Splash Droplets Research Articles

Reduction in the ratio of stemflow to rainfall during heavy rain in two Japanese cedar stands and the influence on rainfall partitioning

Stemflow (SF) and gross rainfall (GR) were measured in two Japanese cedar stands with stand densities (SDs) of 5700 trees/ha and 9700 trees/ha. SF water was collected from four trees in each stand, and SF was derived by dividing the volume by the canopy area of the trees. In previous studies the highest SD for SF measurements in Japanese cedar and cypress was 2500 trees/ha with SF/GR of 23.3%. In this study SF/GR in the 5700-trees/ha and the 9700-trees/ha stands were 34.0% and 53.3%, respectively. Literature data together with the results of this study showed linear relationship between SF/GR and SD. In both cedar stands SF increased linearly with increasing GR on a rain event basis, however, a bending point in linearity that is defined as a threshold appeared for GR > 100 mm in the 9700-trees/ha stand, over which the slopes of the regression line became gentler. The 10-minute analyses also revealed the threshold and gentler slope of the regression line during intense rainfall. The threshold values were not constant but ranged from 4 to 8 mm/10 min. One of the causes of the threshold is splash droplets generated by raindrops hitting canopy and trunks, and another is SF water that breaks away from branches and stems. Both of them become throughfall (TF) and reduce rainwater allocated to SF during intense rainfall. The past measurements on TF and canopy interception (CI) conducted at the same site as this study showed the threshold, over which CI/GR increased. The existence of the threshold in CI/GR means some rainwater that was not allocated to SF but became TF evaporated as splashes before reaching the forest floor. Reduction in SF/GR influences rainfall partitioning in the stands.

Surface wave and splashing of liquid film by oblique water jet impinging on a vertical plate

Liquid film cooling by jet impingement, known as a common thermal protection technique, is extensively applied in liquid rocket engines. The present work investigates the impact of a water jet onto a vertical plate by high-speed visualization system and laser particle analyzer, with a focus on the characteristics of surface waves and droplets splashing during the formation of liquid film. Depending on whether the jet is broken before impact, two regimes are identified, i.e., droplet or continuous jet impingement regime. The frequency and velocity of the surface wave and the splashing rate are jointly affected by the jet angle and the impingement distance. The splashing is mainly caused by the capillary breakup of the crown film that forms at the edge of the surface wave, and most of the splashing droplets are generated from the top of the crown film. The larger the jet angle, the smaller the wave frequency and velocity, but the higher the splashing rate. A large impingement distance causes a small wave frequency and velocity due to the shift of the impingement regime, further resulting in different tendencies in the splashing rate. A parameter expression for the size distributions of splashing droplets is established by the fitting result of the log-normal distribution and the Rosin-Rammler (R-R) distribution, where the parameters are associated with the jet Weber number.

Grouped random splashing droplets model for the rainfall distribution problem in flood discharge atomization

ABSTRACT In this paper, a more accurate and efficient model is proposed for calculating rainfall distribution in flood discharge atomization. The random splashing droplets (RSD) model adopts Monte Carlo method to calculate, which requires a large number of droplets to be generated randomly, resulting in low computational efficiency. Based on the optimization method of particle motion calculation such as parcel particle and super droplet, a new grouped random splashing droplets (GRSD) model is established. The comparison between the analytical solution and the numerical solution shows that there is little difference in the calculation time between the two models under same number of droplets. As the number of droplets increases, the accuracy of both models increases, but the computational efficiency of GRSD model increases more obviously. Therefore, GRSD model has higher computational efficiency than RSD model with same accuracy.

