Particle Tracking Measurements Research Articles

Exploring Electrohydrodynamic Flows Inside Leaky Dielectric Drops: A Laser Velocimetry Approach

A novel experimental methodology based on laser velocimetry for the investigation of electrohydrodynamic (EHD) flows inside a neutrally buoyant leaky dielectric drop of 2.25 mm of radius is proposed. Utilizing fluorescent particles, 2D and 3D Lagrangian Particle Tracking (LPT) measurements of the EHD flows inside an initially spherical drop under a constant electric field are reported. Two leaky dielectric fluids, namely Silicone and Castor oils, were used as either the drop or the medium phases, depending on the case investigated, thus covering oblate and prolate drops both with small and larger deformations. 2D measurements of the trajectories and velocities of the tracer particles in a Lagrangian referential enabled a direct comparison with the leaky dielectric model (LDM), a well-stablished theory covering drops with smaller deformations, showing a good agreement with our measurements. The symmetry inside a drop with increased deformation is investigated from an analysis of the mean 3D velocity field, whose measurements converge to the two-dimensional analysis at the plane of symmetry of the drop, suggesting symmetrical flow structures inside the drop.

Ghost Red Blood Cells For Enhanced Particle Detection With Defocusing PTV In Microcirculation Experiments

Particle tracking of rigid fluorescent particles in microcirculation environments is often found to be difficult, since the presence of red blood cells (RBCs) at physiologically relevant conditions (i.e., hematocrit) masks the particle images. In turn, making particle detection challenging. To overcome this difficulty, we propose to use ghost RBCs for enhanced particle detection. These are quasi-transparent RBCs obtained by removing the hemoglobin from normal RBCs. To understand whether the ghost RBCs can improve the particle detection, we performed defocusing particle tracking measurements with the general defocusing particle tracking (GDPT) method of the rigid fluorescent particles in normal and ghost RBC flows. The measurements were performed at physiological relevant hematocrit and for three different flow rates. From the three-dimensional particle trajectories, we evaluated the correlation coefficient from the GDPT evaluations and compared the fluid dynamic behavior of the two types of cells. Overall, the results show that ghost RBCs are an attractive solution for this type of experiments, since they improve the correlation coefficients up to values similar to cell-free experiments, while exhibiting a fluid dynamic behavior similar to normal RBCs i.e., in terms of velocity and relative viscosity data.

Ghost Red Blood Cells For Enhanced Particle Detection With Defocusing PTV In Microcirculation Experiments

Particle tracking of rigid fluorescent particles in microcirculation environments is often found to be difficult, since the presence of red blood cells (RBCs) at physiologically relevant conditions (i.e., hematocrit) masks the particle images. In turn, making particle detection challenging. To overcome this difficulty, we propose to use ghost RBCs for enhanced particle detection. These are quasi-transparent RBCs obtained by removing the hemoglobin from normal RBCs. To understand whether the ghost RBCs can improve the particle detection, we performed defocusing particle tracking measurements with the general defocusing particle tracking (GDPT) method of the rigid fluorescent particles in normal and ghost RBC flows. The measurements were performed at physiological relevant hematocrit and for three different flow rates. From the three-dimensional particle trajectories, we evaluated the correlation coefficient from the GDPT evaluations and compared the fluid dynamic behavior of the two types of cells. Overall, the results show that ghost RBCs are an attractive solution for this type of experiments, since they improve the correlation coefficients up to values similar to cell-free experiments, while exhibiting a fluid dynamic behavior similar to normal RBCs i.e., in terms of velocity and relative viscosity data.

Effects of cavity pre-detuning on RF power transients at injection into the LHC

At injection into the Large Hadron Collider (LHC), the radio frequency (RF) system is perturbed by beam-induced voltage resulting in strong RF power transients and the instant detuning of the cavities. The automatic tuning system, however, needs time for the mechanical compensation of the resonance frequency to take place. Acting back on the beam, the transients in RF power are expected to limit the maximum injected intensity by generating unacceptable beam loss. Reducing them is therefore essential to reach the target intensity during the High Luminosity (HL) LHC era. At LHC flat bottom, the cavities are operated using the half-detuning beam-loading compensation scheme. As implemented today, the tuner control algorithm starts acting only after the injection of the first longer bunch train which causes the bunches for this injection to experience the largest power spikes. This contribution presents an adapted detuning scheme for the RF cavities before injection. It was proposed as a path to decrease the transients, hence increasing the available intensity margin for the available RF power. The expected gain is evaluated in particle tracking simulations and measurements acquired during operation.

