Values Of Number Density Research Articles

Characterization of Eroded Boron Atoms in the Plume of a Hall Thruster

The erosion of boron particles from Hall thruster channel walls is a concern because it limits the lifetime of the device. A new optical experimental technique that measures path-integrated boron number density in the plume of a Hall effect thruster has recently been tested on an SPT-70 thruster. The data obtained may be used to infer the channel wall erosion rate, but further knowledge of the flow field is required. This development and the need to obtain a complete picture of the thruster channel and plume flows prompted a numerical simulation of the thruster erosion process. The direct simulation Monte Carlo method is used to study the sputtered boron atoms. These particles are introduced to the computational domain with velocities consistent with the sputtering process. The number flux is derived from a Sigmund-Thompson velocity distribution function. The number density values obtained in the plume are processed and compared with the measured values. The experimental and simulation results agree qualitatively, and disagree quantitatively by a factor of three to four, with the experimental results having lower values. Various causes of the discrepancy are discussed.

Tomographic and spectral views on the lifecycle of polar mesospheric clouds from Odin/OSIRIS

AbstractVertical and horizontal structures of Polar Mesospheric Clouds (PMC) have been recovered by tomographic retrieval from the OSIRIS instrument aboard the Odin satellite. The tomographic algorithm has been used to return local scattering coefficients at seven wavelengths in the ultraviolet. This spectral information is used to retrieve PMC particle sizes, number density, and ice mass density. While substantial horizontal variations are found, local vertical structures are overall consistent with the idea of a growth‐sedimentation process leading to a visible cloud. Large numbers of small particles are present near the top of the observed cloud layer. Toward lower altitudes, particle sizes increase while particle number densities decrease. A close relationship is found between the distribution of local PMC scattering coefficient and ice mass density. The bottom of the cloud often features large particles with mode radii exceeding 70 nm that rain out of the cloud before sublimating. The number density of these large particles is small, and they do not contribute significantly to the overall cloud brightness. As a consequence, the presence of these large particles can be difficult to identify for remote sensing techniques that integrate over the entire cloud column. When it comes to deriving absolute values of particle mode radius and number density, there is a strong sensitivity to assumptions on the mathematical form of the particle size distribution. We see a continued strong need to resolve this issue by co‐analysis of various remote sensing techniques and observation geometries.

Temperature and phase-space density of a cold atom cloud in a quadrupole magnetic trap

We present studies on the modifications in temperature, number density and phase-space density when a laser cooled atom cloud from the optical molasses is trapped in a quadrupole magnetic trap. Theoretically it is shown that for a given temperature and size of the cloud from the molasses, the phase-space density in the magnetic trap first increases with magnetic field gradient and then decreases with it, after attaining a maximum value at an optimum value of magnetic field gradient. The experimentally measured variation in phase-space density in the magnetic trap with the magnetic field gradient has shown the similar trend. However, the experimentally measured values of number density and phase-space density are much lower than their theoretically predicted values. This is attributed to the higher experimentally observed temperature in the magnetic trap than the theoretically predicted temperature. Nevertheless, these studies can be useful to set a higher phase-space density in the trap by setting the optimum value of field gradient of the quadrupole magnetic trap.

Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc)

Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ∼65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.

Landslides triggered by the 22 July 2013 Minxian–Zhangxian, China, Mw 5.9 earthquake: Inventory compiling and spatial distribution analysis