Throughfall drop sizes suggest canopy flowpaths vary by phenophase

Water flowpaths caused by incident rainfall onto forest canopy surfaces have a notable effect on the water budgets and chemistry of wooded ecosystems. This study revealed varying canopy flowpaths and residence times at the intra-event scale and across the phenological transition from leafed to leafless states for a set of three American beech (Fagus grandifolia Ehrh.) trees in a multilayered canopy. Simultaneous measurements of raindrops and throughfall drops by fifteen laser disdrometers over five rain events were analyzed during the transition from leafed to leafless phenophases. Throughfall was partitioned into free throughfall, splash throughfall, and canopy drip with four drop size classes. Throughfall drop size distributions and volume of each throughfall type varied at both intra-event and inter-event scales. Smaller canopy drips, <5.5 mm in diameter, were initiated earlier in rain events, whereas more rainfall accumulation was necessary to generate larger canopy drips, >5.5 mm in diameter. Smaller canopy drips were more dominant in the leafed phenophase when some structurally-mediated woody surface drip points were more muted. These results suggested throughfall from foliar surfaces generated smaller-sized canopy drip with shorter residence time, whereas throughfall from structurally-mediated woody surface drip points generated larger-sized canopy drip with longer residence time. There was also an increase in both free throughfall and splash droplets from leafed to leafless states, consistent with increased canopy gaps and direct interaction with woody surfaces in the leafless state. Based on the results, a conceptualization of the genesis and development of leaf and branch flowpaths in canopies is proposed.

Splashing generation by water jet impinging on a horizontal plate

The report of COVID-19 virus in municipal wastewater raises the question of whether viruses can become airborne during wastewater transport in sewer systems. The present work experimentally investigates a water jet impinging vertically onto a horizontal plate and the behaviours of the generated tiny droplets. Depending on whether the jet breaks into primary drops before the impingement, three regimes can be defined: non-splashing, jet-splashing and drop-splashing regimes. The splashing ratio, i.e., the portion of jet flow rate becoming splashing droplets, ranges from 1% to 70% in the drop-splashing regime, while it remains less than 2% in the jet-splashing regime. For the splashing droplets, their size and velocity distributions follow log-normal laws. Their diameters are mainly in the range from 0 to 0.3 of the impact jet or drop diameter with the median less than 0.1. Their velocities mostly range from 0 to 3.0 times of the impact velocity with the median around 1.0. The medians of both the dimensionless diameter and velocity of splashing droplets decrease with the impact Weber number. The ejection angles of splashing droplets obey a bell-shaped distribution with the maximum around 70° and the median ranging from 16° to 30°.

Droplet surface exposure measurement of a large splashing oil fume in the kitchen with a fluorescent 3D microscopic passive testing method

Cooking is a major source of indoor particulate matter, surpassing smoking. Large splashing oil fumes produced by water-oil interactions while cooking have received increasing attention for their common and significant hazards. However, the pollution of large splashing droplets is rather difficult to quantitatively measure with the current techniques. In this study, a fluorescent 3D microscopic passive testing method that is accurate and reliable for large spaces and broad-spectrum droplets is developed by combining fluorescence tracer, passive sampling, microscopic vision and droplet 3D reconstruction technologies. The measurement error for submicron droplets is approximately 22.6%. Because large droplet pollution is not unique to the kitchen, our innovative test method is worthy of reference in many fields, such as cleaning and painting workshops. We tested the surface exposure of a large splashing oil fume of 1–500 μm with this method. The results showed that the particle size of splashing oil droplets presents a bimodal distribution, which matches the two generation mechanisms that have been observed. The surface exposure mass concentration of all droplets of 1–500 μm on the surface at a distance of 0.75 m from the emission source edge is reduced to 3.6% of 0.25 m. Droplets (10–100 μm) are the main pollutants, which have continuous indirect hazards and require more attention.

