Decrease In Kinetic Energy Research Articles

Relativistic multiconfiguration Dirac–Fock calculations of photoionization, electron-impact ionization, Auger decay, and relaxed-orbital oscillations of Cu ions

Core-hole creation and decay are of tremendous importance in both theory and applications. However, different ionization approaches and decay channels, especially at L-shell, bring many challenges to accurate simulation. In order to understand the ionization state effects on the creation and decay at L-shell hole, we performed a theoretical study of photon/electron-impact L-shell ionization of differently charged copper ions and the subsequent Auger electron and fluorescence emission by using the relativistic multiconfiguration Dirac–Fock approach. It was found that photoionization has much larger cross sections than electron-impact ionization near the ionization threshold, but smaller ones at higher impact energies. The ionization cross sections for 2s-sublevels are negligibly small compared with those for 2p-sublevels for both photon and electron impact ionizations. Additionally, the ionization cross sections decrease with the ionization state for both electron- and photon- impact L-shell ionizations. Similar to radiative (fluorescence) decay, Auger kinetic energy and yield decrease with ionization state, and the energy gaps and rate ratios between major Auger transitions change with ionization state. Furthermore, Auger rates from the L-shell decay are much larger than fluorescence rates, and the Auger-to-fluoresce decay ratio slightly decreases with increase in the ionization state of Cu ion.

Instability Processes in Simulated Finite-Width Ocean Fronts

AbstractA simple, isolated front is modeled using a turbulence resolving, large-eddy simulation (LES) to examine the generation of instabilities and inertial oscillations by surface fluxes. Both surface cooling and surface wind stress are considered. Coherent roll instabilities with 200–300-m horizontal scale form rapidly within the front after the onset of surface forcing. With weak surface cooling and no wind, the roll axis aligns with the front, yielding results that are equivalent to previous constant gradient symmetric instability cases. After ~1 day, the symmetric modes transform into baroclinic mixed modes with an off-axis orientation. Traditional baroclinic instability develops by day 2 and thereafter dominates the overall circulation. Addition of destabilizing wind forcing produces a similar behavior, but with off-axis symmetric-Ekman shear modes at the onset of instability. In all cases, imbalance of the geostrophic shear by vertical mixing leads to an inertial oscillation in the frontal currents. Analysis of the energy budget indicates an exchange between kinetic energy linked to the inertial currents and potential energy associated with restratification as the front oscillates in response to the vertically sheared inertial current. Inertial kinetic energy decreases from enhanced mixed layer turbulence dissipation and vertical propagation of inertial wave energy into the pycnocline.

Preparation and Optical Absorption Properties of Ternary Rare Earth Boride LaxPr1-xB6 Submicron Powders.

As multifunctional materials, rare-earth hexaborides (RB6) display many interesting physical properties such as optical absorption, magnetic and thermionic emission. With the wide application of rare earth hexaboride and the continuous extension of its research fields, researchers have studied the synthesis of multi-rare earth hexaboride nano-powders and their thermal emission and light absorption properties. In the present work, ternary Single-phase LaxPr1-xB6 submicron powders are successfully synthesized using a solid-state reaction, in which lanthanum chloride (LaCl₃) and praseodymium chloride (PrCl₃) are used as rare-earth source and NaBH₄ as the boron source under continuous vacuum conditions. The reaction temperature is 1150 °C and the holding time is 2 h. The Pr doping effects on crystal structure, grain morphology, and optical absorption properties were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and ultraviolet-vis absorption measurements. The XRD patterns show that the diffraction peaks are sharp and well-defined, and no other extra impurity peaks were detected, indicating the characteristics of well-crystallized materials. It is found that all the LaxPr1-xB6 solid powders are of single phase. The SEM results demonstrate that the cubicshaped ternary LaxPr1-xB6 submicron crystals with a size of 50~200 nm are obtained. The TEM images reveal the cubic single-crystalline nature, and the FFT patterns implies the lattice fringe d = 0.416 nm which agrees well with the (100) crystal plane. The elements mapping results indicates that the Pr atoms occupied the lattice sites of LaB6. The optical absorption results show that the absorption valley of LaB6 are located at 591 nm. With the increase of Pr doping contents from 0.2 to 0.8, the absorption valley moves from 596.3 nm to 612.9 nm, indicating the characteristics of visible light high transparency. The first-principle calculation results manifest that the move of the absorption valley of LaB6 in the visible region after doping Pr is related to the decrease of kinetic energy of electrons near the Fermi plane. X-ray photoelectron spectroscopy (XPS) analysis shows that the La and Pr exist in the type of La3+ and Pr3+ in LaxPr1-xB6. Therefore, it exists as an efficient optical absorption material. The LaxPr1-xB6 should open up a new route to extend the optical applications of rare-earth hexaborides.

