Droplet Trajectories Research Articles

Mathematical and computer simulation of particle redistribution and inertial swarming in dispersed systems

<span>This work is devoted to modeling the motion of a dispersed phase in a gas flow in two cases: i) the dynamic characteristics of the gas flow (velocity components, stream functions and vorticity, ii) are found by numerically solving the Navier-Stokes equations with the subsequent construction of the droplet trajectory; study of the coalescence of droplets in a given model gas flow with analysis of the swarming of dispersed particles. In the first approach, the size and location of vortex and stagnant zones in a channel with a streamlined plate and with a turning flow are determined by numerically solving the equations of gas dynamics in Helmholtz variables. To study the coalescence of drops that occurs when the trajectories of two or more drops come into contact, a second approach has been developed-a computer model of inertial swarming in dispersed systems using the Delphi10 programming environment based on stochastic lattice algorithms and computable lattice enlargement. This approach is needed to pre-define the media flow rate profile. Also in this article, were analyzed the grouping and redistribution of particles for the manifestation of the "swarm of drops" effect.</span>

Experimentally validated high-fidelity simulations of a liquid jet in supersonic crossflow

Utilizing recent advancements in computational schemes for compressible, multiphase flows , this work features a parametric study of a pure liquid jet in supersonic crossflow that involves simulating the atomization process for four values of momentum-flux ratio. These simulations are validated against experimental results measured with high-speed X-ray imaging, which confirm the accuracy of the numerical approach. Also, the effect of numerical resolution on some flow behavior is investigated, revealing convergence of the jet shape and surface instability wavelength. Analysis of the resulting sprays includes statistical descriptions of the liquid distribution, liquid structures created through breakup, interfacial instabilities, and dominant flow features. As the flowrate increases, the spray penetrates further, it becomes more disperse, and less liquid impacts the wall, but the droplet size distribution changes little. The wavelength of instabilities on the windward side of the jet diverges from measured trends in subsonic crossflows. In a visualization of the time-averaged flow, counter-rotating vortices are observed along the jet core and in the wake, affecting the process of primary atomization and early droplet trajectories. • Large Eddy Simulations of liquid jet in supersonic crossflow at high Weber number. • Quantitative validation against experimental results using equivalent path length. • Shear-driven instability wavelengths differ from subsonic crossflow analogy. • Analysis of spray statistics and mean flow features.

Electric-Field-Assisted Ultrasonic Spray Pyrolysis of Solid State Electrolytes

Solid polymer electrolytes (SPEs) hold the potential to revolutionize energy storage by enabling secondary batteries containing lithium metal anodes, which have ultra-high theoretical capacity of 3860 mAh g−1, compared to a typical graphite anode’s theoretical capacity of 372 mAh g−1. The bulk of SPE research focuses on optimizing SPE chemistry, while the manufacturing process receives little attention. In this interdisciplinary study, as shown in Figure 1, an electric-field-assisted ultrasonic spray pyrolysis (EFAUSP) setup is proposed and integrated with a cyclone-separator, which was simulated ANSYS Fluent, then built based on the results. Digital in-line holography is performed to determine the droplet size distribution and investigate the droplet trajectories near the substrate surface. This newly proposed EFAUSP setup is based on a 3D printer controller, promising macrostructure and microstructure control in future work. Future work will build upon these results to investigate the mechanical and electrochemical properties of the deposited SPEs. Figure 1

Velocity and size quantification of drops in single and collective bursting bubbles experiments

Droplet production from bursting bubbles has been extensively studied for single bubbles but remains sparsely quantified in controlled collective settings. This article explores how the trajectories of droplets produced by interacting bursting bubbles can be used to track back the mode of production, for clean and contaminated water. Noticeably, it compares velocity-size relationships, reviewed for jet and film drops from individual bubbles, with the measurements made in collective experiments.

