Potential Energy Field Research Articles

On the size-dependent magneto/electromechanical buckling of nanobeams

Bending and buckling of piezoelectric and piezomagnetic nanobeams are investigated based upon the Euler-Bernoulli beam model and using the strain gradient theory, which is capable of accounting for higher-order magneto/electromechanical coupling as well as size effects. Governing equations and boundary conditions are extracted using the principle of minimum potential energy, and electrical and mechanical field distributions are computed using the equations obtained. The size effect is accounted for using the strain gradient theory, and nanobeam buckling is analyzed using the nonlinear von Karman strain. Nanobeam bending and buckling are examined through the Galerkin procedure. The impact of parameters such as size effects, length, thickness, material properties, external voltage, and magnetic potential is investigated, and critical load, critical voltage, and critical magnetic potential of buckling are shown to be considerably dependent upon size effects, particularly as thickness increases.

DeePMD-kit: A deep learning package for many-body potential energy representation and molecular dynamics

Recent developments in many-body potential energy representation via deep learning have brought new hopes to addressing the accuracy-versus-efficiency dilemma in molecular simulations. Here we describe DeePMD-kit, a package written in Python/C++ that has been designed to minimize the effort required to build deep learning based representation of potential energy and force field and to perform molecular dynamics. Potential applications of DeePMD-kit span from finite molecules to extended systems and from metallic systems to chemically bonded systems. DeePMD-kit is interfaced with TensorFlow, one of the most popular deep learning frameworks, making the training process highly automatic and efficient. On the other end, DeePMD-kit is interfaced with high-performance classical molecular dynamics and quantum (path-integral) molecular dynamics packages, i.e., LAMMPS and the i-PI, respectively. Thus, upon training, the potential energy and force field models can be used to perform efficient molecular simulations for different purposes. As an example of the many potential applications of the package, we use DeePMD-kit to learn the interatomic potential energy and forces of a water model using data obtained from density functional theory. We demonstrate that the resulted molecular dynamics model reproduces accurately the structural information contained in the original model. Program summaryProgram Title: DeePMD-kitProgram Files doi:http://dx.doi.org/10.17632/hvfh9yvncf.1Licensing provisions: LGPLProgramming language: Python/C++Nature of problem: Modeling the many-body atomic interactions by deep neural network models. Running molecular dynamics simulations with the models.Solution method: The Deep Potential for Molecular Dynamics (DeePMD) method is implemented based on the deep learning framework TensorFlow. Supports for using a DeePMD model in LAMMPS and i-PI, for classical and quantum (path integral) molecular dynamics are provided.Additional comments including Restrictions and Unusual features: The code defines a data protocol such that the energy, force, and virial calculated by different third-party molecular simulation packages can be easily processed and used as model training data.

The Global Mesoscale Eddy Available Potential Energy Field in Models and Observations

AbstractGlobal maps of the mesoscale eddy available potential energy (EAPE) field at a depth of 500 m are created using potential density anomalies in a high‐resolution 1/12.5° global ocean model. Maps made from both a free‐running simulation and a data‐assimilative reanalysis of the HYbrid Coordinate Ocean Model (HYCOM) are compared with maps made by other researchers from density anomalies in Argo profiles. The HYCOM and Argo maps display similar features, especially in the dominance of western boundary currents. The reanalysis maps match the Argo maps more closely, demonstrating the added value of data assimilation. Global averages of the simulation, reanalysis, and Argo EAPE all agree to within about 10%. The model and Argo EAPE fields are compared to EAPE computed from temperature anomalies in a data set of “moored historical observations” (MHO) in conjunction with buoyancy frequencies computed from a global climatology. The MHO data set allows for an estimate of the EAPE in high‐frequency motions that is aliased into the Argo EAPE values. At MHO locations, 15–32% of the EAPE in the Argo estimates is due to aliased motions having periods of 10 days or less. Spatial averages of EAPE in HYCOM, Argo, and MHO data agree to within 50% at MHO locations, with both model estimates lying within error bars observations. Analysis of the EAPE field in an idealized model, in conjunction with published theory, suggests that much of the scatter seen in comparisons of different EAPE estimates is to be expected given the chaotic, unpredictable nature of mesoscale eddies.

