Drop Momentum Research Articles

Coherent spore dispersion via drop-leaf interaction.

The dispersion of plant pathogens, such as rust spores, is responsible for more than 20% of global crop yield loss annually. However, the release mechanism of pathogens from flexible plant surfaces into the canopy is not well understood. In this study, we investigated the interplay between leaf elasticity and rainfall, revealing how a flexible leaf structure can generate a lateral flow stream, with embedded coherent structures that enhance transport. We first modeled the linear coupling between drop momentum, leaf vibration, and the stream flux from leaf surfaces. With Lagrangian diagnostics, we further mapped out the nested coherent structures around the fluttering profile, providing a dynamical description for local spore delivery. We hope the mechanistic details extracted here can facilitate the construction of physically informed analytical models for local crop disease management.

Dual nature of volatility on drop wetting dynamics of acetone–isopropanol mixtures on ultrathin smooth oil films

Volatile drop impacts are commonplace among various industrial and natural processes and are most often studied under Leidenfrost conditions, where a vaporized film sustains the drop weight or reverses drop momentum. The vapor thrust generated is, therefore, a function of many variables such as the enthalpy of vaporization, the superheat, the specific heat capacity of the vapor, vapor thermal diffusivity, and the vapor film thickness. In this study, volatile drop impact and wetting dynamics of acetone and isopropanol mixtures at a normal temperature and pressure were shown to generate enough thrust from evaporation alone during the impact process and allow for unique contact dynamics. Volatility was controlled by varying the acetone concentration in isopropanol mixtures [O(1–10 kPa)] to keep surface tension relatively constant, while the vapor pressure and viscosity increased. Wetting onset was tracked using a high-speed optical total internal reflection microscopy technique where an increase in the vapor pressure enhanced wetting onset (i.e., pure acetone). However, the concentrations between ≈ 49% and 66% isopropanol, which have vapor pressures of ≈11.3–14.8 kPa, respectively, caused drops to rebound at We up to We ≈ 21, beyond the classic disjoining pressure dominant regime of We > 10.

Evaluation on Improvement Zone of Foundation after Dynamic Compaction

Dynamic compaction (DC) is one of the most popular methods for ground improvement. To solve the problem of the factors affecting the sandy soil improvement effect and estimate the effective improvement range under DC, the influences of drop number, drop energy, tamping distance, tamper radius, and drop momentum on the relative degree of improvement were investigated. Three normalized indicators △δz,i, △δA,i, and △δU,i were derived to evaluate the weak zone and corresponding improvement effect. For multipoint tamping, it is found that the improvement depth and the improvement of the weak zone are highly correlated with drop energy and drop momentum, while the influence of the drop number and tamper radius is relatively smaller. The improvement of the weak zone and the improvement depth decrease with tamping distance, whereas the improvement area increases with tamping distance. The soil compacted by the previous impact point will be improved to a lesser extent with impact at subsequent impact points. It is also noted that drop energy had better not exceed the saturated drop energy in DC design. Based on the parametric study, a formula considering the various factors of DC was put forward, with the validation by two field cases of DC.

Ensemble machine learning predicts displacement of cantilevered fibers impacted by falling drops

In this study, we consider a simple system of water drops impacting cantilevered, circular fibers with velocity 1.0–2.4m∕s. The dynamics of the system are complicated because the motion of the drop and deflecting fiber at impact are highly coupled and the outcome is influenced by several continuous variables. The unpredictable dynamics of the system call for the use of computational tools that can reveal relationships between variables and make predictions about the physical outcomes for parameter values for which there is no experimental data. This study considers three fibers of contrasting properties, with hydrophilic and hydrophobic wetting conditions, exposed to a range of falling drop diameters and velocities. We predict maximal fiber deflection due to the impacting drop using an ensemble machine learning algorithm combining three base algorithms: a random forest regressor, a gradient boosting regressor, and a multi-layer perceptron. We train and test our algorithm with experimental datasets comprising 405 total trials using three different fibers and five input variables per fiber. The approach allows the determination of relative dominance of the input features in the prediction, reveals shortcomings of traditional scaling approaches for momentum transfer, and shows that drop momentum plays only a minor role in fiber deflection. Finally, our approach provides another example for the application of machine learning to characterize complex and coupled systems in fluid mechanics.

