Particle Tracking Velocimetry System Research Articles

Experimental analysis of ice crystal size–velocity correlation in icing wind tunnel with digital holographic particle tracking velocimetry

The ice crystal icing in aero-engines poses a significant threat to aircraft flight safety, and investigating the correlation between ice crystal size and velocity in icing wind tunnels is essential for correlating ground tests with flight conditions. In this paper, a digital holographic particle tracking velocimetry system is developed and integrated with an icing wind tunnel, and the ice crystal size and velocity are simultaneously measured in experiments at four different wind speeds. The velocity difference between the ice crystal and wind has been experimentally observed, and it increases with both the ice crystal size and wind speed. The normalized velocities of the ice crystals by the wind speeds decrease with size, and the maximum velocity difference in the experiments reaches 28% of the wind speed. Additionally, quantitative correlations between ice crystal size and normalized velocity based on the power function and polynomial model are established. The relationship between the particle Reynolds number of ice crystal and size is also analyzed, resulting in the development of a model based on the power function, and the power index ranges from 1.51 to 1.64. These findings will provide valuable assistance in the calibration and commissioning of the ice crystal simulation system in icing wind tunnels, as well as in exploring the correlation between ground icing tests and flight and in developing corresponding models.

Experimental Investigation Of Open Channel Flow Around A Hemisphere At Different Relative Depths Using 3D-PTV

In regulated rivers and small streams, the presence of large roughness elements like boulders and concrete structures significantly impacts the local flow field and creates complex hydraulic conditions important for aquatic habitats. This study focuses on investigating and analyzing the complex three-dimensional flow field around an idealized stone (represented as a hemisphere) to understand how the flow field changes for different relative depths i.e. water depth relative to boulder height, and flow rates. Time-resolved 3D Particle Tracking Velocimetry was employed in laboratory experiments conducted in an open-channel flume, to mimic varying river conditions and their impact on flow characteristics. Results reveal that the relative depth significantly influences the flow structure behind the hemisphere, impacting recirculation zones and wake characteristics. The flow field behind the hemisphere was found to be more sensitive to changes in relative depth than bulk velocity. The study also highlights the larger regions of lower streamwise velocity that occurred downstream of the hemisphere caused by the lower submergences combined with a higher magnitude of the downward vertical velocity. Furthermore, this study demonstrates the capability of the measurement technique and the time-resolved 3D Particle Tracking Velocimetry system used to effectively capture the complex flow dynamics around the boulder. This approach allowed for detailed visualization and quantification of flow parameters such as streamwise and vertical velocities within the wake region, which are important factors for aquatic habitats. Understanding the underlying flow dynamics around an idealized stone at low relative depths could yield insights into ecohydraulic engineering and ecological restoration in shallow streams. This can contribute to advancing our understanding of riverine flow dynamics and their ecological implications.

Simultaneous Measurements Of 3D Temperature And Velocity Fields In Gas Flows Using Thermographic Phosphor Tracer Particles

A concept for joined three-dimensional temperature and velocity measurements in turbulent gas flows using sub-micron sized thermographic phosphor tracer particles is presented. Such measurements are needed to resolve flow structures commonly encountered in turbulent flows of scientific and industrial interest. While available techniques for simultaneous 3D thermometry and velocimetry are suitable for microfluidic applications or slow liquid flows, the present concept is applicable to macroscopic turbulent gas flows. The feasibility of these measurements is demonstrated using two turbulent heated gas jets. The concept utilizes two sub-systems: a four-view 3D particle tracking velocimetry system combined with a two-view ratiometric phosphor thermometry system. A double-pulse green laser and a single-pulse UV laser are used for illumination and excitation of phosphor luminescence, respectively. Upon imaging the particle-laden flow using the joined camera system, 3D particle positions and velocities are computed from four double-frame Mie-scattering recordings, while the temperature is inferred from two single-frame luminescence-emission recordings. Measurement results using low particle seeding densities successfully retrieve the expected temperature and velocity distributions. This proof-of-concept is a first step towards the ability of resolving 3D turbulent flow structures from individual single-shot recordings using higher particle seeding densities.