A polynomial model with line-of-sight constraints for Lagrangian particle tracking under interface refraction

This paper introduces an improvement of the ‘shake-the-box (STB)’ (Schanz, Gesemann, and Schröder, Exp. Fluids 57.5, 2016) technique using the polynomial calibration model and the line-of-sight constraints (LOSC) to overcome the refractive interface issues in Lagrangian particle tracking (LPT) measurement. The method (named LOSC-LPT) draws inspiration from the two-plane polynomial camera calibration in tomographic particle image velocimetry (Worth, Nickels, Thesis, 2010) and the STB-based open-source Lagrangian particle tracking (OpenLPT) framework (Tan, Salibindla, Masuk, and Ni, Exp. Fluids 61.2, 2019). The LOSC-LPT introduces polynomial mapping functions into STB calibration in conditions involving gas–solid–liquid interfaces at container walls exhibiting large refractive index variations, which facilitates the realization of particle stereo matching, three-dimensional (3D) triangulation, iterative particle reconstruction, and further refinement of 3D particle position by shaking the LOS. Performance evaluation based on synthetic noise-free images with a particle image density of 0.05 particle per pixel in the presence of refractive interfaces demonstrates that LOSC-LPT can detect a higher number of particles and exhibits lower position uncertainty in the reconstructed particles, resulting in higher accuracy and robustness than that achieved with OpenLPT. In the application to an elliptical jet flow in an octagonal tank with refractive interfaces, the use of polynomial mapping results in smaller errors (mean calibration error <0.1 px) and thus more long trajectories identified by LOSC-LPT (13 000) compared with OpenLPT (4500) which uses the pinhole Tsai model (mean calibration error >1.0 px). Moreover, 3D flow-field reconstructions demonstrate that the LOSC-LPT framework can recover a more accurate 3D Eulerian flow field and capture more complete coherent structures in the flow, and thus holds great potential for widespread application in 3D experimental fluid measurements.

A microrheological examination of insulin-secreting β-cells in healthy and diabetic-like conditions.

Pancreatic β-cells regulate glucose homeostasis through glucose-stimulated insulin secretion, which is hindered in type-2 diabetes. Transport of the insulin vesicles is expected to be affected by changes in the viscoelastic and transport properties of the cytoplasm. These are evaluated in situ through particle-tracking measurements using a rat insulinoma β-cell line. The use of inert probes assists in decoupling the material properties of the cytoplasm from the active transport through cellular processes. The effect of glucose-stimulated insulin secretion is examined, and the subsequent remodeling of the cytoskeleton, at constant effects of cell activity, is shown to result in reduced mobility of the tracer particles. Induction of diabetic-like conditions is identified to alter the mean-squared displacement of the passive particles in the cytoplasm and diminish its reaction to glucose stimulation.

Experiments on large-scale structures in fully developed turbulent pipe flow

The experimental results reported in present paper represent contributions from the Cottbus large pipe facility to DFG-priority program SPP 1881: Turbulent Superstructures. Fundamental questions regarding turbulence statistics and structures in fully developed turbulent pipe flow were addressed, utilizing thermal and optical measuring techniques integrated into the pipe facility. The facility enables unique in-situ calibration for the hot-wire probe and it has full optical access that allows performing Particle Image Velocimetry and 3D Lagrangian Particle Tracking measurements, resulting in consistent and accurate pipe flow data. A streamwise-spanwise field of view up to ≈20×2 pipe radii has been set for both optical techniques with 4–8 high resolution cameras for a friction Reynolds number range of 8⋅102≤R+≤18⋅103. The setup enabled field of measurements with an unprecedented dynamic range in turbulent pipe flow at high Reynolds numbers with spatial resolution better than 1 vector/mm. Proper Orthogonal Decomposition and Shake-The-Box algorithm were used, highlighting energy contents of the large-scale motions in fully developed turbulent pipe flow regime.

