3D Particle Tracking Velocimetry Research Articles

Development of raw-image ray-bundling for focused and unfocused plenoptic 3D-PTV

This paper introduces a new plenoptic 3D particle tracking velocimetry (3D-PTV) technique called raw-image ray-bundling (RIRB), which combines the 3D clustering algorithm of light-field ray-bundling (LFRB) with the raw image segmentation concept of epipolar triangular connectivity (ETC). The combined approach provides flexibility towards accommodating both unfocused and focused plenoptic cameras. Additionally, the RIRB algorithm bypasses the computational step of perspective-view generation in LFRB, thereby allowing lower specification microlens arrays (MLA) to be used without excessive loss of measurement resolution. As part of RIRB’s development, synthetic particle-field data and experimental dot-panel targets were used to demonstrate the viability of RIRB. The latter was performed through a VEO640 high-speed camera retrofitted for plenoptic imaging with a 2.5 mm focal-length, F/10 MLA. For the synthetic dataset, RIRB demonstrated better performance than LFRB in processing low-resolution images, with mean errors below 0.85% of the field-of-view (FOV). Additionally, a synthetic Hagen-Poiseuille flow dataset was used to demonstrate the feasibility of RIRB for 3D flow velocimetry using both high and low-resolution images. For the experimental dataset, x/y errors below 0.15% of the FOV and z error below 1.0% were achieved. The combination of synthetic and experimental validations suggests RIRB is a promising approach for 3D triangulation from both focused and unfocused raw plenoptic images, overcoming the low-resolution issue inherent with perspective-view decoding in plenoptic systems.

Meshless track assimilation (MTA) of 3D PTV data

We propose a data assimilation algorithm to improve the resolution of 3D particle tracking velocimetry (PTV) data and, as a byproduct, it estimates the pressure field. The proposed method, called meshless tracking assimilation (MTA), does not rely on a mesh to compute derivatives. Instead, it exploits the analytical formulation of the obtained field to extract the gradients of the involved quantities in arbitrary points. MTA actively constraints PTV data points to satisfy the Navier–Stokes equations in both time and space at user-defined spatial locations. Three test cases are investigated. The first one is a synthetic dataset reconstructed from a direct numerical simulation and it serves to ensure that the method is properly implemented and working. The second one is the 2020 data assimilation challenge which gives a qualitative comparison with other data assimilation methods. Finally, MTA is used on a Tomographic PTV experiment of a Jet Flow with ReD≃3000 to ensure it can be implemented successfully on experimental data. The first and third test cases show a significant performance boost compared to the classic linear interpolation. This improvement allows for a higher level of structural resolution without sacrificing the temporal and spatial smoothness of the solution. The second test case generally aligns well with other state-of-the-art data assimilation techniques in terms of performance, but there are some noticeable differences.

Experiments on droplet production from solitary waves impacting on a vertical wall

This work presents the results of an experimental investigation of solitary waves impacting on a vertical flat wall. The aim of the study was to correlate the wave characteristics and breaking stage before impact to the droplet production after the impact. The main characteristic of these experiments is that the wall height is lower than the wave height. The droplet sizes were obtained by combining 3D particle tracking velocimetry with an in-house code. Droplet diameters between 0.3–22 mm were measured at a distance of 35 cm and 68 cm from the wall. It was found that the breaking stage before the impact has a large influence on the mean size of the droplets produced, especially when there is air trapped between the wall and the wave at the impact moment. It was found that the mean droplet size decreases with the ratio of water jet velocity after impact and the wave speed. The resulting distributions of droplet sizes were compared to three different theoretical models used in other droplet production studies. Results were also compared to previous experimental studies of droplet formation from breaking waves to determine the best fit to the experimentally obtained distribution.

A method combining half width of point spread function with maximum gradient of image gray for single-camera three-dimensional spray velocity detection

When a defocus image is obtained by direct imaging, it is hard to determine whether the detected object is in the front or rear of the focal plane. This is called the ambiguity problem and it causes great trouble in the 3D-particle tracking velocimetry (PTV) defocus method, which obtains the defocus location by directly obtaining depth information from the intensity along the radial direction of particles. This study proposes a method to obtain the true position of a particle based on the asymmetry feature of the defocus image with respect to the focal plane. The method distinguishes two ambiguous positions with equal width of point spread function (PSF) width by introducing a parameter derived from the maximum gradient of image gray. The method also derives a mathematical expression for the half width of PSF(χ) considering aberration. The method is validated by a calibration technique, which shows that in 19 out of 20 experimental conditions correct results are obtained. We apply the method to measure the 3D velocity of an airflow jet with 3D-PTV and compare it with computational fluid dynamics results, which show a maximum error of 8.8 %. Furthermore, the method is used to measure a real spray as an example, and obtains a 3D vector map and velocity cumulative distribution of the measured area.

