Impact Regime Research Articles

Dynamic Behavior of Droplet Impact on Inclined Surfaces with Acoustic Waves.

Droplet impact on arbitrary inclined surfaces is of great interest for applications such as antifreezing, self-cleaning, and anti-infection. Research has been focused on texturing the surfaces to alter the contact time and rebouncing angle upon droplet impact. In this paper, using propagating surface acoustic waves (SAWs) along the inclined surfaces, we present a novel technique to modify and control key droplet impact parameters, such as impact regime, contact time, and rebouncing direction. A high-fidelity finite volume method was developed to explore the mechanisms of droplet impact on the inclined surfaces assisted by SAWs. Numerical results revealed that applying SAWs modifies the energy budget inside the liquid medium, leading to different impact behaviors. We then systematically investigated the effects of inclination angle, droplet impact velocity, SAW propagation direction, and applied SAW power on the impact dynamics and showed that by using SAWs, droplet impact on the nontextured hydrophobic and inclined surface is effectively changed from deposition to complete rebound. Moreover, the maximum contact time reduction up to ∼50% can be achieved, along with an alteration of droplet spreading and movement along the inclined surfaces. Finally, we showed that the rebouncing angle along the inclined surface could be adjusted within a wide range.

Dynamic behaviors and regime map of a molten blast furnace slag droplet impacting a solid surface

The dynamic morphologies of a molten blast furnace slag droplet impacting a quartz glass surface are studied visually for a wide range of dimensionless initial temperatures (1.604–1.951), Reynolds numbers (17.4–124.4), and Weber numbers (197–1191). Five morphological evolution modes are defined based on the final stage of the slag film as deposition, receding, prompt-splashing, retraction-deposition and retraction-splashing. An impact regime map is established with the Reynolds number and dimensionless initial temperatures as the coordinates and six boundaries dividing the different modes are identified in the regime map. The apparent temperature nephograms of the slag film are extracted by a home-developed optical calibration method. The dynamic spreading processes and temperature distributions of the different regimes are not simply determined by the initial impact parameters, the coupling effect of the flow of the high-viscosity liquid slag and the heat transfer at the interface will obstruct the consecutive morphological evolution. The regulation principle for the centrifugal granulation system is promoted based on the analysis on an energy barrier for the sufficient spreading of the droplet and the impact regime map.

Pressure-Shear Plate Impact Experiments at High Pressures

The pressure shear plate impact (PSPI) experiment, developed over 40 years ago by R. J. Clifton at Brown University enables the study of the dynamic strength of materials at high pressures and strain rates. Traditional PSPI experiments were typically conducted at velocities and corresponding pressures, limiting the experimental conditions not to exceed the Hugoniot Elastic Limit (HEL) of the anvil materials, with typical values of 3–7 GPa. In this work, PSPI experiments are extended to higher pressures, significantly beyond the HEL, approaching 50 GPa, using a powder gun facility at Caltech. The high-pressure and high-velocity impact regimes introduce several experimental challenges which must be overcome: (1) the inelastic behavior of the anvils at high pressures precludes traditional elastic analysis to extract the material’s strength; (2) the potential for slip between impact faces as a result of elevated temperatures and shear forces on the impact surfaces due to higher velocities and pressures; and (3) accurate measurement of the transverse velocity at large normal displacements, resulting from the higher impact velocities. New experimental capabilities have been developed to overcome each of these technical challenges, which include a new all fiber-optic heterodyne transverse velocity interferometer system. New analysis methods that account for the inelastic response of the flyer and anvil plates have also been developed for the accurate extraction of material strength properties from PSPI experiments. Friction and slip were examined, pushing to the limits of their knowledge under these extreme conditions. The new PSPI capabilities have been demonstrated using tungsten carbide and D2 tool steel anvils for measuring the strength of soda–lime glass and magnesium.

