Journal articles on the topic 'Droplet size distributions'
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1
Duthon, Pierre, Michèle Colomb, and Frédéric Bernardin. "Fog Classification by Their Droplet Size Distributions: Application to the Characterization of Cerema’s Platform." Atmosphere 11, no. 6 (June 4, 2020): 596. http://dx.doi.org/10.3390/atmos11060596.
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Fog is one of major challenges for transportation systems. The automation of the latter is based on perception sensors that can be disrupted by atmospheric conditions. As fog conditions are random and non-reproducible in nature, Cerema has designed a platform to generate fog and rain on demand. Two types of artificial fog with different droplet size distributions are generated: they correspond to radiation fogs with small and medium droplets. This study presents an original method for classifying these different types of fog in a descriptive and quantitative way. It uses a new fog classification coefficient based on a principal component analysis, which measures the ability of a pair of droplet size distribution descriptors to differentiate between the two different types of fog. This method is applied to a database containing more than 12,000 droplet size distributions collected within the platform. It makes it possible to show: (1) that the two types of fog proposed by Cerema have significantly different droplet size distributions, for meteorological visibility values from 10 m to 1000 m; (2) that the proposed droplet size distribution range is included in the natural droplet size distribution range; (3) that the proposed droplet size distribution range should be extended in particular with larger droplets. Finally, the proposed method makes it possible to compare the different fog droplet size distribution descriptors proposed in the literature.
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Spiegel, J. K., P. Zieger, N. Bukowiecki, E. Hammer, E. Weingartner, and W. Eugster. "Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100)." Atmospheric Measurement Techniques Discussions 5, no. 3 (May 7, 2012): 3333–93. http://dx.doi.org/10.5194/amtd-5-3333-2012.
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Abstract. Droplet size spectra measurements are crucial to obtain a quantitative microphysical description of clouds and fog. However, cloud droplet size measurements are subject to various uncertainties. This work focuses on the evaluation of two key measurement uncertainties arising during cloud droplet size measurements with a conventional droplet size spectrometer (FM-100): first, we addressed the precision with which droplets can be sized with the FM-100 on the basis of Mie theory. We deduced error assumptions and proposed how to correct measured size distributions for these errors by redistributing the measured droplet size distribution using a stochastic approach. Second, based on a literature study, we derived corrections for particle losses during sampling with the FM-100. We applied both corrections to cloud droplet size spectra measured at the high alpine site Jungfraujoch for a temperature range from 0 °C to 11 °C. We show that Mie scattering led to spikes in the droplet size distributions using the default sizing procedure, while the stochastic approach reproduced the ambient size distribution adequately. A detailed analysis of the FM-100 sampling efficiency revealed that particle losses were typically below 10% for droplet diameters up to 10 μm. For larger droplets, particle losses can increase up to 90% for the largest droplets of 50 μm at ambient windspeeds below 4.4 m s−1 and even to >90% for larger angles between the instrument orientation and the wind vector (sampling angle) at higher wind speeds. Comparisons of the FM-100 to other reference instruments revealed that the total liquid water content (LWC) measured by the FM-100 was more sensitive to particle losses than to re-sizing based on Mie scattering, while the total number concentration was only marginally influenced by particle losses. As a consequence, for further LWC measurements with the FM-100 we strongly recommend to consider (1) the error arising due to Mie scattering, and (2) the particle losses, especially for larger droplets depending on the set-up and wind conditions.
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Spiegel, J. K., P. Zieger, N. Bukowiecki, E. Hammer, E. Weingartner, and W. Eugster. "Evaluating the capabilities and uncertainties of droplet measurements for the fog droplet spectrometer (FM-100)." Atmospheric Measurement Techniques 5, no. 9 (September 20, 2012): 2237–60. http://dx.doi.org/10.5194/amt-5-2237-2012.
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Abstract. Droplet size spectra measurements are crucial to obtain a quantitative microphysical description of clouds and fog. However, cloud droplet size measurements are subject to various uncertainties. This work focuses on the error analysis of two key measurement uncertainties arising during cloud droplet size measurements with a conventional droplet size spectrometer (FM-100): first, we addressed the precision with which droplets can be sized with the FM-100 on the basis of the Mie theory. We deduced error assumptions and proposed a new method on how to correct measured size distributions for these errors by redistributing the measured droplet size distribution using a stochastic approach. Second, based on a literature study, we summarized corrections for particle losses during sampling with the FM-100. We applied both corrections to cloud droplet size spectra measured at the high alpine site Jungfraujoch for a temperature range from 0 °C to 11 °C. We showed that Mie scattering led to spikes in the droplet size distributions using the default sizing procedure, while the new stochastic approach reproduced the ambient size distribution adequately. A detailed analysis of the FM-100 sampling efficiency revealed that particle losses were typically below 10% for droplet diameters up to 10 μm. For larger droplets, particle losses can increase up to 90% for the largest droplets of 50 μm at ambient wind speeds below 4.4 m s−1 and even to >90% for larger angles between the instrument orientation and the wind vector (sampling angle) at higher wind speeds. Comparisons of the FM-100 to other reference instruments revealed that the total liquid water content (LWC) measured by the FM-100 was more sensitive to particle losses than to re-sizing based on Mie scattering, while the total number concentration was only marginally influenced by particle losses. Consequently, for further LWC measurements with the FM-100 we strongly recommend to consider (1) the error arising due to Mie scattering, and (2) the particle losses, especially for larger droplets depending on the set-up and wind conditions.
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Mi, Jia Wei, and Patrick S. Grant. "Numerical Modelling of Spray Formed Grain Size Evolution." Materials Science Forum 561-565 (October 2007): 1991–94. http://dx.doi.org/10.4028/www.scientific.net/msf.561-565.1991.
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A numerical model has been developed to simulate the distribution of polygonal grain size in a sprayed microstructure formed from an alloy droplet spray containing a large number of solid, mushy and liquid droplets. The model takes into account the effects of: (1) the droplet size distribution; (2) its corresponding distribution of solid, mushy and liquid droplets at the instant of deposition; (3) the overall thermal condition of the spray formed preform during final solidification. The model has been validated against experiments of the spray forming of Ni superalloy rings, with modelled grain size distributions giving good agreement with measurements obtained by electron backscatter diffraction.
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Krueger, Steven K. "Technical note: Equilibrium droplet size distributions in a turbulent cloud chamber with uniform supersaturation." Atmospheric Chemistry and Physics 20, no. 13 (July 8, 2020): 7895–909. http://dx.doi.org/10.5194/acp-20-7895-2020.
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Abstract. In a laboratory cloud chamber that is undergoing Rayleigh–Bénard convection, supersaturation is produced by isobaric mixing. When aerosols (cloud condensation nuclei) are injected into the chamber at a constant rate, and the rate of droplet activation is balanced by the rate of droplet loss, an equilibrium droplet size distribution (DSD) can be achieved. We derived analytic equilibrium DSDs and probability density functions (PDFs) of droplet radius and squared radius for conditions that could occur in such a turbulent cloud chamber when there is uniform supersaturation. We neglected the effects of droplet curvature and solute on the droplet growth rate. The loss rate due to fallout that we used assumes that (1) the droplets are well-mixed by turbulence, (2) when a droplet becomes sufficiently close to the lower boundary, the droplet's terminal velocity determines its probability of fallout per unit time, and (3) a droplet's terminal velocity follows Stokes' law (so it is proportional to its radius squared). Given the chamber height, the analytic PDF is determined by the mean supersaturation alone. From the expression for the PDF of the radius, we obtained analytic expressions for the first five moments of the radius, including moments for truncated DSDs. We used statistics from a set of measured DSDs to check for consistency with the analytic PDF. We found consistency between the theoretical and measured moments, but only when the truncation radius of the measured DSDs was taken into account. This consistency allows us to infer the mean supersaturations that would produce the measured PDFs in the absence of supersaturation fluctuations. We found that accounting for the truncation radius of the measured DSDs is particularly important when comparing the theoretical and measured relative dispersions of the droplet radius. We also included some additional quantities derived from the analytic DSD: droplet sedimentation flux, precipitation flux, and condensation rate.