Investigation of bypass-coupled double-pulsed directed energy deposition of Al–Mg alloys

Wire and arc additive manufacturing (WAAM) is a promising directed energy deposition (DED) technique for Al–Mg alloy parts. However, excessive workpiece heat input may limit its high productivity application. Hence, a bypass-coupled double-pulsed WAAM (BCDP-WAAM) method is developed to control the workpiece heat input without deteriorating productivity. High-speed camera images demonstrated that the arc and droplet trajectory were deflected towards the molten pool side in BCDP-WAAM. The observed strong-to-weak change in the arc necessitated a periodic insertion of the bypass-cathode wire into the molten pool for heat absorption. Spray and bridging transfers occurred in the anode and bypass-cathode wires, respectively. Current–voltage waveforms indicated that a dynamic current distribution between the workpiece and bypass-cathode wire was achieved in BCDP-WAAM, decreasing the workpiece current. Consequently, the workpiece heat input was decreased by 38.2 %. Owing to the twin-melted wires, the height and effective area were increased by 134.1 % and 149.0 % in BCDP-WAAM, respectively. The obtained electron back-scattered diffraction (EBSD) results indicated that the average grain size was reduced to 21 µm in BCDP-WAAM compared to a grain size of 36 µm in double-pulsed WAAM. This is attributed to the reduction in the workpiece heat input and the change in solidification induced by the bypass-cathode wire. The BCDP-WAAM enhanced the mechanical strength of the part. Furthermore, the ultimate tensile strength (UTS) and yield strength (YS) of the optimized part increased to 256 and 124 MPa in BCDP-WAAM, representing increments of 7.1 % and 14.8 %, respectively. The average micro-hardness was enhanced to 88.2 HV, representing an increase of 16.1 %. • The arc and droplet trajectory were deflected towards the molten pool side. • Bypass-cathode wire periodically inserted into the molten pool for absorbing heat. • A dynamic current distribution between workpiece and bypass-cathode wire occurred. • Reduced workpiece heat input and improved productivity simultaneously occurred.

Numerical model to study the statistics of whole milk spray drying

Using a numerical model, this study investigates the spray drying process of whole milk by providing statistics on droplet conditions at exit and first impact with the surfaces of the chamber. A comprehensive four-stage droplet evaporation model was validated against an experiment and then coupled to an Euler–Lagrange model for simulating the milk droplet trajectories inside a dryer. Results show that larger droplets remain for a shorter time in the chamber and contain more moisture on exiting. Higher injection angles result in longer residence times, which leads to lower moisture content for the droplets. By increasing the injection velocity of droplets, their relative velocity compared to the airflow increases, raising the rate of evaporation, and consequently more droplets exit the chamber as fully dried. Furthermore, decreasing the airflow rate and humidity, as well as increasing the airflow inlet temperature, results in a lower moisture content in the final powder. When the droplets impacted the wall of the chamber for the first time, nearly 67% of them were fully dried, and 55% had a velocity magnitude less than 1 m/s. A considerable portion of droplets (close to 52%) impacted the wall at an angle of less than 8°. • A modelling approach to better understand the drying of milk droplets in spray dryers is presented. • This approach includes modelling the flow, particle tracking, and a comprehensive drying model. • The validate drying model captures temperature and moisture content distribution. • This model is then used to extract statistical information about the drying state of milk droplet. • The statistical study provides insight into the effects of key operating parameters.

An experimental Stommel Retention Zone facility with an application to oil droplet dispersion

AbstractStommel Retention Zones (SRZs) associated with wind‐driven Langmuir circulation (LC) facilitate the subsurface retention of particles and thus affect processes such as oil spill dispersion, sediment suspension, oil–particle aggregation, and plankton distribution. Since the effect of SRZs on these processes is difficult to study in the field, we present a novel laboratory facility designed to recreate the counterrotating vortex pair flow pattern associated with small‐scale SRZs of various strengths in two dimensions (2D). We then inject an oil jet into this facility to study buoyant oil droplet retention in these SRZs. 2D particle image velocimetry and hybrid Reynolds‐averaged Navier–Stokes/large eddy simulation are used to characterize the flow. Large‐scale oil droplet trajectories in the SRZs are visualized, and oil droplet statistics in the downwelling region and vortex core are measured using high‐magnification brightfield imaging and a droplet detection algorithm. Values of maximum velocity (7.7–32.2 mm s−1), turbulent kinetic energy (10−6 to 10−4 m2 s−2), and turbulent kinetic energy dissipation rate (10−7 to 10−5 m2 s−3) in the downwelling region of the facility fall within the range of values observed in the field. Flow field strength strongly influences oil droplet trajectories, the shape and size of the SRZ, and oil droplet statistics over time, with stronger flows retaining larger oil droplets in the downwelling region over longer periods of time. Indeed, droplets of almost 400 μm diameter are observed in the downwelling region after almost 10 min for the strongest flow. The facility may be used to study the behavior of microplastics, sediments, and zooplankton in LC in the future.