On July 22, 2013, an earthquake of Ms. 6.6 occurred at the junction area of Minxian and Zhangxian counties, Gansu Province, China. This earthquake triggered many landslides of various types, dominated by small-scale soil falls, slides, and topples on loess scarps. There were also a few deep-seated landslides, large-scale soil avalanches, and fissure-developing slopes. In this paper, an inventory of landslides triggered by this event is prepared based on field investigations and visual interpretation of high-resolution satellite images. The spatial distribution of the landslides is then analyzed. The inventory indicates that at least 2330 landslides were triggered by the earthquake. A correlation statistics of the landslides with topographic, geologic, and earthquake factors is performed based on the GIS platform. The results show that the largest number of landslides and the highest landslide density are at 2400m–2600m of absolute elevation, and 200m–300m of relative elevation, respectively. The landslide density does not always increase with slope gradient as previously suggested. The slopes most prone to landslides are in S, SW, W, and NW directions. Concave slopes register higher landslide density and larger number of landslides than convex slopes. The largest number of landslides occurs on topographic position with middle slopes, whereas the highest landslide density corresponds to valleys and lower slopes. The underlying bedrocks consisting of conglomerate and sandstone of Lower Paleogene (Eb) register both the largest number and area of landslides and the highest landslide number and area density values. Correlations of landslide number and landslide density with perpendicular- and along-strike distance from the epicenter show an obvious spatial intensifying character of the co-seismic landslides. The spatial pattern of the co-seismic landslides is strongly controlled by a branch of the Lintan-Dangchang fault, which indicates the effect of seismogenic fault on co-seismic landslides. In addition, the area affected by landslides related to the earthquake is compared to the relationship of “area affected by landslides vs. earthquake magnitude” constructed based on earthquakes worldwide, and it is shown that the area affected by landslides triggered by the Minxian–Zhangxian earthquake is larger than that of almost all other events with similar magnitudes.

Gamma radiation shielding and optical properties measurements of zinc bismuth borate glasses

In this work, the zinc bismuth borate (ZBB) glasses of the composition 10ZnO:xBi2O3:(90−x)B2O3 (where x=15, 20, 25 and 30mol%) were prepared by the melt quenching technique. Their radiation shielding and optical properties were investigated and compared with theoretical calculations. The mass attenuation coefficients of ZBB glasses have been measured at different energies obtained from a Compton scattering technique. The results show a decrease of the mass attenuation coefficient, effective atomic number and effective electron density values with increasing of gamma-ray energies; and good agreements between experimental and theoretical values. The glass samples with Bi2O3 concentrations higher than 25mol% (25 and 30mol%) were observed with lower mean free path (MFP) values than all the standard shielding concretes studied. These results are indications that the ZBB glasses in the present study may be developed as a lead-free radiation shielding material in the investigated energy range.

Computational Study on Melting Process Using Smoothed Particle Hydrodynamics

Recently, smoothed particle hydrodynamics (SPH) method has become popular in computational fluid dynamic and heat transfer simulation. The simplicity offered by this method made some complex system in physics such as moving interface in multiphase flow, heat conductivity jumping in multiple material boundaries and many geometrical difficulties become relative easy to calculate. We will treat a relative easy example of melting process to test the method in solving fluid motion equation coupled by heat transfer process. The main heat transfer processes are caused by solid-liquid (medium to medium) heat diffusion and convection. System interaction with ambient temperature can be modeled by gas surrounding fluid-solid system. For the ambient temperature, we proposed surface particle heat transfer governed by convectional heat flux. Using local particle number density value as surface detection method, we applied cooling and heating to surface particle on the melting ice cube and water system. The simulation result is also verified by experiment.

Phase structure of two-dimensional QED at zero temperature with flavor-dependent chemical potentials and the role of multidimensional theta functions

We consider QED on a two-dimensional Euclidean torus with $f$ flavors of massless fermions and flavor-dependent chemical potentials. The dependence of the partition function on the chemical potentials is reduced to a $(2f-2)$-dimensional theta function. At zero temperature, the system can exist in an infinite number of phases characterized by certain values of traceless number densities and separated by first-order phase transitions. Furthermore, there exist many points in the $(f-1)$-dimensional space of traceless chemical potentials where two or three phases can coexist for $f=3$ and two, three, four or six phases can coexist for $f=4$. We conjecture that the maximal number of coexisting phases grows exponentially with increasing $f$.

A phenomenological bubble number density model developed for simulation of cavitating flows inside high-pressure diesel injection nozzles