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Wittner, Marc, Heike Karbstein, and Volker Gaukel. "Pneumatic Atomization: Beam-Steering Correction in Laser Diffraction Measurements of Spray Droplet Size Distributions." Applied Sciences 8, no. 10 (September 26, 2018): 1738. http://dx.doi.org/10.3390/app8101738.
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Laser diffraction is among the most widely used methods for spray droplet size measurements. However, the so-called beam-steering effect must be considered when pneumatic atomizers are used for droplet generation. The beam-steering effect is a systematic measurement error, leading to the detection of apparent large spray droplets due to gradients in the refractive index of the gas phase. The established correction method is based on the reduction of the laser diffraction system’s measurement range by deactivation of detectors, relevant for the detection of large droplets. As this method is only applicable when size ranges of real and apparent droplet sizes are clearly different, an alternative method for beam-steering correction is introduced in the presented study. It is based on a multimodal log-normal fit of measured spray droplet sizes. The modality representing the largest droplets is correlated to the beam-steering effect and therefore excluded from the measured size distribution. The new method was successfully applied to previously published droplet size distribution measurements of an internal mixing Air-Core-Liquid-Ring (ACLR) atomizer. In measurements where the method of detector deactivation is applicable, excellent accordance of droplet size distributions, gained by both correction methods, was found. In measurements with overlapping real and apparent parts of the distribution, the new correction method led to a significant reduction of overestimated large droplets. As a consequence, we conclude that the new method presented here for beam-steering correction should be applied in laser diffraction measurements of spray droplet sizes, generated by pneumatic atomizers.
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Aiyer, A. K., D. Yang, M. Chamecki, and C. Meneveau. "A population balance model for large eddy simulation of polydisperse droplet evolution." Journal of Fluid Mechanics 878 (September 18, 2019): 700–739. http://dx.doi.org/10.1017/jfm.2019.649.
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In the context of many applications of turbulent multi-phase flows, knowledge of the dispersed phase size distribution and its evolution is critical to predicting important macroscopic features. We develop a large eddy simulation (LES) model that can predict the turbulent transport and evolution of size distributions, for a specific subset of applications in which the dispersed phase can be assumed to consist of spherical droplets, and occurring at low volume fraction. We use a population dynamics model for polydisperse droplet distributions specifically adapted to a LES framework including a model for droplet breakup due to turbulence, neglecting coalescence consistent with the assumed small dispersed phase volume fractions. We model the number density fields using an Eulerian approach for each bin of the discretized droplet size distribution. Following earlier methods used in the Reynolds-averaged Navier–Stokes framework, the droplet breakup due to turbulent fluctuations is modelled by treating droplet–eddy collisions as in kinetic theory of gases. Existing models assume the scale of droplet–eddy collision to be in the inertial range of turbulence. In order to also model smaller droplets comparable to or smaller than the Kolmogorov scale we extend the breakup kernels using a structure function model that smoothly transitions from the inertial to the viscous range. The model includes a dimensionless coefficient that is fitted by comparing predictions in a one-dimensional version of the model with a laboratory experiment of oil droplet breakup below breaking waves. After initial comparisons of the one-dimensional model to measurements of oil droplets in an axisymmetric jet, it is then applied in a three-dimensional LES of a jet in cross-flow with large oil droplets of a single size being released at the source of the jet. We model the concentration fields using $N_{d}=15$ bins of discrete droplet sizes and solve scalar transport equations for each bin. The resulting droplet size distributions are compared with published experimental data, and good agreement for the relative size distribution is obtained. The LES results also enable us to quantify size distribution variability. We find that the probability distribution functions of key quantities such as the total surface area and the Sauter mean diameter of oil droplets are highly variable, some displaying strong non-Gaussian intermittent behaviour.
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Bewley, Jennifer L., and Sonia Lasher-Trapp. "Progress on Predicting the Breadth of Droplet Size Distributions Observed in Small Cumuli." Journal of the Atmospheric Sciences 68, no. 12 (December 1, 2011): 2921–29. http://dx.doi.org/10.1175/jas-d-11-0153.1.
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Abstract A modeling framework representing variations in droplet growth by condensation, resulting from different saturation histories experienced as a result of entrainment and mixing, is used to predict the breadth of droplet size distributions observed at different altitudes within trade wind cumuli observed on 10 December 2004 during the Rain in Cumulus over the Ocean (RICO) field campaign. The predicted droplet size distributions are as broad as those observed, contain similar numbers of droplets, and are generally in better agreement with the observations when some degree of inhomogeneous droplet evaporation is considered, allowing activation of newly entrained cloud condensation nuclei. The variability of the droplet growth histories, resulting primarily from entrainment, appears to explain the magnitude of the observed droplet size distribution widths, without representation of other broadening mechanisms. Additional work is needed, however, as the predicted mean droplet diameter is too large relative to the observations and likely results from the model resolution limiting dilution of the simulated cloud.
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Nishandar, Sanika Ravindra, Yucheng He, Marko Princevac, and Rufus D. Edwards. "Fate of Exhaled Droplets From Breathing and Coughing in Supermarket Checkouts and Passenger Cars." Environmental Health Insights 17 (January 2023): 117863022211482. http://dx.doi.org/10.1177/11786302221148274.
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The global pandemic of COVID-19 has highlighted the importance of understanding the role that exhaled droplets play in virus transmission in community settings. Computational Fluid Dynamics (CFD) enables systematic examination of roles the exhaled droplets play in the spread of SARS-CoV-2 in indoor environments. This analysis uses published exhaled droplet size distributions combined with terminal aerosol droplet size based on measured peak concentrations for SARS-CoV-2 RNA in aerosols to simulate exhaled droplet dispersion, evaporation, and deposition in a supermarket checkout area and rideshare car where close proximity with other individuals is common. Using air inlet velocity of 2 m/s in the passenger car and ASHRAE recommendations for ventilation and comfort in the supermarket, simulations demonstrate that exhaled droplets <20 μm that contain the majority of viral RNA evaporated leaving residual droplet nuclei that remain aerosolized in the air. Subsequently ~ 70% of these droplet nuclei deposited in the supermarket and the car with the reminder vented from the space. The maximum surface deposition of droplet nuclei/m2 for speaking and coughing were 2 and 819, 18 and 1387 for supermarket and car respectively. Approximately 15% of the total exhaled droplets (aerodynamic diameters 20-700 µm) were deposited on surfaces in close proximity to the individual. Due to the non-linear distribution of viral RNA across droplet sizes, however, these larger exhaled droplets that deposit on surfaces have low viral content. Maximum surface deposition of viral RNA was 70 and 1.7 × 103 virions/m2 for speaking and 2.3 × 104 and 9.3 × 104 virions/m2 for coughing in the supermarket and car respectively while the initial airborne concentration of viral RNA was 7 × 106 copies per ml. Integrating the droplet size distributions with viral load distributions, this study helps explain the apparent importance of inhalation exposures compared to surface contact observed in the pandemic.
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Klingebiel, M., A. de Lozar, S. Molleker, R. Weigel, A. Roth, L. Schmidt, J. Meyer, et al. "Arctic low-level boundary layer clouds: in situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the top layer of liquid phase clouds." Atmospheric Chemistry and Physics 15, no. 2 (January 16, 2015): 617–31. http://dx.doi.org/10.5194/acp-15-617-2015.
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Abstract. Aircraft borne optical in situ size distribution measurements were performed within Arctic boundary layer clouds with a special emphasis on the cloud top layer during the VERtical Distribution of Ice in Arctic clouds (VERDI) campaign in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 to 20 μm. In the upper transition zone (i.e., adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions (Mode 1 with 20 μm and Mode 2 with 10 μm diameter) were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating data point populations from different air volumes. The formation of the second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol (which was detected directly above the stratus cloud) might form a second mode of small cloud droplets. However, theoretical considerations and model calculations (adopting direct numerical simulation, DNS) revealed that, instead, turbulent mixing and evaporation of larger droplets are the most likely reasons for the formation of the second droplet size mode in the uppermost region of the clouds.
