This paper describes the principles of a novel 3D PIV system based on the illumination, recording and reconstruction of tracer particles within a 3D measurement volume The technique makes use of several simultaneous views of the illuminated particles and their 3D reconstruction as a light intensity distribution by means of optical tomography The technique is therefore referred to as tomographic particle image velocimetry (tomographic-PIV) The reconstruction is performed with the MART algorithm, yielding a 3D array of light intensity discretized over voxels The reconstructed tomogram pair is then analyzed by means of 3D cross-correlation with an iterative multigrid volume deformation technique, returning the three-component velocity vector distribution over the measurement volume The principles and details of the tomographic algorithm are discussed and a parametric study is carried out by means of a computer-simulated tomographic-PIV procedure The study focuses on the accuracy of the light intensity field reconstruction process The simulation also identifies the most important parameters governing the experimental method and the tomographic algorithm parameters, showing their effect on the reconstruction accuracy A computer simulated experiment of a 3D particle motion field describing a vortex ring demonstrates the capability and potential of the proposed system with four cameras The capability of the technique in real experimental conditions is assessed with the measurement of the turbulent flow in the near wake of a circular cylinder at Reynolds 2,700

Tomographic particle image velocimetry

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Digital holography is an emerging field of new paradigm in general imaging applications. We present a review of a subset of the research and development activities in digital holography, with emphasis on microscopy techniques and applications. First, the basic results from the general theory of holography, based on the scalar diffraction theory, are summarized, and a general description of the digital holographic microscopy process is given, including quantitative phase microscopy. Several numerical diffraction methods are described and compared, and a number of representative configurations used in digital holography are described, including off-axis Fresnel, Fourier, image plane, in-line, Gabor, and phase-shifting digital holographies. Then we survey numerical techniques that give rise to unique capabilities of digital holography, including suppression of dc and twin image terms, pixel resolution control, optical phase unwrapping, aberration compensation, and others. A survey is also given of representative application areas, including biomedical microscopy, particle field holography, micrometrology, and holographic tomography, as well as some of the special techniques, such as holography of total internal reflection, optical scanning holography, digital interference holography, and heterodyne holography. The review is intended for students and new researchers interested in developing new techniques and exploring new applications of digital holography.

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Principles and techniques of digital holographic microscopy

The reliable measurement of the 3D3C velocity field in microfluidic devices becomes more and more important for future optimization and developments for lab-on-a-chip applications or point-of-care medical diagnosis systems. In the past years, different particle-based imaging methods, such as confocal scanning microscopy, holography, stereoscopic and tomographic imaging or approaches based on defocused particle images or optical aberrations have been developed and applied successfully to measure velocity fields in microfluidic systems. The benefits and drawbacks of these methods will be discussed in detail as the proper understanding of the measurement principle is essential to select the most appropriate technique for a desired measurement application. Once an imaging method is chosen, the velocity can be estimated by correlation-based methods or tracking approaches. The advantages and disadvantages of both methods and the importance of image preprocessing will also be discussed in detail.

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Particle imaging techniques for volumetric three-component (3D3C) velocity measurements in microfluidics

Better understanding of particle-particle and particle-fluid interactions requires accurate 3D measurements of particle distributions and motions. We introduce the application of in-line digital holographic microscopy as a viable tool for measuring distributions of dense micrometer (3.2 microm) and submicrometer (0.75 microm) particles in a liquid solution with large depths of 1-10 mm. By recording a magnified hologram, we obtain a depth of field of approximately 1000 times the object diameter and a reduced depth of focus of approximately 10 particle diameters, both representing substantial improvements compared to a conventional microscope and in-line holography. Quantitative information on depth of field, depth of focus, and axial resolution is provided. We demonstrate that digital holographic microscopy can resolve the locations of several thousand particles and can measure their motions and trajectories using cinematographic holography. A sample trajectory and detailed morphological information of a free-swimming copepod nauplius are presented.

