Drop Fall Research Articles

Evaluation of Residual Stress in MEMS Micromirror Die Surface Mounting Process and Shock Destructive Reliability Test

Light Detection and Ranging (LiDAR) devices based on Micro-Electro-Mechanical Systems (MEMS) micromirrors have been promising sensors for automatic driving owing to the advantages of MEMS devices. The packaging plays a vital role in the reliability of MEMS micromirror but introduces the residual stress from the surface mounting process. The shock reliability of micromirrors is influenced by residual stress. In this work, the residual stress from the surface mounting process are investigated through resonant frequency tests. The frequency drift can be 718 Hz and -281 Hz under the residual stress of 107.0 MPa and -25.0 MPa respectively. As the difference of CTE rises, the residual stress proportionally increases but decreases with the thickness of the substrate. In addition, a higher curing temperature can introduce a more significant stress level. The dynamic response in shock environment under different residual stress are analyzed. Drop fall experiments verify the theoretical conclusions that the stiffness hardening effect can improve the shock level tolerance. The tensile stress levels of 41.7 MPa and 107.0 MPa are corresponding to 100 g and 200 g respectively. The influences of residual stress on frequency drift and reliability need to be considered seriously and simultaneously during the packaging design for MEMS micromirrors. This work provides a deep insight into commercial applications of MEMS-based LiDAR in autonomous driving.

Investigation of the free overfall for discharge measurement in stone bed triangular channels

This study utilizes the freefall as a simple and useful tool for measuring end depth discharge EDD in triangular channels having stone beds. For this task, forty-five triangular channel models are constructed and examined all having 360 cm length and end drop fall of 20 cm. In these channel models, three channel side slopes z = 0.7, 1.0 and 1.3, five bed slopes St = 0.00, 0.0033, 0.005, 0.010 and 0.020 and three bed stone diameters ds = 5 mm, 10 mm and 15 mm are tested. The laboratory results demonstrated straight line relationships for the variation of end depth ye with critical depth yc and the variation of end depth with uniform normal depth yn for different values of z, St and ds tested in this study, while, the relation between ye/yc and bed slope St showed a quadratic expression. Moreover, the relationship for the dimensionless discharge with both channel bed slope St and the ratio of end water depth ye to bed stone diameter ds(ye/ds) has been presented by a simple empirical power equation with high determination coefficient which can be applied for subcritical and supercritical states of flow for the range of upstream Froude number Frn between 0.14 and 1.27.

The size distribution metrics and kinetic energy of raindrops above and below an isolated tree canopy in urban environment

Raindrop impact on bare soils is the initial phase of rainfall-induced soil erosion which is altered under any type of vegetation due to the interactions of rainfall with the canopies. This study examines the drop size distribution (DSD) and kinetic energy (KE) of raindrops above and below the birch tree (Betula pendula Roth.) canopy in a research plot in the city of Ljubljana, Slovenia using a one-year observation of 63 rainfall events and the effect of meteorological variables under moderate continental climate. Simultaneous measurement of the microstructures of open rainfall and throughfall was carried out using an optical disdrometer. The result of our analysis revealed that throughfall DSD showed two distinct major peaks (bimodal) occurring primarily on smaller drop sizes while open rainfall has only one. The cumulative drop number, median drop-volume diameter (D50), and drop fall velocity of throughfall were 16.4%, 26.6%, and 5.0% lower than those of open rainfall, respectively. Also, the relative volume percentage of raindrops > 1.5 mm is 1.5 times higher than those observed in throughfall drops which indicates that the presence of the canopy caused the fractionation of larger drops into smaller droplets. These reductions significantly differ depending on the phenoseasons of the canopy with the leafed state being higher than the leafless state. Similarly, the Kruskal-Wallis H test result revealed that birch tree elicits a statistically significant change in the kinetic energy of open rainfall, thus weakening the mean rainfall KE by 33.7%. On the other hand, KE is positively affected by the phenological condition of the canopy with higher attenuation being observed during its leafed state. Also, the correlation analysis demonstrated that vapor pressure deficit, air temperature, and relative humidity have stronger associations with throughfall kinetic energy among meteorological variables considered. These findings underscore the necessity of an optimized selection of tree species for afforestation programs.

Impact Forces of Water Drops Falling on Superhydrophobic Surfaces.