Towards a Model for Simulating Collision of Multiple Water Droplets Flow Down a Leaf Surface

In the current article, a physics-based mathematical model is presented to generate realistic trajectories of water droplets across the Frangipani leaf surface and can be applied on any other kind of leaves, which is the first in the series of two articles that we are going to present the second article later on. In the second article, we will study the collision between the droplet and the liquid streak. The model has many applications in different scientific and engineering fields, such as modelling pesticide movements on leaves surfaces and modeling absorption and nutrition systems. The leaf surface consists of a triangular mesh structure that needs to be constructed using different techniques such as a well-known technique called EasyMesh method. The leaf surface is constructed using surface fitting techniques, such as finite elements methods and Clough-Tocher method, using a set of 3D real-world data points collected by a laser scanner, and the motion of the droplet on each triangle is calculated using a derived equation of motion. The motion of the droplet affected different forces, such as gravity and drag forces. Simulations of the model were verified using Matlab programming, and the results seemed to be real and capture the droplet motion very well.

Real-time Multiple-particle Tracking in Ultrasonic Spray Pyrolysis

Solid polymer electrolytes (SPEs) hold the potential to revolutionize energy storage by enabling secondary batteries containing lithium metal anodes, which have an ultra-high theoretical capacity of 3860 mAh g−1, compared to a typical graphite anode’s theoretical capacity of 372 mAh g−1. The bulk of SPE research focuses on optimizing SPE chemistry, while the manufacturing process receives little attention. In this interdisciplinary study, an electric-field-assisted ultrasonic spray pyrolysis (EFAUSP) setup is proposed and integrated with a cyclone-separator, which was simulated ANSYS Fluent, then built based on the results. Digital in-line holography is performed to determine the droplet size distribution and investigate the droplet trajectories near the substrate surface. This newly proposed EFAUSP setup is based on a 3D printer controller, promising macrostructure and microstructure control in future work. Future work will build upon these results to investigate the mechanical and electrochemical properties of the deposited SPEs.

65 kHz picosecond digital off-axis holographic imaging of 3D droplet trajectory in a kerosene swirl spray flame

It is difficult to track fuel droplets in a swirl spray combustion field due to three-dimensional (3D) movement and turbulent flame influence. The present work carries out an experimental study to investigate and track the dispersed kerosene droplets in a swirl spray combustion field. RP-3 kerosene and air are mixed and burned in open space under different mixture ratios. Pulsed high-speed (65 kHz) digital off-axis holography with an equivalent pixel size of 9.3 μm is adopted to visualize and quantify the droplet size, 3D position and velocity. The measurement uncertainty of the technique is calibrated and discussed. The turbulent flame of hydrocarbon fuel causes some darker speckles in the morphology view, but droplet images are clear and the initial droplet size distribution at 1 mm downstream of the swirler is obtained. A particle tracking algorithm considering x, y and z with different locating errors and droplet diameter is proposed and applied to obtain droplet trajectory. Smoothed droplet trajectories are obtained through multi-frame tracking and trajectory regressing methods. From the perspective of the Lagrangian view, droplet momentum and size oscillation are also observed and discussed.

