Splash Erosion: Some Observations on the Splash‐Cup Technique
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Abstract Experiments under artificial rainfall conditions showed that the initial, excessive loss from the type of splash cup frequently used in splash‐cup experiments was attributable not only to material being pushed sideways over the edge of the cup by the impact of drops near the perimeter of the exposed surface, but also to a decrease in the rate of splash loss. The amount of excess material lost during this initial period was not significantly influenced by the frequency of drop impact, but was significantly influenced by the force of drop impact. Consequently, it is unlikely that the effect was attributable to changes in the hydraulic conditions within the erodible material, but may have been attributable to changes in the roughness of the exposed surface. A calibration, specific to the size, shape and velocity conditions of the impacting drops, may therefore be required where the initial loss makes a significant contribution to the total loss from a splash cup. It is also probable that such a calibration may also be specific to the physical nature of the erodible material and to the hydraulic characteristics of the splash‐cup system.Keywords:
Drop Impact
We simulate the onset and evolution of the earliest splashing of an infinite cylindrical liquid drop on a smooth dry solid surface. A tiny splash is observed to be emitted out of the rim of the lamella in the early stage of the impact. We find that the onset time of the splash is primarily dependent on the characteristic timescale, which is defined by the impact velocity as well as the drop radius, with no strong dependence on either the liquid viscosity or surface tension. Three regimes are found to be responsible for different splashing patterns. The outermost ejected droplets keep extending radially at a uniform speed proportional to the impact speed. Finally, we discuss the underlying mechanism which is responsible for the occurrence of the initial drop splash in the study.
Drop Impact
Solid surface
Liquid drop
Lamella (surface anatomy)
Weber number
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Soil erosion is recognized as one of the most important types of land degradation in the world particularly in many developing countries like Iran. Water erosion is initiated by splash erosion triggered by raindrop impact. Understanding the process of splash erosion under freezing and thawing conditions is essential to unravel soil erosion mechanisms under temperate conditions leading to appropriate planning of soil and water conservation projects. The present study aimed to study the individual effects of freeze-only as well as freezing-thawing cycle on splash erosion in a loess soil from an erosion prone area in mountainous northern regions of Iran. The study was conducted under laboratory conditions using erosion plots. The erosion plots were subjected to freeze only and freeze-thawing treatments by simulating cold conditions using a large cooling compartment system specifically manufactured for this purpose. The splash erosion under a designed simulated rainfall (1.2 mm min−1 for 30 min) was then measured as upward, downward and net splash erosion in splash cups. The results showed that freeze only decreased the upward, downward and net splash erosion by 0.81 ± 0.43, 0.82 ± 0.29 and 0.85 ± 0.23% while freezing-thawing cycle decreased splash erosion to 0.93 ± 0.83, 0.61 ± 0.43 and 0.57 ± 0.36%. This may be attributed to temporary increase in soil strength and stability or surface sealing during freezing process leading to reduced splash erosion.
Water erosion
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Upon impact on a solid surface, a drop expands into a sheet, a corona, which can rebound, stick or splash and fragment into secondary droplets. Previously, focus has been placed on impacts of single drops on surfaces to understand their splash, rebound or spreading. This is important for spraying, printing, and environmental and health processes such as contamination by pathogen-bearing droplets. However, sessile drops are ubiquitous on most surfaces and their interaction with the impacting drop is largely unknown. We report on the regimes of interactions between an impacting drop and a sessile drop. Combining experiments and theory, we derive the existence conditions for the four regimes of drop–drop interaction identified, and report that a subtle combination of geometry and momentum transfer determines a critical impact force governing their physics. Crescent-moon fragmentation is most efficient at producing and projecting secondary droplets, even when the impacting drop Weber number would not allow for splash to occur on the surface considered if the drop were isolated. We introduce a critical horizontal impact Weber number $We_{c}$ that governs the formation of a sheet from the sessile drop upon collision with the expanding corona of the impacting drop. We also predict and validate important properties of the crescent-moon fragmentation: the extension of its sheet base and the ligaments surrounding its base. Finally, our results suggest a new paradigm: impacts on most surfaces can make a splash of a new kind – a crescent-moon – for any impact velocity when neighbouring sessile drops are present.