Discrete particle behavior in solid-liquid concave-wall jet

The aim of this paper is to reveal the solid-liquid separation mechanism in concave-wall jet. The discrete particle experiments and numerical simulations were performed to study the large particle behavior near concave-wall. The results show that the particle behavior is divided into two stages by the first collision point of particle-to-wall and the wall shear stress at the collision point quickly changes from zero to peak. The iterative formulae for particle position coordinates are used and 27.88° is obtained as the maximum of circumferential collision position when the jet width is 20 mm. Before the first collision, the particle tangential velocity is constant and the wall shear stress is 0. After the first collision, the wall shear stress and tangential velocity decrease with time. The distance of particle-to-wall is within 0.1 times of jet width. Low-frequency contact of particle-to-wall is the collision and the particle Reynolds number often exceeds 400. The large particle behavior is affected by the shape of jet inlet cross-section. Comparing to the square inlet cross-section, the particle cumulative probability of rectangle inlet cross-section is lower at the same residence time, the local maximum of turbulent kinetic energy and wall shear stress decrease 5.3 % and 5.9 %, respectively.

On the role of turbulence distortion on leading-edge noise reduction by means of porosity

A possible strategy for the reduction of the aeroacoustic noise generated by turbulence interacting with a wing profile, also referred to as leading-edge noise, is represented by the implementation of a porous medium in the structure of the airfoil. However, the physical mechanisms involved in this noise mitigation technique remain unclear. The present work aims at elucidating these phenomena and particularly how the porosity affects the incoming turbulence characteristics in the immediate vicinity of the surface. A porous NACA-0024 profile equipped with melamine foam has been compared with a solid baseline, both airfoils being in turn subjected to the turbulence shed by an upstream circular rod. The mean wall-pressure distribution along the airfoils shows that the implementation of the porous material mostly preserves the integrity of the NACA-0024 profile's shape. Results of hot-wire anemometry and large-eddy simulations indicate that the porous design proposed in this study allows for a damping of the velocity fluctuations and it has a limited influence on the upstream mean flow field. Specifically, the upwash component of the root-mean-square of the velocity fluctuations turns out to be significantly attenuated in the porous case in contrast to the solid one, leading to a strong decrease of the turbulent kinetic energy in the stagnation region. Furthermore, the comparison between the power spectral densities of the incident turbulent velocities demonstrates that the porosity has an effect mainly on the low-frequency range of the turbulent velocity power spectrum. This evidence is in line with the results of the acoustic beamforming measurements, which exhibit a noise abatement in an analogous frequency range. On the basis of these observations, an interpretation of the phenomena occurring in the turbulence-interaction noise reduction due to a porous treatment of the airfoil is finally given with reference to the theoretical inputs of the Rapid Distortion Theory.

The Basics of Covalent Bonding in Terms of Energy and Dynamics.

We address the paradoxical fact that the concept of a covalent bond, a cornerstone of chemistry which is well resolved computationally by the methods of quantum chemistry, is still the subject of debate, disagreement, and ignorance with respect to its physical origin. Our aim here is to unify two seemingly different explanations: one in terms of energy, the other dynamics. We summarize the mechanistic bonding models and the debate over the last 100 years, with specific applications to the simplest molecules: H2+ and H2. In particular, we focus on the bonding analysis of Hellmann (1933) that was brought into modern form by Ruedenberg (from 1962 on). We and many others have helped verify the validity of the Hellmann–Ruedenberg proposal that a decrease in kinetic energy associated with interatomic delocalization of electron motion is the key to covalent bonding but contrary views, confusion or lack of understanding still abound. In order to resolve this impasse we show that quantum mechanics affords us a complementary dynamical perspective on the bonding mechanism, which agrees with that of Hellmann and Ruedenberg, while providing a direct and unifying view of atomic reactivity, molecule formation and the basic role of the kinetic energy, as well as the important but secondary role of electrostatics, in covalent bonding.

