Digital shearing method for three-dimensional data extraction in holographic particle image velocimetry

We report a new digital shearing method for extracting the three-dimensional displacement vector data from double-exposure holograms. With this method we can manipulate both the phase and the amplitude of the recorded signal, which, like optical correlation analysis, is inherently immune to imaging aberration. However, digital shearing is not a direct digital implementation of optical correlation, and a considerable saving in computation time results. We demonstrate the power of the method by MATLAB simulation and discuss its performance with reference to optical analysis.     

Optical alignment-induced errors in holographic particle image velocimetry

A ray-tracing analysis of point-source imaging in the presence of optical misalignment is used to analyze relative image shift as a source of measurement error in holographic particle image velocimetry (HPIV). Although single-reference-beam HPIV is relatively insensitive to optical misalignment, dual-reference-beam systems may suffer substantial errors because of misalignments of the order of microradians. These systems are particularly sensitive to rotations of the hologram about an axis perpendicular to the film and to reconstruction beam misalignment. In a swirling flow experiment, a proposed error-compensation scheme was able to reduce uncertainty from 130% to 10% of the mean measured velocity.     

Single beam two-views holographic particle image velocimetry

Holographic particle image velocimetry (HPIV) is presently the only method that can measure at high resolution all three components of the velocity in a finite volume. In systems that are based on recording one hologram, velocity components parallel to the hologram can be measured throughout the sample volume, but elongation of the particle traces in the depth direction severely limits the accuracy of the velocity component that is perpendicular to the hologram. Previous studies overcame this limitation by simultaneously recording two orthogonal holograms, which inherently required four windows and two recording systems. This paper introduces a technique that maintains the advantages of recording two orthogonal views, but requires only one window and one recording system. Furthermore, it enables a quadruple increase in the spatial resolution. This method is based on placing a mirror in the test section that reflects the object beam at an angle of 45 degrees. Particles located in the volume in which the incident and reflected beams from the mirror overlap are illuminated twice in perpendicular directions. Both views are recorded on the same hologram. Off-axis holography with conjugate reconstruction and high-pass filtering is used for recording and analyzing the holograms. Calibration tests show that two views reduce the uncertainty in the three-dimensional (3-D) coordinates of the particle centroids to within a few microns. The velocity is still determined plane-by-plane by use of two-dimensional particle image velocimetry procedures, but the images are filtered to trim the elongated traces based on the 3-D location of the particle. Consequently, the spatial resolution is quadrupled. Sample data containing more than 200 particles/mm3 are used for calculating the 3-D velocity distributions with interrogation volumes of 220 x 154 x 250 microm, and vector spacing of 110 x 77 x 250 microm. Uncertainty in velocity is addressed by examining how well the data satisfies the continuity equation. The results show significant improvements compared with previous procedures. Limitations of the technique are also discussed.     

Eliminating background noise effect in micro-resolution particle image velocimetry

A novel method is developed to improve the accuracy of micro-resolution particle image velocimetry (PIV) in microfluidics measurements. This method utilizes the Laplacian of Gaussian method and image-processing techniques to eliminate the background scattering noise. A high signal-to-noise ratio image has been obtained. This technique is especially suitable for improving micro-resolution PIV in micro, two-, or multiphase flow conditions, such as for submicron bubbly flow measurements in a microchannel. The method can easily be implemented with minimal modification of the conventional PlV system. The results of simulation and experiments demonstrated the feasibility of this, to our knowledge, new method.     

In-line holographic particle image velocimetry for turbulent flows

A holographic system has been developed to measure the velocity field in three-dimensional flow regions. The system records the position of small tracer particles on two in-line holograms of the flow obtained simultaneously. Two exposures are recorded on each hologram. The flow velocity is derived from the displacement of the particles between exposures. A general design procedure is described for selecting the particle diameter and the concentration on the basis of the configuration of the flow facility and the resolution characteristics of the holographic imaging system. The system was implemented in a 2 ft x 2 ft (1 ft = 30.48 cm) water channel to measure the velocity field in a turbulent free-surface jet. The spatial resolution of the system is 1 mm, and the field of view is 100 mm, approximately. Measurements performed with this system are compared with results reported in the literature and are found to be in good agreement.     