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Ji, Yun Zhe, Xiao Jie Wang, and Yun Ping Wang. "Comparative Study of Droplet-Nuclei with Pathogen and CO2 Distribution in Infectious Isolation Room." Applied Mechanics and Materials 138-139 (November 2011): 1102–8. http://dx.doi.org/10.4028/www.scientific.net/amm.138-139.1102.
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In this article, the distributions of droplet nuclei with pathogens which was evaporated from the droplets released by cough was and trace gas CO2 were compared to study the airborne characteristics of droplet nuclei. The infectious isolation room model with steady particle source model was built. The particle source was built according to the size distribution of the droplet nuclei with pathogens which was evaporated from the droplets released by cough. The CO2 source was build at the same location with the particle source and released in the same way. The simulation result showed that the distributions of droplet nuclei and CO2 which were simulated by CFD were similar under the same ward airflow. This proved that these droplet nuclei could suspend in air and move with airflow. it was feasible to use tracer gas CO2 as trace gas to solve the validating the numerical simulation of droplet nuclei distribution in infectious isolation room.
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Klingebiel, M., A. de Lozar, S. Molleker, R. Weigel, A. Roth, L. Schmidt, J. Meyer, et al. "Arctic low-level boundary layer clouds: in-situ measurements and simulations of mono- and bimodal supercooled droplet size distributions at the cloud top layer." Atmospheric Chemistry and Physics Discussions 14, no. 10 (June 5, 2014): 14599–635. http://dx.doi.org/10.5194/acpd-14-14599-2014.
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Abstract. Aircraft borne optical in-situ size distribution measurements were performed within Arctic boundary layer clouds, with a special emphasis on the cloud top layer, during the VERtical Distribution of Ice in Arctic Clouds (VERDI) campaign. The observations were carried out within a joint research activity of seven German institutes to investigate Arctic boundary layer-, mixed-phase clouds in April and May 2012. An instrumented Basler BT-67 research aircraft operated out of Inuvik over the Mackenzie River delta and the Beaufort Sea in the Northwest Territories of Canada. Besides the cloud particle and hydrometeor size spectrometers the aircraft was equipped with instrumentation for aerosol, radiation and other parameters. Inside the cloud, droplet size distributions with monomodal shapes were observed for predominantly liquid-phase Arctic stratocumulus. With increasing altitude inside the cloud the droplet mean diameters grew from 10 μm to 20 μm. In the upper transition zone (i.e. adjacent to the cloud-free air aloft) changes from monomodal to bimodal droplet size distributions were observed. It is shown that droplets of both modes co-exist in the same (small) air volume and the bimodal shape of the measured size distributions cannot be explained as an observational artifact caused by accumulating two droplet populations from different air volumes. The formation of a second size mode can be explained by (a) entrainment and activation/condensation of fresh aerosol particles, or (b) by differential evaporation processes occurring with cloud droplets engulfed in different eddies. Activation of entrained particles seemed a viable possibility as a layer of dry Arctic enhanced background aerosol was detected directly above the stratus cloud might form a second mode of small cloud droplets. However, theoretical considerations and a model simulation revealed that, instead, turbulent mixing and evaporation of larger droplets most likely are the main reasons for the formation of the second droplet size mode in the uppermost region of the clouds.
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Bakhtar, F., and A. V. Heaton. "Effects of Wake Chopping on Droplet Sizes in Steam Turbines." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 219, no. 12 (December 1, 2005): 1357–67. http://dx.doi.org/10.1243/095440605x69291.
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This paper is a review article and considers the influence of wake chopping on the size distribution of water droplets formed by homogeneous nucleation in steam turbines. The studies by several investigators are summarized. All the studies show that the fluctuations caused by the wakes can broaden the size distributions of the nucleated droplets substantially and account for the polydisperse nature of the droplet distributions and coarse water observed in turbines. The effect of the presence of impurities in steam on its nucleation behaviour is also discussed.
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Wong, Wing-Hin B., Pieter J. A. Janssen, Martien A. Hulsen, and Patrick D. Anderson. "Numerical simulations of the polydisperse droplet size distribution of disperse blends in complex flow." Rheologica Acta 60, no. 4 (March 6, 2021): 187–207. http://dx.doi.org/10.1007/s00397-021-01258-4.
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AbstractThe blend morphology model developed by Wong et al. (Rheologica Acta, 2019), based on Peters et al. (J Rheol 45(3):659–689, 2001), is used to investigate the development of the polydispersity of the disperse polymer blend morphology in complex flow. First, the model is extended with additional morphological states. The extended model is tested for simple shear flow, where it is found that the droplet size distribution does not simply scale with the shear rate, because this scaling does not hold for coalescing droplets. Subsequently, the model is applied to Poiseuille flow, showing formation of distinct layers, which occurs in realistic pressure-driven flows. Finally, the model is applied on an eccentric cylinder flow, where histograms are made of the average droplet size throughout the domain. It is observed that outer cylinder rotation results in narrow distributions where the small droplets are relatively large, whereas inner cylinder rotation results in broad distributions where the small droplets are significantly smaller than in the case of outer cylinder rotation. Eccentricity seems to only have a minor effect if the maximum shear rate is held constant. The flow profile and history in combination with the maximum shear rate strongly determine how the polydisperse droplet size distribution develops.
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Watanabe, Y., and D. M. Ingram. "Size distributions of sprays produced by violent wave impacts on vertical sea walls." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 472, no. 2194 (October 2016): 20160423. http://dx.doi.org/10.1098/rspa.2016.0423.
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When a steep, breaking wave hits a vertical sea wall in shallow water, a flip-through event may occur, leading to the formation of an up-rushing planar jet. During such an event, a jet of water is ejected at a speed many times larger than the approaching wave’s celerity. As the jet rises, the bounded fluid sheet ruptures to form vertical ligaments which subsequently break up to form droplets, creating a polydisperse spray. Experiments in the University of Hokkaido’s 24 m flume measured the resulting droplet sizes using image analysis of high-speed video. Consideration of the mechanisms forming spray droplets shows that the number density of droplet sizes is directly proportional to a powerpof the droplet radius: wherep=−5/2 during the early break-up stage andp=−2 for the fully fragmented state. This was confirmed by experimental observations. Here, we show that the recorded droplet number density follows the lognormal probability distribution with parameters related to the elapsed time since the initial wave impact. This statistical model of polydisperse spray may provide a basis for modelling droplet advection during wave overtopping events, allowing atmospheric processes leading to enhanced fluxes of mass, moisture, heat and momentum in the spray-mediated marine boundary layer over coasts to be described.
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Desai, N., K. K. Chandrakar, K. Chang, W. Cantrell, and R. A. Shaw. "Influence of Microphysical Variability on Stochastic Condensation in a Turbulent Laboratory Cloud." Journal of the Atmospheric Sciences 75, no. 1 (January 2018): 189–201. http://dx.doi.org/10.1175/jas-d-17-0158.1.
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Diffusional growth of droplets by stochastic condensation and a resulting broadening of the size distribution has been considered as a mechanism for bridging the cloud droplet growth gap between condensation and collision–coalescence. Recent studies have shown that supersaturation fluctuations can lead to a broadening of the droplet size distribution at the condensational stage of droplet growth. However, most studies using stochastic models assume the phase relaxation time of a cloud parcel to be constant. In this paper, two questions are asked: how variability in droplet number concentration and radius influence the phase relaxation time and what effect it has on the droplet size distributions. To answer these questions, steady-state cloud conditions are created in the laboratory and digital inline holography is used to directly observe the variations in local number concentration and droplet size distribution and, thereby, the integral radius. Stochastic equations are also extended to account for fluctuations in integral radius and obtain new terms that are compared with the laboratory observations. It is found that the variability in integral radius is primarily driven by variations in the droplet number concentration and not the droplet radius. This variability does not contribute significantly to the mean droplet growth rate but does contribute significantly to the rate of increase of the size distribution width.
