Digital Holographic Particle Tracking Velocimetry Research Articles

Snapshot Space–Time Holographic 3D Particle Tracking Velocimetry

AbstractDigital inline holography is an amazingly simple and effective approach for 3D imaging, to which particle tracking velocimetry is of particular interest. Conventional digital holographic particle tracking velocimetry techniques are computationally separated in particle and flow reconstruction, plus the expensive computations. Usually, the particle volumes are recovered first, from which fluid flows are computed. Without iterative reconstructions, This sequential space–time process lacks accuracy. This paper presents a joint optimization framework for digital holographic particle tracking velocimetry: particle volumes and fluid flows are reconstructed jointly in a higher space–time dimension, enabling faster convergence and better reconstruction quality of both fluid flow and particle volumes within a few minutes on modern GPUs. Synthetic and experimental results are presented to show the efficiency of the proposed technique.

Quantifying bubble size and 3D velocity in a vortex with digital holographic particle tracking velocimetry (DHPTV)

The 3D characterization of bubbly flow is of wide application in engineering. A double pulse/double frame digital holographic particle tracking velocimetry (DHPTV) system is applied to measure the bubbles in an electrolytic cell. It is found that the actual bubble depth position should be corrected from the reconstructed position by multiplying the refractive index of the surrounding medium. Spherical bubbles and other irregular particles can be discriminated from their shapes. The three-dimensional position, particle size distribution, three-dimensional velocity of over 60,000 bubbles with size ranging from 25 μ m to 400 μ m are detected and analyzed with DHPTV, and statistics on bubble velocities in the y direction reveal bubble acceleration of 6.8 mm/s2.

Experimental investigation of turbulent flow over surfaces of rigid and flexible roughness

This study presents a comparative experiment to investigate turbulent channel flows over surfaces of rigid and flexible roughness. Using digital inline holographic particle tracking velocimetry in a refractive-index-matched flow channel, the turbulent flow and roughness deformation are measured simultaneously. The mean velocity profiles of the two cases match closely, but the flow over flexible roughness shows a substantial reduction of Reynolds stresses with a corresponding decrease of coherence in roughness-induced flow structures. For flexible roughness, the streamwise roughness deformation exhibits higher energy levels at lower frequencies where the fluid velocity spectra peak, while the spanwise deformation shows a more prominent peak at the natural frequency of the roughness elements. Combining the measurements of turbulent flow and roughness kinematics, we show a close match of the strain energy of the flexible roughness with the deficit of turbulent kinetic energy (TKE) due to roughness compliance, and a connection between large roughness deformation and elevated levels of Reynolds stresses. Overall, these analyses demonstrate the anisotropic process of turbulence-roughness interaction, in which the TKE is dampened and primarily converted into streamwise deformation of roughness, and is partially returned to the smaller scale turbulence through spanwise oscillation of roughness at its natural frequency.

Adaptation of reference volumes for correlation-based digital holographic particle tracking

Numerically reconstructed reference volumes tailored to particle images are used for particle position detection by means of three-dimensional correlation. After a first tracking of these positions, the experimentally recorded particle images are retrieved as a posteriori knowledge about the particle images in the system. This knowledge is used for a further refinement of the detected positions. A transparent description of the individual algorithm steps including the results retrieved with experimental data complete the paper. The work employs extraordinarily small particles, smaller than the pixel pitch of the camera sensor. It is the first approach known to the authors that combines numerical knowledge about particle images and particle images retrieved from the experimental system to an iterative particle tracking approach for digital holographic particle tracking velocimetry.

Three-dimensional swimming motility of microorganism in the near-wall region

Microbial organisms are easily observed in geometrically confined environments. The swimming characteristics of these microorganisms are largely influenced by the presence of a solid surface. Their swimming behavior in the near-wall region shows different physical motilities. In this study, digital in-line holographic particle tracking velocimetry technique is used to investigate the three-dimensional (3D) motile characteristics of Prorocentrum minimum, especially in the near-wall region. The effects of the interaction between the microorganism and a solid wall on the 3D swimming characteristics, such as helix parameters, orientation, and attraction to the wall, are experimentally analyzed. As a result, swimming microorganisms are observed to have high motility and thrust generation near the wall, compared with the unrestrained free-swimming ones. In addition, the swimming direction tends to become parallel to the wall and they concentrate near the solid surface.