Scheimpflug stereocamera for particle image velocimetry in liquid flows

A novel stereocamera has been developed based on the angular-displacement method, wherein the two camera axes are oriented in a nonorthogonal manner toward the object plane. The stereocamera satisfies the Scheimpflug condition such that the image plane, the object plane, and the lens plane are nominally colinear. A unique feature of the stereocamera is the introduction of a liquid prism between the object plane and the recording lens, which significantly reduces the radial distortions that arise when imaging through a thick liquid layer. The design of the camera and its computer optimization with geometric modeling are described. Results indicate that the use of a liquid prism reduces the amount of radial distortion by an order of magnitude. The results have been shown to agree very well with experiments.     

Flow diagnostics in a vortex furnace by particle image velocimetry

The internal aerodynamics of a model vortex furnace for a steam generator with a horizontal axis of flow rotation and distributed input of fuel-air mixture jets has been studied. Average characteristics of the flow velocity field in various cross sections have been determined using a digital tracer imaging (particle image velocimetry) technique. Results are compared to data obtained by the method of laser Doppler anemometry.     

Four-dimensional dynamic flow measurement by holographic particle image velocimetry

The ultimate goal of holographic particle image velocimetry (HPIV) is to provide space- and time-resolved measurement of complex flows. Recent new understanding of holographic imaging of small particles, pertaining to intrinsic aberration and noise in particular, has enabled us to elucidate fundamental issues in HPIV and implement a new HPIV system. This system is based on our previously reported off-axis HPIV setup, but the design is optimized by incorporating our new insights of holographic particle imaging characteristics. Furthermore, the new system benefits from advanced data processing algorithms and distributed parallel computing technology. Because of its robustness and efficiency, for the first time to our knowledge, the goal of both temporally and spatially resolved flow measurements becomes tangible. We demonstrate its temporal measurement capability by a series of phase-locked dynamic measurements of instantaneous three-dimensional, three-component velocity fields in a highly three-dimensional vortical flow-the flow past a tab.     

Performance evaluation of a Scheimpflug stereocamera for particle image velocimetry

We describe a novel stereocamera for particle image velocimetry (PIV) applications that incorporates the Scheimpflug condition that the object plane, lens plane, and image plane must be collinear. We examined the governing equations for this system using a computer-based sensitivity analysis to predict the accuracy of the in-plane and out-of-plane measurement. We evaluated the performance of the Scheimpflug PIV system with a three-dimensional uniform translation test. Results indicate that the Scheimpflug PIV stereocamera performs as expected. The larger off-axis angles possible with the Scheimpflug system can provide a higher accuracy in the out-of-plane component when compared with a translation PIV stereocamera.     

Novel particle image velocimetry system based on three-color pulsed lamps and image processing

A novel particle image velocimetry (PIV) measurement system based on flashed lamps as light sources and digital reconstruction of particle positions by image processing is presented. Three different colors are used in order to distinguish, on a same photograph, the corresponding positions of each moving particle at three different instants of time. This solution can give significant advantages in several aerodynamic and hydrodynamic situations. In particular, it allows high flexibility for velocity measurements and independent settings of different acquisition parameters, like pulse powers and timing. Furthermore, the developed PIV measurement system is fully transportable, eye-safe, practical, and economical. The first hydrodynamics measurements achieved by using the proposed velocimetry measurement system are reported.     

Meshfree boundary particle method applied to Helmholtz problems

This paper is concerned with the boundary particle method (BPM), a new boundary-only radial basis function collocation schemes. The method is developed based on the multiple reciprocity principle and applying either high-order nonsingular general solutions or singular fundamental solutions as the radial basis function. Like the multiple reciprocity BEM (MR-BEM), the BPM does not require any inner nodes for inhomogeneous problems and therefore is a truly boundary-only technique. On the other hand, unlike the MR-BEM, the BPM is meshfree, integration-free, symmetric, and mathematically simple technique. In particular, the method requires much less computational effort for the discretization than the MR-BEM. In this study, the accuracy and efficiency of the BPM are numerically demonstrated in some 2D inhomogeneous Helmholtz problems under complicated geometries.     

Eulerian-Lagrangian analysis for particle velocities and trajectories in a pure wave motion using particle image velocimetry

This paper investigates the velocity and the trajectory of water particles under surface waves, which propagate at a constant water depth, using particle image velocimetry (PIV). The vector fields and vertical distributions of velocities are presented at several phases in one wave cycle. The third-order Stokes wave theory was employed to express the physical quantities. The PIV technique's ability to measure both temporal and spatial variations of the velocity was proved after a series of attempts. This technique was applied to the prediction of particle trajectory in an Eulerian scheme. Furthermore, the measured particle path was compared with the positions found theoretically by integrating the Eulerian velocity to the higher order of a Taylor series expansion. The profile of average travelling distance is also presented with a solution of zero net mass flux in a closed wave flume.     