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Aßmann, Simon, Bettina Münsterjohann, Franz J. T. Huber, and Stefan Will. "In Situ Determination of Droplet and Nanoparticle Size Distributions in Spray Flame Synthesis by Wide-Angle Light Scattering (WALS)." Materials 14, no. 21 (November 7, 2021): 6698. http://dx.doi.org/10.3390/ma14216698.
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The investigation of droplet and nanoparticle formation in spray flame synthesis requires sophisticated measurement techniques, as often both are present simultaneously. Here, wide-angle light scattering (WALS) was applied to determine droplet and nanoparticle size distributions in spray flames from a standardized liquid-fed burner setup. Solvents of pure ethanol and a mixture of ethanol and titanium isopropoxide, incepting nanoparticle synthesis, were investigated. A novel method for the evaluation of scattering data from droplets between 2 µm and 50 µm was successfully implemented. Applying this, we could reveal the development of a bimodal droplet size distribution for the solvent/precursor system, probably induced by droplet micro-explosions. To determine nanoparticle size distributions, an appropriate filter and the averaging of single-shot data were applied to ensure scattering from a significant amount of nanoparticles homogeneously distributed in the measurement volume. From the multivariate analysis of the scattering data, the presence of spherical particles and fractal aggregates was derived, which was confirmed by analysis of transmission electron microscopy images. Monte Carlo simulations allowed determining the distribution parameters for both morphological fractions in three heights above the burner. The results showed relatively wide size distributions, especially for the spherical fraction, and indicated an ongoing sintering, from fractal to spherical particles.
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Deng, Shao Yun. "Comparing Simulation for Turbulent Dispersion and Coalescence of Droplets within a Spray with Two Models." Applied Mechanics and Materials 353-356 (August 2013): 2473–76. http://dx.doi.org/10.4028/www.scientific.net/amm.353-356.2473.
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Lagrangian and Eulerian modelling approaches are compared for simulating turbulent dispersion and coalescence of droplets within a spray. Both models predict similar droplet dispersion rates and shifts in droplet size distribution due to coalescence within the spray, over a wide range of droplet and gas flows, and for sprays with different droplet size distributions at the nozzle exit. The computer time required for simulating coalescence within a steady axisymmetric spray is of a similar order of magnitude regardless of which formulation, Eulerian or Lagrangian, is adopted. However, the Lagrangian formulation is more practical in terms of the range of applicability and ease of implementation.
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Wang, Junpeng, Cuicui Xu, Gang Zhou, and Yansong Zhang. "Spray Structure and Characteristics of a Pressure-Swirl Dust Suppression Nozzle Using a Phase Doppler Particle Analyze." Processes 8, no. 9 (September 10, 2020): 1127. http://dx.doi.org/10.3390/pr8091127.
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In order to understand the characteristics of the spray field of a dust suppression nozzle and provide a reference for dust nozzle selection according to dust characteristics, a three-dimensional phase Doppler particle analyzer (PDPA) spray measurement system is used to analyze the droplet size and velocity characteristics in a spray field, particularly the joint particle size–velocity distribution. According to the results, after the ejection of the jet from the nozzle, the droplets initially maintained some velocity; however, the distribution of particles with different sizes was not uniform. As the spray distance increased, the droplet velocity decreased significantly, and the particle size distribution changed very little. As the distance increased further, the large droplets separated into smaller droplets, and their velocity decreased rapidly. The distributions of the particle size and velocity of the droplets then became stable. Based on the particle size-velocity distribution characteristics, the spray structure of pressure-swirl nozzles can be divided into five regions, i.e., the mixing, expansion, stabilization, decay, and rarefied regions. The expansion, stabilization, and decay regions are the effective dust fall areas. In addition, the droplet size in the stabilization region is the most uniform, indicating that this region is the best dust fall region. The conclusions can provide abundant calibration data for spray dust fall nozzles.
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Berghof, M. I. A., G. P. Frank, S. Sjogren, and B. G. Martinsson. "Inversion of droplet aerosol analyzer data for long-term aerosol-cloud interaction measurements." Atmospheric Measurement Techniques Discussions 6, no. 6 (November 29, 2013): 10269–95. http://dx.doi.org/10.5194/amtd-6-10269-2013.
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Abstract. The droplet aerosol analyzer (DAA) was developed to study the influence of aerosol properties on clouds. It measures the ambient particle size of individual droplets and interstitial particles, the size of the dry (residual) particles after the evaporation of water, and the number concentration of the dry (residual) particles. A method was developed for the evaluation of DAA data to obtain the three-parameter dataset: ambient particle diameter, dry (residual) particle diameter and number concentration. First results from in-cloud measurements performed on the summit of Mt. Brocken in Germany are presented. Various aspects of the cloud aerosol dataset are presented, such as the number concentration of interstitial particles and cloud droplets, the dry residue particle size distribution, droplet size distributions, scavenging ratios due to cloud droplet formation and size-dependent solute concentrations. This dataset makes it possible to study clouds and the influence of the aerosol population on clouds.
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Khelifa, Ali, Patricia Stoffyn-Egli, Paul S. Hill, and Kenneth Lee. "Characteristics of Oil Droplets Stabilized by Mineral Particles: The Effect of Salinity." International Oil Spill Conference Proceedings 2003, no. 1 (April 1, 2003): 963–70. http://dx.doi.org/10.7901/2169-3358-2003-1-963.
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ABSTRACT The influence of salinity on the characteristics of oil droplets stabilized by mineral particles (oil-mineral aggregates – OMA) was studied in the laboratory using three different oils and a natural sediment. Size and concentration of oil droplets associated with negatively and positively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that the median droplet size increases rapidly from about 5 μm at zero salinity to double at salinity close to 1.2 ppt; decreases dramatically to about 5 μm at salinity 3.5 ppt and then increases slightly to 6 μm at the seawater salinity. The concentration of oil droplets also increases sharply when the salinity increases from zero to a critical aggregation salinity Scas, after which it stabilizes at its maximum value. The concentration of mineral-stabilized droplets is strongly affected by oil type at any salinity. When normalized to its maximum value, the concentration of droplets correlates well with normalized salinity S/Scas. A relationship is derived to predict the effect of salinity on the concentration of mineral-stabilized droplets. Size distributions of oil droplets follow similar trends, but their magnitudes depend on salinity and oil type. Self-similarity in droplet size distributions was shown when the data were plotted using normalized variables N/Nt and D/D50, where N is the number of droplets of diameter D, Nt is the total number of droplets and D50 the median size of the droplets. With these normalized variables, oil droplet size distributions measured in this study and those measured in field and laboratory under various conditions by different investigators fit the same curve regardless of the formation conditions of the droplets. A function is derived to calculate normalized cumulative size distributions of oil droplets.
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Li, Xiang-Yu, Axel Brandenburg, Gunilla Svensson, Nils E. L. Haugen, Bernhard Mehlig, and Igor Rogachevskii. "Condensational and Collisional Growth of Cloud Droplets in a Turbulent Environment." Journal of the Atmospheric Sciences 77, no. 1 (December 26, 2019): 337–53. http://dx.doi.org/10.1175/jas-d-19-0107.1.
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Abstract We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high-resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water vapor mixing ratio, and the resulting supersaturation fields together with the Navier–Stokes equation. We find that the droplet size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation-fluctuation-induced broadening of droplet size distributions enhances the collisional growth. This is contrary to classical (nonturbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.
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Berghof, M. I. A., G. P. Frank, S. Sjogren, and B. G. Martinsson. "Inversion of droplet aerosol analyzer data for long-term aerosol–cloud interaction measurements." Atmospheric Measurement Techniques 7, no. 4 (April 4, 2014): 877–86. http://dx.doi.org/10.5194/amt-7-877-2014.