Purpose – The present study aims to resolve the adjustment problem of cavitation bubble number density in simulations of the cavitating flows within the diesel injection nozzle holes using a two-fluid cavitation model. Design/methodology/approach – The basic rule that determines the variations of cavitation bubble number density has been checked through the scaling analysis of a two-fluid model under the assumption of hydrodynamic similarity of the cavitating flows. Moreover, a phenomenological model for the number density of cavitation bubbles that takes the hydrodynamic effect into account has been developed through the combined analysis of cavitation bubble dynamics and internal flow characteristics of diesel injection nozzle holes. This new model has also been validated by the discharge coefficient measures in a wide range of injection conditions. Findings – The values of cavitation bubble number density must rationally match changes both in liquid quality effect and in hydrodynamic effect corresponding to different cavitating flows. The validation results show that the two-fluid cavitation model together with this new cavitation bubble number density model predicts well both the cavitation content inside the diesel nozzle hole and the relationship between discharge coefficient and cavitation number, and the new cavitation bubble number density model has the potential to further expand the application range of the two-fluid cavitation model. Originality/value – This study provides insight into hydrodynamic effect corresponding to cavitating flows inside diesel nozzle holes and presents an idea to model the cavitation bubble number density phenomenologically. The model idea and the developed model are useful to researchers and engineers in the area of nozzle internal flow and cavitating flow.

Effect of Copper and Niobium Addition on Crystallization Kinetics in Fe-Cu-Nb-Si-B Alloys

Abstract The activation energies of amorphous Fe 78 Si 11 B 9 and nanocrystalline Fe 73.5 Cu 1 B 7 Si 15.5 Nb 3 alloys were tested by a high-sensitivity differential scanning calorimeter with different heating rates. Kissinger equation was adopted to calculate the activation energies of the primary phase of Fe 73.5 Cu 1 B 7 Si 15.5 Nb 3 alloy (295±5) kJ and the crystal phase of Fe 78 Si 11 B 9 alloy (370±3) kJ. The low activation energy of the primary phase for Fe 73.5 Cu 1 B 7 Si 15.5 Nb 3 alloy is related to the precipitation of Cu clusters. The precipitation kinetics of Cu clusters was analyzed. The calculated result of the mean radius R m of Cu cluster is 3 nm after aging for 3600 s at 773 K in Fe 73.5 Cu 1 B 7 Si 15.5 Nb 3 glass alloy and the maximum value of number density is 3.7 ×10 24 /m 3 after aging for 2.5 h at 773 K, which accords with K. Hono's observation (approximately 3 nm after annealing at 673 K or 3600 s). The density of Cu is in the order of 10 24 m −3 ).

Void statistics and hierarchical scaling in the halo model

We study scaling behaviour of statistics of voids in the context of the halo model of nonlinear large-scale structure. The halo model allows us to understand why the observed galaxy void probability obeys hierarchical scaling, even though the premise from which the scaling is derived is not satisfied. We argue that the commonly observed negative binomial scaling is not fundamental, but merely the result of the specific values of bias and number density for typical galaxies. The model implies quantitative relations between void statistics measured for two populations of galaxies, such as SDSS red and blue galaxies, and their number density and bias.

Impacts of air pressure on the evolution of nanosecond pulse discharge products

Based on the nonequilibrium plasma dynamics of air discharge, a dynamic model of zero-dimensional plasma is established by combining the component density equation, the Boltzmann equation, and the energy transfer equation. The evolution properties of nanosecond pulse discharge (NPD) plasma under different air pressures are calculated. The results show that the air pressure has significant impacts on the NPD products and the peak values of particle number density for particles such as O atoms, O3 molecules, N2(A3) molecules in excited states, and NO molecules. It increases at first and then decreases with the increase of air pressure. On the other hand, the peak values of particle number density for N2(B3) and N2(C3) molecules in excited states are only slightly affected by the air pressure.

Examination of methods to determine free-ion diffusivity and number density from analysis of electrode polarization

Electrode polarization analysis is frequently used to determine free-ion diffusivity and number density in ionic conductors. In the present study, this approach is critically examined in a wide variety of electrolytes, including aqueous and nonaqueous solutions, polymer electrolytes, and ionic liquids. It is shown that the electrode polarization analysis based on the Macdonald-Trukhan model [J. Chem. Phys. 124, 144903 (2006); J. Non-Cryst. Solids 357, 3064 (2011)] progressively fails to give reasonable values of free-ion diffusivity and number density with increasing salt concentration. This should be expected because the original model of electrode polarization is designed for dilute electrolytes. An empirical correction method which yields ion diffusivities in reasonable agreement with pulsed-field gradient nuclear magnetic resonance measurements is proposed. However, the analysis of free-ion diffusivity and number density from electrode polarization should still be exercised with great caution because there is no solid theoretical justification for the proposed corrections.