CO2 - Reinforced nanoporous carbon potential energy field during CO2/CH4 mixture adsorption. A comprehensive volumetric, in-situ IR, and thermodynamic insight

CO2/CH4 mixture adsorption is very important in different fields like, for example, a biogas purification. Using a comprehensive experimental approach based on volumetric and in-situ FTIR measurements the new results of CO2/CH4 mixture separation on a carbon film are reported. The application of this experimental approach makes it possible to elaborate the effect of enhanced CH4 adsorption at low CO2 concentrations in the adsorbed phase. The presence of this effect is proved experimentally for the first time. This effect is responsible for the deviation of Ideal Adsorption Solution model from the experimental data. To discuss separation mechanism the activity coefficients at constant spreading pressure values are calculated. At low spreading pressure, CO2 activity coefficient is strongly disturbed by the presence of CH4 molecules in the surface mixture. In contrast, the CH4 activity coefficients are remarkably influenced by adsorbed CO2 only at higher CO2 surface concentrations. The obtained activity coefficients are successfully described by a new modification of the Redlich-Kister equation. This modification takes into account the interaction between binary mixture components and an adsorbent. Finally we show that the studied carbon possesses very good CO2/CH4 mixture separation properties, comparable to those reported for other adsorbents.

A potential energy field (PEF) approach to the design of a compliant self-guiding statically-balanced straight-line mechanism

For the conception of compliant mechanisms various design aids have been proposed, including some graphical ones. In this work we propose to use the potential energy field of elastic systems to characterise their behaviour over a large motion area. The system and its characterisation can be interactively adapted to obtain a behaviour of interest. The behaviour is fine-tuned by a shape optimization procedure. This approach is elaborated in this paper for systems comprising curved planar beams. A design example is presented that can realise an unconstrained straight-line motion with a constant potential energy level, comprising four symmetrically arranged beams.

Interpenetrated Zirconium–Organic Frameworks: Small Cavities versus Functionalization for CO2 Capture

Porous interpenetrated zirconium–organic frameworks (PIZOFs) with various functional groups are explored for CO2 capture using molecular simulation and experiment. Functionalization enhances the CO2 uptake and selectivity over other gases, but small cavities play an even more important role. Particularly at low pressures, small cavities enhance the CO2 adsorption density nearly 5 times greater than the functionalization. PIZOF-2 outperforms the other PIZOF structures for CO2 separation from methane and nitrogen (related to raw natural gas and postcombustion of coal mixtures) due to the combination of small cavities around 5 A in diameter and functionalized linkers with methoxy groups attached to the central ligand. The small cavities within the interpenetrated structures are crucial for achieving high selectivities, especially for cavities surrounded by a combination of 6 benzene rings, 2 metal clusters, and 4 methoxy groups that offer a tight overlapping potential energy field, ideal for “catching” CO2.

Modeling the potential energy field caused by mass density distribution with Eton approach

Abstract A new approach for modeling real world problems called the “Eton Approach” was presented in this paper. The "Eton approach" combines both the concept of the variable order derivative together with Atangana derivative with memory derivative. The Atangana derivative with memory is used to account for the memory and fractional derivative for its filter effect. The approach was used to describe the potential energy field that is caused by a given charge or mass density distribution.We solve the modified model numerically and present supporting numerical simulations.

An hp-proper orthogonal decomposition–moving least squares approach for molecular dynamics simulation

Abstract The application of model order reduction for molecular dynamic systems exhibits intrinsic complexities due to the highly nonlinear and nonlocal properties of such systems. The most costly computational part of such work is the calculation of potential energy and inter-atomic force fields. We introduce the so-called hp -proper orthogonal decomposition–moving least squares ( hp -POD-MLS) method not only to tackle this force computation, but also to improve the accuracy of reduced displacement approximations in an adaptive and controllable manner. The approach combines the proper orthogonal decomposition (POD) method with the moving least squares (MLS) one to interpolate the inter-atomic force field at an arbitrary time instance; this approximated force field is then fed into the reduced molecular dynamic (MD) governing equation to compute the reduced order displacement field. In fact, this POD–MLS approach is already able to handle efficiently the force computation in the online computational stage (i.e., the online force computational cost is independent of the dimension of the full MD space); however its corresponding approximated displacement error will be very large if the considered total time interval is long. The proposed hp -POD-MLS algorithm will divide adaptively and automatically the global temporal interval into many smaller local subintervals ( h -refinement), and then build the sets of local reduced basis functions over each of these subintervals ( p -refinement). The goal is to reduce the corresponding displacement errors to a desired tolerance over these subintervals. The effectiveness of the algorithm is demonstrated by performing reduced-order simulations of several molecular dynamic systems.