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Abstract. The droplet aerosol analyzer (DAA) was developed to study the influence of aerosol properties on clouds. It measures the ambient particle size of individual droplets and interstitial particles, the size of the dry (residual) particles after the evaporation of water vapor and the number concentration of the dry (residual) particles. A method was developed for the evaluation of DAA data to obtain the three-parameter data set: ambient particle diameter, dry (residual) particle diameter and number concentration. First results from in-cloud measurements performed on the summit of Mt. Brocken in Germany are presented. Various aspects of the cloud–aerosol data set are presented, such as the number concentration of interstitial particles and cloud droplets, the dry residue particle size distribution, droplet size distributions, scavenging ratios due to cloud droplet formation and size-dependent solute concentrations. This data set makes it possible to study clouds and the influence of the aerosol population on clouds.
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Zhang, Jun, and Michael W. Reeks. "A Statistical Model for the Joint Distribution of Droplet Size and Charge Density in an Electrostatic Spray." Advanced Materials Research 97-101 (March 2010): 1438–44. http://dx.doi.org/10.4028/www.scientific.net/amr.97-101.1438.
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A theoretical model to predict the joint distribution of droplet size and charge density for an electrostatic spray is described based on the maximum entropy method. From known values of the electrostatic spray parameters, the model is used to evaluate the joint distribution of droplet size and charge density for a cone-jet mode electrostatic spray. The predicted results of present model show that it has generally a relatively narrow distribution for both droplet and charge density in a cone-jet mode. Comparatively, the droplet size distribution is narrower than that of the charge density. In addition the two distributions are significantly different in shape. The droplet size distribution is nearly symmetric about its peak position, whilst the left side of the charge density distribution curve is noticeably steeper than the right side. The results are also compared with existing experimental data with agreement considering the uncertainties in the data.
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Albert, R., and P. V. Farrell. "Droplet Sizing Using the Shifrin Inversion." Journal of Fluids Engineering 116, no. 2 (June 1, 1994): 357–62. http://dx.doi.org/10.1115/1.2910281.
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A method for measuring droplet size distributions was investigated with an emphasis on limitations related to measurements in real spray environments. The method stores a photographic image of a plane of droplets within a spray and is capable of evaluating particle size distributions within the spray, one small region at a time. The method complements droplet velocity measurements made using Particle Image Velocimetry. In a typical experiment, a plane of the spray was illuminated by a laser light sheet and photographed. After processing, a small laser beam scanned the film and a diffraction pattern was generated for each region illuminated by the small laser. The diffraction pattern was inverted using a Shifrin inversion to solve for the particle size distribution within the illuminated region. In this paper we will discuss some of the limitations of this method and indicate one approach which seems to allow for improved inversions using data signal processing.
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Loureiro, D. D., A. Kronenburg, J. Reutzsch, B. Weigand, and K. Vogiatzaki. "Droplet size distributions in cryogenic flash atomization." International Journal of Multiphase Flow 142 (September 2021): 103705. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2021.103705.
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Han, Z. Y., W. G. Weng, and Q. Y. Huang. "Characterizations of particle size distribution of the droplets exhaled by sneeze." Journal of The Royal Society Interface 10, no. 88 (November 6, 2013): 20130560. http://dx.doi.org/10.1098/rsif.2013.0560.
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This work focuses on the size distribution of sneeze droplets exhaled immediately at mouth. Twenty healthy subjects participated in the experiment and 44 sneezes were measured by using a laser particle size analyser. Two types of distributions are observed: unimodal and bimodal. For each sneeze, the droplets exhaled at different time in the sneeze duration have the same distribution characteristics with good time stability. The volume-based size distributions of sneeze droplets can be represented by a lognormal distribution function, and the relationship between the distribution parameters and the physiological characteristics of the subjects are studied by using linear regression analysis. The geometric mean of the droplet size of all the subjects is 360.1 µm for unimodal distribution and 74.4 µm for bimodal distribution with geometric standard deviations of 1.5 and 1.7, respectively. For the two peaks of the bimodal distribution, the geometric mean (the geometric standard deviation) is 386.2 µm (1.8) for peak 1 and 72.0 µm (1.5) for peak 2. The influences of the measurement method, the limitations of the instrument, the evaporation effects of the droplets, the differences of biological dynamic mechanism and characteristics between sneeze and other respiratory activities are also discussed.
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Salum, Pelin, Onur Güven, Levent Yurdaer Aydemir, and Zafer Erbay. "Microscopy-Assisted Digital Image Analysis with Trainable Weka Segmentation (TWS) for Emulsion Droplet Size Determination." Coatings 12, no. 3 (March 9, 2022): 364. http://dx.doi.org/10.3390/coatings12030364.
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The size distribution of droplets in emulsions is very important for adjusting the effects of many indices on their quality. In addition to other methods for the determination of the size distribution of droplets, the usage of machine learning during microscopic analyses can enhance the reliability of the measurements and decrease the measurement cost at the same time. Considering its role in emulsion characteristics, in this study, the droplet size distributions of emulsions prepared with different oil/water phase ratios and homogenization times were measured with both a microscopy-assisted digital image analysis technique and a well-known laser diffraction method. The relationships between the droplet size and the physical properties of emulsions (turbidity and viscosity) were also investigated. The results showed that microscopic measurements yielded slightly higher values for the D(90), D[3,2], and D[4,3] of emulsions compared to the laser diffraction method for all oil/water phase ratios. When using this method, the droplet size had a meaningful correlation with the turbidity and viscosity values of emulsions at different oil/water phase ratios. From this point of view, the usage of the optical microscopy method with machine learning can be useful for the determination of the size distribution in emulsions.
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Wang, Y., and L. Bourouiba. "Unsteady sheet fragmentation: droplet sizes and speeds." Journal of Fluid Mechanics 848 (June 13, 2018): 946–67. http://dx.doi.org/10.1017/jfm.2018.359.
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Understanding what shapes the drop size distributions produced from fluid fragmentation is important for a range of industrial, natural and health processes. Gilet & Bourouiba (J. R. Soc. Interface, vol. 12, 2015, 20141092) showed that both the size and speed of fragmented droplets are critical to transmission of pathogens in the agricultural context. In this paper, we study both the size and speed distributions of droplets ejected during a canonical unsteady sheet fragmentation from drop impact on a target of comparable size to that of the drop. Upon impact, the drop transforms into a sheet which expands in the air bounded by a rim on which ligaments grow, continuously shedding droplets. We developed high-precision tracking algorithms that capture all ejected droplets, measuring their size and speed, as well as the detachment time from, and link to, their ligament of origin. Both size and speed distributions of all ejected droplets are skewed. We show that the polydispersity and skewness of the distributions are inherently due to the unsteadiness of the sheet expansion. We show that each ligament sheds a single drop at a time throughout the entire sheet expansion by a mechanism of end-pinching. The droplet-to-ligament size ratio $R\approx 1.5$ remains constant throughout the unsteady fragmentation, and is robust to change in impact Weber number. We also show that the population mean speed of the fragmented droplets at a given time is equal to the population mean speed of ligaments one necking time prior to detachment time.
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Shum, Sam C. K., Steve K. Johnson, Ho-Ming Pang, and R. S. Houk. "Spatially Resolved Measurements of Size and Velocity Distributions of Aerosol Droplets from a Direct Injection Nebulizer." Applied Spectroscopy 47, no. 5 (May 1993): 575–83. http://dx.doi.org/10.1366/0003702934067108.
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Aerosol droplet sizes and velocities from a direct injection nebulizer (DIN) are measured with radial and axial spatial resolution by phase Doppler particle analysis (PDPA). The droplets on the central axis of the spray become finer and their size becomes more uniform when ≍ 20% methanol is added to the usual aqueous solvent. This could explain why the analyte signal is a maximum at this solvent composition when the DIN is used for inductively coupled plasma-mass spectrometry (ICP-MS). Mean droplet velocities are 12 to 22 ms−1 with standard deviations of ±4 to ±7 ms−1. The outer fringes of the aerosol plume tend to be enriched in large droplets. The Sauter mean diameter ( D3,2) and velocity of the droplets also vary substantially with axial position in the aerosol plume.