High fidelity digital inline holographic method for 3D flow measurements.

Among all the 3D optical flow diagnostic techniques, digital inline holographic particle tracking velocimetry (DIH-PTV) provides the highest spatial resolution with low cost, simple and compact optical setups. Despite these advantages, DIH-PTV suffers from major limitations including poor longitudinal resolution, human intervention (i.e. requirement for manually determined tuning parameters during tracer field reconstruction and extraction), limited tracer concentration, and expensive computations. These limitations prevent this technique from being widely used for high resolution 3D flow measurements. In this study, we present a novel holographic particle extraction method with the goal of overcoming all the major limitations of DIH-PTV. The proposed method consists of multiple steps involving 3D deconvolution, automatic signal-to-noise ratio enhancement and thresholding, and inverse iterative particle extraction. The entire method is implemented using GPU-based algorithm to increase the computational speed significantly. Validated with synthetic particle holograms, the proposed method can achieve particle extraction rate above 95% with fake particles less than 3% and maximum position error below 1.6 particle diameter for holograms with particle concentration above 3000 particles/mm3. The applicability of the proposed method for DIH-PTV has been further validated using the experiment of laminar flow in a microchannel and the synthetic tracer flow fields generated using a DNS turbulent channel flow database. Such improvements will substantially enhance the implementation of DIH-PTV for 3D flow measurements and enable the potential commercialization of this technique.

Simultaneous measurement of 3D velocity and 2D rotation of irregular particle with digital holographic particle tracking velocimetry

The dispersed irregular particle can have a complicated both translational and rotational motions in multiphase flows, and the experimental evaluation of the interactions of a particle with its surrounding relies on the quantitative measurement methods. Digital inline holographic particle tracking velocimetry (DHPTV) is proposed to measure the projected morphology, 3D translational as well as 2D rotational (ωz) motion of an irregular particle simultaneously. In order to measure the particle rotation and translation in the double pulses/double frame DHPTV, the relation among the rotation speed, translational velocity and characteristic length of the particle is formulated. The rotation angle around z axis of the irregular particle in the two frames can be determined by pairing the feature points of the reconstructed in-focus particle images. The 2D rotations of an irregular coal particle in an acoustic levitator (20r/s to 500r/s) and nonspherical droplet ellipsoids (up to 200r/s) in a spray were measured. Results show that the recovered particle image from the second frame overlaps the original one in the first frame, evidencing the accurate particle rotation ωz measurements. This work demonstrates that DHPTV can measure the particle shape, 3D position and 3D velocity as well as 2D rotation simultaneously, which provides a tool to get a deeper understanding of particle dynamics in a multiphase flow.

Direct measurement of particle size and 3D velocity of a gas-solid pipe flow with digital holographic particle tracking velocimetry.

The 3D measurement of the particles in a gas-solid pipe flow is of great interest, but remains challenging due to curved pipe walls in various engineering applications. Because of the astigmatism induced by the pipe, concentric ellipse fringes in the hologram of spherical particles are observed in the experiments. With a theoretical analysis of the particle holography by an ABCD matrix, the in-focus particle image can be reconstructed by the modified convolution method and fractional Fourier transform. Thereafter, the particle size, 3D position, and velocity are simultaneously measured by digital holographic particle tracking velocimetry (DHPTV). The successful application of DHPTV to the particle size and 3D velocity measurement in a glass pipe's flow can facilitate its 3D diagnostics.

Characterization of atomization and breakup of acoustically levitated drops with digital holography

A digital holographic particle tracking velocimetry system is applied to quantitatively study the drop atomization induced by capillary waves, and the breakup caused by increased sound pressure levels. A wavelet-based algorithm is used for particle detection and autofocusing with a wide size range of 20μm-2mm. To eliminate the influence of large particles on small particles, a two-step detection method is adopted. Large drops are first characterized and simulated by a diffraction-based model. Then the contributions of the drops are subtracted from the original hologram followed by the detection of small droplets. Finally, the velocity and size distribution of the secondary droplets are obtained from the experimental holograms. The results demonstrate the validity of the digital in-line holographic technique for the atomization and breakup study of acoustically levitated drops.

High-accuracy measurement of depth-displacement using a focus function and its cross- correlation in holographic PTV.