A falling liquid drop, after impact on a rigid substrate, deforms and spreads, owing to the normal reaction force. Subsequently, if the substrate is nonwetting, the drop retracts and then jumps off. As we show here, not only is the impact itself associated with a distinct peak in the temporal evolution of the normal force, but also the jump-off, which was hitherto unknown. We characterize both peaks and elucidate how they relate to the different stages of the drop impact process. The time at which the second peak appears coincides with the formation of a Worthington jet, emerging through flow focusing. Even low-velocity impacts can lead to a surprisingly high second peak in the normal force, even larger than the first one, namely when the Worthington jet becomes singular due to the collapse of an air cavity in the drop.

Effect of the Geometrical Structure on the Superhydrophobicity and Self-Cleaning Properties of Plasma-Treated Polyvinylidene Fluoride Fabrics.

The purpose of this study is to develop superhydrophobic polyvinylidene fluoride (PVDF) fabrics to increase their water repellency and self-cleaning properties and to investigate the effects of the inherent fabric roughness on these properties. A PVDF fabric, composed entirely of electrospun PVDF filament yarns, and two PVDF/polyester (PET) fabrics with different weave densities are used. After treatment with O2 plasma for 12 min and CF4 plasma for 4 min, superhydrophobicity is achieved in all fabrics, resulting in an increase in water repellency and self-cleaning efficiency. The PVDF fabric with the lowest shedding angle exhibits the most pronounced droplet rebound behavior and the highest self-cleaning efficiency. Increases in surface inclination angle and droplet volume and a decrease in the drop fall height all contribute to conditions more favorable for water droplet repellency. The self-cleaning efficiencies of the plasma-treated PVDF fabric and high-density PVDF/PET fabric are higher for hydrophilic dust, in contrast to those of the untreated ones. The findings of this study are expected to enable the design of weaving or nano-structuring conditions that enhance the water repellency and self-cleaning properties of PVDF fabrics, for the development of stable energy-harvesting smart textiles.

Effect of Vertical Downdraft on Rain Attenuation Modeling Over an Earth-Space Path

The impact of vertical downdraft on rain attenuation estimation from rain rate measurements has been studied. The rain rate (<i>R</i>) and specific attenuation (&#x03B3;) are derived from rain drop size distributions (DSD) by considering the widely accepted Gunn&#x2013;Kinzer (G-K) relationship between drop size and drop fall velocity that is valid for stagnant air. In the real atmosphere, drops are carried with the vertical wind draft, and the actual fall velocity of the raindrops deviates from the G&#x2013;K formulation. The ITU-R estimated rain attenuation values, based on DSDs, which are mostly valid for temperate regions, can deviate from actual rain attenuation measurements in tropical regions, especially at higher rain rates when downdraft is prominent. The present study uses disdrometer measurements of DSD, optical rain gauge measurements of rain rates, and Ku-band rain attenuation data over an earth-space path at a tropical location Kolkata (22&#x00B0;34&#x2032; N, 88&#x00B0;29&#x2032; E) to show that vertical downdrafts impact the rain attenuation estimation from rain rate measurements. At the present location, an overall representative vertical downdraft of 2 m&#x002F;s needs to be considered to match the experimental attenuation measurements with the estimated values using the modified &#x03B3;&#x2013;<i>R</i> relationship in the ITU-R model.

Scale invariant relationship between rainfall kinetic energy and intensity in Paris region: An evaluation using universal multifractal framework

To calculate the effect of rainfall in detaching particles and initiating soil erosion, or in eroding wind turbine leading edge, it is important to measure recorded drop size distributions (DSD) and fall velocity over long period. Commonly used relationships between kinetic energy (KE) and rainfall rate (R) exhibit strong dependence on the temporal resolution at which the analysis is carried out. Here we aim at developing a new scale invariant relationship relying on the framework of Universal Multifractals (UM), which is widely used to analyze and characterize geophysical fields that exhibit extreme variability across wide range of scales.Rainfall data is collected using three optical disdrometers working on different underlying technologies (one Campbell Scientific PWS100 and two OTT Parsivel2 instruments) and operated by the Hydrology Meteorology and Complexity laboratory of École des Ponts ParisTech in Paris area (France). They provide access to the size and velocity of drops falling through sampling areas of few tens of cm2. Such data enables estimation of rainfall DSD, R and KE at various resolutions. The temporal variations of this geophysical data over wide range of scales are then characterized in the UM framework, which was never done for KE. A new power law relation is developed and tested against the theoretical framework assuming gamma DSD for describing the dependence between KE and R. The developed equation using scale invariant features of UM does not rely on gamma DSD assumption, performs as well as the existing tools, and is valid not only at a single scale, but also across scales.

