Effect Of Initial Density Research Articles

Plasma dynamics at the Schwinger limit and beyond.

Strong field physics close to or above the Schwinger limit are typically studied with vacuum as initial condition or by considering test particle dynamics. However, with a plasma present initially, quantum relativistic mechanisms such as Schwinger pair creation are complemented by classical plasma nonlinearities. In this work we use the Dirac-Heisenberg-Wigner formalism to study the interplay between classical and quantum mechanical mechanisms in the regime of ultrastrong electric fields. In particular, the effects of initial density and temperature on the plasma oscillation dynamics are determined. Finally, comparisons with competing mechanisms such as radiation reaction and Breit-Wheeler pair production are made.

Influence of compaction on small-strain shear modulus of iron ore tailings

Understanding the geotechnical properties of iron ore tailings is one of the biggest challenges currently faced by the mining industry. The brittle behaviour of these tailings has brought the importance of small-strain stiffness to the geotechnical forefront. However, lack of knowledge and information about the behaviour of iron ore tailings still exists. The results and analysis of a laboratory programme aimed at assessing the small-strain stiffness of tailings materials are presented in this paper. The materials tested were produced during the iron ore treatment process. Bender elements were used to measure shear wave velocities and evaluate dynamic shear moduli at different effective stress levels resulting from isotropic consolidation tests. Three types of iron ore tailings with different grain size distributions were used – flotation tailings, slimes tailings and blended tailings. Reconstituted specimens were prepared at different densities (loose and dense conditions) to assess the effects of initial density (percentage compaction) on the shear modulus. The laboratory results were compared with empirical correlations for other soil types. The equations were ineffective at representing tailings materials containing large amounts of fines (slimes). The advantages and limitations of the equations are discussed and a new empirical equation that includes the degree of compaction is suggested.

Molecular dynamics simulation of shock-induced structural change and permanent densification of silica glass: The effect of initial density

This work investigates the Hugoniot relation, structural change and permanent densification of initial densified silica glass under shock compression-release by atomistic simulations. The Hugoniot results show a good agreement with the experimental data. Moreover, the shock velocity and shock pressure are found to increase with the initial density of sample, and the pressure gap between different samples will be increased with the impact velocity. The microstructural analysis shows the decrease of Si–Si, O–O nearest-neighbor distance, Si–O–Si bond angle and the increase of 5-fold, 6-fold Si atoms during the plastic region. Also, those microscopic changes will get smaller with the increase of initial densification below 14 GPa, and all the samples undergo similar structural changes after complete densification identified by the splitting of Si–Si peak, the new O–O peak and 6-fold Si atoms. Considering the release from the shock pressure above 14 GPa, the initial incomplete densified samples can reach the maximum densification of 2.56 g/cm3, where Si–O–Si bond angle decreases, and 11% 5-fold Si appears. It is found that the shock release cannot make the sample obtain the maximum densification of 20%. Besides, HCP or FCC O atoms are observed in all samples when the shock pressure is over 30 GPa, indicating the crystallization tendency of silica glass. The initial densification seems to promote the transformation to stishovite phase.

Effect of Initial Density, Particle Shape, and Confining Stress on the Critical State Behavior of Weathered Gap-Graded Granular Soils

Weathered gap-graded soils are a common geological body in mountainous regions, and they are widely used as construction materials. The shear strength is the controlling parameter for the design of civil projects; however, there is still a controversy on the coarse fraction effect on the shear strength of gap-graded soils. To this end, 22 triaxial shear tests are performed on gap-graded soils, and the factors affecting the coarse fraction effect have been analyzed, including the confining stress, the particle shape of aggregates, and the initial density of sand matrix. Partial contacts and sand bridges between aggregates are responsible for the transmission of loading and thus affect the coarse fraction effect. The results of triaxial tests reveal: (1) The overall shear strength of sand-beads mixtures is rather independent of the confining stress and the coarse fraction, and even the volume of aggregates is as high as 44.5%. (2) The effect of the shape of aggregates is effective only at a high coarse volume fraction (44.5%) for loose-sand-gravel mixtures, where the partial contacts between aggregates play an important role in forming the interaggregate structure. (3) The overall shear strength of gap-graded soils with a denser matrix increases continuously with a rising coarse fraction. The formation of a densified sand bridge is correlated with the initial density of sand matrix, contributing to the loading transmission in the interaggregate structure and, in turn, affecting the overall critical state behavior of gap-graded soils. The insights drawn from this study provide a reference for assessing the deformation behavior of weathered residual soils.