A kinetic energy budget on the severe wind production that causes a serious state grid failure in Southern Xinjiang China

AbstractBased on the European Centre for Medium‐Range Weather Forecasts (ECMWF) ERA5 reanalysis data, in this study, formation mechanisms of a severe windstorm that caused successive trippings of the transmission lines in Southern Xinjiang were investigated. The strong windstorm occurred within a lower‐tropospheric warm region due to adiabatic heating of the descending motions ahead of a shortwave trough in the westerly wind (the blocking effects of high mountain was a key reason for the strong descending motions). The kinetic energy (KE) budget indicates two typically different stages appeared in the variation of the windstorm. The former stage showed a rapid wind KE enhancement in the lower troposphere. The KE increase was mainly governed by the downward stretching of high KE (i.e., downward momentum transportation) from the middle troposphere (rather than from the upper‐level jet) and the KE production due to the work on rotational wind by the pressure gradient force. The latter stage showed a rapid KE decrease mainly due to the transport of KE by the rotational wind and the pressure gradient force's negative work on the rotational wind. In contrast, the vertical advection of KE still acted as transporting high KE from middle troposphere to lower troposphere, which resisted the KE reduce at the lower levels.

Investigation of the influence of the gas pipeline tee geometry on hydraulic energy loss of gas pipeline systems

CFD simulation investigated turbulent flows in equal gas pipeline tees, in which the gas flow completely moves from the main line to the branch. The study was performed for tees of different geometry – stamped with different bending radii of the transition from the branch to the main line and weld, where the main line and branch connection is made at right angles. The outer diameter of the tees varied from 219 mm to 1,420 mm, the bending radius of the transition from the branch to the main line from the minimum permissible to the maximum possible, the pressure in the gas pipeline at the tee location from 3 MPa to 7 MPa. The mathematical model is based on the solution of the Navier-Stokes and energy transfer equations closed by a two-parameter high-Reynolds k – e Launder-Sharma turbulence model. To describe the processes occurring at the wall, the wall function was used. It was found that the bending of the transition from the branch to the main line, the increase in the bending radius lead to a decrease in the intensity of flow separation at the bending point and a decrease in turbulence kinetic energy in recirculation areas. The velocity field of the gas flow after it moves from the main line to the branch becomes more uniform. All this greatly affects the magnitude of hydraulic energy loss of the gas flow in the tees. In this case, the greatest energy losses were observed in the tees located at the lowest pressure points in the gas pipeline system. An analysis of the results showed that if the ratio of the bending radius of the main line and branch connection to the outer diameter is more than 0.25, then the influence of such a tee on the energy loss of the gas pipeline system is minimal. Local resistance coefficients of equal gas pipeline tees are calculated and the resulting equation for their calculation will be useful for specialists designing gas pipeline systems

Fully Correlated Stochastic Inter-Particle Collision Model for Euler\u2013Lagrange Gas\u2013Solid Flows

In Lagrangian stochastic collision models, a fictitious particle is generated to act as a collision partner, with a velocity correlated to the velocity of the real colliding particle. However, most often, the fluid velocity seen by this fictitious particles is not accounted for in the generation of the fictitious particle velocity, leading to a de-correlation between the fictitious particle velocity and the local fluid velocity, which, after collision, leads to an unrealistic de-correlation of the real particle velocity and the fluid velocity as seen by the particle. This de-correlation, in turn, causes a spurious decrease of the particle kinetic energy, even though the collisions are assumed perfectly elastic. In this paper, we propose a new model in which the generated fictitious particle velocity is correctly correlated to both the real particle velocity and the local fluid velocity at the particle, hence preventing the spurious loss of the total particle kinetic energy. The model is suitable for small inertial particles. Two algorithms for integrating the collision frequency are also compared to each other. The models are validated using large eddy simulation (LES) of mono-dispersed particle-laden stationary homogeneous isotropic turbulence. Simulations are conducted with spherical particles with different turbulent Stokes number, St_t = [0.75 - 5.8], and volume fractions, alpha _p = [0.014 - 0.044], and are compared to the results of the LES using a deterministic discrete particle simulation model.