Understanding the Effect of Granulation and Milling Process Parameters on the Quality Attributes of Milled Granules

Milling is an essential unit operation used for particle size reduction in solid oral dosage manufacturing. The breakage of particles in a comil is due to the intense shear applied on the particles between impeller and the screen. Breakage also occurs due to the impact from a rotating impeller. Particles exit the mill based on their size relative to the aperture size of the screen bores. This study was set up to understand the working of the comil better. A new CPP (Critical Process Parameter), in the form of batch loading was identified. It was found that there are two different regimes (quasi static regime and impact regime) in which a comil generally operates, and the effect of the CPP’s (batch loading and impeller speed) on these regimes was studied. Knowledge of the effect of upstream operations on a particular unit operation is of significant importance, especially for pharmaceutical industry. For this reason, the effect of granulation variables such as liquid-to-solid ratio, granulator impeller speed and the amount of binder in the formulation were analyzed. Milled particle size distribution and other critical quality attributes such as bulk density, friability, and porosity were studied. Batch loading and the interaction effect of batch loading with impeller speed are significant parameters that affect the quality attributes of the mill. Predictive regression models were developed for throughput of the mill, milled product bulk density and milled product tapped density (with an R2 of 0.987, 0.953, 0.995 respectively) to enable their use in downstream process modeling.

Sticking dust and micrometeorite particles on to ices at high impact velocities - Implications for astrochemical ice enrichment

Impact events are inevitable to date both within the inner and outer regions of the Solar System. Such impact events dominate the surface modifications of most of the airless bodies. Regardless of the destructive nature of impact events, the birth of few moons in the Solar System are known to be the by-products of impacts. Moreover, particle aggregation from relatively low velocity impacts (from nm to μm sized dust particles) are thought to be the reason behind the growth of planetesimals. While considering impact events in the colder regions of the Solar System the role of molecular ices in planetesimal aggregation cannot be neglected. Therefore, to understand the role of such small particle impacts over icy bodies, we investigated the sticking of dust particles on to ices in the higher velocity impact regime, 100–300 ​m ​s−1, using a modified hand driven shock tube (Reddy Tube). Grains of brick, basalt and powdered turmeric, graphite and fullerene soot particles were fired on to the CO2 ice targets. Meteorite samples (Sulagiri and Allende) were used to mimic the real micrometeorite impacts on to dry ice. The particles of different sizes and impact angle are found to significantly affect the sticking pattern. The impact area was observed to be coated by the impacting material. Lesser micron sized particles were observed to penetrate into the ice layers and the larger ones eroding them. Results suggest that astrochemical ices can be chemically enriched by high velocity dust/micrometeoroid impacts.

The role of beach state and the timing of pre-storm surveys in determining the accuracy of storm impact assessments

Dune erosion principally occurs when water level exceeds the elevation of the beach and predicting erosion is progressively becoming more important for management as coastal populations increase, sea level rises, and storms become more powerful. This study assesses storm impacts using a simple model from Stockdon et al. (2007) configured with oceanographic information from the ADCIRC + SWAN model and frequently collected beach profiles. We applied that model to barrier islands in North Carolina including: Core Banks with a more dissipative beach morphology and Shackleford Banks and Onslow Beach with intermediate beach morphologies. The study periods captured 10 events where wave collision with the dunes and/or overwash were either predicted or observed, including large multiple-day events caused by hurricanes and smaller events caused by onshore winds and high tide. Comparing model output with a time series of beach photographs shows the predictive power and sensitivity of the model was consistently high at the Core Banks Site with its wide and low-gradient beach, high-elevation dunes (2.58 m), and high resistance to overwash. Model predictive power and sensitivity was lowest at the Shackleford Banks Site because frequent and large changes to beach slope and intermediate dune elevation (0.54–1.25 m) caused small variations of modeled total water level to either overpredict or underpredict storm impacts. In addition, storm impacts were always overpredicted during hurricanes at the Shackleford Banks Site, which was likely due to storm waves decreasing the beach slope from what was measured prior to the event and used as model input. Like Shackleford Banks, the beach slope of the Onslow Beach Site was steep and variable, but the low-elevation dunes (0.24–0.28 m) made resistance to overwash low and the predictive power and sensitivity of the model higher than at the Shackleford Banks Site. Results suggest that storm impacts and the associated potential for dune erosion is predicted more accurately at beaches where the threshold for overwash is high or low because total water level during most events will commonly fall short of or exceed the overwash threshold, respectively. The accuracy of predicting the storm impact regime is sensitive to beach slope. The slope of intermediate beaches is more variable than dissipative beaches and requires frequent measurement if it is to be represented accurately in the model, but this can be impractical and costly even using the latest drone-surveying methods. To maximize the accuracy of predicting storm impacts, intermediate beach morphology should be constrained by surveying at seasonal or yearly time scales and used as input to numerical models that estimate beach slope over short time scales (hours during an event or daily), configured with the latest wave and water-level forecasts.

