. The implementations of these methods are explained, and direct comparisons using the same GPI data sets are presented to highlight the discrepancies in measured velocities. In order to understand the discrepancies, we present a code that generates synthetic sequences of images that mimic features of the experimental GPI images, with user-specified input values for structure (blob) size and velocity. This allows quantitative comparison of the TDE and Fourier analysis methods, which reveals their strengths and weaknesses. We found that the methods agree for structures of any size as long as all structures move at the same velocity and disagree when there is significant nonlinear dispersion or when structures appear to move in opposite directions. Direct Fourier methods used to extract poloidal velocities give incorrect results when there is a significant radial velocity component and are subject to the barber pole effect. Tracking TDE techniques give incorrect velocity measurements when there are features moving at significantly different speeds or in different directions within the same field of view. Finally, we discuss the limitations and appropriate use of each of methods and applications to the relationship between blob size and velocity. C 2016 AIP Publishing LLC. [http://dx

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Particle image velocimetry with optical flow

Cited by 172 publications

References 14 publications

Phase change problems with free convection: fixed grid numerical simulation

Phase change problems with free convection: fixed grid numerical simulation

Turbulence velocimetry of density fluctuation imaging data

Comparison of velocimetry techniques for turbulent structures in gas-puff imaging data

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