Actual Physical Process Research Articles

Phase-Field Micro-Solidification Simulation for Dendrite Growth in Ni-Cu Binary Alloy

By optimizing the relevant dendrite growth parameters of Ni-Cu alloy undercooling melt, it has studied the effect that the dendrite evolution process of undercooled melt and the degree of undercooling melt have on the dendrite growth of undercooling melt. In the isothermal and non-isothermal solidification condition, relatively accurate result is obtained by applying the phase field method to simulate Ni-Cu alloy. Simulation results show non-isothermal simulation with Neuman boundary condition suit to the actual physical process better.

Simulation of cuttings transport with foam in deviated wellbores using computational fluid dynamics

Foam is non-Newtonian pseudo-plastic fluid, which is used for drilling, well intervention, and stimulation. Predicting the cutting transport efficiency of foam in the wellbore annulus is very important to optimize the drilling process. In this paper, the cuttings transport process with foam is numerically simulated using an Eulerian two-phase model in inclined wellbores. A computational fluid dynamics (CFD) software package called FLUENT was used for this goal. The effect of foam quality, foam velocity, drill pipe rotation, and wellbore inclination on cuttings transport phenomena in both concentric and eccentric annulus was investigated. The simulation results are compared to the experimental data from previous studies, with a relative error less than 8 %. This study shows the reliability of the CFD simulation in replicating the actual physical process.

Viscoplastic Regularization of Local Damage Models: A Latent Solution

Local damage models are known to produce pathological mesh dependence in finite element simulations. The solution is to either use a regularization technique or to adopt a non-local damage model. Viscoplasticity is one technique which can regularize the mesh dependence of local damage model by incorporating a physical phenomenon in the constitutive model i.e. rate effects. A detailed numerical study of viscoplastic regularization is carried out in this work. Two case studies were considered i.e. a bar with shear loading and a sheet metal under tensile loading. The influence of hardening / softening parameters, prescribed deformation rate and mesh size on the regularization was studied. It was found that the primary viscoplastic length scale is a function of hardening and softening parameters but does not depend upon the deformation rate. Mesh dependency appeared at higher damage values. This mesh dependence can be reduced by mesh refinement in the localized region and also by increasing the deformation rates. The viscoplastic regularization was successfully used with a local anisotropic damage model to predict failure in a cross die drawing process with the actual physical process parameters.

Analytical Model of Raman Enhancement by Metal Nanoparticles

AbstractWe present an analytical model for the enhancement of molecular Raman process by metal nanoparticles. The result is compared with that of a PL process. Although both processes are similar in the sense that they are both a two-photon process in which one photon is absorbed and another photon at a different frequency is emitted, the SP enhancement mechanisms for these two processes have some rather distinct features. In addition to stronger enhancement, the Raman process shows no sign of quenching ever taking place even when the molecule is placed right at the surface of the metal nanoparticle – a situation that will lead to strong quenching effect in PL measurement. Significant advantages of our analytical approach include not just predications that are consistent with experimental observations, but rather a clear insight for the actual physical process at work.

Artefacts in CBCT: a review

Artefacts are common in today's cone beam CT (CBCT). They are induced by discrepancies between the mathematical modelling and the actual physical imaging process. Since artefacts may interfere with the diagnostic process performed on CBCT data sets, every user should be aware of their presence. This article aims to discuss the most prominent artefacts identified in the scientific literature and review the existing knowledge on these artefacts. We also briefly review the basic three-dimensional (3D) reconstruction concept applied by today's CBCT scanners, as all artefacts are more or less directly related to it.

An analysis of the thermodynamic conditions for solid powder particles spreading over liquid surface

The spreading of solid powder over a liquid surface is a prevalent phenomenon encountered in many industrial processes such as food and pharmaceutical processes. The driving force for powder spreading over a liquid surface is not clearly understood. The Marangoni effect due to a temperature gradient and the spreading coefficient for solid powder over liquid ( λ S/L) have both been proposed as causes for powder spreading over liquids. The proposed λ S/L was based on the same form of the spreading coefficient for a liquid over a solid surface ( λ L/S). Whereas λ L/S has a clear thermodynamic definition, the spreading coefficient of solid powder over liquid, λ S/L, which was defined by simply interchanging the subscripts of the interfacial energy terms, has not been thoroughly analysed. Our experimental results showed that the spreading behaviour of solid powders over liquids cannot be explained or predicted by λ S/L. In this study we focus on problems associated with the λ S/L. Through a thermodynamic analysis we conclude that the existing parameter λ S/L is unable to predict the spreading behaviour of solid powder on liquid surface, since the interfacial energy approach does not capture the actual physical process of powder spreading over liquid surface. A closer examination of the powder spreading process reveals the fundamental different between liquid spreading over solid surface and solid powder spreading over liquid. This work shows that further research is required to identify and analyse the physical mechanisms which are responsible for powder particles spreading over liquid surfaces.

