Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z35: Atomization and Sprays IV |
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Chair: G. Agbaglah, University of Ottawa Room: 243 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z35.00001: Fragmentation of thin liquid sheet through hole nucleation G. Gilou Agbaglah The formation of holes in the dynamics of thin liquid sheets is commonly seen in nature and industrial applications. This study focuses on aerodynamics effects in the perforation and disintegration of a thin liquid sheet and the outcomes of the collision of two distinct holes using three-dimensional numerical simulations. Thin liquid sheets with thicknesses ranging from 25 to 150 µm are considered using air/water conditions. The liquid sheet, sandwiched by a top and bottom fast gas streams, oscillates and small holes, preceded by craters surrounded by ripples, are formed. The thinning of the liquid sheet, spanwise corrugations, and vortex separations within the liquid-gas boundary layer are shown to govern the dynamics of the liquid sheet. A localized pressure jump is observed inside the liquid sheet and precedes the rupture of the liquid sheet by pushing the liquid away in the span direction. The holes formed subsequently grow, collide, merge with each other and break the liquid sheet into multiple droplets. |
Tuesday, November 22, 2022 1:03PM - 1:16PM Author not Attending |
Z35.00002: Study of the fragmentation of the liquid core in high-speed planar jets Mohan Ananth, Mario F Trujillo The current study focuses on the complete fragmentation of the liquid core when a liquid jet is injected into a quiescent gas medium. By considering the spatial linear stability analysis for a planar jet, a large-scale sinuous mode is observed at large wavelengths when the gas shear layer thickness is high. When these modes were imposed in a VoF simulation to analyze their manifestation in the non-linear regime, they were found to be dominant even in the non-linear regime leading to the liquid core fragmentation. Further analysis of the energy budget, the pressure fluctuations in the gas are found to be the main factor that causes these large-scale modes to grow leading to the liquid core fragmentation. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z35.00003: Direct numerical simulation of secondary atomization of a vaporizing drop Bradley Boyd, Yue Ling The secondary breakup of a freely moving volatile drop in a uniform high-speed and high-temperature gas stream is investigated through direct numerical simulation. The sharp liquid-gas interface is tracked using the geometric Volume-of-Fluid (VOF) method. The incompressible Navier-Stokes equations are solved in conjunction with a two-fluid model for energy of each phase, with an immersed Dirichlet boundary condition at the interface to implicitly account for the latent heat absorption. The model is implemented in the open-source solver, Basilisk, which uses adaptive octree mesh for spatial discretization and will allow for adaptive mesh refinement. The code has been validated by a series of test cases, including the vaporization of a spherical water drop with a very low Weber number. The simulation results agree well with the experimental results. The validated code were then used to simulate the secondary breakup of a volatile drop at moderate Weber numbers. The drop is initially stationary and at saturation temperature and is suddenly exposed to a uniform high-speed superheated gas stream. Through the simulation results, we will investigate the correlation between droplet deformation and the rate of droplet vaporization and will also characterize the effect of vaporization on the drop breakup dynamics. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z35.00004: A multi-scale computational model of respiratory droplet formation and dispersion Joseph Giliberto, Olivier Desjardins The need to study the mechanisms behind the formation of biological aerosols has become increasingly important as researchers continue to search for ways to limit the spread of COVID-19. Biological aerosols are formed from a continual mixing of the air that rapidly moves through the respiratory tract and oral cavity during breathing, speaking, coughing and sneezing with the saliva and phlegm that line the trachea and mouth. While the underlying processes that generate biological aerosols are quite complex, this study takes a simplified approach to modeling their formation by simulating the atomization of a thin liquid layer subjected to a high-speed shearing gas flow. Being able to properly reproduce the behavior of human saliva is challenging due to is inherent non-Newtonian behavior, as such this work explores various approaches to model the viscoelastic properties of saliva. To capture the formation and dispersion of the biological aerosol, a two-domain, multi-scale computational approach based on volume-of-fluid and Lagrangian droplet tracking is used to study two regions of interest: (1) a highly simplified oral cavity to observe the formation of fluid ligaments and sheets as well as the atomization of these liquid structures into droplets, and (2) a broader environment beyond the mouth where the ejected droplets are tracked in a Lagrangian fashion as they disperse. Results of the simulation allow for the observation of droplet formation and analysis of the size distribution of the generated droplets. The impact of non-Newtonian modeling on the break-up dynamics is explored. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z35.00005: High-fidelity simulations of a rotary bell atomizer with electrohydrodynamic effects Venkata Krisshna, Mark F Owkes Rotary Bell Atomizers (RBA) are extensively used as paint applicators in the automotive industry. Atomization of paint is achieved by a high speed rotating bell cup in the presence of a background electric field. Automotive paint shops account for 70% of the total energy costs [Galitsky et. al., 2008], 50% of the electricity demand [Leven et. al., 2001] and up to 80% of the environmental concerns [Geffen et al., 2000] in an automobile manufacturing facility. The atomization process in an RBA affects the size and the velocity distribution of the droplets which subsequently control the transfer efficiency and the surface finish quality. Optimal spray parameters used in industry are often obtained from expensive trial-and-error experimentation methods. In this work, three-dimensional near-cup atomization is simulated computationally using a high-fidelity