Boltzmann analysis of electron swarm parameters in binary CF4+ Ar mixtures

The electron swarm parameters, namely electron mean energy, drift velocity, effective ionization coefficient and limiting values of number density reduced electric fields, E/N, are calculated in CF4+Ar mixtures for various CF4 concentrations that vary from 2 to 100% over a range of E/N from 50Td to 600Td by solving Boltzmann's equation. In this study Boltzmann's equation was solved using finite difference method under steady-state Townsend condition. One of the most important results of this study indicated that at higher E/N values inelastic processes due to argon began to control the swarm energy thus leading this binary mixture act essentially like pure argon.

THE ROLE OF THE VELOCITY DISTRIBUTION IN THE DSMC PRESSURE BOUNDARY CONDITION FOR GAS MIXTURES

A prescribed pressure is the most common flow boundary condition used in flow simulations. In the Direct Simulation Monte Carlo (DSMC) method, boundary pressure is controlled by the number flux of the simulating molecules entering the domain. In the conventional DSMC algorithm, this number flux is calculated iteratively using sampled values of velocity and number density by means of an expression derived from the Maxwell distribution function. It is known that this procedure does not work well for low speed flows which are of interest in most micro-flow applications and the statistical scatter of the DSMC results is generally stated to be the main reason. However, the Maxwell distribution used in the pressure boundary treatment is valid for equilibrium conditions, and therefore, current implementations of the DSMC pressure boundary treatment are limited to boundaries with sufficiently small rarefaction effects. This is not the case for some practical problems in which highly rarefied flows through the boundaries lead to considerable nonequilibrium effects. In this study, an expression for the species number flux is derived using the Chapman–Enskog velocity distribution to improve the pressure boundary condition. The resulting algorithm is then used for modeling a micro-channel binary gas mixture flow with prescribed pressure boundary conditions.

The kinetics of clustering in Al–Mg–Si alloys studied by Monte Carlo simulation

Abstract The kinetics of clustering in Al–Mg–Si alloy is studied by means of Kinetic Monte Carlo simulations. The simulations take into account the probability of vacancy jumping to nearest neighbour sites. This probability is calculated by considering both the activation energies for single vacancy migration and the difference of interaction energies before and after jumping. The simulations show that clustering in Al–Mg–Si is fast and takes place in three stages. In the initial stage, dimers, trimers and small co-clusters form. The number density of such clusters increases rapidly and solute atoms aggregate to those clusters until a maximum number density value is reached after 2 min. In the second stage, from 2 min to around 100 min, a decrease in the number density of clusters is accompanied by an increase in the fraction of solute contained in all the clusters. Finally, a further reduction in the cluster number density is observed, while the amount of solute atoms in the clusters remains constant. We discuss how robust the results are with respect to changes in the input parameters.

On the Emmenthal distribution of highly inclined asteroids

Highly inclined asteroids are objects with sin (i) > 0.3. Among highly inclined asteroids, we can distinguish between objects with inclinations smaller than that of the centre of the ν6= g − g6 secular resonance and objects at higher inclinations. Using the current mechanisms of dynamical mobility, it is not easy to increase the values of an asteroid with an initial small inclination to values higher than that of the centre of the ν6 resonance. The presence of highly inclined objects might therefore be related to the early phases of the Solar system. It has been observed that several dynamically stable regions are characterized by a very low number density of objects, unlike low-inclined bodies that tend to occupy all the dynamically viable regions. The distribution of asteroids at a high inclination in the domain of proper elements in dynamically stable regions resembles an Emmenthal cheese, with regions of low number density close to highly populated areas. While this phenomenon has been observed qualitatively in the past, no quantitative study has yet been carried out on the extent and long-term stability of these regions. In this paper, we identify two dynamically stable regions characterized by very low values of number density and permanence times of 100 Myr or more when the Yarkovsky force is considered. We show that the low number density of objects in these areas cannot be produced as a statistical fluctuation of any simple one-dimensional statistical distribution, such as the Poissonian, uniform and Gaussian distributions, or of a tri-dimensional distribution, such as the tri-variate normal distribution. The presence of unoccupied dynamically stable regions could indicate that the primordial asteroidal population might not have reached all available zones at high-i. This sets constraints on the scenarios for the early phases of the history of our Solar system.