Investigation on strain dependence of dynamic recrystallization behavior using an inverse analysis method

Since it is difficult to accurately describe the nucleation behavior through analytical calculation or experimental observation, an inverse analysis method is used to evaluate the reasonability of two different nucleation models, i.e., a conventional constant nucleation model where the nucleation rate is assumed to be independent of strain and a dislocation-related nucleation model where it is strain dependent. In order to accurately estimate the parameters in the above two models, a flow stress-based inverse analysis method is developed. This method is implemented by coupling a cellular automata (CA) model with an adaptive response surface method (ARSM). Then the reasonability of each nucleation model is evaluated by comparing the level of agreement between the simulations and the experiments. Taking a low carbon steel deformed at 1173 K and 0.1 s −1 as an example, it is shown that the flow stress-based inverse analysis method is able to estimate the nucleation parameters accurately and the introduction of the dislocation-related nucleation model makes the simulation even closer to the actual physical process.

On cone‐beam computed tomography artifacts induced by titanium implants

To briefly review the mathematical background of beam-hardening artifacts in cone-beam computed tomography (CBCT)-reconstruction and to investigate geometrical properties relevant for these reconstruction errors. By means of simulated and experimental results, beam-hardening effects caused by titanium implants are evaluated. The geometrical and physical properties of the acquisition process of the projections used for 3D reconstruction are investigated and their effects on the CBCT images in the presence of titanium implants are derived. Beam-hardening effects are computed for a simplified polychromatic situation (three energy subsets of 80 and 110 kV) and compared with experimental results from a hard-plaster phantom containing two 'implants' (pure titanium rods; 4 mm diameter) exposed in two CBCT machines. Massive absorption within a typical implant body (diameter: 4 mm) was computed for the low-energy subset of both energies (80 kV: 99.7%; 110 kV: 90.9%), whereas the high-energy subsets are only marginally absorbed (80 kV: 14.8%; 110 kV: 11.3%). Accordingly, phantom data revealed drastically reduced gray values in artifact-affected regions (3DAccuitomo: -46% to -51%) or (3DExam: -55%) plus increased noise (+67% vs. +73%), when compared with unaffected regions. Our theoretical and experimental results prove massive beam-hardening artifacts for a typical implant diameter and typical energies of up-to-date CBCT machines. Meaningful artifact reduction has to be based on more sophisticated mathematical modeling of the actual physical image acquisition process rather than on postprocessing of the erroneous results obtained from the rather crude reconstruction algorithms used presently.

Alternative interpretations of horizontal to vertical spectral ratios of ambient vibrations: new insights from theoretical modeling

Different positions exist about the physical interpretation of horizontal to vertical spectral ratios (HVSR) deduced from ambient vibrations. Two of them are considered here: one is based on the hypothesis that HVSR are mainly conditioned by body waves approaching vertically the free surface, the other one assumes that they are determined by surface waves (Rayleigh and Love, with relevant upper modes) only. These interpretations can be seen as useful approximations of the actual physical process, whose reliability should be checked case-by-case. To this purpose, a general model has been here developed where ambient vibrations are assumed to be the complete wave field generated by a random distribution of independent harmonic point sources acting at the surface of a flat stratified visco-elastic Earth. Performances of the approximate interpretations and complete wave field models have been evaluated by considering a simple theoretical subsoil configuration and an experimental setting where measured HVSR values were available. These analyses indicate that, at least as concerns the subsoil configurations here considered, the surface-waves approximation seems to produce reliable results for frequencies larger than the fundamental resonance frequency of the sedimentary layer. On the other hand, the body waves interpretation provides better results around the resonance frequency. It has been also demonstrated that the HVSR curve is sensitive to the presence of a source-free area around the receiver and that most energetic contribution of the body waves component comes from such local sources. This dependence from the sources distribution implies that, due to possible variations in human activities in the area where ambient vibrations are carried on, significant variations are expected to affect the experimental HVSR curve. Such variations, anyway, only weakly affect the location of HVSR maximum that confirms to be a robust indicator (in the range of 10%) of the local fundamental resonance frequency.