On the combined flow and structural measurements via robotic volumetric PTV

This study describes a novel measurement approach for combined flow and structural measurements in wind tunnels using robotic volumetric particle tracking velocimetry (PTV). The measurement approach is based on the application of a particle tracking algorithm on images including flow or structure tracers, where the latter are implemented by means of fiducial markers. The main steps of the measurement procedure comprise the simultaneous acquisition of flow and structure tracers in the same images, the distinction of the tracers leading to separate flow and structure image sets, the application of Lagrangian particle tracking and the further post-processing, and recombination of the obtained data. The approach is applied to the fluid-structure interaction between a flexible plate with a span of 1.2 m and a periodic gust. The total measurement volume amounts approximately to 150 liters. A phase-averaged description of the FSI problem is presented, with the focus on the effects of the spatio-temporal averaging of the flow information. The structural displacements obtained from the PTV system are validated against a scanning vibrometer. The phase-averaged displacement of the markers is also analyzed, assessing both the validity of the phase-averaged approach and the physical coherence of their motion with respect to a structural model of the plate. It is found that robotic volumetric PTV is suitable for the measurement of large-scale structural displacements, while it should not be used to measure small-scale vibrations. Finally, a visualization of the combined measurement is presented, together with an analysis of the consistency between the measured structure and flow field.

Impact Region of Nonbuoyant Orthogonal Discharge

The results of experimental studies into the behavior of nonbuoyant discharges impacting a solid boundary are presented. The discharges were released perpendicular to the boundary at nondimensional heights (H/d) ranging from to 36 to 173, where H/d is the ratio of the jet discharge height above the boundary to the jet diameter. Although the primary focus was on concentration field measurements using a laser-induced fluorescence system, additional velocity field data are presented from a recent study that used a similar discharge configuration and a particle tracking velocimetry system. Integral models provided a relatively simplistic framework for quantifying and interpreting the flow behavior in the vicinity of the boundary. The new data sets enabled defining the scale of the impact region based on the ability of integral techniques to model the flow entering and leaving this region. These data sets also offered insights into the flow behavior in the impact region and provided the basis for determining the influence of the impact region on the flow behavior.

Role of low-order proper orthogonal decomposition modes and large-scale coherent structures on sediment particle entrainment

This study presents experimental investigations of entrainment events of a single sediment particle resting in a small pocket on a smooth bed in an open channel flow. The data were acquired with a tomographic particle tracking velocimetry system. The shake the box algorithm, a spatially high resolved Lagrangian tracking method, was applied to determine time-resolved and three-dimensional flow velocities in a volume during particle entrainment. The proper orthogonal decomposition (POD) method was applied to identify motions in the flow field carrying the most turbulent kinetic energy (TKE). Based on the POD method, the most energetic flow structures were linked with the occurring quadrant events at particle entrainment. It was shown that particle entrainment follows TKE peaks based on low-order POD modes caused by large sweeps. Two streamwise elongated counter rotating vortices with emerging sweeps in between were observed during particle entrainment. Here, most of the TKE exists in the first POD mode. This signature is suggested to be part of very large-scale coherent motions.

Methodological aspects of using high-speed cameras to quantify soil splash phenomenon