A new type of high-pressure low-flow metering valve for continuous decompression: First experimental results on degassing of rhyodacitic melts

A novel type of high-pressure low-flow metering valve has been designed for experiments at continuous decompression in internally heated pressure vessels (IHPV) at pressures up to 500 MPa and temperatures up to 1500 °C. It consists of a modified high-pressure valve coupled with a piezoelectric nanopositioning system. The piezoelectric actuator (1) positions the needle of the valve statically with very high precision on a nanometer scale for infinitesimally slow decompression and, alternatively, (2) opens and closes the valve completely, within milliseconds, to achieve a fast pressure drop only limited by the diameter of the high-pressure tubing and the entire volume of the autoclave system. The valve connected to an IHPV has been successfully tested to simulate continuous decompression of volatile-bearing magma. Water saturated rhyodacitic melt was synthesized at 1050 °C and 300 MPa and subsequently decompressed to 50 MPa at an integrated decompression rate of 0.28 MPa/s using continuous decompression as well as single-step and multi-step decompression techniques. The experimental results of the three methods show significant differences, having important implications for the interpretation of textures observed in natural volcanic rocks. Bubble number density (BND) values increase from continuous to multi-step and to single-step decompression by two orders of magnitude. Bubble size distribution (BSD) also differs significantly. The BSD curve of the single-step decompression shows largest variation both in size and in population number density, whereas the BSD trend for the sample of multi-step decompression shows the smallest variation between size and population density of bubbles. The BSD of the continuously decompressed sample indicates similar proportions of bubbles with different size. Channel formation in the sample of single-step decompression may indicate fluid escape from over-pressurized bubbles through the melt. Single-step and continuous decompression style represent two extreme cases of magma degassing in which bubble nucleation and bubble growth are predominant processes, respectively, whereas multi-step decompression resembles the intermediate case.

Development of microstructure and irradiation hardening of Zircaloy during low dose neutron irradiation at nominally 358 °C

Wrought Zircaloy-2 and Zircaloy-4 were neutron irradiated at nominally 358 °C in the high flux isotope reactor (HFIR) at relatively low neutron fluences between 5.8 × 10 22 and 2.9 × 10 25 n/m 2 ( E > 1 MeV). The irradiation hardening and change in microstructure were characterized following irradiation using tensile testing and examinations of microstructure using Analytical Electron Microscopy (AEM). Small increments of dose (0.0058, 0.11, 0.55, 1.08, and 2.93 × 10 25 n/m 2) were used in the range where the saturation of irradiation hardening is typically observed so that the role of microstructure evolution and 〈 a〉 loop formation on irradiation hardening could be correlated. An incubation dose between 5.8 × 10 23 and 1.1 × 10 24 n/m 2 was needed for loop nucleation to occur that resulted in irradiation hardening. Increases in yield strength were consistent with previous results in this temperature regime, and as expected less irradiation hardening and lower 〈 a〉 loop number density values than those generally reported in literature for irradiations at 260–326 °C were observed. Unlike previous lower temperature data, there is evidence in this study that the irradiation hardening can decrease with dose over certain ranges of fluence. Irradiation induced voids were observed in very low numbers in the Zircaloy-2 materials at the highest fluence.

Hinode extreme-ultraviolet imaging spectrometer observations of a limb active region

Aims. We investigate the electron density and temperature structure of a limb active region. Methods. We have carried out a study of an active region close to the solar limb using observations from the Extreme-ultraviolet Imaging Spectrometer (EIS) and the X-ray telescope (XRT) on board Hinode. The electron density and temperature distributions of the coronal emission have been determined using emission line intensity ratios. Differential emission measure (DEM) analysis and the emission measure (EM) loci technique were used to examine the thermal structure of the emitting plasma as a function of distance from the limb. Results. The highest temperature and electron density values are found to be located in the core of the active region, with a peak electron number density value of 1.9 × 10 10 cm −3 measured using the Fe XII 186.887 A to 192.394 A line intensity ratio. The plasma along the line of sight in the active region was found to be multi-thermal at different distances from the limb. The EIS and XRT DEM analyses appear to be in agreement in the temperature interval from log T = 6.5−6.7. Conclusions. Our results provide new constraints for models of coronal heating in active regions.