High-speed volumetric particle tracking measurements of unstable helical vortex pairs

We present results from an experimental study investigating pairs of helical vortices generated by a one-bladed rotor in hovering conditions. Time-resolved volumetric Lagrangian particle tracking measurements are conducted in a water tunnel to analyze the three-dimensional development of the vortex system. The vortex pairs are generated by a specific tip design, which allows splitting the single tip vortex into two vortices, whose characteristics depend on the geometric fin parameters. The objective of this procedure is the modification of the tip vortex structure, in order to minimize negative effects caused by fluid–structure interactions in applications involving rotors. Certain vortex configurations are affected by centrifugal instabilities, which result in an immediate pronounced growth of the vortex cores. As a consequence of the instability, secondary vortex structures are formed between the unstable cores. The presence of these structures results in an accelerated break-up of the cores, causing them to merge. In order to investigate the influence of the trailing vorticity layer shed from the inner part of the blade, two blade designs with different radial circulation distributions are considered. The measurements are able to track the evolution of the vorticity layer and the secondary structures, providing new insights into the instability of closely spaced vortex pairs with varying circulation ratios.

Magnetohydrodynamics in free surface liquid metal flow relevant to plasma-facing components

While flowing Liquid Metal (LM) Plasma-Facing Components (PFCs) represent a potentially transformative technology to enable long-pulse operation with high-power exhaust for fusion reactors, Magnetohydrodynamic (MHD) drag in the conducting LM will reduce the flow speed. Experiments have been completed in the linear open-channel LMX-U device [Hvasta et al 2018 Nucl. Fusion 58 01602] for validation of MHD drag calculations with either insulating or conducting walls, with codes similar to those used to design flowing LM PFCs for a Fusion Nuclear Science Facility [Kessel et al 2019 Fusion Sci. Technol. 75 886]. We observe that the average channel flow speed decreased with the use of conducting walls and the strength of the applied transverse magnetic field. The MHD drag from the retarding Lorentz force resulted in an increase of the LM depth in the channel that ‘piled up’ near the inlet, but not the outlet. As reproduced by OpenFOAM and ANSYS CFX calculations, the magnitude and characteristics of the pileup in the flow direction increased with the applied traverse magnetic field by up to 120%, as compared to the case without an applied magnetic field, corresponding to an average velocity reduction of ∼45%. Particle tracking measurements confirmed a predicted shear in the flow speed, with the surface velocity increasing by 300%, despite the 45% drop in the average bulk speed. The MHD effect makes the bulk flow laminarized but keeps surface waves aligned along the magnetic field lines due to the anisotropy of MHD drag. The 3D fringe field and high surface velocity generate ripples around the outlet region. It was also confirmed that the MHD drag strongly depends on the conductivity of the channel walls, magnetic field, and volumetric flow rate, in agreement with the simulations and a developed analytical model. These validated models are now available to begin to determine the conditions under which the ideal LM channel design of a constant flow speed and fluid depth could be attained.

Effect of Retrofit Design Modifications on the Macroturbulence of a Three-Phase Flotation Tank-Flow Characterization Using Positron Emission Particle Tracking (PEPT).

Turbulence in stirred tank flotation tanks impacts the bulk transport of particles and has an important role in particle-bubble collisions. These collisions are necessary for attachment, which is the main physicochemical mechanism enabling the separation of valuable minerals from ore in froth flotation. Modifications to the turbulence profile in a flotation tank, therefore, can result in improvements in flotation performance. This work characterized the effect of two retrofit design modifications, a stator system and a horizontal baffle, on the particle dynamics of a laboratory-scale flotation tank. The flow profiles, residence time distributions, and macroturbulent kinetic energy distributions were derived from positron emission particle tracking (PEPT) measurements of tracer particles representing valuable (hydrophobic) mineral particles in flotation. The results show that the use of both retrofit design modifications together improves recovery by increasing the rise velocity of valuable particles and decreasing turbulent kinetic energy in the quiescent zone and at the pulp-froth interface.

Complementary mass transport investigations in open-cell foams: Full-field CFD simulation with random-walk microscopic particle tracking and PFG NMR displacement measurements

Complementary mass transport investigations in open-cell foams: Full-field CFD simulation with random-walk microscopic particle tracking and PFG NMR displacement measurements

Three-dimensional streaming around an obstacle in a Hele-Shaw cell

Driving oscillatory flow around an obstacle generates, due to inertial rectification, a steady ‘streaming’ flow that is useful in a host of microfluidic applications. While theory has focused largely on two-dimensional flows, streaming in many practical microfluidic devices is three-dimensional due to confinement. We develop a three-dimensional streaming theory around an obstacle in a microchannel with a Hele-Shaw-like geometry, where one dimension (depth) is much shorter than the other two dimensions. Utilizing inertial lubrication theory, we demonstrate that the time-averaged streaming flow has a three-dimensional structure. Notably, the flow reverses direction across the depth of the channel, which is a feature not observed in less confined streaming set-ups. This feature is confirmed by our experiments of streaming around a cylinder sandwiched in a microchannel. Our theory also predicts that the streaming velocity decays as the inverse cube of the distance from the cylinder, faster than that expected from previous two-dimensional approaches. We verify this velocity decay quantitatively using particle tracking measurements from experiments of streaming around cylinders with different aspect ratios at different driving frequencies.