Application of particle image velocimetry for measuring He II thermal counterflow jets

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component flow velocity.     

Bayesian decision method applied to a problem in gear train design

The problem of backlash in gear mechanisms is approached from the point, of view of a decision-maker, The elements of Bayesian Decision Theory are described and their significance is related to the backlash problem. The Normal statistical model is motivated as the basis for decision analysis with and without observational data. Each step of the analysis is examined in detail for the backlash problem, and is illustrated by a simple but realistic numerical example.     

The complex aerodynamic footprint of desert locusts revealed by large-volume tomographic particle image velocimetry

Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animal''s entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.     

Digital particle image velocimetry measurements of the downwash distribution of a desert locust Schistocerca gregaria.

Actuator disc models of insect flight are concerned solely with the rate of momentum transfer to the air that passes through the disc. These simple models assume that an even pressure is applied across the disc, resulting in a uniform downwash distribution. However, a correction factor, k, is often included to correct for the difference in efficiency between the assumed even downwash distribution, and the real downwash distribution. In the absence of any empirical measurements of the downwash distribution behind a real insect, the values of k used in the literature have been necessarily speculative. Direct measurement of this efficiency factor is now possible, and could be used to compare the relative efficiencies of insect flight across the Class. Here, we use Digital Particle Image Velocimetry to measure the instantaneous downwash distribution, mid-downstroke, of a tethered desert locust (Schistocerca gregaria). By integrating the downwash distribution, we are thereby able to provide the first direct empirical measurement of k for an insect. The measured value of k = 1.12 corresponds reasonably well with that predicted by previous theoretical studies.     

Two-dimensional wavelet transform and application to holographic particle velocimetry

The goal of holographic particle velocimetry is to infer fluid velocity patterns from images reconstructed from doubly exposed holograms of fluid volumes seeded with small particles. The advantages offered by in-line holography in this context usually make it the method of choice, but seeding densities sufficient to achieve high spatial resolution in the sampling of the velocity fields cause serious degradation, through speckle, of the signal-to-noise ratio in the reconstructed images. The in-line method also leads to a great depth of field in paraxial viewing of reconstructed images, making it essentially impossible to estimate particle depth with useful accuracy. We present here an analysis showing that these limitations can be circumvented by variably scaled correlation, or wavelet transformation. The shift variables of the wavelet transform are provided automatically by the optical correlation methodology. The variable scaling of the wavelet transform derives, in this case, directly from the need to accommodate varying particle depths. To provide such scaling, we use a special optical system incorporating prescribed variability in spacings and focal length of lenses to scan through the range of particle depths.

Calculation shows, among other benefits, improvement by approximately two orders of magnitude in depth resolution. A much higher signal-to-noise ratio together with faster data extraction and processing should be attainable.

     

Particle image velocimetry: improving fringe quality with a negative-mask method

The photographic negative-mask method of neutralizing the diffraction-halo effect in speckle photography is applied to enhance the quality of Young's fringes obtained in particle-image-velocimetry studies. The improvement of the fringes achieved with the negative-mask method is compared with improvements by the commonly used method of analyzing a contact copy of particle-image-velocimetry specklegrams. Theoretical analysis and experimental results are presented.     

A direct method for porous particle density characterization applied to activated carbons

A new method for setting the porous particle overall density is proposed. The apparent density of the particle including the solid matter as well as the different internal porosities (from microporous to macroporous pore size ranges) is currently needed for industrial applications as gas storage, heterogeneous catalysis… The technique proposed is based on the principle own immersion fluid applied to a granular medium consisting of extremely fine particles playing the role of non-wetting liquid. Here are presented the restrictive theoretical and experimental conditions, which have to be fulfilled by any granular medium before applying this principle. The conventional technique of mercury porosimetry has been used to characterize apparent grain density and validate the obtained results. The technique has been calibrated to be used with particles, which exhibit a different density, size, shape, and surface roughness. In a second part, the proposed method has been applied to a crop of materials including modified activated carbons previously obtained by oxidative transient treatments. The corresponding change in microporous properties has been studied by N₂ physisorption at 77 K. Moreover, the size ranges of the particle over which each technique leads to efficient measurements have been identified.