Shock Destructive Reliability Analysis of Electromagnetic MEMS Micromirror for Automotive LiDAR

Light Detection and Ranging (LiDAR) devices are preferred sensors in the field of automatic and intelligent driving. LiDAR based on Micro-Electro-Mechanical Systems (MEMS) micromirrors has gained vital attention owing to the advantages of MEMS devices. In a harsh environment, the shock reliability of micromirrors has significantly constrained automotive applications, but such studies are rarely reported. In this work, the shock destructive reliability of electromagnetic MEMS micromirror in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -direction is investigated to accelerate the commercialization of MEMS-LiDAR. The drop fall tests cover from 100 to 800 g in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -direction, and the fracture emerges at the shock level of around 700 g. To comprehensively explore the factors of shock destructive reliability, finite element and dynamic models are completed. Finite element analysis is employed to investigate the stress distribution under shock loads, and the fracture is located at the position of maximum stress. The dynamic response and peak stress with shock pulses are explored numerically by the established dynamic model. The calculated results demonstrate that the response reaches a peak when the frequency of shock is the same as the natural frequency of micromirror. The results of peak stress with varied shock levels can be utilized to quickly predict the shock resistance. This work provides a deep insight into the optimization for the design of MEMS micromirror, which may promote the applications of MEMS-based LiDAR in autonomous driving. [2021-0200]

Single Contaminated Drops Falling through Stagnant Liquid at Low Reynolds Numbers

Numerical simulations of contaminated spherical drops falling through a stagnant liquid at low Reynolds numbers are carried out using the finite difference method. The numerical results are used to describe the behavior of the surfactant concentrations and to understand the surfactant effects on the fluid motions in detail. The predicted interfacial surfactant concentration, Γ, is almost zero for angles, θ, below a certain value (the stagnant-cap angle, θcap), whereas it steeply increases and reaches a large value for θ&gt;θcap (the stagnant-cap region). The increase in the initial surfactant concentration, C0, in the drop enhances the adsorption from the drop to the interface, which results in the increase in Γ and the decrease in θcap. Peaks appear in the predicted Marangoni stresses around θcap, which causes similar peaks in the pressure distribution. The high-pressure spots prevent the fluid motion along the interface, which results in the formation of the stagnant-cap region and the attenuation of the tangential velocity in the continuous phase. The surfactant flux from the bulk to the interface decreases C in the vicinity of the interface for θ&lt;θcap and the weak diffusion cannot compensate for the reduction in C by adsorption, which results in C at the interface smaller than C0. The pattern of the low C region is determined by the advection and does not smear out because of a small diffusive flux.

Influence of Reduced Anthropogenic Activities on Rain Microphysical Properties and Related Atmospheric Parameters Over an Urban Tropical Location

This letter reveals the prevailing scenario of raindrop size distribution (DSD) in terms of mass-weighted mean drop diameter ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> ) over a tropical metropolis, Kolkata (22.57°N, 88.37°E), India, in a contrasting aerosol environment that occurred during the COVID-19 pandemic in the absence of usual human activities. In the premonsoon months (March–May), the probability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D_{m}$ </tex-math></inline-formula> values exceeding 2 mm has increased in 2020, indicating the dominance of large raindrops, compared to the years 2017–2019. Increased number densities of larger drops have influenced the drop fall velocity spectrum as measured by a laser precipitation monitor in terms of the percentage occurrence of high-velocity small drops (superterminal) and low-velocity large drops (subterminal) for both convective and stratiform precipitations. As measured from a Ka-band microrain Doppler radar, the mean melting layer altitude during stratiform rain has decreased by ~800 m during the premonsoon of 2020 compared to 2017–2019. According to the ERA-5 reanalysis data, changing rain microphysical characteristics are related to decreasing zero-degree isotherm height and reduced wind shear.

