Secondary Coalescence Research Articles

Toward Building a Virtual Laboratory to Investigate Rainfall Microphysics at Process Scales

Abstract A 3D numerical model was built to serve as a virtual microphysics laboratory (VML) to investigate rainfall microphysical processes. One key goal for the VML is to elucidate the physical basis of warm precipitation processes toward improving existing parameterizations beyond the constraints of past physical experiments. This manuscript presents results from VML simulations of classical tower experiments of raindrop collisional collection and breakup. The simulations capture large raindrop oscillations in shape and velocity in both horizontal and vertical planes and reveal that drop instability increases with diameter due to the weakening of the surface tension compared with the body force. A detailed evaluation against reference experimental datasets of binary collisions over a wide range of drop sizes shows that the VML reproduces collision outcomes well including coalescence, and disk, sheet, and filament breakups. Furthermore, the VML simulations captured spontaneous breakup, and secondary coalescence and breakup. The breakup type, fragment number, and size distribution are analyzed in the context of collision kinetic energy, diameter ratio, and relative position, with a view to capture the dynamic evolution of the vertical microstructure of rainfall in models and to interpret remote sensing measurements. Significance Statement Presently, uncertainty in precipitation estimation and prediction remains one of the grand challenges in water cycle studies. This work presents a detailed 3D simulator to characterize the evolution of drop size distributions (DSDs), without the space and functional constraints of laboratory experiments. The virtual microphysics laboratory (VML) is applied to replicate classical tower experiments from which parameterizations of precipitation processes used presently in weather and climate models and remote sensing algorithms were derived. The results presented demonstrate that the VML is a robust tool to capture DSD dynamics at the scale of individual raindrops (precipitation microphysics). VML will be used to characterize DSD dynamics across scales for environmental conditions and weather regimes for which no measurements are available.

Flow state transition and hydrodynamic characteristics of droplets on fiber in liquid phase environment

The movement of droplets on fiber is crucial for industrial applications. In this work, the motion behavior and flow state transformation mechanism of droplets on vertical fiber in liquid phase environment is systematically investigated by using image processing technology. The effect of different factors on the hydrodynamic characteristics is analyzed during the droplet movement process. The experimental results show that the three flow states are observed during the flow of droplets along the fiber in liquid phase environment, including stable state, oscillation state, and coalescence state. Among them, the transverse oscillation of the droplets has a certain periodicity, and the primary coalescence rate of the droplets is 16.22% faster than secondary coalescence. Increasing the fiber diameter can expand the flow rate range of droplets in the stable state and significantly enhance the critical flow rate. Moreover, with the increase of nozzle diameter, the flow rate range of the oscillation state is increased and the flow rate range of the coalescence state is decreased. During the stable state, the hydrodynamic characteristics of droplets are influenced by factors such as fiber diameter, nozzle diameter, and flow rate. The study results not only contribute to deeply understand the movement mechanism of droplets on fiber surface, but also have an important significance in promoting the development of liquid–liquid extraction technology and droplet microfluidics technology.

Modeling of aluminum agglomeration at solid propellant burning surface by combining aluminum ignition with LDP-SC cluster analysis

Aluminum particles at burning surface may agglomerate during the combustion process of composite solid propellant, involving the aggregation and ignition of aluminum particles as well as the secondary coalescence of agglomerates. So far it is still a huge challenge to capture the agglomeration process and obtain the size distribution of agglomerates accurately. Modeling of aluminum agglomeration was carried out by constructing several submodels, including propellant combustion, cluster analysis of aluminum particles, aluminum ignition and coalescence based on the packed three-dimensional propellant microstructure unit. Dynamic combustion process of propellant microstructure unit was simulated firstly, which characterizes the temperature distribution above the burning surface. Next, aluminum clusters were grouped using a new local density peaks-spectral clustering (LDP-SC) algorithm for initial aluminum particles in microstructure unit. The ignition of aluminum particles in clusters exposed from burning surface was then examined by considering their distributions in the flame of propellant. These exposed aluminum particles in a cluster yield an agglomerate once an aluminum particle is successfully ignited. Secondary mergence of multiple agglomerates was also considered when the distance between two particles in different agglomerates was less than a critical separation distance. With these submodels, the agglomeration process of a typical propellant and properties of aluminum clusters and agglomerates were estimated. The predicted size distribution of agglomerates shows reasonable agreement with experimental data, indicating that the present model is capable of predicting aluminum particle agglomeration during composite propellant combustion.