Viscoplastic drop impact on thin films

We use high-speed imaging to study the effect of thixotropic aging in drop impact of yield-stress fluids on pre-coated substrates. Our results reveal that drop splashing is suppressed for "aged" compared to "unaged" samples, indicating that thixotropic breakdown timescales during impact are long enough to affect the dynamics. We propose and test several hypotheses for modifying the dimensionless group ${\rm IF}(D/t)$ [1,2] to account for thixotropic aging. The main challenge is that the steady flow properties (Herschel-Bulkley model parameters) used in the current dimensionless group cannot be defined or measured for thixotropically aged samples, because any deformation inherently rejuvenates and breaks down the microstructure. We find the most suitable hypothesis is to only increase the yield stress ($\sigma_{\rm y}$, plastic component) based on the storage modulus of aged samples, while keeping the viscous parameters ($K$ or $\eta_{\infty}$) constant. The work reveals fundamental insight into rarely studied short-timescale flow conditions with thixotropic effects. These results are important for applications such as fire suppression or spray coating that involve complex fluids of varying degrees of thixotropic aging.

Leidenfrost behavior in drop-wall impacts at combustor-relevant ambient pressures

Liquid-fueled combustion systems demand optimal performance over a range of operating conditions—requiring predictable fuel injection events, spray breakup, and vaporization across a range of temperatures and pressures. In direct injection combustors, these sprays impinge directly on combustion chamber surfaces. Although the outcome of fuel droplets impacting a wall is primarily driven by the wall temperature and the Leidenfrost effect, the shifting liquid-vapor saturation point with pressure may influence the droplet-wall heat transfer rate and transition from nucleate to film boiling. In this paper, the role of ambient pressure on the droplet impact regimes, spreading rate, and droplet rebound velocity during impact are explored for representative low boiling point and high boiling point pure hydrocarbon liquids (n-heptane and n-decane). High-speed image sequences of the drop-wall impact were acquired for ambient pressures of 1–20 bar and wall temperatures ranging from 35–300 ∘C with a drop Weber number of ~ 50. Droplet impact sequences were recorded using a high-speed CMOS camera and were processed to measure the droplet spread, droplet rebound velocity and track the droplet centroid motion. The dynamics of the drop spreading and rebound show similar behavior across a range of ambient pressures with the largest differences observed for wetted versus non-wetted cases (above the Leidenfrost temperature). For both fluids, the onset of drop rebound remains bounded by the saturation temperature (shifting with ambient pressure) and the thermodynamic limit of liquid superheat. This leads to a decrease in the superheat temperature above the saturation point as the critical pressure is approached.

Pressure transient analysis in the child well with complex fracture geometries and fracture hits by a semi-analytical model

Recently, the model development of pressure transient analysis in wells with large-scale fracturing treatments has attracted more and more attention. However, a little work has involved the complex fracture networks with fracture hits. In this study, the transient pressure behaviors of a child well with complex fracture geometries and fracture hits are studied, using an efficient semi-analytical model. The semi-analytical model has the capability to simulate shale well performance by considering complex non-planar hydraulic fractures and fracture hits.The pressure transient solution of the semi-analytical model is obtained by the incorporation of nodal analysis and Laplace transforms. After the numerical verification of the proposed semi-analytical model, we apply it to analyze the pressure testing data of a child well and a parent well. Through conducting a series of history matching, the most likely fracture parameters of the child well is determined. Results show that the flow regimes of complex fracture geometries with fracture hits include wellbore storage, skin effect, fracture bilinear flow, “fluid feed”, pseudo-boundary dominated flow, unconnected fracture (UF) impact, and pseudo radial flow. During the flow regime of UF impact, the pressure derivative curves exist a second “V-shape”, as the UF improve the matrix permeability and the pressure depletion will be reduce once the flow reaches the UF. A case study in the Lucaogou shale oil reservoir from the Junggar Basin show that complex fracture geometries is developed along the child wellbore. After estimation, the fracture hits between the parent well and child well may via five hitting fractures. The length and conductivity of the hitting fractures are evaluated as 20 m and 120 mD∙m, respectively. This work could be beneficial to performance forecast and well spacing optimization in unconventional reservoirs.