Change of network structure in agarose solution during gelation studied by multiple particle tracking and NMR measurements

The gelation mechanism of agarose solution on cooling has been successfully elucidated by measurements from macroscopic to microscopic scales using a dynamic rheometer, the multiple particle tracking with two different diameters and the pulsed-field-gradient stimulated echo (PGSTE) 1H NMR. The dynamic rheometer detected the gradual increase of G’ in the temperature range from 55 to 40 °C before the steep increase at the gelation temperature. The time-averaged mean square displacement (msd) obtained by multiple particle tracking for particles with diameter of 270 nm and 1000 nm revealed the generation of micro domains with the network structure obstructing particle diffusion. Further, the NMR measurements gave detailed information about the change of molecular mobility for agarose chains during the formation of the micro domains and the macroscopic overall network structure. These results suggest a gelation mechanism and gel network structure including the formation of micro domains in the temperature range from 55 to 40 °C and the connection among the micro domains by agarose aggregates to form the overall network structure at the gelation temperature.

Settling of localized particle plumes in a quiescent water tank

Plumes of particles falling in still water transfer kinetic energy to the fluid and set it in motion. This article presents experimental results of simultaneous particle tracking and fluid flow measurements of dilute particles plumes with varying particle size and density. While the settling speed of the plumes is found to be enhanced by an increase of the particle concentration, their motion relative to the fluid is entirely unaffected. The energy transfer from particles to the fluid scales linearly with their concentration and is more effective for smaller and lighter particles.

Evaluation of coarse-grained CFD-DEM models with the validation of PEPT measurements

Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) is a commonly used numerical method to model gas-solid flow in fluidised beds and other multiphase systems. A significant limitation of CFD-DEM is the feasibility of the realistic simulation of large numbers of particles. Coarse-graining (CG) approaches, through which groups of multiple individual particles are represented by single, larger particles, can substantially reduce the total number of particles while maintaining similar system dynamics. As these three CG models have not previously been compared, there remains some debate, however, about the best practice in the application of CG in CFD-DEM simulations. In this paper, we evaluate the performance of three typical CG methods based on simulations of a bubbling fluidised bed. This is achieved through the use of a numerical validation framework, which makes full use of the high-resolution 3D positron emission particle tracking (PEPT) measurements to rigorously validate the outputs of CFD-DEM simulations conducted using various different coarse-graining models, and various different degrees of coarse-graining. The particle flow behaviours in terms of the particle occupancy field, velocity field, circulation time, and bubble size and velocity, are comprehensively analysed. It is shown that the CG simulation starts to fail when the size ratio between the bed chamber and the particles decreases to approximately 20. It is also observed, somewhat surprisingly, that the specific CG approach applied to interparticle contact parameters does not have a substantial effect on the simulation results for the bubbling bed simulations across a wide range of CG factors.

A data-driven machine learning framework for modeling of turbulent mixing flows

A novel computationally efficient machine learning (ML) framework has been developed for constructing the turbulent flow field of single-phase or two-phase particle-liquid flows in a mechanically agitated vessel by feeding a very short-term experimental Lagrangian trajectory. Using a supervised k-nearest neighbors regressor learning algorithm coupled with a Gaussian process, the framework predicts the mean flow and turbulent fluctuations by sharing the statistical features learned from experimental data. The capability of the ML framework is evaluated by comparing the flow dynamics of predicted trajectories to extensive Lagrangian particle tracking measurements under various flow conditions. Local velocity distributions, Lagrangian statistical analysis, solid concentration distributions, and phase flow numbers show very good agreement between ML-predictions and experiments. Being accurate, efficient, and robust, the ML framework is a powerful tool for analyzing and modeling multiphase flow systems using a minimal amount of driver data input, which can equally be provided from any reliable numerical simulation, thus avoiding costly experimental measurements.