We propose a method using a focus function and its cross-correlation to measure depth-position and precise depth-displacement. The focus function provides acceptable results in the determination of depth-position of a transparent particle, an opaque particle, and a red blood cell. However, positional errors and a short time interval can cause unreliable results in identifying depth-displacement (Δz) and depth-directional velocity in digital holographic particle tracking velocimetry (DHPTV). To minimize the errors in Δz, we propose a method that directly obtains depth displacement from the cross-correlation of focus values between consecutive holograms. The feasibility of this method is demonstrated by quantitatively visualizing a 3D flow using HPTV.

Wavelet-based depth-of-field extension, accurate autofocusing, and particle pairing for digital inline particle holography

Depth-of-field extension and accurate 3D position location are two important issues in digital holography for particle characterization and motion tracking. We propose a method of locating the axial positions of both opaque and transparent objects in the reconstructed 3D field in the wavelet domain. The spatial-frequency property of the reconstructed image is analyzed from the viewpoint of the point spread function of the digital inline holography. The reconstructed image is decomposed into high- and low-frequency subimages. By using the variance of the image gradient in the subimages as focus metrics, the depth-of-field of the synthesis image can be extended with all the particles focalized, and the focal plane of the object can be accurately determined. The method is validated by both simulated and experimental holograms of transparent spherical water droplets and opaque nonspherical coal particles. The extended-focus image is applied to the particle pairing in a digital holographic particle tracking velocimetry to obtain the 3D vector field.

Effect of ambient medium viscosity on the motility and flagella motion of Prorocentrum minimum (Dinophyceae)

The effect of ambient medium viscosity on the motility and flagellar beating motion of the dinoflagellate Prorocentrum minimum was experimentally investigated. Prorocentrum minimum is a thecate desmokont dinoflagellate with two flagella that emerge from the flagellar pore at the anterior end of the cell. A digital holographic particle tracking velocimetry technique was employed to capture the swimming trajectories of hundreds of P. minimum cells and to extract the kinematic motion parameters. The viscosity of the surrounding medium was changed from 1.12 cP (natural sea water at 228C) to 11.31 cP by adding methylcellulose to increase fluid viscosity. In natural sea water, the swimming speeds of P. minimum cells (n 1⁄4 77) in a helical motion ranged from 5.4 to 138.4 mm/s with a mean speed of 51.3+27.9 mm/s. The helix radius and pitch of the swimming trajectories are 3.08+0.64 and 25.34+7.96 mm/s, respectively. The longitudinal flagellum beats with a planar wave at a beating frequency of 87.10+ 10.96 Hz. On the other hand, the transverse flagellum beats with a helical wave at a beating frequency of 45.38+13.61 Hz. As the ambient medium viscosity increases, the beating frequency of flagella decreased and consequently, the swimming speed of P. minimum was reduced.

Measurement of microchannel flow with digital holographic microscopy by integrated nearest neighbor and cross-correlation particle pairing

A micro digital in-line holographic particle tracking velocimetry (micro-DHPTV) system has been developed and applied to investigate the three-dimensional flow field in straight and Y-junction microchannels. The micro-DHPTV system comprises a cooled frame-transfer CCD camera and a double-pulsed laser. The processing algorithm introduced to evaluate the three-dimensional velocity is based on the combination of integrated cross-correlation and nearest neighbor matching algorithms, taking advantage of information from both the reconstructed particle field and the original holograms fringes patterns. Tests on simulated pairs of holograms show that the particles can be detected, located, and paired with high probability and accuracy. Results obtained in the straight and Y-junction microchannels show that the superimposed vector field is physically reasonable.

Detection of microbubble position by a digital hologram

This paper reports on a new technique of measurements of microbubble position in three dimensions with high time-resolution. The technique is based on micro digital holographic particle tracking velocimetry. In this technique, an intensity profile is constructed from a holographic image of a microbubble where the profile results in showing two peaks. The distance between the two peaks appears to relate to the size of the microbubble's diameter. The three-dimensional position of the bubble can be detected by the center of the two peaks and the center point of the bubble image focused by a digital hologram. We also theoretically obtained the intensity profile of a microbubble by considering a refraction of light on a bubble surface to a ring-shaped aperture model. The theoretically obtained distance between the two peaks is found to be in good agreement with the values obtained experimentally.