Soil splash is the first step of water erosion. There are many methods for splash investigations however the use of high-speed cameras is becoming increasingly popular. This study describes the methodological aspects of using high-speed cameras and a PTV (Particle Tracking Velocimetry) system to investigate the soil splash phenomenon. The purpose of the research was to examine the impact of image processing and the choice of settings for particle tracking software on the effectiveness of detecting and identifying splashed/ejected particles (with their flight trajectories). Experiments were carried out on moisturized Haplic Luvisol soil samples and the impact of single water-drop onto the surface was recorded by three calibrated Phantom Miro M310 high-speed cameras. Registered frames were subjected to image processing in Dantec DynamicStudio software, where the selection of the optimal set of parameters/settings was validated, in the context of maximum detection of the splashed particles. The resulted images provided the input data for the Volumetric 3D PTV module (DynamicStudio) to identify/track the ejected particles and then determine their trajectories. The study presents the use of different settings (variants) of tracking modules, enabling the most efficient identification and description of the particles’ trajectories (76%). The incomplete trajectories detected (excluding starting and landing fragments), were subjected to a reconstruction process based on mathematical extrapolation. The analyze of the registered images and reconstructed trajectories of splashed particles gave the possibility to determine the following parameters describing soil splash phenomenon: a) number of splashed particles, b) ejection angle (for each particle), c) ejection velocity, d) displacement range, e) altitude (the maximal height reached by the particle).

Color contamination correction based on light intensity correlation in two-color, double-exposure particle tracking velocimetry

An algorithm for correcting color contamination is proposed for two-color, double-exposure particle tracking velocimetry (PTV). A two-color PTV system used in this study consists of blue and green laser diodes and a consumer digital camera, where a laser pulser with an avalanche transistor is developed for achieving optical pulses of 50 ns for application in airflows. In the PTV, the camera captures images of tracer particles illuminated by a sequence of green and blue light pulses with a certain time interval. Because of spectral characteristics of a Bayer filter, camera sensors in a blue channel respond to green light scattered by the particles. This color contamination results in pseudo-particles in the blue channel. Pixels occupied by the pseudo-particles have very high correlation of light intensity between green and blue channels. In the proposed method, the pseudo-particles caused by the color contamination are detected and removed based on the high correlation. The present color contamination correction hardly affects real particles illuminated by the blue laser diodes. Measurements of an airflow induced by DC fans confirm that the proposed system with the color contamination correction works well for PTV.

A low‐cost underwater particle tracking velocimetry system for measuring in situ particle flux and sedimentation rate in low‐turbulence environments

AbstractWe describe a low‐cost three‐dimensional underwater particle tracking velocimetry system to directly measure particle settling rate and flux in low‐turbulence aquatic environments. The system consists of two waterproof cameras that acquire stereoscopic videos of sinking particles at 48 frames s−1 over a tunable sampling volume of about 45 × 25 × 24 cm. A dedicated software package has been developed to allow evaluation of particle velocities, concentration and flux, but also of morphometric parameters such as particle area, sinking angle, shape irregularity, and density. Our method offers several advantages over traditional approaches, like sediment trap or expensive in situ camera systems: (1) it does not require beforehand particle collection and handling; (2) it is not subjected to sediment trap biases from turbulence, horizontal advection, or presence of swimmers, that may alter particulate load and flux; (3) the camera system enables faster data processing and flux computation at higher spatial resolution; (4) apart from the particle settling rates, the particle size distribution, and morphology is determined. We tested the camera system in Lake Stechlin (Germany) in low turbulence and mean flow, and analyzed the morphological properties and settling rates of particles to determine their sinking behavior. The particle flux assessed from conventional sediment trap measurements agreed well with that determined by our system. By this, the low‐cost approach demonstrated its reliability in low turbulence environments and a strong potential to provide new insights into particulate carbon transport in aquatic systems. Extension of the method to more turbulent and advective conditions is also discussed.