Angle dependence of dissociative tunneling ionization of NO in asymmetric two-color intense laser fields

Dissociative tunneling ionization of nitric oxide (NO) in linearly polarized phase-locked two-color femtosecond intense laser fields (45 fs, $\ensuremath{\lambda}=800$ and 400 nm, total field intensity $I=1\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$) has been studied by three-dimensional ion momentum imaging. The ${\mathrm{N}}^{+}$ fragment produced by the dissociative ionization, $\mathrm{NO}\ensuremath{\rightarrow}\mathrm{N}{\mathrm{O}}^{+}+{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{N}}^{+}+\mathrm{O}+{e}^{\ensuremath{-}}$, exhibits a butterflylike momentum distribution peaked at finite angles with respect to the laser polarization direction. In addition, a clear dependence on the relative phase between the two laser fields is observed, showing that the tunneling ionization occurs efficiently when the electric field points from N to O. For the highest kinetic energy component, the observed orientation dependence is well explained with theoretical calculations by the weak-field asymptotic theory for the 2\ensuremath{\pi} highest occupied molecular orbital (HOMO). On the other hand, the peak angle shifts toward the laser polarization direction as the kinetic energy decreases, indicating that pathways other than direct ionization from the HOMO contribute to the dissociative ionization.

On the Impact of Trees on Ventilation in a Real Street in Pamplona, Spain

This paper is devoted to the quantification of changes in ventilation of a real neighborhood located in Pamplona, Spain, due to the presence of street trees Pollutant dispersion in this urban zone was previously studied by means of computational fluid dynamic (CFD) simulations. In the present work, that research is extended to analyze the ventilation in the whole neighborhood and in a tree-free street. Several scenarios are investigated including new trees in the tree-free street, and different leaf area density (LAD) in the whole neighborhood. Changes between the scenarios are evaluated through changes in average concentration, wind speed, flow rates and total pollutant fluxes. Additionally, wind flow patterns and the vertical profiles of flow properties (e.g., wind velocity, turbulent kinetic energy) and concentration, horizontally-averaged over one particular street, are analyzed. The approach-flow direction is almost perpendicular to the street under study (prevailing wind direction is only deviated 4º from the perpendicular direction). For these conditions, as LAD increases, average concentration in the whole neighborhood increases due to the decrease of wind speed. On the other hand, the inclusion of trees in the street produces an increase of averaged pollutant concentration only within this street, in particular for the scenario with the highest LAD value. In fact, the new trees in the street analyzed with the highest LAD value notably change the ventilation producing an increase of total pollutant fluxes inward the street. Additionally, pollutant dispersion within the street is also influenced by the reduction of the wind velocity along the street axis and the decrease of turbulent kinetic energy within the vegetation canopy caused by the new trees. Therefore, the inclusion of new trees in a tree-free street should be done by considering ventilation changes and traffic emissions should be consequently controlled in order to keep pollutant concentration within healthy levels.

Texture and Stress Evolution in HfN Films Sputter-Deposited at Oblique Angles

In this study, polycrystalline hafnium nitride (HfN) thin films were grown by oblique angle deposition (OAD) technique to investigate the relationship between column tilt angle, texture development and residual stress evolution with varying inclination angle α of the substrate. The films (~1 μm thickness) were grown at various angles (α = 5°, 25°, 35°, 65°, 75°, and 85°) with respect to the substrate normal by reactive magnetron sputtering at 0.3 Pa and 300 °C. The film morphology, crystal structure and residual stress state were characterized by scanning electron microscopy and X-ray diffraction (XRD), including pole figure and sin2ψ measurements. All HfN films had a cubic, NaCl-type crystal structure with an [111] out-of-plane orientation and exhibited a biaxial texture for α ≥ 35°. XRD pole figures reveal that the crystal habit of the grains consists of {100} facets constituting triangular-base pyramids, with a side and a corner facing the projection of the incoming particle flux (indicative of a double in-plane alignment). A columnar microstructure was formed for α ≥ 35°, with typical column widths of 100 nm. It is observed that the column tilt angle β increases monotonously for α ≥ 35°, reaching β = 34° at α = 85°. This variation at microscopic scale is correlated with the tilt angle of the (111) crystallographic planes, changing from −24.8 to 11.3° with respect to the substrate surface. The residual stress changes from strongly compressive (~−5 GPa at α = 5°) to negligible or slightly tensile for α ≥ 35°. The observed trends are compared to previous works of the literature and discussed based on existing crystal growth and stress models, as well as in light of energy and angular distribution of the incident particle flux calculated by Monte Carlo. Importantly, a decrease of the average kinetic energy of Hf particles from 22.4 to 17.7 eV is found with increasing α due to an increase number of collisions.