Modeling marine mud: A four phase system

Fine-grained marine sediment consists of up to four phases: (1) clay Minerals often with some silt and sand, (2) saline water, (3) free gas, and (4) organic matter (OM). Shallow-water, shelf, and slope muds often have all four phases present. The clay minerals are the “building blocks” of mud deposits; these consist of multi-plate face-to-face domains. There are only three possible modes of domain contact (termed signatures): (1) edge-to-face (EF), (2) edge-to-edge (EE), and (3) face-to-face (FF), usually offset. The domain faces are negatively charged and have a large surface area and the edges are both negative and positive charged. OM is usually expressed as total organic carbon (TOC) in percent total dry mass and can range from <0.5% to several percent and the OM can occupy a significant portion of pore volume. OM is usually attached to clay domain signatures and faces with the location partially dictated by electrostatic potential energy fields. When several percent TOC is present, the OM drives up water contents and porosity considerably. When free gas is present, it replaces part of the seawater in the pore space. Several published organo-clay fabric models for mud deposits may improve prediction of acoustic properties.

Potential Energy Field Function in Medical Image Processing Based on the Characterization of Overall and Partial Area

Potential Energy Field Function in Medical Image Processing Based on the Characterization of Overall and Partial Area

Collision cross section calculations for polyatomic ions considering rotating diatomic/linear gas molecules

Structural characterization of ions in the gas phase is facilitated by measurement of ion collision cross sections (CCS) using techniques such as ion mobility spectrometry. Further information is gained from CCS measurement when comparison is made between measurements and accurately predicted CCSs for model ion structures and the gas in which measurements are made. While diatomic gases, namely molecular nitrogen and air, are being used in CCS measurement with increasingly prevalency, the majority of studies in which measurements are compared to predictions use models in which gas molecules are spherical or non-rotating, which is not necessarily appropriate for diatomic gases. Here, we adapt a momentum transfer based CCS calculation approach to consider rotating, diatomic gas molecule collisions with polyatomic ions, and compare CCS predictions with a diatomic gas molecule to those made with a spherical gas molecular for model spherical ions, tetra-alkylammonium ions, and multiply charged polyethylene glycol ions. CCS calculations are performed using both specular-elastic and diffuse-inelastic collisions rules, which mimic negligible internal energy exchange and complete thermal accommodation, respectively, between gas molecule and ion. The influence of the long range ion-induced dipole potential on calculations is also examined with both gas molecule models. In large part we find that CCSs calculated with specular-elastic collision rules decrease, while they increase with diffuse-inelastic collision rules when using diatomic gas molecules. Results clearly show the structural model of both the ion and gas molecule, the potential energy field between ion and gas molecule, and finally the modeled degree of kinetic energy exchange between ion and gas molecule internal energy are coupled to one another in CCS calculations, and must be considered carefully to obtain results which agree with measurements.

High Resolution HIRLAM Simulations of the Role of Low-Level Sea-Breeze Convergence in Initiating Deep Moist Convection in the Eastern Iberian Peninsula

We present a numerical investigation of a non-forecast sea-breeze-initiated thunderstorm that occurred unexpectedly in the eastern complex area of the Iberian Peninsula (Spain) on 7 August 2008. A high horizontal (2.5-km grid spacing) and vertical (60 sigma levels) resolution set-up of the hydrostatic HIRLAM model is used to simulate the evolution of isolated convection associated with sea breezes. The convective inhibition, convective available potential energy, vertical velocity, wind and precipitation fields are examined here in order to analyze the role of low-level sea-breeze convergence and sea-breeze front development in initiating intense convective activity (heavy rainfall, hail and gusty winds) under weakly defined synoptic disturbances. We observe that convective inhibition layers are eroded by sea-breeze frontal zones, increasing the convective available potential energy due to the enhanced vertical motion, forcing parcels to lift to the level of free convection. We show that the location of thunderstorms along the east coast of the Iberian Peninsula is partly controlled by the impact of sea-breeze fronts on modulating the diurnal spatio-temporal evolution of convective inhibition, convective available potential energy and updrafts.