Holographic analysis of colloidal spheres sedimenting in horizontal slit pores.

The mobility of a colloidal particle in a slit pore is modified by the particle's hydrodynamic coupling to the bounding surfaces and therefore depends on the particle's position within the pore and its direction of motion. We report holographic particle tracking measurements of colloidal particles' diffusion and sedimentation between parallel horizontal walls that yield the mobility for motions perpendicular to the walls, including its dependence on height within the channel. These measurements complement previous studies that probed colloidal mobility parallel to confining surfaces. When interpreted with effective-medium theory, holographic characterization measurements yield estimates for the sedimenting spheres' densities that can be compared with kinematic values to draw insights into the spheres' compositions. This comparison suggests, for example, that the silica spheres used in this study are slightly porous, but that their pores are too small for water to penetrate.

Dense interpolations of LPT data in the presence of generic solid objects

Three-dimensional Lagrangian particle tracking measurements with helium filled soap bubbles provide quantitative flow visualizations in large measurement volumes up to the cubic metre scale. However, the instantaneously available fluid information density is severely restricted by the finite spatial resolution of the measurements. Therefore, the use of experimental data assimilation approaches are utilized to exploit the temporal information of the flow measurements, along with the governing equations of the fluid motion, to increase the measurement spatial resolution. Nevertheless, only in the last years, attempts to apply data assimilation methods to enhance the Lagrangian particle tracking (LPT) resolution in proximity of solid boundaries have been performed. Thus, in order to handle generic solid body intrusions within the densely interpolated LPT data, two different approaches based on the computational fluid-structure interaction frameworks are proposed. The introduced variants of the state of the art physics-driven data assimilation methods are assessed with a high fidelity numerical test case of flow over periodic hills. The accuracy superiority of the flow field reconstructions with the proposed approaches are denoted especially in close proximity of the interaction surface. An experimental application of the introduced methods is demonstrated to compute the pressure distribution over an unsteadily moving elastic membrane surface, revealing the time-resolved interaction between the flow structures and the membrane deformations.

Three-Dimensional Sag Tracking in Falling Liquid Films

Coating defects often arise during application in the flash stage, which constitutes the ∼10 min interval immediately following film application when the solvent evaporates. Understanding the transient rheology and kinematics of a coating system is necessary to avoid defects such as sag, which results in undesirable appearance. A new technique named variable angle inspection microscopy (VAIM) aimed at measuring these phenomena was developed and is summarized herein. The essence of this new, non-invasive, rheological technique is the measurement of a flow field in response to a known gravitational stress. VAIM was used to measure the flow profile through a volume of a liquid thin film at an arbitrary orientation. Flow kinematics of the falling thin film was inferred from particle tracking measurements. Initial benchmarking measurements in the absence of drying tracked the velocity of silica probe particles in ∼140 μm thick films of known viscosity, much greater than water, at incline angles of 5° and 10°. Probe particles were tracked through the entire thickness of the film and at speeds as high as ∼100 μm/s. The sag flow field was well resolved in ∼10 μm thick cross sections, and in general the VAIM measurements were highly reproducible. Complementary profilometer measurements of film thinning were utilized to predict sag velocities with a known model. The model predictions showed good agreement with measurements, which validated the effectiveness of this new method in relating material properties and flow kinematics.

Correlated ionisations in two spatially separated nanometric volumes within the track structure of 241Am alpha particles: comparison with Monte Carlo simulations

The production of two double strand breaks in spatially separated locations on the DNA molecule can cause the loss of a whole DNA loop. This loss, which can be of substantial length depending on the geometrical position of the two damaged sites, depends on the degree of correlation between ionisation clusters formed in sites of several nanometres in size. The first part of this paper reported on nanodosimetric measurements of alpha particle tracks in 1.2 mbar H2O, 1.2 mbar C3H8 and 1.2 mbar C4H8O with the PTB ion counter nanodosimeter. In this second part, the focus is on the geometrical characterisation of the two sites simulated with the nanodosimeter in the three target gases and on the comparison of the measurement results with Monte Carlo simulations. The measurements in 1.2 mbar C3H8 were simulated with PTra, a track structure code dedicated to modelling the PTB ion counter nanodosimeter. Further simulations were performed with Geant4-DNA for 241Am alpha particle tracks in liquid water. Simulations of the experiment were found to be in good agreement with the measurements for the investigated irradiation geometries.