Damage mechanics and energy absorption capabilities of natural fiber reinforced elastomeric based bio composite for sacrificial structural applications

The present study deals with the experimental, finite element (FE) and analytical assessment of low ballistic impact response of proposed flexible ‘green’ composite make use of naturally available jute and rubber as the constituents of the composite with stacking sequences namely jute/rubber/jute (JRJ), jute/rubber/rubber/jute (JRRJ) and jute/rubber/jute/rubber/jute (JRJRJ). Ballistic impact tests were carried out by firing a conical projectile using a gas gun apparatus at lower range of ballistic impact regime. The ballistic impact response of the proposed flexible composites are assesses based on energy absorption and damage mechanism. Results revealed that inclusion of natural rubber aids in better energy absorption and mitigating the failure of the proposed composite. Among the three different stacking sequences of flexible composites considered, JRJRJ provides better ballistic performance compared to its counterparts. The damage study reveals that the main mechanism of failure involved in flexible composites is matrix tearing as opposed to matrix cracking in stiff composites indicating that the proposed flexible composites are free from catastrophic failure. Results obtained from experimental, FE and analytical approach pertaining to energy absorption and damage mechanism agree well with each other. The proposed flexible composites due to their exhibited energy absorption capabilities and damage mechanism are best suited as claddings for structural application subjected to impact with an aim of protecting the main structural component from being failed catastrophically.

Hyperbolic Meteoroids Impacting the Moon

Abstract Since the discovery of the Moon’s asymmetric ejecta cloud, the origin of its sunward-canted density enhancement has not been well understood. We propose impact ejecta from meteoroids on hyperbolic trajectories (β-meteoroids) that hit the Moon’s sunward side could explain this unresolved asymmetry. β-meteoroids are submicron in size, comparable to or smaller than the regolith particles they hit, and can impact the Moon at very high speeds ∼100 km s−1. Therefore, their impact regime may differ from the significantly larger and slower sporadic meteoroids responsible for generating the bulk of the lunar impact ejecta cloud. We compare lunar impact ejecta production to β-meteoroid fluxes observed by multiple spacecraft. If β-meteoroids are able to liberate similar sized submicron particles, orbital dust detector measurements from the Lunar Dust Experiment on board the Lunar Atmosphere and Dust Environment Explorer spacecraft only need to detect one ejecta grain out of every 106 β-meteoroid impacts to the lunar surface to explain the sunward asymmetry with this additional population. This finding suggests β-meteoroids may also contribute to the evolution of other airless surfaces in the inner solar system, and by extension, at exozodiacal systems.

Filling the gap between hypervelocity and low velocity impacts

During more than one decade of studying initiation phenomenology numerous papers were published by the authors. A multitude of experimental data on hypervelocity impact initiation of plastic bonded high explosive charges by shaped charge jets (SCJ) and some data in the ordnance velocity impact regime obtained with STANAG projectiles and explosively formed projectiles (EFP) were generated. Amongst other findings the results showed that the established assumption that the critical stimulus is constant was wrong and that a new initiation model is needed taking the new results into account. Towards such a new model further investigation was necessary trying to make a link between the initiation phenomenology of shaped charge jet impacts in the hypervelocity regime and of projectile impacts in the lower velocity regime. To bridge the existing data gap a new approach was taken applying newly designed “simplified shaped charges” (SSC). This paper describes the way to this new approach and summarizes the related test results as well as their implications.