Digital holographic microscope for measuring three-dimensional particle distributions and motions

Holographic particle image velocimetry (HPIV) offers potentially the best solution to volumetric measurements of the three-dimensional velocity fields in complex flows. However, the traditional film-based HPIV measurement is rather cumbersome, limiting its use to only a handful of groups worldwide. The newly emerged digital HPIV revolutionizes flow measurement science by providing a practical 3D velocimetry tool. It commands simple hardware that is similar to regular two-dimensional particle image velocimetry (PIV), yet it provides continuous (time-series) three-dimensional, three-component flow field data. Not only is the need for chemical processing eliminated, but also the cumbersome optical reconstruction is completely replaced by numerical reconstruction algorithms. Several breakthroughs have led to the development of the first practical and integrated digital HPIV systems. To explain the transition from film to digital recording, fundamental issues in HPIV are reviewed in this paper. Axial accuracy in HPIV measurement is ultimately limited by an inherent depth-of-focus problem, while information capacity is limited by inherent speckle noise. Information capacity is an important concept in HPIV, comprising the maximum acceptable seeding density multiplied by the sample volume depth along the optic axis. Both the axial accuracy and the information capacity are limited by the effective hologram aperture. The pursuit of a large hologram aperture in the past has resulted in further complexity in film-based HPIV systems. Digital HPIV, on the other hand, enjoys great simplicity of implementation and operation. A digital HPIV is also far more compact and rugged compared to existing film-based HPIV systems, making it suitable for duplication and commercialization. However, since digital sensors suffer from inferior pixel resolutions compared to films, the effective hologram aperture is much smaller in digital HPIV than that achievable in film-based HPIV. Alleviating this problem, digital HPIV also presents new possibilities in data processing such as the use of the complex amplitude of the reconstructed light wave to improve depth sensitivity and signal-to-noise ratio. Two examples of digital HPIV systems and measurement results are given. We believe digital HPIV can revitalize holographic particle imaging and bring it into the mainstream in much the same way that digital PIV brought PIV into widespread use a decade ago.

http://iopscience.iop.org/article/10.1088/0957-0233/15/4/009/pdf

Holographic particle image velocimetry: from film to digital recording

Digital holography appears to be a strong contender as the next-generation technology for holographic diagnostics of particle fields and holographic particle image velocimetry for flow field measurement. With the digital holographic approach, holograms are directly recorded by a digital camera and reconstructed numerically. This not only eliminates wet chemical processing and mechanical scanning, but also enables the use of complex amplitude information inaccessible by optical reconstruction, thereby allowing flexible reconstruction algorithms to achieve optimization of specific information. However, owing to the inherently low pixel resolution of solid-state imaging sensors, digital holography gives poor depth resolution for images, a problem that severely impairs the usefulness of digital holography especially in densely populated particle fields. This paper describes a technique that significantly improves particle axial-location accuracy by exploring the reconstructed complex amplitude information, compared with other numerical reconstruction schemes that merely mimic traditional optical reconstruction. This novel method allows accurate extraction of particle locations from forward-scattering particle holograms even at high particle loadings.

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Digital holography of particle fields: reconstruction by use of complex amplitude.

Digital holography of particle fields: Reconstruction by use of complex amplitude

Turbulent scalar mixing in the near field of a tee mixer (transverse jet in a pipe) was studied using particle image velocimetry and planar laser induced fluorescence. Two jet-to-pipe velocity ratios r = 3.06 and r = 5.04 were investigated for a turbulent pipe flow at a Reynolds number of 20,850. Laser measurement techniques are described, typical flow structures in the near field of tee mixer are examined, and turbulence statistics and scalar statistics are calculated. The general behavior of mixing is analyzed from the jet expansion and the concentration decay along the jet centerline, and the flow condition for jet impingement on the pipe wall is estimated. The mixing mechanism in the near field is revealed from the variation of the scalar PDF across the jet. The mixing on the upstream side of the jet is dominated by large-scale structures and on the downstream side by small eddies and turbulent diffusion, where the β-PDF is an excellent approximation.

Experimental study of turbulent mixing in a tee mixer using PIV and PLIF

To apply digital holography to the measurement of three-dimensional dense particle fields in large facilities, we have developed a hybrid digital holographic particle-imaging system. The technique combines the advantages of off-axis (side) scattering in suppressing speckle noise and on-axis (in-line) recording in lowering the digital sensor resolution requirement. A camera lens is attached to the digital sensor to compensate for the weak object wave from side scattering over a large recording distance. A simple numerical reconstruction algorithm is developed for holograms recorded with a lens without requiring complex and impractical mathematical corrections. We analyze the effect of image sensor resolution and off-axis angle on system performance and quantify the particle positioning accuracy of the system. The holographic system is successfully applied to the study of inertial particle clustering in isotropic turbulence.

Gang Pan

Papers

Holographic particle image velocimetry: from film to digital recording

Digital holography of particle fields: reconstruction by use of complex amplitude.

Digital holography of particle fields: Reconstruction by use of complex amplitude

Experimental study of turbulent mixing in a tee mixer using PIV and PLIF

Hybrid digital holographic imaging system for three-dimensional dense particle field measurement.