Electron spin and probability current density in quantum mechanics

This paper analyzes how the existence of electron spin changes the equation for the probability current density in the quantum-mechanical continuity equation. A spinful electron moving in a potential energy field experiences the spin-orbit interaction, and that additional term in the time-dependent Schrödinger equation places an additional spin-dependent term in the probability current density. Further, making an analogy with classical magnetostatics hints that there may be an additional magnetization current contribution. This contribution seems not to be derivable from a non-relativistic time-dependent Schrödinger equation, but there is a procedure described in the quantum mechanics textbook by Landau and Lifschitz to obtain it. We utilize and extend their procedure to obtain this magnetization term, which also gives a second derivation of the spin-orbit term. We conclude with an evaluation of these terms for the ground state of the hydrogen atom with spin-orbit interaction. The magnetization contribution is generally the larger one, except very near an atomic nucleus.

First use of synoptic vector magnetograms for global nonlinear, force-free coronal magnetic field models

The magnetic field permeating the solar atmosphere is generally thought to provide the energy for much of the activity seen in the solar corona, such as flares, coronal mass ejections (CMEs), etc. To overcome the unavailability of coronal magnetic field measurements, photospheric magnetic field vector data can be used to reconstruct the coronal field. Currently there are several modelling techniques being used to calculate three-dimension of the field lines into the solar atmosphere. For the first time, synoptic maps of photospheric vector magnetic field synthesized from Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) are used to model the coronal magnetic field and estimate free magnetic energy in the global scale. The free energy (i.e., the energy in excess of the potential field energy) is one of the main indicators used in space weather forecasts to predict the eruptivity of active regions. We solve the nonlinear force-free field equations using optimization principle in spherical geometry. The resulting three-dimensional magnetic fields are used to estimate the magnetic free energy content E_{free}=E_{nlfff}-E_{pot}, i.e., the difference of the magnetic energies between the nonpotential field and the potential field in the global solar corona. For comparison, we overlay the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the Atmospheric Imaging Assembly on board SDO. For a single Carrington rotation 2121, we find that the global NLFFF magnetic energy density is 10.3% higher than the potential one. Most of this free energy is located in active regions.

Sedimentation of a Charged Porous Particle in a Charged Cavity

The sedimentation of a charged porous sphere at the center of a charged spherical cavity filled with an electrolyte solution is analyzed. The thickness of the electric double layers around the particle and cavity wall is arbitrary, and their relaxation effect is considered. Through the use of a set of linearized electrokinetic equations and a perturbation method, the ionic electrochemical potential energy, electric potential, and velocity fields in the fluid are solved with the fixed space charge density of the particle and surface charge density of the cavity as the small perturbation parameters, and an explicit formula for the sedimentation velocity is obtained. Due to the electroosmotic enhancement on the fluid recirculation in the cavity caused by the sedimentation-induced electric field, the presence of the surface charges on the cavity wall increases the sedimentation velocity of the porous particle. For the sedimentation of a porous particle in a cavity with their fixed charges of the same sign, the effect of electric interaction between the particle and cavity wall in general increases the sedimentation velocity. For the case of their fixed charges with opposite signs, the sedimentation velocity is increased/reduced if the magnitude of the fixed charge density of the cavity wall is relatively large/small. The effect of the surface charges at the cavity wall on the sedimentation of the porous particle increases with an increase in the permeability for fluid flow within the particle and with a decrease in the particle-to-cavity radius ratio (i.e., an increase in the surface area of the cavity wall relative to a given size of the particle, which enhances the fluid recirculation effect).

Numerical analysis of feedforward position control for non-pulley musculoskeletal system: a case study of muscular arrangements of a two-link planar system with six muscles

In a musculoskeletal system like a tendon-driven robot, redundant actuation is necessary because muscles (or mechanical parts such as tendons) can transmit tension only unidirectionally. This redundancy yields internal force among muscles, which has a particular field of potential energy. Using internal force as a feedforward input, a musculoskeletal system can achieve feedforward position control with no sensory feedback. This paper studies the feedforward position control coming from the redundancy for a non-pulley musculoskeletal system. Targeting a planar two-link system with six muscles as a case study, the motion convergence depending on the muscular arrangement is examined quasi-statically. The results point out that the convergence is extremely sensitive to the muscular arrangement, and adding small offsets for the muscular connected points can remarkably improve the positioning performance.

An observable for vacancy characterization and diffusion in crystals.