Impact of Nonuniform Land Surface Warming on Summer Anomalous Extratropical Cyclone Activity Over East Asia

AbstractAs one of the typical midlatitude synoptic‐scale disturbances, extratropical cyclones (ECs) can exert significant impacts on the atmospheric general circulation through its interaction with the time mean flow. Under the background of global warming, the Eurasian continent exhibits evident nonuniform warming, which has the potential to alter the atmospheric baroclinicity by changing the meridional temperature gradient and further affect the ECs activity. In this study, we investigated the possible connection between the land surface thermal anomaly over Eurasia and the summer ECs activity over East Asia together with relevant mechanisms. We found that the land surface warming (cooling) near 50°N of East Asia is associated with anomalous weak (strong) summer ECs activity over East Asia. Warm (cool) land surface usually reduces (increases) the meridional temperature gradient and further the atmospheric baroclinicity in the key area of cyclone activity, resulting in low (high) frequency of the extratropical cyclogenesis and weakened (intensified) ECs activity. The land surface warming (cooling) can also depress (benefit) the associated baroclinic conversion between the time mean effective potential energy and eddy effective potential energy, resulting in decrease (increase) in the eddy kinetic energy. As a result, the energy obtained by the synoptic‐scale eddy from the time mean flow has been reduced (increased), which favors (hampers) the extratropical cyclogenesis, causing weak (strong) cyclone activity in the middle latitude of East Asia.

On the decrease of kinetic energy with depth in wave\u2013current interactions

We study wave–current interactions in two-dimensional water flows with constant vorticity over a flat bed. We establish decay rates for the velocity beneath spatially periodic surface waves without any restrictions on the wave amplitude. The approach relies on complex function theory and overcomes the intricacies inherent to nonlinear flow patterns by taking advantage of specific structural properties of the governing equations for water waves.

Rate of decay of turbulent kinetic energy in abruptly stabilized Ekman boundary layers

In the late afternoon, the surface temperature drops below air temperature and the lower atmosphere's density stratification becomes stable. A simple model that predicts the rate at which turbulence kinetic energy and mixing decrease after the onset of stable stratification is presented.

Full-scale CFD investigation of gas-particle flow, interactions and combustion in tangentially fired pulverized coal furnace

Investigations suggest the need for better understanding of reactive gas-particle turbulent flow phenomena in full-scale energy systems. Numerical study was done in 350 MWe utility boiler tangentially fired furnace to clarify selected issues, such as turbulence modulation, particles dispersion, energy transfer between phases, combustion process and flame, by using an in-house developed combustion code. Numerical experiments demonstrated remarkable complexity of flow and interphase exchange. Maximal decrease in average turbulence kinetic energy of 33% due to dispersed phase was predicted for representative monodispersed coal; augmentation obtained for large particles could become attenuation due to the particles size change during combustion. Grinding fineness of polydispersed coal affected the flow, combustion and flame considerably. Fine grinding (R90 = 48.40%) provided ascending flame, higher furnace exit temperature and decrease in turbulence energy, compared with coarse grinding (R90 = 73.85%). Combustion of each particle size class of coal is completed at different vertical levels, influencing the flame position. Diagrams based on numerical predictions were proposed to enable efficient estimations of combustion and flame characteristics in the case-study furnace, for various coal qualities and mass fractions and changed distributions of coal particle size classes over the burner tiers, while necessity for further investigation was pointed out as well.