Influence of torsion on icing process of transmission lines

Icing threatens the safe and stable operation of transmission lines. Accurate prediction of the icing process of conductor can help the work of anti-icing and disaster reduction of transmission lines. Generally, the icing of conductor is accompanied by the torsion process of conductor. Because of the interaction of conductor torsion and icing, there is a big difference of the icing shape and rate between different positions on a transmission line. In order to improve the accuracy of numerical calculation of conductor icing, a dynamic numerical calculation model of conductor icing torsion is established based on hydrodynamics and basic mechanics in this paper. Comprehensively, the model takes the effects of torsion angle and torsion icing shape on the icing process into consideration, including the effects on airflow filed and trajectories of water droplets in the air. Through the simulation, the change law of the torsion angle at different positions of the conductor, the influence characteristics of the conductor torsion on the icing process, and the differences of the icing torsion of conductor under different environmental parameters are analysed. The results show that, compared with the condition without torsion, the collision range of water droplets on the conductor is larger under the torsion condition, the average value of collision efficiency β1 is higher, and the icing rate is faster. The larger the wind velocity and the median volume diameter of water droplets in the air, the faster the conductor icing torsion development.

The effects of indoor temperature and humidity on local transmission of COVID-19 and how it relates to global trends.

During the COVID-19 pandemic, analyses on global data have not reached unanimous consensus on whether warmer and humid weather curbs the spread of the SARS-CoV-2 virus. We conjectured that this lack of consensus is due to the discrepancy between global environmental data such as temperature and humidity being collected outdoors, while most infections have been reported to occur indoors, where conditions can be different. Thus, we have methodologically investigated the effect of temperature and relative humidity on the spread of expired respiratory droplets from the mouth, which are assumed to be the main cause of most short-range infections. Calculating the trajectory of individual droplets using an experimentally validated evaporation model, the final height and distance of the evaporated droplets is obtained, and then correlated with global COVID-19 spread. Increase in indoor humidity is associated with reduction in COVID-19 spread, while temperature has no statistically significant effect.

A rapid method for prediction of airborne disease infection risks in an intercity bus

In an intercity bus, respiratory infectious diseases put passengers at high risk of getting infected by the droplets exhaled by an infected person, and the risk increases when exposed to more droplets. Here, to quickly determine the concentration distribution of droplets, to predict the infection risks in a closed space, and to enhance the reliability of the conventional steady-state particle tracking method for predicting the trajectory of droplets released by coughing or sneezing, an improved steady-state particle tracking method is proposed. In it, the momentum of released droplets previously ignored in the conventional steady-state particle tracking method was specifically incorporated using experimental data. Then, the improved method was combined with a random walk model and applied to investigate all possible trajectories of droplets released by different passengers inside a bus. Consequently, the concentration distribution of droplets was obtained from the trajectory information. Finally, the Wells–Riley equation was used to predict the infection risk of every passenger based on the evaluated number of droplets inhaled per passenger. The results show that the improved steady-state tracking method performs more accurately at predicting the concentration field of droplets and associated infection risk than the conventional steady-state particle tracking method. Furthermore, the relative cost of the improved steady-state tracking method is just 1% of the transient calculation method currently considered the most accurate.

Experimental study on droplet breakup and droplet particles diffusion of a pressure nozzle based on PIV

• The turbulence of dust fall spray field are studied for the first time. • The I in the axial direction has an exponential relationship with the distance. • Droplet breakup mainly occurs in the range of 0–4 mm area near strong turbulence. • The droplet size and turbulence intensity change linearly in the range of 300 mm. • When the I reaches 3.35 or more, the droplets are broken more completely. The x-swirl nozzles has been widely used in the field of spray technology for dust reduction. In this study, experimental studies on the droplet breakup and droplet particles diffusion of X-swirl nozzles by particle image velocimetry (PIV). Based on the experimental results, the velocity fields at five different water supply pressures were compared, and 6 MPa was chosen as the water supply pressure to study droplet breakup and droplet particles diffusion. The results show that, turbulence can change the trajectory of droplets and have an important impact on the atomization process. The variation law of turbulence intensity in axial and radial directions is revealed. The turbulence intensity ( I ) in the spray field axial direction continuously decreased as the distance increased and had an overall exponential relationship. I within 2.5 mm in the radial direction of the spray field’s center rapidly decreased, but the downward trend slightly decelerated in the area exceeding 2.5 mm. Combined with the parameters of strain rate ( S ) and vorticity ( Ω ), the comparative analysis is carried out. When the gas and liquid are fully mixed, the turbulence intensity increases rapidly. This promotes the growth, deformation and breakup of droplets, and affects the diffusion of droplets. Through the probability density function, it is revealed that droplet breakup mainly occurs in the range of 0–4 mm near strong turbulence. The droplet size at different locations was measured by Phase Doppler Interferometer (PDI), which revealed the mechanism of turbulent flow on the droplet size. In the range of 0–300 mm, the droplets with larger initial particle size and velocity gradually evolve into droplets with small particle size, small velocity and large number over time. But in the 300–450 mm range, the turbulent flow’s effect on the breakup gradually decreased with the droplet movement time. Most of the droplets have finished breaking. Due to coalescence between small droplets, the particle sizes tended to slowly increase.