Experimental study on flow and turbulence characteristics of jet impinging on cylinder using three-dimensional Lagrangian particle tracking velocimetry

When a round jet impinges on a convex cylindrical surface, complex three-dimensional (3D) flow structures occur, accompanied by the Coanda effect. To characterize the flow and turbulence properties of the general system, ensemble averages of 3D Lagrangian particle tracking velocimetry measurements were taken. The radial bin-averaging method was used in post-processing the tracked particles and corresponding instantaneous velocity vectors to generate appropriate ensemble-averaged statistics. Two impinging angles were selected, and at a fixed Reynolds number, the ensemble-averaged volumetric velocity field and turbulent stress tensor components were measured. The flow and turbulence characteristics of the impinging jet on the cylinder were notably different based on the impinging angle, especially in the downstream region. Surprisingly, the attached wall jet with a half-elliptic shape was abruptly thickened in the wall-normal direction, similar to the axis switching phenomenon observed in elliptic jets in the case of oblique impingement. In the jet-impinging region, the flow spread in all directions with high mean vorticity values. With the development of a 3D curved wall jet, both the Coanda effect and centrifugal force played a significant role in the flow behavior. A notable feature of the self-preserving region was the similarity of mean velocity profiles with scaling by the maximum velocity and the jet half-width for both impinging angle cases. Local isotropy of turbulent normal stresses was observed in this region, supporting the existence of self-preservation in the 3D curved wall jet. The volumetric ensemble-averaged Reynolds stress tensor revealed strong inhomogeneous turbulence in the boundary layer region and the curvature effect on the Reynolds shear stress in the free shear layer.

On the characterization of bias errors in defocusing-based 3D particle tracking velocimetry for microfluidics

In this work, we provide a systematic theoretical and experimental characterization of bias errors in defocusing particle tracking (DPT) methods based on a single calibration function, with respect to microfluidic applications, in which it is not possible to use calibration targets inside the measurement volume. This approach is widely used in microfluidic experiments, but bias errors are often neglected and to date only few works reported empirical procedures to compensate for that. A systematic characterization of the impact of such error in DPT measurements is still lacking. We show that the field curvature aberration and the refractive index mismatch are the main sources of bias error in these applications. We present a correction methodology for the bias error based on the determination of a reference surface, and in addition we propose a procedure based on a reference measurement of a Poiseuille flow to determine the reference surface on microfluidic channels with constant cross section. We discuss the impact of the refractive index mismatch and how to correctly compensate for it. We validated our methodology and quantified the bias errors on 10 different experimental setups, using different working fluids, materials, geometries, and microscope objective lenses ranging from 5times to 40times magnification. Our results indicate that the impact of this type of bias errors is in general not predictable and must be evaluated case by case. The proposed methodology allows to estimate and minimize the bias error in most microfluidic setups and is suitable for any single-camera DPT approach.

Thermographic 3D particle tracking velocimetry for turbulent gas flows

Turbulent flows are characterized by diverse and unsteady three-dimensional (3D) features that require 3D measurements to study. In case of non-isothermal flows, combined 3D measurements of temperature and velocity are necessary. In this paper, a thermographic 3D particle tracking velocimetry (thermographic 3D-PTV) concept is introduced for simultaneous 3D temperature and velocity measurements in turbulent gas flows. It is based on sub-micron thermographic phosphor particles seeded into the flow as flow tracers with low response times of a few microseconds. To obtain each tracer’s position and velocity, the measurement region is illuminated volumetrically using a double-pulse green laser and Mie-scattered light is imaged by four double-frame cameras. Following the pinhole model-based calibration of all cameras, 3D particle positions are computed for both laser pulse-times using a fast minimum line of sight reconstruction code. Three-component velocities are derived from tracking individual particles between these time steps. For simultaneous 3D thermometry, temperature dependent luminescence emissions from the same phosphor particles are exploited. These emissions are excited using a UV laser synchronized with the first green laser pulse and imaged using two cameras equipped with spectral filters for ratiometric phosphor thermometry. As a result, instantaneous 3D fields of discrete temperature and velocity measurements are obtained throughout the volume. The concept is demonstrated in a turbulent heated gas jet emerging from a circular nozzle at a particle image concentration of 0.005 particle per pixel, where the symmetry of the velocity and temperature distributions about the jet axis is successfully reconstructed.