Silica particle deposition in superheated steam in an annular flow: Computational modeling and experimental investigation

The work presents computational modeling and an experimental investigation of silica deposition occurring in geothermal systems with superheated steam flows. An advection diffusion model was implemented for predicting silica particle transport and deposition in turbulent flows. The model includes the effect of Brownian diffusion, turbulent diffusion, turbophoresis, Saffman lift force, and drag force on particle motion. Simulation results for deposition of silica particles in superheated steam flow were validated experimentally. An experimental setup consisting of a superheated steam generating system, silica injection system and test section assembly was designed and constructed. Deposition velocity was measured for silica fume particles ranging from 1 to 20 μm in diameter. The measured data shows agreement with the implemented model simulation results showing an increase in deposition velocity with increase in particle relaxation time in diffusion impaction regime.

Spatiotemporal Storm Impact on the Northern Yucatan Coast during Hurricanes and Central American Cold Surge Events

We investigate the storm impact associated with historical events in the northern Yucatan Peninsula. The study area is prone to coastal flooding due to both its geographical location and low-lying areas. Extreme events associated with tropical cyclones and Central American cold surge (CACS; locally known as Nortes) are ubiquitous in this region, and coastal development in the study area has exacerbated the erosion of the sand beach-dune system. This study aims to assess the impact on the northern coast of Yucatan associated with different types of storms and to investigate the role of the dune in its spatial variability. Nearshore hydrodynamics, associated with hurricanes (Gilbert: 14 September 1988; Isidore: 22 September 2002) and energetic Nortes (Norte A: 12 March 1993; Norte B: 25 December 2004), were computed using a numerical model. The beach and dune characteristics were extracted from a LIDAR flight with a spatial resolution of 1 m conducted in 2011. Furthermore, the extreme water levels and the spatiotemporal variability of the storm-impact regime (swash, collision, overwash, or inundation), along a 41.5 km stretch of coast, were derived using both runup parametrizations and the modeling results. On the one hand, the predominant storm impact regimes for Hurricanes Gilbert and Isidore were inundation and overwash, respectively. The flood that propagated from east to west in the northern Yucatan was due to westerly-directed hurricane tracks. On the other hand, for the Norte events, the predominant impact regimes were collision and overwash for Nortes A and B, respectively. This difference in the impact regime between Norte events can be ascribed to tidal differences. Moreover, during the passages of Nortes A and B, the flood was propagated from west to east in the northern Yucatan, consistent with cold-front paths. The results suggest that the western part of the study area presented a stronger impact regime due to the dune degradation caused by coastal infrastructure and settlements established in those areas. This work highlights the important role of sand dunes in providing natural coastal protection during Norte events.

МАТЕМАТИЧЕСКАЯ МОДЕЛЬ ЗАВИСИМОСТИ РАСПРЕДЕЛЕНИЯ СУЛЬФИДНЫХ ВКЛЮЧЕНИЙ В ОТЛИВКЕ ИЗ УГЛЕРОДИСТОЙ СТАЛИ ПРИ ГАЗОДИНАМИЧЕСКОМ ВОЗДЕЙСТВИИ НА РАСПЛАВ, ЗАТВЕРДЕВАЮЩИЙ В КОКИЛЕ

Приведены результаты анализа цифровых изображений серных отпечатков темплетов отливок из стали 35Л, полученных литьем в кокиль по традиционной технологии и с применением газодинамического воздействия. Предложена математическая модель для оценки влияния технологических режимов на количество и размеры сульфидных включений.

Neutral helium line profiles through the simulation of local interactions

AbstractFor the past 25 years, we have been considering the Stark effect for neutral helium lines in DB white dwarfs using the standard Stark broadening theory in both the impact regime (in the center of the lines) and the quasi-static regime (in the wings) for the electrons, while neglecting the effect of ions in motion. Although this is probably a good approximation based on previous theoretical work, the transition between the two regimes for the electrons and the contribution of the ions very near the core might be poorly represented. To better represent these particularities, we report here the results of a new series of simulations that treat the local dynamics and interactions of both electrons and ions around a neutral helium atom. From these simulations, we produce new improved line profiles, which we compare with our previous analytical results.