Analysis of Numerical Methods for Droplet Impingement Characteristics under Aircraft Icing Conditions

The investigation of super-cooled droplet impingement characteristics is the most important step for aircraft icing and anti-icing/de-icing analyses. The Lagrangian method and the Eulerian method are widely used to compute the droplet motion and collection efficiency, and the two methods are considered to obtain almost the same results for surface impingement characteristics under icing conditions. The models and implementation approaches of the two methods were established in this work, and the simulations of droplet motion were carried out for a NACA 0012 airfoil, a 2D section of an A320 head, a multi-element airfoil, and an icing wind tunnel. The collection efficiencies of the NACA 0012 airfoil obtained by the present Lagrangian and Eulerian methods show good agreement with the results in the literature, validating the established methods. The droplet impingement characteristics of the two methods are consistent for the aircraft surfaces without upstream trajectory deflections. However, when the droplet motion is deflected by the frontal body before hitting the rear surfaces, the results obtained by the two methods are different whether the droplet trajectories intersect or not, which subverts the traditional opinion that the Lagrangian and Eulerian methods would obtain the same result of the droplet impingement characteristics. The reason is studied in detail according to the droplet motion results in the icing wind tunnel. The findings of this work are helpful for the accuracy of aircraft icing and anti-icing/de-icing simulations, and useful for the development of airworthiness certification.

Overload wave-memory induces amnesia of a self-propelled particle

Information storage is a key element of autonomous, out-of-equilibrium dynamics, especially for biological and synthetic active matter. In synthetic active matter however, the implementation of internal memory in self-propelled systems is often absent, limiting our understanding of memory-driven dynamics. Recently, a system comprised of a droplet generating its guiding wavefield appeared as a prime candidate for such investigations. Indeed, the wavefield, propelling the droplet, encodes information about the droplet trajectory and the amount of information can be controlled by a single scalar experimental parameter. In this work, we show numerically and experimentally that the accumulation of information in the wavefield induces the loss of time correlations, where the dynamics can then be described by a memory-less process. We rationalize the resulting statistical behavior by defining an effective temperature for the particle dynamics where the wavefield acts as a thermostat of large dimensions, and by evidencing a minimization principle of the generated wavefield.

Large-Eddy Simulation of two secondary air supply strategies in kraft recovery boilers

Kraft recovery boilers are large scale combustion applications operating on black liquor, a common side-product of the pulp industry. Here, a simplified boiler model utilizing Computational Fluid Dynamics (CFD) with Large-Eddy Simulation (LES) and Lagrangian Particle Tracking (LPT) is explored to better understand the secondary air supply system and the dispersion of sprayed droplets. The unsteady nature of the air jets and droplet dispersion in such context advocates the usage of scale-resolving simulations such as LES. In the present exploratory study, the usage of LES in recovery boiler air jet simulations is piloted for the first time. The air supply system is modeled as high-momentum-flux jets injected to a uniform cross-flow. First, the set-up was verified by performing a mesh sensitivity study. The main observed global flow features included the mixing zones, wall jet and jet impact regions, jet bending in cross-flow, and reverse flow downstream of the jets. Two different engineering relevant air supply systems, a staggered and an in-line configuration, were studied and compared in terms of mixing and droplet dispersion. The main findings of the present study are as follows. First, out of the two studied configurations, the staggered one was observed to provide a more uniform downstream temperature field. Second, the in-line configuration was noted to outperform the staggered configuration in droplet dispersion by having 22% less spray-wall impingement and 15% more droplets landing at the bottom of the furnace. Third, different modes for droplet trajectories were identified based on the global flow structures.