Numerical investigation of droplet condensation and self-propelled jumping on superhydrophobic microcolumned surfaces

This paper investigates the processes of droplet condensation and self-propelled jumping on microcolumn-structured superhydrophobic surfaces with various size parameters. Using a three-dimensional (3D) multiphase lattice Boltzmann method, a novel phenomenon of secondary coalescence jumping is identified, and the underlying mechanisms are analyzed in detail. The simulation results show that wettability has a significant influence on droplet jumping. As the hydrophobicity of the surface increases, the droplets tend to jump from the substrate. However, structure parameters, such as the microcolumn spacing and height, have non-monotonic effects on droplet jumping. The structure parameters determine whether droplet coalescence occurs under the bottom–bottom droplet coalescence mode or the bottom–top droplet coalescence mode. Bottom–bottom droplet coalescence is shown to promote droplet jumping. Based on the simulation results and kinetic analysis, the optimal spacing-to-width and height-to-width ratios of the microcolumns for droplet jumping are found to be approximately 0.6 and 1.0, respectively. We believe the results of this work will provide valuable guidance in the design of self-cleaning surfaces and enhancing heat transfer efficiency.

Capillary instability of a two-layer annular film: an airway closure model

Capillary instability of a two-layer liquid film lining a rigid tube is studied computationally as a model for liquid plug formation and closure of human airways. The two-layer liquid consists of a serous layer, also called the periciliary liquid layer, at the inner side and a mucus layer at the outer side. Together, they form the airway surface liquid lining the airway wall and surrounding an air core. Liquid plug formation occurs due to Plateau–Rayleigh instability when the liquid film thickness exceeds a critical value. Numerical simulations are performed for the entire closure process, including the pre- and post-coalescence phases. The mechanical stresses and their gradients on the airway wall are investigated for physiologically relevant ranges of the mucus-to-serous thickness ratio, the viscosity ratio, and the air–mucus and serous–mucus surface tensions encompassing healthy and pathological conditions of a typical adult human lung. The growth rate of the two-layer model is found to be higher in comparison with a one-layer equivalent configuration. This leads to a much sooner closure in the two-layer model than that in the corresponding one-layer model. Moreover, it is found that the serous layer generally provides an effective protection to the pulmonary epithelium against high shear stress excursions and their gradients. A linear stability analysis is also performed, and the results are found to be in good qualitative agreement with the simulations. Finally, a secondary coalescence that may occur during the post-closure phase is investigated.

Experimental investigation on bubble coalescence regimes under non-uniform electric field

In this study, the characteristics of interactions between bubbles in ethanol were examined using a needle-ring electrode rig in the presence of a direct current electric field, using high-speed photography and with a focus on the effect of the electric field strength on the bubble coalescence. The results show bubble coalescence associated with four regimes in the manipulated parameter regions-namely, the coalescence of growing bubbles, single coalescence of rising bubbles, secondary coalescence of rising bubbles, and multiple coalescences of bubbles. The bubble interaction dynamics-which are characterized by the bubble trajectories, height positions, inclination angles, and bubble sizes-show that the bubble approach, collision, and coalescence in the electric field proceed in ways different from those reported in the literature. Furthermore, the cross-sectional average bubble diameter values corresponded to the bubble coalescence regimes. In addition, the increase in the Weber number (We) and electrical Bond number (BoE) gives rise to the bubble coalescence frequency. The We works to strengthen the wake induction, while the BoE works to accelerate the generation of smaller bubbles.

Dynamic analysis of the primary breakup of an annular energetic liquid jet injecting into a confined space

The Large Eddy Simulation (LES) with the Volume of Fluid (VOF) method was carried out to investigate the primary breakup dynamic mechanism of an annular energetic liquid jet injecting into a confined space. The gas vortex structure and the liquid jet formation were visualized by tracking the evolution of the phase velocity and the interface location, and hence the coalescence process of the goblet-shaped liquid jet from the annular liquid jet was reproduced. Simulation results indicate that the first coalescence is caused by the Rayleigh-Taylor instability, which is generated by the interaction between the liquid jet head and the gas in the confined space. Two kinds of large-scale vortex structure expedite the secondary coalescence and the final goblet-shaped liquid jet form. Cavities are observed on the dense liquid jet column as a result of the coalescence on the upstream. Consequently, the droplet breakup occurs more frequently on the downstream liquid jet under the gas phase disturbance. The viscous shear is created by the aerodynamic interaction, and the regular fish-scale structure is presented under the Kelvin-Helmholtz instability, which promotes the bulge surface to break into droplets. The research reveals the dynamic mechanism of the annular energetic liquid jet structural destabilization observed in the previous experiments. The combination of the structural instability and the surface instability facilitate the primary breakup of the annular energetic liquid jet, which is beneficial to improve the atomization and the combustion performance.