Dynamics of Evaporating and Non-Evaporating Drops Falling in Air

Dynamics of Evaporating and Non-Evaporating Drops Falling in Air

Fall and break-up of viscous miscible drops in a Hele-Shaw cell

We study the fall of pancake-shaped drops in a Hele-Shaw cell filled with a more viscous ambient fluid in the case where the drop and ambient fluid are miscible. We propose a theoretical expression for the falling velocity of the drops which takes into account elongated drops with width smaller than the gap of the cell. We also provide experimental evidence for the break-up of miscible drops, which can be caused by internal fluid motion within the drop, or by an instability in the shape of the drop.

A Liquid drop Falling in Another Fluid: A Two-Phase Flow Phenomenon

Liquid drops exist everywhere in daily life. However, such a common phenomenon involves complex mechanisms, and therefore has never been investigated thoroughly—the achievements of the studies are limited and unsatisfactory. In this article, the phenomenon of a liquid drop falling in another immiscible fluid (mostly its settling speed) was investigated. A series of experiments, in which liquid drops emerging from a syringe falls inside a measuring cylinder filled another fluid, was performed. Fluids used in the experiment include ethanol solutions and rice bran oil; for rice bran oil, its temperature-dependent viscosity was approximated by a function, which was proved effective on other similar oils. The dependence of the settling speed of the liquid drop on the density difference between the liquid drop and the surrounding fluid, as well as on the temperature of the surrounding fluid, was measured. Finite-Element Modeling (FEM) simulations were then carried out to model these situations, and lead to results that agreed with the experiments. It also visualized the flow field and revealed more details of the two-phase flow that were not detectable by our devices. Furthermore, inspired by some previous formulae, quantitative equations were derived semi-analytically by modifying an approximate drag formula originally developed for a rigid object at moderate Reynolds numbers. The form of the modification was never introduced in any other previous works. The equations were validated by simulations under ideal conditions.

THEORY AND PRACTICE OF AGRICULTURAL MATERIALS MOVEMENT IN LIQUID AND AIR ENVIRONMENTS UNDER CONDITIONS OF GRAVITATIONAL FIELD

Existing technologies and modern technical means do not always provide the required quality of technological processes of growing crops. The growing demands in a competitive market environment necessitate the constant improvement of existing and the creation of new high-performance machines and machine units to ensure the quality of technological processes in precision farming systems. The article presents improved theoretical principles of practical applied direction of agricultural materials MOVEMENT in air and liquid environments under the action of gravitational field. As a result of research using a fundamental analytical and mathematical apparatus, an algorithmic model has been developed that describes the movement of: seeds or mineral fertilizers on the way from seed drills to furrows, which are formed in the soil after the passage of seed drill openers; grain that moves from top to bottom in the cleaning systems of combine harvesters or during unloading from the hopper, etc. Such processes and phenomena occur during the production operation of seeders and planters, machines for post-harvest processing of grain, harvesting of roots, fruits, berries, etc. Particular attention is paid to the fall of raindrops or drops created by irrigation systems. The influence of the impact of drops on the field surface is analyzed, which provides the development of practical recommendations for the prevention of soil erosion, which is especially relevant in fields with difficult terrain. Some technical systems of creating "artificial rain" are considered, their technological schemes are substantiated and constructive and technological parameters are optimized. The application of the algorithm of mathematical analysis and calculation of technological processes and technical systems provides the necessary conditions for seed germination, growth and development of crops in both plain fields and on sloping lands. In addition, the proposed innovative methods of theoretical analysis and calculation of processes and phenomena characteristic of agro-industrial production can be effectively and used in the educational process during the preparation of agricultural engineers for project professional activities.

Rainfall drop arrival rate at the ground: A potentially informative parameter in the experimental study of infiltration, soil erosion, and related land surface processes

Rainfall drop arrival rate (DAR) quantifies the number of impacts at the ground of drops of varying size per unit area and time. It can be calculated from the drop size distribution and fall speed data used for rain integral parameters such as total kinetic energy or rainfall intensity. However, the DAR can also be calculated for individual drop size classes, revealing the relative number of impacts of small and large drops. New calculations of DAR are developed, and show that for increasingly intense rain, impacts by drops of large diameters become more common, and contribute an increasing proportion of the total kinetic energy. This may exert an important influence on many landsurface processes. A comparison with the kinds of rainfall arrival rates present in simulated rainfall is made, and suggests that although simulated rainfall may be generated with an intensity corresponding to natural rain, the DAR, especially for small and large drops, is often significantly different to the value expected in natural rain. For research on soil splash, interrill erosion, infiltration, and other important land surface processes, it is suggested that attention to DAR may prove informative, both as a potentially useful explanatory variable in field studies made under natural rainfall, and as a target parameter in the design of rainfall simulation experiments.