Effect of initial density on thermal conductivity of new micro‐pipeline heat conduction C/SiC composites

AbstractC/SiC composites prepared by chemical vapor infiltration technique (CVI) have been regarded as thermal structural materials widely. However, these composites still suffer from poor functional properties like low thermal conductivity, especially in thickness direction of the composites, limiting their large‐scale applications. Herein, mesophase pitch based carbon fiber (MPCF) and continuous wave laser machining were utilized to construct highly effective heat conductive micro‐pipelines within CVI C/SiC composite. The effect of initial density on the final density and thermal conductivity of the as‐obtained MPCF‐C/SiC composites were investigated. The results revealed that higher initial density would directly enhance the thermal conductivity and reduce the negative impact of the bottle‐neck effect. At temperatures between 100°C and 500°C, MPCF‐C/SiC composites preserved more than threefold of the thermal conductivity (340%) when compared to reference C/SiC composites. This work provides a highly effective route for enhancing the thermal conductivity of C/SiC, which would broaden their future applications.

Modeling stand basal area growth of Cryptomeria japonica D. Don under different planting densities in Taiwan

ABSTRACT Stand basal area (BA) is an important parameter, yet significantly influenced by initial planting density, in describing the developments of plantation forests. The effects of initial density on the growth of sugi (Cryptomeria japonica D. Don), an important plantation tree species in Taiwan, have seldom been studied. The goals of this study are to (1) model sugi stand BA growth under different initial densities, and (2) quantify model forecast accuracy. The data were from a spacing trial established in 1950 at the National Taiwan University Experimental Forest. The initial densities were 2500, 1111, 625, and 400 trees ha−1. Ten measurements were carried out up to stand age 65. Using a mixed-effects modeling approach, we fitted a Gompertz model with the initial density as the covariate to describe stand BA growth trends. Results showed that stand BA was the largest for stands with the highest initial density. The maximum current annual increment of the highest initial density was also the largest and reached earlier in stand development than that of the other spacings. Data up to stand age 60 were needed to yield a forecast accuracy within 6% for BA at stand age 65. This study showed that the fitted Gompertz model could adequately capture the general growth trends in stand BA despite unequal measurement intervals. The results agreed with other spacing trials in other regions and tree species, and the model accuracy was acceptable given the availability of data. The prediction accuracy results underlined the importance of long-term growth monitoring.

A Suction-Controlled Ring Device to Measure the Coefficient of Lateral Soil Pressure in Unsaturated Soils

A suction-controlled ring device has been developed to continuously measure the coefficient of lateral soil pressure in deformable unsaturated soil samples from the at-rest to the active condition under application of increasing vertical pressure and controlled matric suction. The device incorporates a thin aluminum specimen ring equipped with horizontal strain gages for recording the lateral soil strains. In addition, a sensor recording water volume changes is utilized to continuously monitor the degree of saturation of the soil sample during tests. The matric suction within the soil texture is controlled using the axis translation technique. In order to verify the performance of the ring device, twenty suction-controlled tests under application of five different constant matric suctions have been performed on two different unsaturated soil types, namely Firouzkouh Clay and Sand-Kaolin mixture, each with two different initial densities. During tests, the lateral soil pressure and corresponding lateral strains have been continuously recorded from the at-rest to the active state of the soil. Accordingly, the coefficients of the lateral soil pressure for the at-rest and the active states of the soil are calculated based on the experimental data and by considering the effective stress concept in unsaturated soils. Finally, the observed responses have been analyzed and the effects of initial density and matric suction on the ratio of lateral to vertical soil pressure have been discussed.

Effects of initial density during laser machining assisted CVI process and its influence on strength of C/SiC composites

Novel laser-assisted chemical vapor infiltration (LA-CVI) technique has been used to improve the density and strength of carbon fiber reinforced SiC composites (C/SiC). Initial density of C/SiC before laser machining played an important role in determining the final density and strength of composites. Results show that final density and bending strength of lower initial density composites were better than that of higher initial density samples after LA-CVI process, while the porosity exhibited opposite behavior. Micro-CT and COMSEL simulation results verify that after LA-CVI process, dense band width of C/SiC with initial density of 1.5 g/cm3 was twice as that of C/SiC with initial density of 1.8 g/cm3. This led to crack propagation bypassing the micro-hole. In conclusion, low initial density when laser machining was carried out resulted in better properties of final composites.