An In-Vitro Study of the Flow Past a Transcatheter Aortic Valve Using Time-Resolved 3D Particle Tracking.

The performance of a transcatheter aortic valve (TAV) can be evaluated by analyzing the flow field downstream of the valve. However, three dimensional flow and pressure fields, and particle residence time, a quantity closely related to thrombosis risk, are challenging to obtain. This experimental study aims to provide a comprehensive 3D measurement of the flow field downstream of an Edwards SAPIEN 3 using time-resolved 3D particle tracking velocimetry (3D PTV) with Shake-the-Box (STB) algorithm. The valve was deployed in an idealized aorta model and tested in a left heart simulator under physiological conditions. Detailed 3D vortical structures, pressure distributions, and particle residence time were obtained by analyzing the 3D particle tracks. Results have shown large-scale retrograde flow entering the sinuses of the TAV at systole, reducing flow stasis there. However, the 3D particle tracks reveal that the retrograde flow has a high residence time and might have already experienced high shear stress near the main jet. Thus by only focusing on the flow in the sinus region is not sufficient to evaluate the leaflet thrombosis risk, and the flow downstream of the valve should be taken into consideration. The unique perspectives offered by 3D PTV are important when evaluating the performance of the TAVs.

Low-Cost 3D-PTV Utilizing Rainbow Light Illumination And Artificial Neural Network Applied To Rayleigh-Bénard Convection

Building on the novel methodology for three-dimensional (3D) color particle tracking velocimetry (PTV) proposedearlier by our research group, an improvement and an evaluation of the method is presented in this study. The method utilizes only a color camera and a consumer-grade liquid crystal display projector (LCDP), and thus cost for implementation is greatly lower than that required for a tomographic system. Employment of artificial neural network system enables to eliminate human decisions in constructing a color-to-depth conversion function. That is, the calibration can be performed automatically once relationship between particle color and depth are associated in actual measurement environments. The method was tested in a Rayleigh–Bénard convection of water, which is expected to have horizontally isotropic structures. Two different flow states were successfully measured. The results showed that the 3D color PTV can measure the same statistics of velocity components in the in-plane and out-of-plane directions. Considering all the results, the proposed method using only a single camera and an LCDP has a potential to be an alternative to the state-of-art tomographic measurements.

Simultaneous Particle Tracking Velocimetry And Pyrometry Of Spatter Particles Ejected During The Laser Power-Bed Fusion Process Using A Spectral Plenoptic Camera

This paper aims to provide a method for simultaneous 3D particle tracking velocimetry and dual-wavelength pyrometry using a single spectral plenoptic camera for uses in spray and flow diagnostics. The spectral plenoptic camera can capture spatial, angular, and spectral information of light from the scene using a single main lens and sensor. Particle tracking is conducted using the Light-Field Ray Bundling algorithm paired with a four-frame best estimate-enhanced track initialization algorithm to generate 3D high-resolution spatial locations and velocity of particles. Secondly, a dual-wavelength pyrometry method was developed to use particle locations from Light-Field Ray Bundling to find the particle spectra for each wavelength, and then calculate temperature using the spectral intensities. This method achieves 3D particle tracking and temperature measurements using a single lens and sensor, which can be applicable for many different flow measurements. The method was applied to the spatter particle generation caused by the Laser Powder-Bed Fusion process, where spatter particles have been found to be detrimental to the manufactured parts. The calculated speeds of the spatter particles are within the expected range, and the tracks show a complex 3D process. The temperature measurements indicate that the detected particles are in the liquid phase, with temperatures greater than 2000ºC. The simultaneous measurements demonstrate an overall deceleration and cooling of particles during their flight. Therefore, this is a viable technique for simultaneous tracking and temperature measurements.