Characteristics of cavity collapse behind a high-speed projectile entering the water

We investigate cavity collapse regimes behind high-speed projectiles entering the water. Using numerical simulations, we confirmed two different collapse phenomena: deep pinch-off and consecutive collapse. We performed a theoretical analysis to develop the relationship between projectile motion and cavity evolution. We found that projectile acceleration is the most significant factor determining the initial cavity collapse. There is a critical acceleration determining the direction of cavity collapse at the location of deep pinch-off. The pressure field is obviously affected by the collapse. The increase in pressure induced by surface pinch-off accelerates the collapse at the cavity tail. Because of the impact of surface pinch-off, consecutive collapse can be seen if a projectile reaches critical acceleration near a free surface. Otherwise, the cavity will pinch off at a distance from the surface of the water and form a deep pinch-off. Particular attention is paid to the impact of consecutive collapse on the projectile. Numerical calculations show that cavity collapse and a high-speed water jet have an obvious impact on the stability of the projectile. The jet exerts great pressure on the projectile, and the accompanying splash droplets contaminate the cavity wall. Cavity pulsation and the asymmetric geometry of the projectile-cavity system aggravate attitude deflection.

Water and energy balance of canopy interception as evidence of splash droplet evaporation hypothesis

ABSTRACT Canopy interception (I) can be divided into three processes: evaporation during rainfall (IR ), during storm break time (ISbt ) and after cessation of rainfall. Those values were measured using plastic Christmas tree stands, and it was found that IR was much larger than ISbt . Although it is commonly accepted that wet canopy evaporation is a major process of I, if so, ISbt must be greater than IR because of higher potential evaporation during storm break time. It was also demonstrated that measured IR was greater than calculated IR using the Penman-Monteith (PM) equation that assumes wet surface evaporation. Splash droplet evaporation (SDE), described as splash droplets generated by a raindrop hitting the canopy evaporate, was an alternative mechanism of I. SDE can elucidate ISbt « IR ; for a larger rainfall amount, the number of splash droplets becomes higher. Calculated IR was smaller than measured IR because the PM equation does not include SDE.

Droplet generation during spray impact onto a downward-facing solid surface

Experiments were conducted to explore the droplet generation process when an ascending liquid spray impacts on a downward-facing solid surface. The droplets generated in the present experiments were classified into the two types: one is the splashing droplets generated by the impacts of droplets in spray and the other is the falling droplets produced from a liquid film formed on the solid surface. The falling droplets were further classified to the first falling droplets and the satellite droplets. It was found that the ratio of the splashing droplets increases with an increase in the impact Weber number. As for the droplet size, the splashing droplets were much smaller than the falling droplets. Hence, the oil immersion method and the image analysis were used to measure the sizes of the splashing droplets and the falling droplets, respectively. The size of the splashing droplets was in the same order with the impacting droplets in spray. On the other hand, the size of the falling droplets was proportional to the wavelength of the most unstable wave in the Rayleigh-Taylor instability. Interestingly, the diameters of the falling droplets (first falling and satellite droplets) were inversely proportional to their order. Using the experimental data accumulated in this work, dimensionless correlations were developed for the rates and sizes of the splashing and falling droplets.

Evolution of anodic erosion components and heat transfer efficiency for W and W80Ag20 in atmospheric-pressure arcs

Based on the in situ diagnostic system and the digital image post-processing technique, quantifications of the anodic erosion components and heat transfer efficiency for tungsten and the W80Ag20 material in arcs of argon, helium, and nitrogen were achieved. The leading vaporization of silver has been found to result in evidently lower anodic heat transfer efficiency and much smaller molten pools compared with those of the tungsten anode, but the intensive generation of silver vapor under the melt layer causes higher splashing erosion of the W80Ag20 anode. However, these characteristics can be strongly changed by the cathodic blowing, which leads to a much higher splashing erosion of tungsten anode but a lower erosion rate of W80Ag20 anode. Through the visualization of the erosion behavior and the post microanalysis of the anode surface, the possible driving mechanisms for the generation of the splashing droplet were discussed. The result indicates that the local high pressure caused by the vapor blasting during the bubble explosion plays a key role in the detachment of splashing droplets. Furthermore, as for the tungsten-based anode, the comparison of erosion behavior between the W80Ag20 anode and the previous-studied W70Cu30 anode makes it clear that increasing the temperature difference between the melting point (MP) of tungsten and the boiling point (BP) of the other material has the pros and cons. It is effective for the decrease of the anode erosion when the anode heat energy is relatively weak, while it can promote the splashing erosion when the liquid tungsten layer has been formed on the anode tip. These results are beneficial for the optimal design of the electrode system in circuit breakers.