A PTV method based on ultrasound imaging and feature tracking in a low-concentration sediment-laden flow

This study aims to provide a particle tracking velocimetry (PTV) method based on ultrasound imaging and feature-tracking in a low-concentration sediment-laden flow. A phased array probe is used to generate a 2D ultrasound image at different times. Then, the feature points are extracted to be tracked instead of the centroids of the particle image. In order to better identify the corresponding feature point, each feature is described by an oriented angle and its location. Then, a statistical interpolation procedure is used to yield the displacement vector on the desired grid point. Finally a correction procedure is adopted because the ultrasound image is sequentially acquired line by line through the field of view. A simple test experiment was carried out to evaluate the performance. The ultrasound PTV system was applied to a sediment-laden flow with a low concentration of 1‰, and the speed was up to 10 cm s−1. In comparison to optical particle image velocimetry (PIV), ultrasound imaging does not have a limitation in optical access. The feature-tracking method does not have a binarisation and segmentation procedure, which can result in overlapping particles or a serious loss of particle data. The feature-tracking algorithm improves the peak locking effect and measurement accuracy. Thus, the ultrasound PTV algorithm is a feasible alternative and is significantly more robust against gradients than the correlation-based PIV algorithms in a low-concentration sediment-laden fluid.

Development of multi-spectral three-dimensional micro particle tracking velocimetry11Originally presented in 11th International Symposium on Particle Image Velocimetry, Santa Barbara, California, September 14–16, 2015.

The color-coded 3D micro particle tracking velocimetry system (CC3DμPTV) is a volumetric velocimetry technique that uses the defocusing digital particle image velocimetry (DDPIV) approach to reconstruct the 3D location of the particle. It is suited for microscopic flow visualization because of the single camera configuration. However, several factors limit the performance of the system. In this study, the limitation of the CC3DμPTV is discussed in detail and a new concept of a multi-camera 3D μ-PTV system is proposed to improve the performance of the original CC3DμPTV system. The system utilizes two dichroic beam splitters to separate the incoming light into 3 spectral ranges, and image with three monochrome image sensors. The use of a color-matched light source, off-center individual pinhole and monochrome image sensors allow the system to achieve better sensitivity and optical resolution. The use of coherent lasers light sources with high-speed cameras improves the velocity measurement dynamic range. The performance of the proposed multi-spectral system is first evaluated with a simulation model based on the finite element method (FEM). The performance is also compared numerically with the CC3DμPTV system. The test results show significant improvements on the signal to noise ratio and optical resolution.

Velocity measurements in inclined negatively buoyant jets

Experiments were performed with a particle tracking velocimetry system to investigate the behaviour of inclined negatively buoyant jets with source angles of 15°, 30°, 45°, 60°, 65°, 70°, and 75° in stationary ambient conditions. Velocities were measured in a plane aligned with the central axis of the flow and the experiments were designed such that the flow did not interact with boundaries in the region were the flow behaviour was measured. The results of this study complement previous research, which has largely focused on the mean geometric characteristics and the mean dilution of the discharged fluid. Geometric characteristics, spreading rates, and time-averaged (mean) centreline velocity results are compared with relevant experimental results from previous studies and integral model predictions. Axial and transverse mean velocity profiles at maximum height and the return point provide additional insights into the detrainment of discharged fluid due to the unstable density gradient on the inner side of the flow.

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.

A study of particle splash on developing ripple forms for two bed materials

A custom-designed Particle Tracking Velocimetry (PTV) system was developed to measure particle splash dynamics in two materials: mixed quartz sand and ground acrylic particles approximating the density of snow. The strengths and limitations of the PTV system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation. Analysis of particle collisions on the bed surface (splash) demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand. They also eject a greater number of particles, though at lower velocities and angles than observed for the sand particles. These relations appear to be highly sensitive to the porosity of the bed surface, while particle shape and elasticity play a secondary role. With regard to PTV measurements carried out along the wind-aligned axis of migrating aeolian ripples, particles ejected from the lee slope tend to be slightly larger than those on the stoss slope, and in general are ejected with a lower speed and greater angle to the surface. Irrespective of the type of granular material, when the average impact angle is close to or greater than the negative slope of the evolving bedform, the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope. This empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate. However, systematic under-prediction of this rate by an early model suggests that additional processes, inclusive of surface creep, must also be accounted for.