To locate the position and characterize the dynamics of a vacancy in a crystal, we propose to represent it as the ground state density of a pseudo-quantum probe particle associated to the Hamiltonian which has, for the potential energy, the field generated by the atoms in the sample. In this description, the coefficient of the kinetic energy term is a tunable parameter controlling the density localization in the regions of the relevant minima of the potential energy field. Based on this description, we derive a set of collective variables that we use in rare event simulations to identify some of the vacancy diffusion paths in a 2D crystal. Our simulations reveal that the vacancy migrates according to local and non-local mechanisms, the second involving several lattice sites and producing a long range migration. We also observed a vacancy induced crystal reorientation process.

The Energy Cycle Associated to the Pacific Walker Circulation and Its Relationship to ENSO

In this paper we study the Lorenz energy cycle of the Walker circulation associated with ENSO. The robust formulation of the energetics allows drawing a clear picture of the global energy and conversion terms associated with the three dimensional domains appropriate to qualify the large scale transfers that influence, and are influenced by, the anomalies during ENSO. A clear picture has emerged in that El Nino and La Nina years have approximately opposite anomalous energy fluxes, regardless of a non-linear response identified in the potential energy fields (zonal and eddy). During El Ninos the tropical atmosphere is characterized by an increase of zonal available potential energy, decrease of eddy available potential energy and decrease of kinetic energy fields. This results in weaker upper level jets and a slowingdown of the overall Walker cell. During La Ninas reversed conditions are triggered, with an acceleration of the Walker cell as observed from the positive anomalous kinetic energy. The potential energy in the Walker circulation domain during the cold phase is also reduced. An equally opposite behavior is also experienced by the energy conversion terms according to the ENSO phase. The energetics-anomalous behavior seem to be triggered at about the same time when ENSO starts to manifest for both the positive and negative phases, suggesting a coupled mechanism in which atmospheric and oceanic anomalies interact and feed back onto each other.

Variance of the quantum dwell time for a nonrelativistic particle

Muñoz, Seidel, and Muga [Phys. Rev. A 79, 012108 (2009)10.1103/PhysRevA.79.012108], following an earlier proposal by Pollak and Miller [Phys. Rev. Lett. 53, 115 (1984)10.1103/PhysRevLett.53.115] in the context of a theory of a collinear chemical reaction, showed that suitable moments of a two-flux correlation function could be manipulated to yield expressions for the mean quantum dwell time and mean square quantum dwell time for a structureless particle scattering from a time-independent potential energy field between two parallel lines in a two-dimensional spacetime. The present work proposes a generalization to a charged, nonrelativistic particle scattering from a transient, spatially confined electromagnetic vector potential in four-dimensional spacetime. The geometry of the spacetime domain is that of the slab between a pair of parallel planes, in particular, those defined by constant values of the third (z) spatial coordinate. The mean Nth power, N = 1, 2, 3, …, of the quantum dwell time in the slab is given by an expression involving an N-flux-correlation function. All these means are shown to be nonnegative. The N = 1 formula reduces to an S-matrix result published previously [G. E. Hahne, J. Phys. A 36, 7149 (2003)10.1088/0305-4470/36/25/316]; an explicit formula for N = 2, and of the variance of the dwell time in terms of the S-matrix, is worked out. A formula representing an incommensurability principle between variances of the output-minus-input flux of a pair of dynamical variables (such as the particle's time flux and others) is derived.

Theoretical analysis of ion-enhanced thermionic emission for low-temperature, non-equilibrium gas discharges

When thermionic emission is used in a gas discharge, the ions exiting the discharge will influence the local electric field at the cathode. This effect is similar to the Schottky effect, as the potential field due to the ion can effectively reduce the potential barrier (work function) at the cathode and increase emission. In this work, this enhancement phenomenon is treated theoretically to understand how Schottky emission is enhanced due to the presence of an ion—so-called ion-enhanced Schottky emission. The effect of a stationary ion is determined through the analytical and numerical solution of the potential energy fields involved in the problem, including the applied field, electron image charge, and the ion potential field. The time-averaged enhancement due to an ion drifting towards the cathode is calculated, and an effective emission coefficient is predicted. These analyses show that a single stationary ion can significantly enhance thermionic emission, more than doubling the emission current depending on its location, but the enhancement is less significant (∼5–10%) when the motion of the ion is considered. However, the enhancement effect is more pronounced at moderate electric fields (∼105 V m−1) due to the residence time of the ion near the cathode. These results reveal the impact ions in the discharge may have on emission and how this needs to be considered when designing devices that integrate thermionic emission and discharges.