The significance of drop non-sphericity in sprays

This paper presents a new framework to model drop dynamics in Lagrangian sprays. The framework builds on the Taylor Analogy Breakup (TAB) model. Real-fluid (gas-liquid) thermodynamics applicable to multicomponent systems are combined with Gradient Theory to facilitate detailed calculations of drop surface tension forces, oscillations, and breakup processes. This is combined with a more detailed treatment of deforming drop dynamics to construct more accurate representations of the local interfacial exchanges of mass, momentum, and energy. The framework is derived using an energy balance equation that explicitly enforces drop momentum conservation during the breakup process. This facilitates development of a refined set of drop equations that address current shortcomings in the prediction of drop properties over a wide range of relevant breakup conditions. The resulting drag forces, evaporation, and heating rates deviate significantly from the predictions given by contemporary drop models used in modern simulations. These deviations are quantified using Large Eddy Simulation (LES) with a Lagrangian-Eulerian modeling approach. The analysis demonstrates how the model improvements in the new framework provides a more detailed representation of physical complexities that are largely neglected in modern studies.

Analysis of a pressure equalized rainscreen wall as a periodic array of Helmholtz resonators.

One of the primary purposes of the building enclosure is to separate occupants from the natural elements such as rain and snow. Water can penetrate this enclosure through four primary mechanisms: air pressure differences between interior and exterior, rain drop momentum, surface tension, and capillary action. The pressure equalized rainscreen (PER) wall is a two layer wall construction that places a vented cavity between an inner wall and outer cladding. When properly designed, the cavity pressure equalizes to that of the external wind on the building, keeping rain from entering the cavity by air pressure differences and the cavity space eliminates momentum, surface tension, and capillary action as water transport mechanisms. A typical building using PER walls will have a large number of these cavities with a regular spacing. In this paper the wind response of the PER wall will be analyzed as a periodic array of Helmholtz resonators exposed to periodic excitation.

IRRIGATION NOZZLE HEIGHT INFLUENCES WATER CAPTURED BY CONTAINERS

Rain drop momentum, based on the height from which it falls, is an important factor in drop penetration of plant canopy. This may explain why nursery operators report that substrates appear wetter from rain than from an equivalent amount of water applied with overhead irrigation. We investigated the influence of irrigation nozzle height on amount of water captured by Rhododendron sp. `Formosa' grown in 10-liter containers. A Wobbler® (#8, 7.6 liters·min–1) irrigation nozzle was positioned 1.2, 2.4, 3.6, 4.8, or 6.0 m above grade. Plants were placed in a circle 3.6 m from the riser base for the 1.2-m-high nozzle, 4.5 m from riser base for the 2.4-m-high nozzle, and 5.4 m from riser base for all other heights and irrigated for 3 hours. Preweighed disposable diapers were placed on substrate surface of each container with and without (control) plants. Diapers were weighed after irrigation and water captured was calculated and expressed as percentage of control containers. Capture increased from 144% at 1.2 m to 178% at 3.6 m then declined with increasing height. The decline was likely due to small drops with low momentum striking plants because plants remained 5.4 m from the riser base.

WATER DROP ENERGY AND SOIL SEAL PROPERTIES

Rain properties (depth, drop size, and impact velocity) affect the infiltration rate (IR) curve and final IR (FIR) of soils. Because the IR is not a unique function of rain depth or rain energy, the objective of this study was to find a unique function of rain properties that determines the IR of the soil. Simulated rain of constant intensity (40 mm h−1), with 2.53− and 3.37-mm-diameter drops, was applied from heights of 0.4, 1.0, 2.0, 6.0, and 10.0 m on two soil samples: Ruppin hamra (sandy loam, mixed, Typic Rhodoxeralf), and Ruhama loess (silty loam, mixed, Calcic Haploxeralf). The FIR of the two soils decreased with increasing kinetic energy (KE) of the drops. The sandy loam was less stable than the silty loam, and seal formation in it was more susceptible to the KE of the drops. The infiltration decay process was better correlated with rain momentum than with rain depth or KE. Thus, prediction of infiltration rate decay for a given soil exposed to rains of various drop sizes and velocities is best based on drop momentum and the soil stability constant.