A comprehensive methodology for characterizing sprinkler sprays

Sprinklers are widely used in fire suppression applications. The suppression performance of these sprays is determined by their ability to penetrate the fire to reach burning surfaces below, while dispersing water throughout the hot fire environment. Spray penetration and dispersion are governed by the initial drop size and velocity characteristics of the spray, which depend on the injection conditions and sprinkler configuration. In this study, the initial spray is fully characterized using a laser-based shadowgraphy and particle tracking velocimetry system producing nearly a million simultaneous drop size/velocity realizations for each sprinkler spray. Near-field spray characteristics are established from local measurements, which are mapped in a spherical coordinate system consistent with the kinematics of the spray. A novel data compression scheme is introduced to generate analytical functions describing the sprinkler spray based on the measurements. These functions are useful for initiating the sprinkler spray in computational fluid dynamics (CFD) based spray dispersion and fire suppression modeling. This framework also reveals physical characteristics of the initial spray not easily recognized from raw data. The near-field spray measurements and associated data compression approach are validated by comparing volume density measurements 1 m below the sprinkler with volume density predictions generated from spray dispersion calculations initiated with the analytical spray functions.

Instability waves in a low-Reynolds-number planar jet investigated with hybrid simulation combining particle tracking velocimetry and direct numerical simulation

Instability waves in a laminar planar jet are extracted using hybrid unsteady-flow simulation combining particle tracking velocimetry (PTV) and direct numerical simulation (DNS). Unsteady velocity fields on a laser sheet in a water tunnel are measured with time-resolved PTV; subsequently, PTV velocity fields are rectified in a least squares sense so that the equation of continuity is satisfied, and they are transplanted to a two-dimensional incompressible Navier–Stokes solver by setting a multiple of the computational time step equal to the frame rate of the PTV system. As a result, the unsteady hybrid velocity field approaches that of the measured one over time, and we can simultaneously acquire the unsteady pressure field. The resultant set of flow quantities satisfies the governing equations, and their resolution is comparable to that of numerical simulation with the noise level much lower than the original PTV data. From hybrid unsteady velocity fields, we extract eigenfunctions using bi-orthogonal decomposition as a spatial problem for viscous instability. We also investigate stability/convergence characteristics of the hybrid simulation referring to linear stability analysis.

In situ droplet size and speed determination in a fluid-bed granulator

The droplet size affects the final product in fluid-bed granulation and coating. In the present study, spray characteristics of aqueous granulation liquid (purified water) were determined in situ in a fluid-bed granulator. Droplets were produced by a pneumatic nozzle. Diode laser stroboscopy (DLS) was used for droplet detection and particle tracking velocimetry (PTV) was used for determination of droplet size and speed. Increased atomization pressure decreased the droplet size and the effect was most strongly visible in the 90% size fractile. The droplets seemed to undergo coalescence after which only slight evaporation occurred. Furthermore, the droplets were subjected to a strong turbulence at the event of atomization, after which the turbulence reached a minimum value in the lower halve of the chamber. The turbulence increased as speed and droplet size decreased due to the effects of the fluidizing air. The DLS and PTV system used was found to be a useful and rapid tool in determining spray characteristics and in monitoring and predicting nozzle performance.

A comparison of collisions of saltating grains with loose and consolidated silt surfaces

A particle tracking velocimetry system was used to study the trajectories of saltating sand particles as they impacted either consolidated (solid) or unconsolidated (loose) silt surfaces in a wind tunnel. The tunnel friction velocity varied between 0.26 and 0.3 m s−1. The average coefficient of restitution, defined as the ratio of postcollision velocity to precollision velocity, was found to be ε = 0.64 for the loose bed and ε = 0.79 for the solid bed, respectively. The average ratio of energy loss to impact energy was found to be EL/E0 = 0.58 for the loose bed and EL/E0 = 0.37 for the solid bed, respectively. These indices demonstrate that the loose bed absorbs more momentum and energy from the impacting sand particle. The average ejection angle was lower at 18° for the loose bed than 23° for the solid bed. In the loose surface, crater formations were observed to form with each impact. Surface profile measurements suggest an average crater volume of >0.1 mm3. For both beds, the coefficient of restitution decreases with the particle impact speed. In the case of the solid bed, the dependence on impact speed is in good agreement with a model of two colliding spheres with identical material properties. With regard to the loose bed, a model of the impacting particle motion as it plows through and plastically deforms the bed material is tested. If the ratio of the horizontal to vertical forces on the particle as it plows through the bed is taken as a linear function of impact speed, the model is in good agreement with measured data. This suggests that compaction of the surface may occur along with the plowing and displacement of loose bed material.