Groups of Sprays from the Impact of a Water Drop Falling Freely into a Melted Metal

Groups of Sprays from the Impact of a Water Drop Falling Freely into a Melted Metal

Raindrop fall velocity in turbulent flow: an observational study

Abstract. Laboratory measurements of drop fall speeds by Gunn–Kinzer under still air conditions with pressure corrections of Beard are accepted as the “gold standard”. We present measured fall speeds of 2 and 3 mm raindrops falling in turbulent flow with 2D-video disdrometer (2DVD) and simultaneous measurements of wind velocity fluctuations using a 3D-sonic anemometer. The findings based on six rain events are, (i) the mean fall speed decreases (from the Gunn–Kinzer terminal velocity) with increasing turbulent intensity, and (ii) the standard deviation increases with increase in the rms of the air velocity fluctuations. These findings are compared with other observations reported in the literature.

Dynamical evolution of vertical profile of rain structures observed using ground-based radar over a tropical station

In this study, measurements of vertical profiles of rain parameters have been made using vertically pointing micro rain radar (VPMRR) at Akure (7.30° N, 5.13° E). Rain parameter data collected over seven-month rainfall episodes during the intense rainy season (April to October) have been analyzed for a dynamical evolutionary trend over the site. Nearly all the episodes observed followed a similar pattern, hence, a single continuous rainfall episode occurring between 20:45:00 h and 21:14:00 h Greenwich Meridian Time (GMT) local time on 6th August 2018 is presented in this report. The results show no significant changes to the rain parameters (such as rain rate and liquid water content) nor contributed to the raindrop size distribution, based on average fall velocity of 6.55 m s−1 and rain rates within 1.3 and 2.6 mm h−1. This is to enable a stable fall for the dominant drops during the period. Further, the results revealed the transformation and collision of smaller drops to enhance a stable fall of larger drops during the rain event. The information from the study will be useful for radar meteorologists and microwave engineers in their designs.

Study of rain classification and the tendency of gamma DSD parameterizations in Mexico

Precipitation is a key component of the hydrologic cycle, so the knowledge and comprehension of its processes of formation and evolution are of paramount importance. This study presents an analysis of microphysical data and drop size distributions (DSD) collected with PWS100 optical disdrometers at four sites in Mexico. Bulk parameters (accumulated water and rainfall intensity) estimated from microphysical data (drop size and fall speed) showed good agreement with observations obtained using rain gauges installed next to the optical devices. Estimates of accumulated water were used to evaluate the efficiency of four different methods of moments for the fitting of the DSD gamma parameters. The outcomes reveal that the second, fourth, and sixth DSD moments method has a better performance (by showing the lowest RMSE value with respect to the measurements) and yields the closest results to those for the Γ–normalized DSD parameterization. Also, a new method for rain classification based on the variation of accumulated water and median volume diameter (D0), which is used as an estimator of the DSD width, is proposed. The method allows for the characterization of rain into three types (convective, stratiform, and shallow), and results can be related to previous observations from DSD parameters (N0 jumps). The study shows that all three gamma DSD parameters (N0, Λ and μ) have large values when the spectra are dominated by large numbers of small and medium-sized drops (low R values), and smaller values when DSD shifts toward large diameters as R increases. Furthermore, the averages of the gamma parameters reveal lower, intermediate and greater values as rain is classified as convective (intense), stratiform or shallow (very light), respectively. The opposite behavior is observed for D0, which agrees with the previous results. These outcomes reveal that continuous research is necessary to refine the tendencies of DSD parameters, even at different places, with rainfall intensity or type of rain and for the improvement of our knowledge of rain processes.

Falseiling

Based on an urban myth in India, where people often question the use of drop (fall) ceiling in households wondering if they are hiding money up there; this series was my interpretation of what we are hiding. For me, this meant a literal exposure of what was exactly above each drop ceiling tile. My work does not happen to you, you happen upon it through subtle interventions I create. These interventions demand the viewer to introspect the space and my work makes visible the unnoticed elements of that space. After the viewers suspension of disbelief, there is a discrepancy in the moment of realisation, that my work is trying to be something it's not while simultaneously blending into the environment. This discrepancy is not alarming, but grows exponentially as one spends time with it, leaving little grasp of what is real and what is not.