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Brandvik, Per Johan, Øistein Johansen, and Umer Farooq. "Subsea Release of Oil & Gas – A Downscaled Laboratory Study Focused on Initial Droplet Formation and the Effect of Dispersant Injection." International Oil Spill Conference Proceedings 2014, no. 1 (May 1, 2014): 283–98. http://dx.doi.org/10.7901/2169-3358-2014.1.283.
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ABSTRACT This article describes the SINTEF Tower Basin (located in Trondheim, Norway) and its use for examining droplet formation and the effectiveness of dispersant injection. The Tower Basin is 6 m high and 3 m in diameter, containing 42 m3 of natural sea water. Oil is injected from the base of the basin and oil droplets are monitored by laser diffraction and in-situ camera techniques. Size distributions of oil droplets formed in deep water oil & gas blowouts have a substantial impact on the fate of the oil in the environment. However, very limited data on droplet size distributions from subsurface releases exist. The objective of this study has been to establish a laboratory facility to examine droplet size versus release conditions (flow rates and nozzle diameters), oil properties and injection of dispersants (injection techniques and dispersant types). Changes in the size of oil droplets that result from injection of dispersant are used to assess the effectiveness of the dispersant application (dosage and injection method). This comprehensive dataset is used to develop and calibrate existing algorithms to predict droplet sizes from subsurface releases, and the effect of dispersant treatment. The improved algorithms are implemented in current operational models where they are used to describe subsurface use of dispersant and fate of the dispersed oil in the water column.
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Darwish Ahmad, Adnan, Binit B. Singh, Mark Doerre, Ahmad M. Abubaker, Masoud Arabghahestani, Ahmad A. Salaimeh, and Nelson K. Akafuah. "Spatial Positioning and Operating Parameters of a Rotary Bell Sprayer: 3D Mapping of Droplet Size Distributions." Fluids 4, no. 3 (September 5, 2019): 165. http://dx.doi.org/10.3390/fluids4030165.
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In this study, we evaluated the fundamental physical behavior during droplet formation and flow from a rotary bell spray in the absence of an electrostatic field. The impact of a wide range of operating parameters of the rotary bell sprayer, such as flow rates, rotational speeds, and spatial positioning, on droplet sizes and size distributions using a three-dimensional (3-D) mapping was studied. The results showed that increasing the rotational speed caused the Sauter mean diameter of the droplets to decrease while increasing flow rate increased the droplet sizes. The rotational speed effect, however, was dominant compared to the effect of flow rate. An increase in droplet size radially away from the cup was noted in the vicinity of the cup, nevertheless, as the lateral distances from the cup and rotational speed were increased, the droplet sizes within the flow field became more uniform. This result is of importance for painting industries, which are looking for optimal target distances for uniform painting appearance. Furthermore, the theoretical formulation was validated with experimental data, which provides a wider range of applicability in terms of environment and parameters that could be tested. This work also provides an abundance of measurements, which can serve as a database for the validation of future droplet disintegration simulations.
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Li, Jiarong, Xinfeng Wang, Jianmin Chen, Chao Zhu, Weijun Li, Chengbao Li, Lu Liu, et al. "Chemical composition and droplet size distribution of cloud at the summit of Mount Tai, China." Atmospheric Chemistry and Physics 17, no. 16 (August 23, 2017): 9885–96. http://dx.doi.org/10.5194/acp-17-9885-2017.
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Abstract. The chemical composition of 39 cloud samples and droplet size distributions in 24 cloud events were investigated at the summit of Mt. Tai from July to October 2014. Inorganic ions, organic acids, metals, HCHO, H2O2, sulfur(IV), organic carbon, and elemental carbon as well as pH and electrical conductivity were analyzed. The acidity of the cloud water significantly decreased from a reported value of pH 3.86 during 2007–2008 (Guo et al., 2012) to pH 5.87 in the present study. The concentrations of nitrate and ammonium were both increased since 2007–2008, but the overcompensation of ammonium led to an increase in the mean pH value. The microphysical properties showed that cloud droplets were smaller than 26.0 µm and most were in the range of 6.0–9.0 µm at Mt. Tai. The maximum droplet number concentration (Nd) was associated with a droplet size of 7.0 µm. High liquid water content (LWC) values could facilitate the formation of larger cloud droplets and broadened the droplet size distribution. Cloud droplets exhibited a strong interaction with atmospheric aerosols. Higher PM2. 5 levels resulted in higher concentrations of water-soluble ions and smaller sizes with increased numbers of cloud droplets. The lower pH values were likely to occur at higher PM2. 5 concentrations. Clouds were an important sink for soluble materials in the atmosphere. The dilution effect of cloud water should be considered when estimating concentrations of soluble components in the cloud phase.
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Levis, Aviad, Yoav Y. Schechner, Anthony B. Davis, and Jesse Loveridge. "Multi-View Polarimetric Scattering Cloud Tomography and Retrieval of Droplet Size." Remote Sensing 12, no. 17 (September 1, 2020): 2831. http://dx.doi.org/10.3390/rs12172831.
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Tomography aims to recover a three-dimensional (3D) density map of a medium or an object. In medical imaging, it is extensively used for diagnostics via X-ray computed tomography (CT). We define and derive a tomography of cloud droplet distributions via passive remote sensing. We use multi-view polarimetric images to fit a 3D polarized radiative transfer (RT) forward model. Our motivation is 3D volumetric probing of vertically-developed convectively-driven clouds that are ill-served by current methods in operational passive remote sensing. Current techniques are based on strictly 1D RT modeling and applied to a single cloudy pixel, where cloud geometry defaults to that of a plane-parallel slab. Incident unpolarized sunlight, once scattered by cloud-droplets, changes its polarization state according to droplet size. Therefore, polarimetric measurements in the rainbow and glory angular regions can be used to infer the droplet size distribution. This work defines and derives a framework for a full 3D tomography of cloud droplets for both their mass concentration in space and their distribution across a range of sizes. This 3D retrieval of key microphysical properties is made tractable by our novel approach that involves a restructuring and differentiation of an open-source polarized 3D RT code to accommodate a special two-step optimization technique. Physically-realistic synthetic clouds are used to demonstrate the methodology with rigorous uncertainty quantification.
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Skenderović, Ivan, Gregor Kotalczyk, and Frank Kruis. "Dual Population Balance Monte Carlo Simulation of Particle Synthesis by Flame Spray Pyrolysis." Processes 6, no. 12 (December 6, 2018): 253. http://dx.doi.org/10.3390/pr6120253.
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The Dual Population Balance Monte Carlo Method (DPBMC) takes into account the full size spectrum of the droplet and particle phase. Droplet and particle size distributions are rendered by weighted simulation particles. This allows for an accurate description of particle nucleation and coagulation and droplet combustion, simultaneously. Internal droplet properties such as temperature and concentrations fields are used to define criteria for the onset of droplet breakage in the framework of weighted Monte Carlo droplets. We discuss the importance of droplet polydispersity on particle formation in metal oxide particle synthesis, which is shown to strongly affect particle formation and growth. The method is applied to particle synthesis from metal nitrate precursor solutions with flame spray pyrolysis (FSP) and compared to experiments from literature.
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Trujillo, M. F., W. S. Mathews, C. F. Lee, and J. E. Peters. "Modelling and experiment of impingement and atomization of a liquid spray on a wall." International Journal of Engine Research 1, no. 1 (February 1, 2000): 87–105. http://dx.doi.org/10.1243/1468087001545281.
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An experimental and computational investigation of spray impingement on a flat surface is presented. Different angles of incidence are studied, namely 30, 45 and 60°C measured from the normal to the surface. Back-lit photographs of the wall spray taken at various times during the impingement period give a qualitative view of the secondary atomization process. A stochastic model based on the sampling of velocity and size distributions of secondary droplets is used to simulate the creation of incident droplet fragments created by the numerous splashing events occurring during the impingement period. Size characteristics of the secondary droplet cloud are computed at various points in the impingement region and these are compared against phase/Doppler particle analyser (P/DPA) measurements yielding reasonable agreement. The effect of surface roughness is incorporated into the model and is found to play a major role in affecting the splashing threshold and the sizes of splashing fragments. The secondary droplet distributions are virtually unchanged among the different angles of incidence. This behaviour is explained by considering the shift in the splashing droplet distribution as a function of incident angle.