Dynamics of splashed droplets impacting wheat leaves treated with a fungicide.

Wheat is threatened by diseases such as leaf rust. One significant mechanism of disease spread is the liberation and dispersal of rust spores due to rainsplash. However, it is unclear to what extent the spore-laden splashed droplets can transmit the disease to neighbouring leaves. Here, we show that splashed droplets either bounce or stick, depending on the orientation of the leaf and whether the surface of the leaf has been treated with a fungicide. A scaling model revealed that bouncing was enabled when the droplet's kinetic energy exceeded its pinning energy to the surface. Our findings indicate that, ironically, the application of fungicide to protect a wheat plant may also facilitate pathogen spread and infection by making leaves sticky to spore-laden droplets.

Vortex-induced dispersal of a plant pathogen by raindrop impact.

Raindrop impact on infected plants can disperse micron-sized propagules of plant pathogens (e.g., spores of fungi). Little is known about the mechanism of how plant pathogens are liberated and transported due to raindrop impact. We used high-speed photography to observe thousands of dry-dispersed spores of the rust fungus Puccinia triticina being liberated from infected wheat plants following the impact of a single raindrop. We revealed that an air vortex ring was formed during the raindrop impact and carried the dry-dispersed spores away from the surface of the host plant. The maximum height and travel distance of the airborne spores increased with the aid of the air vortex. This unique mechanism of vortex-induced dispersal dynamics was characterized to predict trajectories of spores. Finally, we found that the spores transported by the air vortex can reach beyond the laminar boundary layer of leaves, which would enable the long-distance transport of plant pathogens through the atmosphere.

Mathematical model for decarburization of ultra-low carbon steel during RH treatment

ABSTRACT A mathematical model was established to investigate the decarburization behaviour of ultra-low carbon steel during Ruhrstahl-Heraeus (RH) refining process. In this model, it was assumed that the decarburization reactions occur at four sites: Ar bubble surface, bath surface, bulk steel and splash droplet surface. The effect of Ar bubble behaviour and splash droplet behaviour on decarburization rate was taken into consideration specifically. The calculated results are basically consistent with the industrial experimental observations. Carbon concentration was mainly eliminated at bath surface at early stage of RH treatment, and was mostly removed in bulk steel at middle stage, and was removed at splash droplet surface in the final stage. The decarburization rate at different sites and its contribution to total decarburization value were evaluated in detail. The decarburization rate during RH treatment could be effectively improved by accelerating the drop of vacuum pressure at early stage. Appropriate increasing Ar gas flow rate was beneficial to deep decarburization by increasing the reaction area of droplet surface and bath surface.

Effect of temperature on fuel adhesion under spray-wall impingement condition

In direct-injection spark-ignition (DISI) engines, spray-wall impingement affects the formation of the air–fuel mixture as well as combustion and exhaust emissions. In this study, the characteristics of fuel adhesion injected by a mini-sac gasoline injector with a single hole were investigated under the evaporation and non-evaporation conditions. The fuel spray was observed using Mie scattering, and fuel adhesion was measured using the refractive index matching (RIM) method under high injection and ambient pressures. The results showed that when evaporation occurs, the thin fuel adhesion evaporates quickly, whereas the thicker adhesion barely evaporates in a short period of time, resulting in a more uniform fuel adhesion on the wall. The maximum adhesion thickness continues to increase even after the end of injection, likely because of the re-deposition of the splashing droplets. Various mechanisms are purposed to explain the spray-wall interaction: fuel droplets primarily impinge on the wall forming primary impingement region, followed by the re-depositing droplets which form secondary impingement region.