Modeling of Turbulent Flow around Bubbles in Molten Steel

Turbulent flow around bubbles in the molten steel is predicted by using a two‐dimensional axisymmetric model. The dependence of bubble terminal velocity and bubble shape on its size is taken into account. The recirculation region is not observed for the free spherical and spheroidal bubbles, while it is found at the rear of the free spherical‐cap bubbles and the rigid bubbles. For the free spherical and spheroidal bubbles, the velocities at the rear of bubbles and at the bubble surface are raised with increasing turbulence kinetic energy. For free spherical‐cap bubbles and rigid bubbles, the wake flow is more easily formed in the weaker turbulence and the recirculation volume increases as the turbulent kinetic energy decreases.

Daytime Temporal Variation of Surface-Layer Parameters and Turbulence Kinetic Energy Budget in Topographically Complex Terrain Around Umiam, India

We present the temporal variation of surface-layer parameters and the turbulence kinetic energy budget over complex terrain during daytime. Data from three-dimensional fast response sonic anemometers at heights 6 m, 18 m, and 30 m above the ground are used for the analysis. Mountainous topography induces wind-direction variability over the measurement site throughout the day, which significantly influences the diurnal evolution of sensible heat flux, momentum flux, and turbulence kinetic energy. The increase and decrease of turbulence kinetic energy rely on the various production and consumption terms. Buoyancy dominates in the morning whereas in the afternoon both buoyancy and shear production contribute equally for the turbulence kinetic energy. The turbulence kinetic energy budget is also influenced by horizontal advection at the station. The correlation between turbulent decay and buoyancy (shear production) is negative (positive) in free convective conditions in the morning, whereas no correlation is observed for mixed convective conditions in the afternoon.

Heat transfer intensification by low or high frequency ultrasound: Thermal and hydrodynamic phenomenological analysis

The aim of this work is to quantitatively demonstrate the intensification of heat transfer in forced convection by mean of ultrasonic irradiation at low (25 kHz) or high (2 MHz) frequency. High frequency ultrasound induces convective acoustic streaming while low frequency ultrasonic waves produce mainly cavitation effects. These hydrodynamic phenomena are at the origin of strongly different observations in terms of flow pattern modification and thereby in Nusselt number values. A link is tentatively established between hydrodynamic behaviors at both ultrasonic frequencies and corresponding thermal results. Hydrodynamic approach was performed with a 2D-2C PIV device while thermal one was carried out under uniform heat flux conditions. It seems that thermal enhancement effect of acoustic streaming (2 MHz) decreases as flow rate increases. This behavior is consistent with the decrease of turbulent kinetic energy produced by acoustic streaming in the same conditions. In the contrary, thermal enhancement effect produced by acoustic cavitation (25 kHz), increases as flow rate increases. This result could be due to the increase of relative size of acoustic bubbles with respect to laminar boundary layer thickness as flow rate increases. In conclusion, the two different ultrasound frequencies, which lead to two different hydrodynamic effects, also lead to two different thermal and turbulence trend with respect to flow rate modifications.

Numerical simulation of two coalescing turbulent forced plumes in linearly stratified fluids

A computational fluid dynamic model that can solve the Reynolds-averaged Navier-Stokes equations and the species transport equation is developed to simulate two coalescing turbulent forced plumes, which are released with initial momentum and buoyancy flux into a linearly stable stratified environment. The velocity fields, turbulence structures, and entrainment of two plumes with different source separations and source buoyancy fluxes are analyzed quantitatively, in comparison with a series of physical experiments. An empirical parameterization is proposed to predict the amplification of the maximum rise height of two coalescing forced plumes caused by superposition and mutual entrainment. The maximum values of both turbulent kinetic energy and turbulence dissipation rate decrease monotonically with the increase in source separation of the two turbulent plumes. However, the trajectory of the maximum turbulent viscosity attained in the plume cap region presents two notable enhancements. This variation may be attributed to the turbulence transported from the touching region and the strong mixing around the neutrally buoyant layer between two plumes, while the mixing is caused by the lateral convection and the rebound after overshooting. The plume entrainment coefficient in near vent stems has a positive relationship with the source Richardson number. A transition of flow regimes to plume-like flows would occur when the contribution of initial momentum is important. The entrainment coefficient will decrease in the touching region of two plumes due to mutual entrainment, while the superposition of plumes can lead to distortion of the boundary of plume sectors.