Sprays from droplets impacting a mesh

In liquid spray applications, the sprays are often created by the formation and destabilization of a liquid sheet or jet. The disadvantage of such atomization processes is that the breakup is often highly irregular, causing a broad distribution of droplet sizes. As these sizes are controlled by the ligament corrugation and size, a monodisperse spray should consist of ligaments that are both smooth and of equal size. A straightforward way of creating smooth and equally sized ligaments is by droplet impact on a mesh. In this work we show that this approach does however not produce monodisperse droplets, but instead the droplet size distribution is very broad, with a large number of small satellite drops. We demonstrate that the fragmentation is controlled by a jet instability, where initial perturbations caused by the injection process result in long-wavelength disturbances that determine the final ligament breakup. During destabilization the crests of these disturbances are connected by thin ligaments which are the leading cause of the large number of small droplets. A secondary coalescence process, due to small relative velocities between droplets, partly masks this effect by reducing the amount of small droplets. Of the many parameters in this system, we describe the effect of varying the mesh size, mesh rigidity, impact velocity and wetting properties, keeping the liquid properties the same by focusing on water droplets only. We further perform lattice Boltzmann modelling of the impact process that reproduces key features seen in the experimental data.

Analysis of pneumatic nozzle operation with the stochastic Euler-Lagrange model

The aim of this study was to develop a CFD model for initial qualitative analysis of the operation of the pneumatic nozzle used for dry coating. The calibration consisted in comparing the droplets velocity yielded by the model with our own experimental data. The impact of the sheet and ligament constants as well as of the application of secondary coalescence and breakup models on dispersed phase velocity were taken into consideration. It turned out that the secondary phenomena had the largest impact on the dispersed phase velocity. Only after they had been taken into consideration was the agreement between the computation results and empirical data satisfactory.Afterwards, the best set-up of the model was used for describing the plasticizer atomization in a draft tube of a spout-fluid bed apparatus. It was concluded that this process should be carried out with the atomizing gas at a pressure of 2 ∙ 105 Pa.

Mechanism of the secondary liquid—liquid droplet—film rupture on inclined plate

The mechanism of the secondary liquid—liquid droplet—film rupture on an inclined plate has been examined theoretically and experimentally. A synergistic mixture, D2EHPA—TOPO—kerosene, and phosphoric acid were used during the investigations. A fundamental approach has been used to analyse a special stability and rupture problem of droplet—film structures during the secondary coalescence. In rupture adhesion processes, instead of viscous forces, some kind of electrical forces are supposed to be dominant. The stability and rupture problem of the secondary droplet—film structure on the inclined plate was analysed using mechanical and electrical principles. Formation and rupture processes of the secondary liquid—liquid droplet—film structures were discussed considering the interfacial potential, its spontaneous and incident oscillations, impedance and impedance structure.

Finspine morphogenesis in squalid and heterodontid sharks

Modern selachian finspines develop within a single, enlarged dermal papilla much earlier than any other papillary derived structure, and rapidly enlarge beyond the confines of the papilla. Within this, the spine primordium differentiates into two parts (mantle and trunk). Both components elongate basally and become mineralized by odontoblasts, but only the mantle has an enameloid layer. Mantle and trunk are reunited secondarily by dentine. The finspine vascular supply becomes blocked in adults, possibly to avoid loss of body fluids should the finspine (which is not shed) be damaged. Finspines probably have both defensive (perhaps mainly in juveniles) and locomotory (unctions. Odontocytes sometimes occur in the finspine base of Squalus, although the rest of the spine consists of acellular metadentine. The innermost dentine layers are centripetally deposited, resembling lamellar orthodentine of a tooth pulp. Over this is a continuous layer of centrifugally deposited lamellar dentine. Apically there is another centripetal layer (the mantle), which occurs only on certain parts of the spine. Enamelled tubercles can occur in Squalus, as a result of scleroblast disorganization, not as a result of secondary coalescence.