Unsteady PTV velocity field past an airfoil solved with DNS: Part 1. Algorithm of hybrid simulation and hybrid velocity field at Re ≈ 103

We develop a hybrid unsteady-flow simulation technique combining direct numerical simulation (DNS) and particle tracking velocimetry (PTV) and demonstrate its capabilities by investigating flows past an airfoil. We rectify instantaneous PTV velocity fields in a least-squares sense so that they satisfy the equation of continuity, and feed them to the DNS by equating the computational time step with the frame rate of the time-resolved PTV system. As a result, we can reconstruct unsteady velocity fields that satisfy the governing equations based on experimental data, with the resolution comparable to numerical simulation. In addition, unsteady pressure distribution can be solved simultaneously. In this study, particle velocities are acquired on a laser-light sheet in a water tunnel, and unsteady flow fields are reconstructed with the hybrid algorithm solving the incompressible Navier–Stokes equations in two dimensions. By performing the hybrid simulation, we investigate nominally two-dimensional flows past the NACA0012 airfoil at low Reynolds numbers. In part 1, we introduce the algorithm of the proposed technique and discuss the characteristics of hybrid velocity fields. In particular, we focus on a vortex shedding phenomenon under a deep stall condition (α = 15°) at Reynolds numbers of Re = 1000 and 1300, and compare the hybrid velocity fields with those computed with two-dimensional DNS. In part 2, the extension to higher Reynolds numbers is considered. The accuracy of the hybrid simulation is evaluated by comparing with independent experimental results at various angles of attack and Reynolds numbers up to Re = 104. The capabilities of the hybrid simulation are also compared with two-dimensional unsteady Reynolds-Averaged Navier–Stokes (URANS) solutions in part 2. In the first part of these twin papers, we demonstrate that the hybrid velocity field approaches the PTV velocity field over time. We find that intensive alternate vortex shedding past the airfoil, which is predicted by the two-dimensional DNS, is substantially suppressed in the hybrid simulation and the resultant flow field is similar to the PTV velocity field, which is projection of the three-dimensional velocity field on the streamwise plane. We attempt to identify the motion that originates three-dimensional flow patterns by highlighting the deviation of the PTV velocity field from the two-dimensional governing equations at each snapshot. The results indicate that the intensive spots of the deviation appear in the regions in which three-dimensional instabilities are induced in the shear layer separated from the pressure side.

Particle Tracking Experiments in Match‐Index‐Refraction Porous Media

A low-cost, noninvasive, three-dimensional (3D), particle tracking velocimetry system was designed and built to investigate particle movement in match-index-refraction porous media. Both a uniform load of the glass beads of the same diameter and a binary load of the glass beads of two diameters were used. The purpose of the experiments is to study the effect of the two loads on the trajectories, velocity distribution, and spreading of small physical particles. A total of 35 particles were released and tracked in the uniform load and 46 in the binary load. The 3D trajectory of each particle was recorded with two video camcorders and analyzed. It is found that the particle's velocity, trajectory, and spreading are very sensitive to its initial location and that the smaller pore size or heterogeneity in the binary load increases the particles' velocity and enhances their spreading as compared with the uniform load. The experiments also verified the previous finding that the distribution of the particle velocities are lognormal in the longitudinal direction and Gaussian in two transverse directions and that the particle spreading is much larger along the longitudinal direction than along the traverse directions.