Droplet generation during liquid jet impingement onto a horizontal plate

In sodium-cooled fast reactors, liquid sodium leakage from piping may lead to fire accident. In the case that the liquid sodium is discharged as a liquid jet, a number of droplets are produced during liquid jet impingement on the structures; the surfaces of splashed droplets serve as a main reaction field of sodium combustion. In the present work, a liquid jet was emanated vertically downward from a circular nozzle hole onto a horizontal disk to measure the total amount and the maximum size of splashed droplets. It was found that the splashing rate was negligibly small when the liquid jet impinged as a continuous jet whilst a significant amount of liquid was splashed when the liquid jet impinged as a broken jet. Thus, a prediction method was first developed for the impact frequency of the primary droplets produced due to the jet breakup. It was then shown that a phenomenological model using the impact frequency and the impact Weber number as the important variables can predict the splashing rate well. It was also indicated that the size of the maximum splashed droplets was fairly proportional to the size of primary droplets.

Contrasting plant height can improve the control of rain-borne diseases in wheat cultivar mixture: modelling splash dispersal in 3-D canopies.

Growing cultivars differing by their disease resistance level together (cultivar mixtures) can reduce the propagation of diseases. Although architectural characteristics of cultivars are little considered in mixture design, they could have an effect on disease, in particular through spore dispersal by rain splash, which occurs over short distances. The objective of this work was to assess the impact of plant height of wheat cultivars in mixtures on splash dispersal of Zymoseptoria tritici, which causes septoria tritici leaf blotch. We used a modelling approach involving an explicit description of canopy architecture and splash dispersal processes. The dispersal model computed raindrop interception by a virtual canopy as well as the production, transport and interception of splash droplets carrying inoculum. We designed 3-D virtual canopies composed of susceptible and resistant plants, according to field measurements at the flowering stage. In numerical experiments, we tested different heights of virtual cultivars making up binary mixtures to assess the influence of this architectural trait on dispersal patterns of spore-carrying droplets. Inoculum interception decreased exponentially with the height relative to the main inoculum source (lower diseased leaves of susceptible plants), and little inoculum was intercepted further than 40 cm above the inoculum source. Consequently, tall plants intercepted less inoculum than smaller ones. Plants with twice the standard height intercepted 33 % less inoculum than standard height plants. In cases when the height of suscpeptible plants was doubled, inoculum interception by resistant leaves was 40 % higher. This physical barrier to spore-carrying droplet trajectories reduced inoculum interception by tall susceptible plants and was modulated by plant height differences between cultivars of a binary mixture. These results suggest that mixture effects on spore dispersal could be modulated by an adequate choice of architectural characteristics of cultivars. In particular, even small differences in plant height could reduce spore dispersal.

A coupled Euler-Lagrange CFD modelling of droplets-to-film

ABSTRACTIn this paper, a droplet to film interaction model technique is presented. In the proposed approach, the liquid and gas continua are modelled using an enhanced Volume-of-Fluid (VoF) technique while the droplets are tracked using a Lagrangian framework and are coupled to the Eulerian phases using source terms. The eventual target application is an aeroengine bearing chamber in which oil is found as droplets, shed from the bearings, splashing on impact, separated from wall surfaces at obstacles or simply re-entrained, and as a continuum oil film coating the bearing chamber outer walls which it also cools. In finite volume Computational Fluid Dynamics (CFD) techniques, a prohibitively large number of cells would be required to describe the details of the droplet impact phenomenon. Based on published correlations, the splashing droplets are created and tracked as Lagrangian particles. The flowing film and the gas continua are handled with an enhanced VoF technique.