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Li, Xiang-Yu, Gunilla Svensson, Axel Brandenburg, and Nils E. L. Haugen. "Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds." Atmospheric Chemistry and Physics 19, no. 1 (January 17, 2019): 639–48. http://dx.doi.org/10.5194/acp-19-639-2019.
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Abstract. Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations (DNS) with droplets being modeled as Lagrangian particles. The supersaturation field is calculated directly by simulating the temperature and water vapor fields instead of being treated as a passive scalar. Thermodynamic feedbacks to the fields due to condensation are also included for completeness. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as t1∕2. Also, for the first time, we explicitly demonstrate that the time evolution of the size distribution is sensitive to the Reynolds number, but insensitive to the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increase with increasing Reynolds number; therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the mean updraft velocity is almost zero.
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Khain, A., V. Arkhipov, M. Pinsky, Y. Feldman, and Ya Ryabov. "Rain Enhancement and Fog Elimination by Seeding with Charged Droplets. Part I: Theory and Numerical Simulations." Journal of Applied Meteorology 43, no. 10 (October 1, 2004): 1513–29. http://dx.doi.org/10.1175/jam2131.1.
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Abstract A new method of droplet collision acceleration, with the purpose of rain enhancement and fog elimination, is proposed. According to the method, some fraction of the droplets is taken from clouds (or fog) themselves, charged, and then injected back into clouds (or fog). To verify the efficiency of the method, a novel model has been developed, allowing simulation of droplet spectrum evolution by collision in case a certain fraction of the droplets in a droplet spectrum is charged. Simulations of droplet spectra evolution include several steps: (a) The forces arising between charged and neutral droplets, as well as between charged droplets, are calculated as the function of the value of the charges, droplet size, and distance between droplets. It is shown that because of the induction effect, significant attraction forces arise between charged and neutral droplets. (b) The results obtained have been used to calculate the collision efficiencies between charged and neutral, as well between charged droplets. As a result, a “four dimensional” table of the collision efficiencies (the collision efficiency is the function of the droplet size and charge) was calculated. The collision efficiencies between charged and neutral droplets turn out to be significantly higher than the pure gravity-induced values. (c) To accomplish these simulations, a novel numerical method of solving the stochastic collision equation has been developed. Cloud droplets are described by a two-dimensional size distribution function in which droplets are characterized by both their mass and charge. (d) This model, with the implemented table of the collision efficiencies, has been used to simulate droplet spectra evolution in clouds and fog in case some fraction of these droplets was charged. Simulations of the effects of seeding by charged droplets have been performed. Evolution of initially narrow droplet size spectra (typical of extremely continental clouds in highly smoky air), in the case of seeding and under natural conditions, has been simulated. It was shown that although a natural droplet spectrum does not develop and no raindrops are formed, the injection of just a small fraction of charged particles rapidly triggered the collision process and lead to raindrop formation a few minutes after the injection. Significant acceleration of raindrop formation has been found in the case of a maritime wide-droplet spectrum. Simulations of fog seeding were conducted using droplet spectra distributions of typical fog. Seeding by charged fog droplets of one or both polarities was simulated. In both cases a significant increase in fog visibility was found. The advantages of the seeding method proposed are discussed.
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Yang, Fan, Pavlos Kollias, Raymond A. Shaw, and Andrew M. Vogelmann. "Cloud droplet size distribution broadening during diffusional growth: ripening amplified by deactivation and reactivation." Atmospheric Chemistry and Physics 18, no. 10 (May 25, 2018): 7313–28. http://dx.doi.org/10.5194/acp-18-7313-2018.
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Abstract. Cloud droplet size distributions (CDSDs), which are related to cloud albedo and rain formation, are usually broader in warm clouds than predicted from adiabatic parcel calculations. We investigate a mechanism for the CDSD broadening using a moving-size-grid cloud parcel model that considers the condensational growth of cloud droplets formed on polydisperse, submicrometer aerosols in an adiabatic cloud parcel that undergoes vertical oscillations, such as those due to cloud circulations or turbulence. Results show that the CDSD can be broadened during condensational growth as a result of Ostwald ripening amplified by droplet deactivation and reactivation, which is consistent with early work. The relative roles of the solute effect, curvature effect, deactivation and reactivation on CDSD broadening are investigated. Deactivation of smaller cloud droplets, which is due to the combination of curvature and solute effects in the downdraft region, enhances the growth of larger cloud droplets and thus contributes particles to the larger size end of the CDSD. Droplet reactivation, which occurs in the updraft region, contributes particles to the smaller size end of the CDSD. In addition, we find that growth of the largest cloud droplets strongly depends on the residence time of cloud droplet in the cloud rather than the magnitude of local variability in the supersaturation fluctuation. This is because the environmental saturation ratio is strongly buffered by numerous smaller cloud droplets. Two necessary conditions for this CDSD broadening, which generally occur in the atmosphere, are as follows: (1) droplets form on aerosols of different sizes, and (2) the cloud parcel experiences upwards and downwards motions. Therefore we expect that this mechanism for CDSD broadening is possible in real clouds. Our results also suggest it is important to consider both curvature and solute effects before and after cloud droplet activation in a cloud model. The importance of this mechanism compared with other mechanisms on cloud properties should be investigated through in situ measurements and 3-D dynamic models.
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Guler, Huseyin, Zhihong Zhang, Heping Zhu, Matthew Grieshop, and Mark A. Ledebuhr. "Spray Characteristics of Rotary Micro Sprinkler Nozzles Used in Orchard Pesticide Delivery." Transactions of the ASABE 63, no. 6 (2020): 1845–53. http://dx.doi.org/10.13031/trans.13445.
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HighlightsDroplet sizes were determined for rotary micro sprinkler nozzles used in solid set canopy delivery systems.An empirical multiple-variable model was developed to predict volume median diameters in spray patterns.Sprinkler nozzles produced medium to coarse droplets to minimize pesticide drift in orchards and trellised systems.Droplet size information can be used to select optimal nozzles for either irrigation or pesticide delivery systems.Abstract. Rotary micro sprinkler nozzles can be used for both irrigation and pesticide applications in orchard systems, but little to no information is available on their droplet size distributions. In this study, the droplet size distributions were investigated and described for rotary micro sprinkler nozzles with five different orifice diameters. A particle/droplet laser image analysis system was used to measure droplet spectra at two pressures (207 and 310 kPa) and two radial distances (0.25 and 0.85 m) from the sprinkler nozzle center. Nozzle orifice sizes, rotational speeds, and flow rates were also measured. Droplet sizes varied with the orifice size, operating pressure, and sampling location. Spiral-shaped spray patterns formed due to the spinning discharge port, within which droplet densities varied with location, orifice diameter, and operating pressure. The volume medium diameters (Dv0.5) for green-black, orange-blue, black-black, blue-black, and red-gray nozzles were respectively 317, 338, 379, 352, and 218 µm at 207 kPa and 283, 250, 283, 270, and 222 µm at 310 kPa. An empirical multiple-variable regression model was developed to predict Dv0.5 in the spray patterns discharged from the nozzles. Test results demonstrated that the rotary micro sprinkler nozzles produced medium to coarse droplets that could be used to minimize spray drift while maintaining efficacy in orchard pesticide applications. Keywords: Chemical application, Droplet size, Irrigation, Rotary nozzle, Spray drift reduction.
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J. Li, H. Kawano, and K. Yu. "Droplet Size Distributions from Different Shaped Sprinkler Nozzles." Transactions of the ASAE 37, no. 6 (1994): 1871–78. http://dx.doi.org/10.13031/2013.28278.
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Simmons, M. J. H., P. A. Langston, and A. S. Burbidge. "Particle and droplet size analysis from chord distributions." Powder Technology 102, no. 1 (April 1999): 75–83. http://dx.doi.org/10.1016/s0032-5910(98)00197-1.