volume-of-fluid transport scheme that includes electrohydrodynamic (EHD) effects. Atomization quality metrics such as droplet size and droplet charge density distributions are analyzed for different operating conditions. This cost-effective method of research aims to identify the effect of EHD on the spray to help maximize the transfer efficiency of the device. |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z35.00006: Characterizing thin liquid sheet morphology and dynamics through topological skeletons Graham Garcia, Yue Ling Thin liquid sheets are commonly seen in multiphase flows. One example is the bag breakup of a drop subjected to a uniform air stream. In the moderate Weber number regime, the originally spherical drop will deform to a bag and then break into small children drops. The liquid sheet thickness decreases rapidly over time as the bag is inflated. Measuring the thickness of the deforming, non-uniform, and curved liquid sheet obtained in interface-resolved simulation is challenging. A novel approach based on topological skeleton is proposed in this study. Skeletonization is a medial axis transformation process in which the topological features of 2D/3D objects can be captured with precision. The skeleton is the loci of centers of maximal inscribed sphere into the liquid structures, like liquid sheet. The transform takes the interfacial points and the corresponding normal from simulations and generates skeleton points and the corresponding radius of the inscribed ball. The radius for the skeleton points provides is unique and natural way to characterize local thickness of non-uniform liquid sheet of arbitrary shapes. The technique has been applied to a direct numerical simulation of aerobreakup of water drop in the bag breakup regime. The ordered skeleton points are shown to be useful to reveal morphological evolution of liquid sheets. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z35.00007: An adaptive-mesh Cahn-Hilliard Navier-Stokes (CH-NS) model for resolving film- and filament- breakup in primary jet atomization Kumar Saurabh, Makrand A Khanwale, Masado Ishii, Hari Sundar, Baskar Ganapathysubramanian The development of scalable and accurate numerical methods to model primary jet atomization over a long time horizon is challenging. A viable method must capture complex interfacial phenomena (breakup/coalesce) at disparate scales across long time horizons while preserving structure (mass, interface, droplet statistics) and exhibiting computational scalability. Here, we address the challenge of preserving the interface's structure by dynamically adapting resolution in a CH-NS model, thereby preventing artifacts from polluting the breakup statistics. The CH-NS model is endowed with mass conservation. However, when droplet sizes become comparable to the interface thickness (Cn number), there is a coalescence of smaller droplets with nearby bigger droplets (Ostwald ripening). Reducing the Cn number everywhere is infeasible as it increases compute requirements. Therefore, it becomes important to selectively identify the regions of the small drops/filaments/films to enforce lower Cn, ensuring a balance between computational cost and accuracy. We showcase improved results of primary jet atomization simulations that incorporate on-the-fly detection of small drops and filaments to reduce the Cn number, coupled with improved numerical schemes that enforce degenerate mobility. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z35.00008: PIV Measurements in a Planar Two-Phase Countercurrent Shear Layer Upom L Costa, Alison B Hoxie Efficient atomization of viscous liquids has widespread applications in different industries. Numerous research articles have focused on trying to make the atomization process more efficient and less energy consuming. While conventional atomizers have struggled with the efficiency of the atomization of highly viscous liquids, a counterflowing nozzle recently developed at the University of Minnesota has shown remarkable results in spraying liquids ranging in viscosity from 54 cP to over 1000 cP and producing sauter mean diameters of less than 50 microns at Air to Liquid Ratios ranging from 0.1 to 0.5. Upon closer investigation, it is assumed that extremely thin shear layers might be formed on liquid and air streams leading to interfacial instabilities of very short wavelength. The work presented here is an attempt to look deeper into the physics at work by studying the dynamics and spatiotemporal evolution of interfacial instabilities in a planar two-phase countercurrent mixing layer. The counterflowing shear layer was set up by two opposing momentum-driven streams, water and air. High speed imaging and Particle Image Velocimetry (PIV) were used to investigate the governing flow physics. Finally, a parametric study was also performed by varying the flow rates and velocities in the streams to gain a better understanding. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z35.00009: The ocean fine spray Alfonso M Ganan-Calvo An important fraction of the atmospheric aerosols come from the ocean spray originated by the bursting of surface bubbles. A theoretical framework that incorporates the latest knowledge on film and jet droplets from bubble bursting is here proposed, suggesting that the ejected droplet size in the fine and ultrafine (nanometric) spectrum constitute the ultimate origin of primary and secondary sea aerosols through a diversity of physicochemical routes. In contrast to the latest proposals on the mechanistic origin of that droplet size range, when bubbles of about 10 to 100 microns burst, they produce an extreme energy focusing and the ejection of a fast liquid spout whose size reaches the free molecular regime of the air. Simulations show that this spout yields a jet of sub-micrometer and nanometric scale droplets whose number and speed can be far beyond any previous estimation, overcoming by orders of magnitude other mechanisms recently proposed. The model proposed can be ultimately reduced to a single controlling parameter to predict the global probability density distribution (pdf) of the ocean spray. The model fits remarkably well most published experimental measurements along five orders of magnitude of spray size, from about 5 nm to about 0.5 mm. According to this proposal, the majority of ocean aerosols would have their extremely elusive birth in the collapsing uterus-like shape of small bursting bubbles on the ocean surface. |
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