Experimental Investigation of Fastener Pull-out Response in Composite Joints under Static and Dynamic Loading Rates

A novel device is adopted in order to experimentally investigate the effect of various loading rates on the pull-out response of a fastened composite joint configuration. The joint coupons comprise a composite plate made of the carbon/epoxy AS4/8552 material system and a centrally located titanium lockbolt. Tensile-type (pull) loading was applied to the specimens in a velocity range from quasi-static to 2.1 m/s. Both quasi-static and dynamic tests were conducted using the same specimen geometry and boundary conditions, which conform to international and industrial standards. The experimental work expands the limited literature and understanding of the mechanical response of composite pull-out joints under the action of dynamic loading. The main experimental observations revealed an increase of 15% regarding maximum load values when loading rate shifts from the static to the impact regime, while the failure patterns derived from static and dynamic tests were similar, although the latter presented a more intense damage zone.

New sources of housing market risk: Asset pricing for the US state‐level housing markets

AbstractThis study develops three novel housing risk factors to explore the sources of risk in US state‐level housing markets. The asset‐pricing models enrich our investigations into housing markets from three perspectives: housing boom–bust, volatile–calm, and factors’ time‐varying impact regimes. The findings indicate that state‐level housing returns all show significant exposure to systemic risk of housing markets, but the demographic and economic factors’ explanatory powers for housing returns differ across markets. Although the liquidity factor is significant under all three frameworks, the inventory and credit factors show significant impacts only in the impact‐switching specification. There are differences in the responses of housing returns to various risk factors and shifts in housing regimes across state housing markets, depending on whether the states exhibited housing bubbles. The results suggest that an effective housing stabilization mechanism must be capable of tracking shifts in the relations between housing markets and risk factors.

Multicomponent drop breakup during impact with heated walls

Understanding multicomponent drop breakup during impact with walls is critical to the prediction and optimization of fuel-air mixture distribution in combustion systems. In combustors, drops impact walls over a range of wall temperatures and drop velocities, resulting in complex outcomes. In this paper, the regimes of drop impact are characterized for bicomponent and multicomponent fuel drops impacting hot walls. Mixtures of n-heptane and n-decane were used to represent low and high boiling point fuel components, respectively. The wall surface temperatures were varied from 27 to 400 °C with drop Weber numbers ranging up to ∼700 for a range of mixing ratios of n-heptane and n-decane. The drop impact events were recorded using high-speed imaging, allowing the identification of impact outcomes and classification into regimes of film deposition, nucleate boiling, and rebound at low Weber numbers, and splashing and breakup at high Weber numbers. In bicomponent mixtures of n-heptane and n-decane, increasing the fraction of the volatile component (n-heptane) results in secondary film breakup at low wall temperatures and a shift in the classified regimes of impact. The droplet size distribution following this secondary breakup was determined, and results show a decrease in the mean droplet size with increasing volatile concentration. Finally, commercial gasoline and diesel fuel drops were examined over similar wall temperature and Weber number ranges. The impact regimes are comparable to those identified for the well-defined bicomponent liquid mixture, in which more volatile components promote rebound and disintegration of the liquid.

The response of vegetated dunes to wave attack

For many coastal communities, dunes serve as the primary defense against tropical and extratropical events. Vegetation is believed to increase the stability of dunes during wave attack, but limited data is available. A physical model study was performed to evaluate changes in the dune stability with and without biomass, both above and belowground. The above and belowground biomass was modeled using wooden dowels and coir fibers, respectively. For both the collision and overwash storm impact regimes, the results of this study clearly demonstrate that the inclusion of biomass in the model dune reduces the erosion and overwash. The combination of both above and belowground biomass was the most effective at reducing erosion followed by belowground biomass, with aboveground biomass providing the smallest benefit regardless of the wave condition and water level. Additionally, the overwash of sediment and water was decreased with the inclusion of biomass, following the same trends as the erosion. As the dune eroded, the storm impact regime transitioned from collision to overwash. The inclusion of biomass delays this transition in storm impact regime, providing greater protection to coastal communities. This study highlights the need to consider dune vegetation for dune construction and coastal planning.