Camera calibration algorithm development for a two-view collimated shadowgraph system

Shadowgraph imaging is a promising technique for volumetric velocity measurements, which features a high framing rate, long depth focus, and a cheap light source. The main objective of the current study is to develop a camera calibration algorithm for collimated shadowgraph systems, which is an essential procedure for 3D particle tracking velocimetry (PTV) strategies. First, the optical model of a two-view collimated shadowgraph system is established, which can be described by the orthographic projection model. The image distortion effect is also taken into consideration. Then, the calibration algorithm is developed using a flexible planar-target-based method. Aiming towards 3D PTV applications, the extrinsic parameters, including rotation and translation relationships between the two camera imaging coordinates, have been derived. The ambiguity around the sign confirmation of the extrinsic parameters has been solved by introducing extra information from the relative positions of the two views. Moreover, extrinsic parameters self-calibration (EPSC) has been implemented to deal with unavoidable camera drifts during the experiments. The results indicate that the EPSC is effective in removing the global system error in the current two-view system. The proposed calibration algorithm has been verified using synthetic images, which has shown a mean reprojection error of less than 0.1 pixels. In a water jet experiment, the mean reprojection error is around 0.3 pixels (about 0.019 mm in reality) after the board calibration. The relative error evaluated from the reconstruction points is less than 1%. The results indicate that the proposed calibration procedure is effective and feasible for collimated shadowgraph imaging systems. The 3D-particle positions of a sample frame have been reconstructed successfully. It is believed that the high quality shadowgraph images can offer high precision measurements for further implementations of 3D PTV algorithms.

The Presence of a Substrate Strengthens The Jet Generated by Upside-Down Jellyfish

Upside-down jellyfish, Cassiopea, are prevalent in warm and shallow parts of the oceans throughout the world. They are unique among jellyfish in that they rest upside down against the substrate and extend their oral arms upwards. This configuration allows them to continually pull water along the substrate, through their oral arms, and up into the water column for feeding, nutrient and gas exchange, and waste removal. Although the hydrodynamics of the pulsation of jellyfish bells has been studied in many contexts, it is not clear how the presence or absence of the substrate alters the bulk flow patterns generated by Cassiopea medusae. In this paper, we use three-dimensional (3D) particle tracking velocimetry and 3D immersed boundary simulations to characterize the flow generated by upside-down jellyfish. In both cases, the oral arms are removed, which allows us to isolate the effect of the substrate. The experimental results are used to validate numerical simulations, and the numerical simulations show that the presence of the substrate enhances the generation of vortices, which in turn augments the upward velocities of the resulting jets. Furthermore, the presence of the substrate creates a flow pattern where the water volume within the bell is ejected with each pulse cycle. These results suggest that the positioning of the upside-down jellyfish such that its bell is pressed against the ocean floor is beneficial for augmenting vertical flow and increasing the volume of water sampled during each pulse.

Measurement of Electrokinetically induced hydrodynamics at Ion-selective interfaces using 3D Micro particle tracking velocimetry (µPTV)

Electrokinetic flow phenomena are ubiquitous in electrical systems for desalination, chemical conversion, or mixing at a micro-scale. However, the important features of resulting 3D flow fields are only accessible through cost-intensive numerical simulations. Experimental 2D recording of the chaotic three-dimensional velocity fields developing for example at currents exceeding the limiting current density does not capture the complex 3D structures present in such flow fields. Additionally, numerical 3D studies are limited to dimensions three orders of magnitude smaller as found in real applications and only short run times due to the enormous computational effort. To apply the theoretical knowledge in real-world systems and create the possibility for detailed parameter studies, we present the first experimental method for recording and quantifying the time-resolved velocity field in an electrochemical microfluidic cell in 3D with dimensions found in industrial applications. We utilize this method in a co-submitted paper to record the 3D velocity field of electroconvection at a cation-exchange membrane.•Cell design suitable for simultaneous electrochemical experiments with optical 3D velocity quantification•Method optimized for velocity reconstruction of membrane-to-membrane distances found in industrial cells•Highly adaptable cell design, for optical characterization of electrochemical systems