Rain measurement on buoys using hydrophones

The impact of rain and spray on the ocean disturbs the sea surface and generates underwater ambient noise. The short scale roughness is influenced by impacting drops due to the momentum transfer. Radar and sonar signals are scattered by the short elements of the sea surface. Spray and rain impact change their characteristics, and consequently affect radar and sonar backscatter. In situ measurements of rain and spray impact are necessary to study their effects on the sea surface. Accurate sea measurements of rain momentum fluxes and drop size distributions are a complex problem, especially on buoys. A new measuring technique has been developed using hydrophones. Exposed to precipitation, these instruments are affected directly by the impact of rain. A drop falling on the hydrophone deforms its surface and is sensed by a piezoelectric transducer. The voltage output of the sensor is a rapidly decaying oscillation. The integral value of this signal is a measure of the drop momentum, and the drop size can be deduced. Laboratory studies of defined drops as well as field measurements of natural rain have shown that hydrophones can be used to determine drop momentums and drop size distributions. Based on simultaneous rain measurements by a Joss-Waldvogel Disdrometer and a hydrophone, an analytical function has been derived which relates drop size and hydrophone voltage output. >

Water Drop Kinetic Energy and Momentum Measurement Considerations

The impulse applied to a surface from water drop impact can be measured experimentally by recording and integrating impact force over time. A momentum number, defined as measured impulse divided by the drop's momentum, is a dimensionless physically-based parameter which can be used for modeling soil movement due to water drop impact. Impact forces were measured and integrated over time and the momentum number equals one for a smooth-dry surface and is greater than one for a smooth-wet or a rough surface. Accurate impulse measurements do not require sensors with fast dynamic response or calibration, but reasonant frequencies must be sufficient to minimize signal amplification. The factors to be considered when measuring impulse or work from water drop impact are identified.

Gravity–capillary rings generated by water drops

A theory for water waves created by the impact of small objects such as raindrops on an initially quiescent body of water is established. Capillary and dissipative viscous effects are taken into account in addition to gravity. It is shown that the prevailing waves are in a mixed capillary–gravity regime around a wavenumber km which corresponds to the minimum value of the group velocity. The waves are described as function of time and distance by the linear superposition of two transient wave components, a ‘sub-km’ (k < km) component and a ‘super-km’ (k > km) component. The super-km components prevail at a short distance from the drop, whereas only the sub-km ones remain at a larger distance. The relative time history of the wavetrain is independent of the size of the drop, and its amplitude is proportional to the drop momentum when it hits the free surface. The wave pattern is composed of a multiplicity of rings of amplitude increasing towards the drop location and is terminated by a trailing wave with an exponential decay. The number of rings increases with time and distance.

Droplet deposition and interchange in annular two-phase flow

A mathematical description of droplet entrainment and deposition is presented. This enables both unidirectional deposition and interchange experiments to be analysed. This approach permits description of deposition by both the diffusion like mechanism conventionally used and by the direct impaction mechanism which has recently been identified. A criterion has been derived to differentiate between the two mechanisms of deposition based on the balance between the initial drop momentum and the drag force of a turbulent eddy. The transition criterion has been substantiated with experimental data derived from unidirectional deposition experiments. A preliminary examination of interchange data has been carried out.

Progress with an Electrostatic Device for enhancing the Selectivity of Herbicides in Cereal Crops1

Apparatus is described that produces uniformly sized and charged drops that can be propelled to the target at a range of speeds. Results from the use of this equipment suggest that drop momentum, rather than just drop size, may have an effect on spray deposition. The magnitude of this effect is dependent on the characteristics of leaf surfaces. For example, spray deposition on cereal leaves is sensitive to changes in drop momentum but this is not the case for Raphanus sp. This factor, if exploited, could be the basis for altering selectivity between plant species.

Corona produced by splashing of water drops on a water surface in a strong electric field

Investigations of corona discharges associated with water drops splashing on a water surface in the presence of a strong vertical electric field are reported. When the ambient electric field strength equals or exceeds a critical value, a hydrodynamic instability occurs at the tip of the jet thrown up during the splash. This instability causes the tip suddenly to assume a conical shape from which corona, characteristic of impulse excitation, is produced. The observed critical field strength is inversely related to the falling drop momentum and is independent of the electric field polarity. The minimum critical field for freely falling drops is approximately 1.8×105 V m−1 for 3-mm-radius drops falling at terminal velocity. A quantity of charge in the range 10−7−10−6 C is liberated by each splash depending on the drop momentum and the field strength. This process should be important in determining the response of the ocean to large rapid electric field changes where it is raining.