Drop Impact
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Solid surface
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Dimensionless quantity
Drop Impact
Weber number
Deposition
Liquid drop
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The impact process of fuel drops on a solid dry surface under engine relevant conditions were studied using a numerical method based on smoothed particle hydrodynamics (SPH). The post-impingement properties (mass, velocity, and location) of the splashed secondary drops were analyzed. A drop/wall interaction model was developed on the basis of the SPH simulations. Numerical results show that the mass of the secondary drop will increase as the kinetic energy of the incident drop and surface temperature increase. For contact-splash, the radial location of the secondary drop increases linearly as the kinetic energy of the incident drop increases, while no clear trend was found for film-splash. The height of the secondary drop is also randomly distributed for both contact-splash and film-splash. The velocities of secondary drops will increase first and then decrease as the kinetic energy of the incident drop increases. The effects of impact angle on the impact outcomes were also characterized and incorporated into the model. It was found that the impact angle affects the distribution of the secondary drops. The proposed drop/wall interaction model derived from the present SPH study can be readily implemented for engine spray/wall impingement simulation.
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Smoothed Particle Hydrodynamics
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The impact and splash of liquid drops on solid substrates are ubiquitous in many important fields. However, previous studies have mainly focused on spherical drops while the non-spherical situations, such as raindrops, charged drops, oscillating drops, and drops affected by electromagnetic field, remain largely unexplored. Using ferrofluid, we realize various drop shapes and illustrate the fundamental role of shape in impact and splash. Experiments show that different drop shapes produce large variations in spreading dynamics, splash onset, and splash amount. However, underlying all these variations we discover universal mechanisms across various drop shapes: the impact dynamics is governed by the superellipse model, the splash onset is triggered by the Kelvin-Helmholtz instability, and the amount of splash is determined by the energy dissipation before liquid taking off. Our study generalizes the drop impact research beyond the spherical geometry, and reveals the potential of using drop shape to control impact and splash.
Drop Impact
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We investigate drop impact dynamics near both closed pits and open- ended pores experimentally. The resulting impact phenomena differ greatly for a pit or a pore. For the first, we observe three phenomena: a splash, a jet and an air bubble, whose appearance depends on the distance between impact location and pit. Furthermore, we found that splash velocities can reach up to seven times the impact velocity. Drop impact near a pore, however, results solely in splashing. Surprisingly, two distinct and disconnected splashing regimes occur, with a region of plain spreading in-between. For pores, splashes are less pronounced than in the pit case. We state that, for the pit case, the presence of air inside the pit plays a crucial role: it promotes splashing and allows for air bubbles to appear.
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Air entrainment
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We investigate drop impact dynamics near closed pits and open-ended pores experimentally. The resulting impact phenomena differ greatly in each case. For a pit, we observe three distinct phenomena, which we denote as a splash, a jet and an air bubble, whose appearance depends on the distance between impact location and pit. Furthermore, we found that splash velocities can reach up to seven times the impact velocity. Drop impact near a pore, however, results solely in splashing. Interestingly, two distinct and disconnected splashing regimes occur, with a region of planar spreading in between. For pores, splashes are less pronounced than in the pit case. We state that, for the pit case, the presence of air inside it plays the crucial role of promoting splashing and allowing for air bubbles to appear.
Drop Impact
Air entrainment
Air bubble
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We investigate drop impact dynamics near both closed pits and open- ended pores experimentally. The resulting impact phenomena differ greatly for a pit or a pore. For the first, we observe three phenomena: a splash, a jet and an air bubble, whose appearance depends on the distance between impact location and pit. Furthermore, we found that splash velocities can reach up to seven times the impact velocity. Drop impact near a pore, however, results solely in splashing. Surprisingly, two distinct and disconnected splashing regimes occur, with a region of plain spreading in-between. For pores, splashes are less pronounced than in the pit case. We state that, for the pit case, the presence of air inside the pit plays a crucial role: it promotes splashing and allows for air bubbles to appear.
Drop Impact
Air bubble
Air entrainment
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Liquid drop impact on dry, solid surfaces has been studied to elucidate the role of control parameters, such as drop size, impact velocity, liquid properties, surface roughness, and wettability, on the mechanism of splashing phenomenon. It has been shown more recently that ambient gas plays a pivotal role in initiating the disintegration mechanisms leading to the ejection of secondary droplets from an impacting drop. Through systematic experiments, the role of target surface temperature in altering the morphology of a splash outcome of impacting fuel drops is investigated in the present work. It is observed that at elevated surface temperatures, the heated air film present very close to the hot surface suppresses splashing and consequently raises the splash threshold Weber number of the impacting fuel drop. For a given Weber number, the morphology of the impacting drop shifts from splashing to spreading with a rise in the surface temperature through an intermediate transition regime, characterized by the tendency of the liquid sheet to recontact the drop lamella without ejecting any secondary droplets. The experimental observations are compared with theoretical model predictions reported in the literature, and fair agreement is found in terms of both the observed splash suppression and the underlying mechanisms that govern the identified morphological regimes.
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Liquid drop
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