Large-scale 3D-PTV measurement of Ahmed-body wake in crossflow

The near wake of the Ahmed body at zero-yaw has been extensively studied in the literature, but comparatively little is known about the wake flow in crossflow condition. In this investigation, we characterize the near wake of a square-back Ahmed body at a Reynolds number of ReH = 9.2 × 104 for three yaw angles of β = 0°, 5°, and 10°. The measurements are carried out using 3D particle tracking velocimetry utilizing an innovative helium-filled soap bubble (HFSB) system for seeding a large domain of approximately 35 × 30 × 18 cm3. The results showed that increasing the yaw angle shifted the separation bubble towards the windward side and enlarged the recirculating region of the leeward side. The downwash motion from the top-edge and the spanwise motion from the leeward edge of rear-face also strengthened. A vortex ring formed in the periphery of the mean separation bubble. With increasing the yaw, the ring skewed toward the leeward edge and its windward potion became weaker. For the yaw cases, streamwise vortices appeared originating from the windward portion of the vortex ring. Spectral analysis showed that the overall energy of oscillations increased with yaw while the Strouhal numbers of the spanwise and wall-normal flow motions remained relatively constant. Proper orthogonal decomposition of the velocity field showed that the first spatial mode contained 32.8% of the total kinetic energy for the zero-yaw case. The energy of the first spatial mode reduced to approximately 10% for the β = 5° and 10° wakes due to the disappearance of the bi-stability mode. The energetic flow motions within the yawed wakes were primarily on the windward side of the body.

DNS of a turbulent boundary layer using inflow conditions derived from 4D-PTV data

Unsteady, 3D particle tracking velocimetry (PTV) data are applied as an inlet boundary condition in a direct numerical simulation (DNS). The considered flow case is a zero pressure gradient (ZPG) turbulent boundary layer (TBL) flow over a flat plate. The study investigates the agreement between the experimentally measured flow field and its simulated counterpart with a hybrid 3D inlet region. The DNS field inherits a diminishing contribution from the experimental field within the 3D inlet region, after which it is free to spatially evolve. Since the measurement does not necessarily provide a spectrally complete description of the turbulent field, the spectral recovery of the flow field is analyzed as the TBL evolves. The study summarizes the pre-processing methodology used to bring the experimental data into a form usable by the DNS as well as the numerical method used for simulation. Spectral and mean flow analysis of the DNS results show that turbulent structures with a characteristic length on the order of one average tracer particle nearest neighbor radius {bar{r}}_{text {NN}} or greater are well reproduced and stay correlated to the experimental field downstream of the hybrid inlet. For turbulent scales smaller than {bar{r}}_{text {NN}}, where experimental data are sparse, a relatively quick redevelopment of previously unresolved turbulent energy is seen. The results of the study indicate applicability of the approach to future DNS studies in which specific upstream or far field boundary conditions (BCs) are required and may provide the utility of decreasing high initialization costs associated with conventional inlet BCs.Graphic abstract

Time-resolved sphere and fluid motions in turbulent boundary layers

This paper extends the study by Tee et al. (2020) to investigate the effect of large coherent structures on motion of spheres with specific gravities of 1.006 (P1) and 1.152 (P3) at Reτ = 670 and 1300 (d+ = 56 and 116). The sphere and fluid motions are tracked simultaneously via 3D particle tracking and stereoscopic particle image velocimetry over the streamwise-spanwise plane, respectively. With sufficient mean shear, sphere P1 lifts off of the wall upon release before descending back towards the wall at both Reτ. It typically accelerates strongly over a streamwise distance of less than one boundary layer thickness before approaching an approximate terminal velocity. By contrast, the denser sphere P3 does not lift off upon release but mainly slides along the wall. At lower Reτ where wall friction is stronger, this sphere translates with unsteady velocity, significantly lagging the local fluid. The streamwise velocities of both spheres correlate strongly with the fast- and slow-moving zones that approach and move over them. In most runs, both spheres lag the local coherent structures and travel with either fast- or slow-moving zones throughout the observed trajectories. Vortex shedding, which is most prevalent for sphere P3 at Reτ = 670, is also important. The sphere spanwise motion is prompted by wall friction, spanwise fluid motion, and/or meandering of the coherent structures, and spheres do not appear to migrate preferentially into slow-moving zones.