Advances in digital holographic micro-PTV for analyzing microscale flows

The accurate three-dimensional (3D) velocity field measurement technique has been receiving large attention in the study of microfluidics. DHM-PTV technique was developed by combining the digital holographic microscopy and particle tracking velocimetry technique. DHM-PTV is an ideal method for measuring three-component-three-dimensional (3C-3D) velocity field in a microscale flow with a fairly good spatial resolution. The advances in the DHM-PTV technique enable the measurement of various microscale flows, such as transport of red blood cells in a microtube and 3D flows in microfluidic devices. DHM-PTV is also applied in studying the motile behavior of swimming microorganisms. DHM-PTV would play an important role in ascertaining the undiscovered basic physics in various microscale and biofluid flow phenomena. In the current study, the basic principle of the DHM-PTV technique and its typical applications to microscale flows are introduced and discussed.

Three-dimensional simultaneous measurements of micro-fluorescent-particle position and temperature field via digital hologram

This paper reports on a new technology of simultaneous measurements of micro-fluorescent-particle position in 3D and of temperature field with high time-resolution. The technology to measure both velocity and temperature field is based on the simultaneity of micro digital holographic particle tracking velocimetry (micro-DHPTV) and laser-induced fluorescence (LIF).

Determination of the swimming trajectory and speed of chain-forming dinoflagellate Cochlodinium polykrikoides with digital holographic particle tracking velocimetry

The marine dinoflagellate Cochlodinium polykrikoides is a harmful and highly motile algal species. To distinguish between the motility characteristics of solitary and chain-forming cells, the swimming trajectories and speeds of solitary cells and 2- to 8-cell chains of C. polykrikoides were measured using a digital holographic particle tracking velocimetry (PTV) technique. C. polykrikoides cells exhibited helical swimming trajectories similar to other dinoflagellate species. The swimming speed increased as the number of cells in the chain increased, from an average of 391 μm s−1 (solitary cells) to 856 μm s−1 (8-cell chain). The helix radius R and pitch P also increased as the number of cells in the chain increased. R increased from 9.24 μm (solitary cell) to 20.3 μm (8-cell chain) and P increased from 107 μm (solitary cell) to 164 μm (8-cell chain). The free thrust-generating motion of the transverse flagella and large drag reduction in the chain-forming cells seemed to increase the swimming speed compared to solitary cells. The measured swimming speeds agreed with those from field observations. The superior motility of chain-forming C. polykrikoides cells may be an important factor for its bloom, in addition to the factors reported previously.

Special purpose computer system with highly parallel pipelines for flow visualization using holography technology

We have designed a PC cluster system with special purpose computer boards for visualization of fluid flow using digital holographic particle tracking velocimetry (DHPTV). In this board, there is a Field Programmable Gate Array (FPGA) chip in which is installed a pipeline for calculating the intensity of an object from a hologram by fast Fourier transform (FFT). This cluster system can create 1024 reconstructed images from a 1024 × 1024 -grid hologram in 0.77 s. It is expected that this system will contribute to the analysis of fluid flow using DHPTV.

Measurement of three-dimensional stress field of RP model having a notch by using digital holography

We measure three-dimensional stress field of RP model having a V-notch. RP model (Elastic modulues E = 3317 MPa, 9.3 × 7.7 × 50 mm 3 having a V-notch) is made of acrylic transparent resin and tracer particle (averaged diameter: 60μm) are dispersed. The model is subjected to the static load (100 N) at the middle. Firstly, three-dimensional deflection is measured by using digital holographic PTV (Particle Tracking Velocimetry). Finally, three-dimensional stress field of RP model is visualized as a differential value of the deflection field.

J0503-1-3 座屈する長柱のホログラフィックPTVによる観察([J0503-1]多次元計測技術の展開と応用(1))

The present study shows the observation of a buckling column by using digital holographic Particle-Tracking Velocimetry (PTV). The column is made of acrylic resin and tracer particles are uniformly dispersed in the column. The displacement of the column is obtained by each dispersed particle's positions between unloaded and loaded beam. Three-dimensional deflection of the particle is measured by digital holographic PTV. The column is installed in the water tank which is filled with a refractive-index matching liquid. Aqueous solution of Sodium iodide is used as a matching liquid. Experimental results on the three-dimensional deflection field and the comparison of present method and CAE analysis are presented.