Evacuation flow of pedestrians considering compassion effect**Project supported by the Key Research and Development Program of China (Grant No. 2016YFC0802508) and the National Natural Science Foundation of China (Grant Nos. 11672289 and 11422221). MBH acknowledges the support of Chinese Scholarship Council.

By means of game theory, the effect of compassion mechanism on the evacuation dynamics of pedestrians from a room is studied based on a cellular automaton model. Pedestrians can choose to cooperate or defect in a snowdrift game during the movement. With the compassion mechanism, pedestrians share their payoff to the poorest peer when several pedestrians compete for the same empty cell. Simulation results show that the escape time grows with fear degree r of the snowdrift game, and the compassion mechanism will have a different effect on the system compared with the situation of a spatial game with fixed population. By payoff redistribution, the compassion can help the minor strategy to survive. When the fear degree r is large, the compassion can sustain the cooperative behavior, and spontaneously decreases the escape time. When the fear degree r is small, the compassion will decrease the cooperation frequency, and slightly increase the escape time. The phenomenon is explained by the evolution and competition of defectors and cooperators in the system. Finally, the effect of initial cooperator proportion, the effect of two exits, and the effect of “Richest-Following” strategy, and the effect of initial density are also discussed.

Effect of initial densities in the lattice Boltzmann model for non-ideal fluid with curved interface

The effect of initial densities in a free energy based two-phase-flow lattice Boltzmann method for non-ideal fluids with a curved interface was investigated in the present work. To investigate this effect, the initial densities in the liquid and gas phases coming from the saturation points and the equilibrium state were adopted in the simulation of a static droplet in an open and a closed system. For the purpose of simplicity and easier comparison, the closed system is fabricated by the implementation of the periodic boundary condition at the inlet and outlet of a gas channel, and the open system is fabricated by the implementation of a constant flux boundary condition at the inlet and a free-out boundary condition at the outlet of the same gas channel. By comparing the simulation results from the two types of initial densities in the open and closed systems, it is proven that the commonly used saturation initial densities setting is the reason for droplet mass and volume variation which occurred in the simulation, particularly in the open system with a constant flux boundary condition. Such problems are believed to come from the curvature effect of the surface tension and can be greatly reduced by adopting the initial densities in the two phases from equilibrium state.

Effects of initial density, nutrient, and water level regime on the seedling survival and growth of Typha orientalis Presl

Seedlings’ successful establishment is of importance in the preemption process of pioneers in wetlands. Although Typha orientalis Presl has been reported as a pioneer in Asia countries, studies on the seedling phase of T. orientalis are not available yet. A mesocosm experiment was conducted to understand the effects of biotic (initial density) and abiotic (nutrient and water level regime) factors on the seedling survival and growth of T. orientalis. Most seedlings survived under low initial density (93.8%) and eutrophic (95.5%) rather than high initial density (64.3%) and ombrotrophic (62.5%). Seedlings under low initial density, eutrophic, and flooded conditions showed relatively higher growth in shoot height. The final number of ramets showed an adverse tendency compared to the survival rate and shoot height particularly depending on the water level regime. T. orientalis compensated its biomass production with producing less but longer shoots under the flooded condition, indicating the phenotypic plasticity of T. orientalis as a deep water species. However, the compensation seemed to be guaranteed only under the condition of sufficient nutrients. Asian T. orientalis seemed not to be a pioneer but a weak-competitor not only in mature plant stage but also in juvenile seedling stage unless sufficient nutrients are guaranteed.

Soil–structure cyclic direct shear tests: a new interpretation of the direct shear experiment and its application to a series of cyclic tests

This paper presents a series of cyclic 2D direct shear tests on sand–rough material interfaces under constant normal load (CNL) and constant normal stiffness (CNS) conditions. The aim of these tests is to describe the behavior of the soil–pile contact subjected to a large number of cycles due to environmental or anthropic loadings. These cycles (typically 104 or less due to an early rupture) are small (10, 20 and 40 kPa in terms of shear stress). A new interpretation of the direct shear tests is proposed. The sample of soil is schematically composed of a sheared interface and of a buffer under oedometric load. The problem of sand leakage between the shear box and the rough plate, classical phenomenon in this type of test, is focused. The effect of initial density, position of “center of cycles” in stress plane (mean cyclic variables) and cyclic amplitude is investigated. The cycles are defined by the initial mean cyclic normal stress, the level of initial mean cyclic stress ratio and the normalized cyclic amplitude. Under CNL condition, either dilation or contraction is exhibited, in agreement with the characteristic state developed by Luong (International symposium on soils under cyclic and transient loading, Swansea, 7–11 January, pp 315–324, 1980). The influence of a prescribed normal stiffness is especially considered. It can be highlighted that CNS cyclic paths are always contractive. This contraction results in a drop of mean cyclic normal stress often called degradation of friction.