3D Particle Tracking Velocimetry applied to droplets generated by breaking waves

One of the environmental difficulties of exploring the polar regions is marine icing. The understanding of this phenomenon is important for the safety of installations, ships and people that operates in these environments. One of the main sources of marine icing is wave breaking. Therefore, experimental and field work has been conducted to understand the break-up of waves in different situations and some explanation have been proposed to the instabilities that create the spray formation. In this work, two different situations of wave breaking were studied: 1. Solitary waves were created and steepened by the use of a beach. The waves impacted on a vertical wall with different wall heights. 2. Violent plunging breakers were created by a focusing wave train and a sloping beach. The main objective of these experiments was to quantify the production of droplets from the impact by using Particle Tracking Velocimetry in 3 dimensions. It was found that the initial distribution of droplet sizes is similar in both experiments. These distributions are compared with previous studies, where the distribution of droplet sizes in different experimental cases were approximated by lognormal, Weibull or G-distributions respectively.

On the uncertainty of defocus methods for 3D particle tracking velocimetry

Defocus methods have become more and more popular for the estimation of the 3D position of particles in flows (Cierpka and Kahler, 2011; Rossi and K ¨ ahler, 2014). Typically the depth positions of particles are ¨ determined by the defocused particle images using image processing algorithms. As these methods allow the determination of all components of the velocity vector in a volume using only a single optical access and a single camera, they are often used in, but not limited to microfluidics. Since almost no additional equipment is necessary they are low-cost methods that are meanwhile widely applied in different fields. To overcome the ambiguity of perfect optical systems, often a cylindrical lens is introduced in the optical system which enhances the differences of the obtained particle images for different depth positions. However, various methods are emerging and it is difficult for non-experienced users to judge what method might be best suited for a given experimental setup. Therefore, the aim of the presentation is a thorough evaluation of the performance of general advanced methods, including also recently presented neural networks (Franchini and Krevor, 2020; Konig et al., 2020) based on typical images.

3D-PTV flow measurements of Newtonian and non-Newtonian fluid blending in a batch reactor in the transitional regime

Lagrangian trajectories obtained through 3D Particle Tracking Velocimetry (3D-PTV) measurements have been used to visualize the flow field of Newtonian and non-Newtonian fluids in a flat-bottomed vessel. The vessel, of diameter T=180 mm, was equipped with a 6-blade Rushton turbine of diameter D=T/3 and four baffles of width b=T/10. The experiments were carried out in the transitional flow regime (73≤Re≤1,257). The velocities and Lagrangian accelerations in the flows have been calculated from the time-resolved tracer coordinates. Non-Newtonian fluids exhibited a smaller impeller flow number compared to Newtonian fluids. The distributions of shear rate have been obtained via interpolation of the Lagrangian velocity data in a 3D Eulerian grid. In the impeller region, the mean shear rate was, to a first approximation, proportional to the impeller rotational speed, although a more detailed analysis revealed influences of both rheology and Reynolds number. The mean Lagrangian acceleration scaled with the mean shear rate raised to the power of 1.8.

Light-scattering of tracer particles for liquid flow measurements

A variety of modern flow measurement techniques use tracer particles that should accurately follow fluid motions and should scatter sufficient light to be detectable by imagining systems. These two requirements are at odds if they are to be full-filled by varying the tracer size. For this reason, other particle properties such as material, structure, and coating are also considered. While the effect of these properties on the particle response time can be estimated, it is challenging to quantify their effect on the scattered light using the Mie scattering theory. To address this issue, we investigated the light scattering properties of several commercially available tracer particles and provided simple guidelines for selecting appropriate particles. The investigations were carried out using particle images recorded in forward, side, and backward-scatter angles that are typically used in 3D-particle tracking velocimetry. The selected particles represent a wide spectrum of particle sizes and included glass, polymer, and fluorescent particles used in liquid flows. Other properties such as hollow structures and metallic coatings were also investigated. The results showed that glass particles had greater light scattering in the forward-scatter direction, while the polystyrene particles scattered more light in the back-scatter direction. The fluorescent particles had a relatively narrow intensity distribution with a strong side-scatter. We found that silver-coated glass particles had two to four times higher image intensity in the side and back-scatter cameras when compared with uncoated glass particles. The hollow glass particles had a higher forward-scatter compared with the solid glass particles. The recorded images were also used to obtain 3D particle tracks. A large intensity variation was observed along the 3D tracks that was mainly associated with the discretization of particle images on the camera sensor.