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Jones, D. P., and A. P. Watkins. "Droplet size and velocity distributions for spray modelling." Journal of Computational Physics 231, no. 2 (January 2012): 676–92. http://dx.doi.org/10.1016/j.jcp.2011.09.030.
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Chandrakar, Kamal Kant, Izumi Saito, Fan Yang, Will Cantrell, Toshiyuki Gotoh, and Raymond A. Shaw. "Droplet size distributions in turbulent clouds: experimental evaluation of theoretical distributions." Quarterly Journal of the Royal Meteorological Society 146, no. 726 (December 6, 2019): 483–504. http://dx.doi.org/10.1002/qj.3692.
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Eroglu, H., and N. Chigier. "Initial Drop Size and Velocity Distributions for Airblast Coaxial Atomizers." Journal of Fluids Engineering 113, no. 3 (September 1, 1991): 453–59. http://dx.doi.org/10.1115/1.2909517.
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Initial drop size and velocity distributions, after complete disintegration of coaxial liquid jets, were determined by phase Doppler measurements. The measured radial distributions of Sauter mean diameter (SMD) were compared with the photographs of the disintegrating liquid jet. The SMD distribution was found to be strongly affected by the structure and behavior of the preceding liquid intact jet. The results showed that SMD increases with increasing liquid supply pressure as well as with decreasing air supply pressure. The axial measurement stations were determined from the photographs of the coaxial liquid jet at very short distances (1–2 mm) downstream of the observed break-up locations. The droplets accelerated at these regions under the influence of the air velocity. Smaller droplets were found to reach higher velocities because of their larger drag-to-momentum ratio. In general, minimum droplet mean velocities were found at the center, and the maximum velocities were near the spray boundary. Size velocity correlations show that the velocity of larger drops did not change with drop size. Drop rms velocity distributions have double peaks whose radial positions coincide with the maximum mean velocity gradients.
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Rozynek, Z., R. Bielas, and A. Józefczak. "Efficient formation of oil-in-oil Pickering emulsions with narrow size distributions by using electric fields." Soft Matter 14, no. 24 (2018): 5140–49. http://dx.doi.org/10.1039/c8sm00671g.
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We propose a new bulk approach to fabricating Pickering emulsions. We used electric fields not only to facilitate coalescence but also to manipulate surface particles and to induce droplet rotation, each contributed to formation of stable particle-covered droplets.
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Chandrakar, Kamal Kant, Will Cantrell, Kelken Chang, David Ciochetto, Dennis Niedermeier, Mikhail Ovchinnikov, Raymond A. Shaw, and Fan Yang. "Aerosol indirect effect from turbulence-induced broadening of cloud-droplet size distributions." Proceedings of the National Academy of Sciences 113, no. 50 (November 28, 2016): 14243–48. http://dx.doi.org/10.1073/pnas.1612686113.
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The influence of aerosol concentration on the cloud-droplet size distribution is investigated in a laboratory chamber that enables turbulent cloud formation through moist convection. The experiments allow steady-state microphysics to be achieved, with aerosol input balanced by cloud-droplet growth and fallout. As aerosol concentration is increased, the cloud-droplet mean diameter decreases, as expected, but the width of the size distribution also decreases sharply. The aerosol input allows for cloud generation in the limiting regimes of fast microphysics (τc<τt) for high aerosol concentration, and slow microphysics (τc>τt) for low aerosol concentration; here, τc is the phase-relaxation time and τt is the turbulence-correlation time. The increase in the width of the droplet size distribution for the low aerosol limit is consistent with larger variability of supersaturation due to the slow microphysical response. A stochastic differential equation for supersaturation predicts that the standard deviation of the squared droplet radius should increase linearly with a system time scale defined as τs−1=τc−1+τt−1, and the measurements are in excellent agreement with this finding. The result underscores the importance of droplet size dispersion for aerosol indirect effects: increasing aerosol concentration changes the albedo and suppresses precipitation formation not only through reduction of the mean droplet diameter but also by narrowing of the droplet size distribution due to reduced supersaturation fluctuations. Supersaturation fluctuations in the low aerosol/slow microphysics limit are likely of leading importance for precipitation formation.
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Wang, Zhaoyuan, Jianming Yang, and Frederick Stern. "High-fidelity simulations of bubble, droplet and spray formation in breaking waves." Journal of Fluid Mechanics 792 (March 3, 2016): 307–27. http://dx.doi.org/10.1017/jfm.2016.87.
Full textAbstract:
High-fidelity simulations of wave breaking processes are performed with a focus on the small-scale structures of breaking waves, such as bubble/droplet size distributions. Very large grids (up to 12 billion grid points) are used in order to resolve the bubbles/droplets in breaking waves at the scale of hundreds of micrometres. Wave breaking processes and spanwise three-dimensional interface structures are identified. It is speculated that the Görtler type centrifugal instability is likely more relevant to the plunging wave breaking instabilities. Detailed air entrainment and spray formation processes are shown. The bubble size distribution shows power-law scaling with two different slopes which are separated by the Hinze scale. The droplet size distribution also shows power-law scaling. The computational results compare well with the available experimental and computational data in the literature. Computational difficulties and challenges for large grid simulations are addressed.
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Leister, Nico, Chenhui Yan, and Heike Petra Karbstein. "Oil Droplet Coalescence in W/O/W Double Emulsions Examined in Models from Micrometer- to Millimeter-Sized Droplets." Colloids and Interfaces 6, no. 1 (February 8, 2022): 12. http://dx.doi.org/10.3390/colloids6010012.
Full textAbstract:
Water-in-oil-in-water (W1/O/W2) double emulsions must resist W1–W1, O–O and W1–W2 coalescence to be suitable for applications. This work isolates the stability of the oil droplets in a double emulsion, focusing on the impact of the concentration of the hydrophilic surfactant. The stability against coalescence was measured on droplets ranging in size from millimeters to micrometers, evaluating three different measurement methods. The time between the contact and coalescence of millimeter-sized droplets at a planar interface was compared to the number of coalescence events in a microfluidic emulsion and to the change in the droplet size distributions of micrometer-sized single and double emulsions. For the examined formulations, the same stability trends were found in all three droplet sizes. When the concentration of the hydrophilic surfactant is reduced drastically, lipophilic surfactants can help to increase the oil droplets’ stability against coalescence. This article also provides recommendations as to which purpose each of the model experiments is suited and discusses advantages and limitations compared to previous research carried out directly on double emulsions.
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Navarro-Martinez, Salvador, Giovanni Tretola, Mohammad Reza Yosri, Robert L. Gordon, and Konstantina Vogiatzaki. "An investigation on the impact of small-scale models in gasoline direct injection sprays (ECN Spray G)." International Journal of Engine Research 21, no. 1 (November 26, 2019): 217–25. http://dx.doi.org/10.1177/1468087419889449.
Full textAbstract:
The work presents a numerical investigation of gasoline direct injection and the resulting early development of spray plumes from an eight-hole injector (Engine Combustion Network Spray G). The objective is to evaluate the impact on the droplet size distribution (DSD) statistics from the assumed model physics, particularly for the small scales. Two modelling approaches are compared: Eulerian–Lagrangian spray atomisation with adaptive mesh refinement and a stochastic fields transported probability density function method. The two models simulate the small scales and sub-grid droplet physics with different approaches, but based on the same concept of transport of liquid surface density. Both approaches predict similar liquid distributions in the near-field comparable to experimental measurements. The spray break-up patterns are very similar and both models reproduce quasi-log-normal droplet distributions, with same overall Sauter mean diameters. The Eulerian–Lagrangian spray atomisation with probability density function approach shows different break-up behaviour between droplets originating from the dilute region and those originating from the dense core region. The transition from Eulerian to Lagrangian can be observed in the Eulerian–Lagrangian spray atomisation with adaptive mesh refinement predicted distribution with an abrupt change in the DSD. Both methods are able to produce similar DSD below filter width/grid size resolution.