Model-based Characterization of the Parameters of Dissimilatory Sulfate Reduction Under the Effect of Different Initial Density of Desulfovibrio piger Vib-7 Bacterial Cells.

The objective of this study was to design a model of dissimilatory sulfate reduction process using the Verhulst function, with a particular focus on the kinetics of bacterial growth, sulfate and lactate consumption, and accumulation of hydrogen sulfide and acetate. The effect of the initial density (0.12±0.011, 0.25±0.024, 0.5±0.048 and 1.0±0.096 mg cells/ml of medium) of the sulfate-reducing bacteria Desulfovibrio piger Vib-7 on the growth and dissimilatory sulfate reduction was studied. The exponential growth phase of the D. piger Vib-7 was observed for 72 hours of cultivation at the (0.12 and 0.25 mg/ml) initial concentration of bacterial cells. Sulfate and lactate were consumed incompletely during this time. The increase in the initial concentration of cells to 0.5 and 1 mg/ml led to a shortening of the exponential bacterial growth phase and a shift to the stationary phase of the growth. In the case of 0.5 mg/ml seeding, the stationary growth phase was observed in the 36th hour of cultivation. The increase in the initial concentration of cells to 1 mg/ml led to the beginning of the stationary growth phase in 24th hours of cultivation. Under these conditions, sulfate and lactate were consumed completely in the 48th hour of cultivation. The kinetic analysis of the curves of bacterial growth and the process of dissimilatory sulfate reduction by D. piger Vib-7 was carried out.

Structural and magnetic properties of single-stage hot deformed NdFeB magnets prepared by different initial densities

The effects of initial density on the magnetic properties of NdFeB magnets prepared by single-stage hot deformation were investigated in this work. The results show that the values of maximum energy product (BH)m and coercivity H cj decrease with the increase of initial density. Under optimum condition, an anisotropic magnet with a maximum energy product of 264 kJ·m−3 was produced using the initial density of 4.4 g·cm−3. The influence of initial density on the magnetic properties was discussed on the basis of microstructure and strain energy. It is concluded that the thicker platelet grains are obtained along the Nd2Fe14B base plane with the initial density increasing. It is mainly because that grain rotation is restricted by high strain energy, which results from high initial density.

Discrete element modeling of shear band in granular materials

Numerical biaxial tests using discrete element method (DEM) were carried out to study the shear band formation in granular materials and the microstructures of the specimen at different strain levels were carefully examined. The effect of initial density and confining pressure on the characteristics of shear band were investigated. It was found that the progressive formation of the shear band was successfully captured by the DEM simulation with flexible lateral boundaries. During the shearing, several potential shear bands are formed at the peak deviatoric shear stress. With further shearing, the potential shear bands enlarge and softening behavior happens. Finally, a persistent shear band forms and other potential shear bands deteriorate. Generally, loose state and high confining pressure “delay” the onset of shear banding. The simulation also showed that the shear band thickness decreases with increasing confining pressure and density. Meanwhile, the inclination angle of shear band in dense specimen is generally higher than that in loose specimen. The void ratio of shear band is significantly larger than other parts of the specimen. At microscopic level, the overall coordination number of the specimen decreases significantly. The coordination number in the shear band is significantly lower than that outside the shear band, which is consistent with the large porosity in the shear band. The formation of shear band is due to the bulking of lateral confining force chains and the shear band is characterized by large void ratio, low coordination number, and high particle sliding and rotation compared with other part of the specimen.