An experimental investigation of the oil film distribution in an oil–gas cyclone separator

The film flow behavior in an oil–gas cyclone separator was experimentally studied to improve the separation efficiency in terms of the effect of the oil film on the cylinder wall. The oil film flow pattern was captured using a high-speed camera, and the cylinder wall was divided into seven regions to analyze according to the different oil film flow patterns. Along the cyclone cylinder height, the central part of the cylinder was the main flow area, in which droplet–wall collisions and oil film splashing were severe. Additionally, the oil film’s distribution characteristics under inlet velocities of 14.0, 16.0, and 18.0 m/s were compared, and the results showed that more splashing oil droplets were generated under higher inlet velocity. Moreover, changing the structure of the central channel and outer cylinder slightly changed the oil film’s area and flow pattern but exhibited a weak effect on the oil film thickness and re-entrainment. Then, an improved structure was proposed by adding a porous cylinder to the outer cyclone to avoid the generation of small splashing droplets from the oil film. The performance of the modified separator was measured in a real oil-injected compressor system, which demonstrated higher separation efficiency with no increase in static pressure loss. The separation efficiency increased by up to 2.7%, while the pressure loss decreased by up to 10%. Thus, the improved structure can improve the performance of oil–gas separators by changing the distribution and thickness of the oil film on the cylinder wall.

Rainfall interception and the coupled surface water and energy balance

Evaporation from wet canopies (E) can return up to half of incident rainfall back into the atmosphere and is a major cause of the difference in water use between forests and short vegetation. Canopy water budget measurements often suggest values of E during rainfall that are several times greater than those predicted from Penman–Monteith theory. Our literature review identified potential issues with both estimation approaches, producing several hypotheses that were tested using micrometeorological observations from 128 FLUXNET sites world-wide. The analysis shows that FLUXNET eddy-covariance measurements tend to provide unreliable measurements of E during rainfall. However, the other micrometeorological FLUXNET observations do provide clues as to why conventional Penman–Monteith applications underestimate E. Aerodynamic exchange rather than radiation often drives E during rainfall, and hence errors in air humidity measurement and aerodynamic conductance calculation have considerable impact. Furthermore, evaporative cooling promotes a downwards heat flux from the air aloft as well as from the biomass and soil; energy sources that are not always considered. Accounting for these factors leads to E estimates and modelled interception losses that are considerably higher. On the other hand, canopy water budget measurements can lead to overestimates of E due to spatial sampling errors in throughfall and stemflow, underestimation of canopy rainfall storage capacity, and incorrect calculation of rainfall duration. There are remaining questions relating to horizontal advection from nearby dry areas, infrequent large-scale turbulence under stable atmospheric conditions, and the possible mechanical removal of splash droplets by such eddies. These questions have implications for catchment hydrology, rainfall recycling, land surface modelling, and the interpretation of eddy-covariance measurements.

Splash dispersal of Phyllosticta citricarpa conidia from infected citrus fruit.

Rain-splash dispersal of Phyllosticta citricarpa (syn. Guignardia citricarpa) conidia (pycnidiospores) from infected oranges was studied in still air and combined with wind. High power microscopy demonstrated the presence of conidia in splash droplets from diseased oranges, which exuded conidia for over one hour during repeated wetting. The largest (5 mm) incident drops produced the highest splashes (up to 41.0 cm). A linear-by-quadratic surface model predicted highest splashes to be 41.91 cm at a horizontal distance of 25.97 cm from the target orange. Large splash droplets contained most conidia (4–5.5 mm splashes averaged 308 conidia), but were splashed <30 cm horizontal distance. Most (80–90%) splashes were <1 mm diameter but carried only 0–4 conidia per droplet. In multiple splash experiments, splashes combined to reach higher maxima (up to 61.7 cm; linear-by-quadratic surface model prediction, 62.1 cm) than in the single splash experiments. In combination with wind, higher wind speeds carried an increasing proportion of splashes downwind travelling horizontally at least 8 m at the highest wind speed tested (7 m/s), due to a small proportion of droplets (<1 mm) being aerosolised. These experiments suggest that P. citricarpa conidia can be dispersed from infected oranges by splashes of water in rainfall events.