Safe Distance of Virus Quantitative Analysis and Simulation of the Trajectory of Pathogen-Containing Droplets in the Air Respiratory Airways

Droplets of mucosalivary ejecta emitted by sneezing or coughing are a major carrier of numerous types of bacterial and viral diseases. This study develops a numerical model to estimate the spread distance for inhalable droplets (1–50 μm) in the air, the inhalability of the particles, and the trajectory as well as velocity of these pathogen-containing droplets in human respiratory airways. Moreover, particularly for droplets with diameters of 1 μm, 5 μm, 10 μm, and 50 μm, specific comparisons between their inhalability and transmission velocities are made. Data extracted from previous experiments proceeded by other researchers discussing the visualization of sneeze ejecta and deposition features of inhaled drops were used to obtain parameters to fit the model prediction of this work. Currently, research on similar topics was mostly based on either experiments or theoretical calculations only on one specific clan of pathogen, while the novel contribution of this paper is the combination and comparison of these two distinct methodologies that can be applied to solve a general practical problem aiming to all types of viruses by considering the pathogen-containing droplets as a whole entity.

Binary droplet interactions in shear water-in-oil emulsion: A molecular dynamics study

The shear field and binary droplet interaction regimes of water-in-oil emulsions between parallel gold plates were investigated by molecular dynamics (MD) methods. The developed MD model was subjected to dimensional analysis and the dimensionless numbers that guarantee the similarity between the model and the actual system were identified as the capillary number (Ca), Reynolds number (Re), Weber number (We), and dimensionless time (τ). The effects of different pressures were compared by applying different forces perpendicular to the shear direction to the parallel plates. The heptane formed adsorbed layers on the gold plates, exhibited a shear thinning characteristic, and formed a no-slip uniform shear field driven by the parallel plates. The interactions between gold atoms and a heptane molecule have been investigated in depth by quantum chemical methods and it has been found that the nature of the adsorption is a van der Waals interaction. Three main binary droplet interaction regimes are produced in the presence of shear fields: coalescence, a temporary bridge formation, and sliding. The effects of capillary forces and shear effects on liquid bridges were discussed. Droplet trajectories and deformations under different pressure were analyzed. Finally, the effect of initial offset (ΔYin/2R) and Ca on the droplet interaction regime was investigated through regime maps. It was found that the coalescence regime has upper critical points for ΔYin/2R and Ca; droplet slipping occurs mainly at higher ΔYin/2R; the temporary bridge formation regime appears mainly at higher Ca. This study provides a new idea for understanding binary droplet interactions in shear emulsions.

Phase Doppler Anemometer - The Shift From Simulation To The Real World

A Monte Carlo type simulation program has been developed to model phase Doppler systems. The work was divided into four main sections, the development of a launch strategy of a known range of droplet sizes and trajectories, the determination of the scattered light fields using Mie theory, the generation of the photomultiplier signals, and then their processing to generate drop size estimates followed by the comparison of these data with the launched data. The signal processing portion of the simulation measured the Doppler frequency in each signal to estimate the droplet velocity. Rather than using measurements of the phase differences between the multiple Doppler-shifted signals, the time shift between the arrival times of each signal was used to estimate the dropsize. This provided a linear relationship between the dropsize and the signal time shift. The missing link in the simulation is the ability to guide the experimentalist as to the best way to acquire the signal bursts and determine how accurate the measurements would be when using a PDA system to determine the droplet size distributions in real spray flows. Thus, the only way to determine measurement accuracy is to include that in the signal processing of an actual PDA system and spray. In an assessment of the capabilities of the signal processing technique, a sequence of Doppler bursts from an actual spray characterization were recorded and used as input to the software signal processor. The Time Shift Technique was based on the use of Weighted Average calculations to determine the exact middle of each of the three photomultiplier signal bursts. However, it was found that good quality signal bursts are required. Since cycle measurement techniques are less effected by noise, a cycle technique based on the samples captured by the AD converter was added to the software. Whereas this technique did show an improvement in measurement accuracy it led to the development of a cycle technique based on the signal burst frequency. This involved a reconstruction of the original captured bursts. The measurement errors were found to be in the micron range.