A PHENOMENOLOGICAL CONSTITUTIVE MODEL OF ALUMINUM ALLOY FOAMS AT VARIOUS STRAIN RATES

A nonlinear elasto-plastic phenomenological constitutive model for aluminum alloy foams subjected to quasi-static and dynamic compression is proposed. The six-parameter model can fully capture the three typical features of stress-strain response, i.e., linearity, plasticity-like stress plateau, and densification phases. Moreover, the parameters of the model can be systematically varied to describe the effect of initial density of foams that may be responsible for changes in yield stress and hardening-like or softening-like behavior at various strain rates. The experimental results at various loading rates are provided to validate the model. It is shown that the proposed model can be used in the selection of the optimal-density and energy absorption foam for a specific application based on certain design criteria.

Community convergence in a simple microbial food web

AbstractPrevious studies of communities implicate many potential mechanisms that can create alternate stable states. These include density‐dependent foraging behavior, size refuges reached by early colonists, environmental feedback following disturbance, and different initial densities of intraguild predators. Previous work shows that alternate states of varying stability can occur in food webs containing the intraguild predators Blepharisma americanum and Tetrahymena vorax. Differences in colonization history could create the alternate states, consisting of dominance by either Blepharisma or Tetrahymena, but it was unclear whether results depended on effects of initial density or only on changes in the resource base. We manipulated initial densities of both species to determine if density effects alone could create alternate stable states. Convergence of these communities over time indicated that differences in initial density did not create alternate stable states. By default, other factors influenced by colonization history, such as resource availability, may produce alternate states. Models of alternate stable‐state phenomena should incorporate differences in resource availability in addition to direct competitive and predatory interactions to provide a more complete depiction of the causes of differences in community composition in otherwise similar habitats.

The partitioning of density‐dependent dispersal, growth and survival throughout ontogeny in a highly fecund organism

The way in which density‐dependent effects are partitioned amongst survival, growth and dispersal are key in determining the temporal and spatial dynamics of populations. Here we propose a mechanistic approach to understanding how the relative importance of these sources of density dependence can change over ontogeny through changes in dispersal abilities, energy stores and mortality risks. Whereas the potential for active dispersal typically increases over ontogeny as a function of body size, susceptibility to starvation and predation decreases. The joint effect of these mechanisms suggests a general model for the ontogenetic sequence of how density dependence is manifested, with density dependence early in ontogeny being primarily expressed as mortality on local spatial scales, whereas later stages respond to local density in terms of dispersal and potentially growth. Here we test this model by manipulating the densities of juvenile Atlantic salmon (Salmo salarL.) at two life‐history stages in the wild. Density‐dependent mortality during the early juvenile stage (i.e. fry at onset of exogenous feeding) was accompanied by no effects on body size and weak effects on dispersal. In contrast, dispersal of older juveniles (i.e. parr 2–3 months after onset of feeding) was strongly density‐dependent, with more individuals emigrating from high‐density release sites, and with no effect of initial density on mortality. This dispersal, however, appeared insufficient to produce an ideal free distribution within the study stream, as indicated by the effect of spatial variation in density on body size by the end of the first growth season. These results demonstrate that the way density‐dependent effects are partitioned amongst survival, growth and dispersal changes throughout ontogeny. Furthermore, these changes occur in correlation with changes in individual mortality risks and dispersal abilities, and suggest a general paradigm for the way in which juvenile density‐dependence is manifest in spatially structured populations of highly fecund organisms.

Laboratory, Prototype, and In-Service Accelerated Pavement Testing to Model Permanent Deformation

State departments of transportation began using the Superpave® mixture design system and the associated material specifications without long-term validation. This design system is based on the Strategic Highway Research Program performance models, which need further calibration. Within this context, the National Center for Asphalt Technology built and tested several test sections on a full-scale oval test track in Opelika, Alabama. One purpose of the track was to provide validation of the Superpave mixture design method. The test track also provided a unique opportunity to verify the relationships among the PURWheel laboratory wheel tracker, the Indiana Department of Transportation/Purdue University accelerated pavement tester (APT), and the in-service track performance. The verification was accomplished by replicating eight hot-mix asphalt mixtures from the test track in the APT. Samples were also taken from the APT facility and the test track and tested in the PUR-Wheel. A simple power model was used to relate the rutting performance in both the APT and the PURWheel. The results demonstrate a strong correlation between the two and their capabilities to predict the in-service rutting performance at the test track. The effect of initial density and varying in-service temperatures on rutting performance is also presented and explained.

Effects of Initial Density of Green Compact on Hot-Press Sintering and Superplastic Forming of Nano 3Y-TZP

Effects of Initial Density of Green Compact on Hot-Press Sintering and Superplastic Forming of Nano 3Y-TZP