Propagation of Photons in Spacetime

A quantum field theory QED formalism is systematically developed to describe photon propagation in spacetime as a time evolution process based on the actual physical process of propagation between emitters and detectors as applied to the reflection of pho- tons. This development, as well as early studies by Feynman, clearly show that a practical, computational and predictive dynamical formalism in spacetime was lacking. The present one generalizes to different experimental situations and other interacting field theories as well emphasizing the practicality of the problem treated here.

Health Monitoring of Composite Plates using Acoustic Wave Propagation, Continuous Sensors and Wavelet Analysis

Health monitoring of aerospace structures can be done passively by listening for acoustic waves generated by cracks, impact damage and delaminations, or actively by propagating diagnostic stress waves and interpreting the parameters that characterize the wave travel. This paper investigates modeling of flexural wave propagation in a plate and the design of sensors to detect damage in plates based on stress wave parameters. To increase understanding of the actual physical process of wave propagation, a simple model is developed to simulate wave propagation in a plate with boundaries. The waves can be simulated by applied forces and moments in the model either to represent passive damage growth or active wave generation using piezoceramic actuators. For active wave generation, the model considers a piezoceramic patch bonded perfectly to a quasi-isotropic glass-epoxy composite plate. Distributed sensors are used on the plate and are modeled as being constructed using active fiber composite and piezoceramic materials. For active wave generation, a moment impulse is generated by the actuation of a piezoceramic patch. The waves generated from the patch are detected by the distributed sensor. For passive sensing of acoustic waves, a step function is used to simulate an acoustic emission from a propagating damage. The resulting acoustic wave is measured by the distributed sensor and produces micro-strains in the sensor nodes. The strains produce a single voltage signal output from the distributed sensor. Computational simulations and animations of acoustic wave propagation in a plate are discussed in the article. A new method to locate the source of an acoustic emission using the time history of the dominant lower frequency components of the flexural wave mode detected by continuous sensors is also presented.

Erratum: Spontaneous excitation of an accelerated atom in a spacetime with a reflecting plane boundary [Phys. Rev. DPRVDAQ0556-282172, 064022 (2005)10.1103/PhysRevD.72.064022

We study a two-level atom in interaction with a real massless scalar quantum field in a spacetime with a reflecting boundary. The presence of the boundary modifies the quantum fluctuations of the scalar field, which in turn modifies the radiative properties of atoms. We calculate the rate of change of the mean atomic energy of the atom for both inertial motion and uniform acceleration. It is found that the modifications induced by the presence of a boundary make the spontaneous radiation rate of an excited inertial atom to oscillate near the boundary and this oscillatory behavior may offer a possible opportunity for experimental tests for geometrical (boundary) effects in flat spacetime. While for accelerated atoms, the transitions from ground states to excited states are found to be possible even in vacuum due to changes in the vacuum fluctuations induced by both the presence of the boundary and the acceleration of atoms, and this can be regarded as an actual physical process underlying the Unruh effect.

Spontaneous excitation of an accelerated atom in a spacetime with a reflecting plane boundary

We study a two-level atom in interaction with a real massless scalar quantum field in a spacetime with a reflecting boundary. The presence of the boundary modifies the quantum fluctuations of the scalar field, which in turn modifies the radiative properties of atoms. We calculate the rate of change of the mean atomic energy of the atom for both inertial motion and uniform acceleration. It is found that the modifications induced by the presence of a boundary make the spontaneous radiation rate of an excited inertial atom oscillate near the boundary and this oscillatory behavior may offer a possible opportunity for experimental tests for geometrical (boundary) effects in flat spacetime. While for accelerated atoms, the transitions from ground states to excited states are found to be possible even in a vacuum due to changes in the vacuum fluctuations induced by both the presence of the boundary and the acceleration of atoms, and this can be regarded as an actual physical process underlying the Unruh effect.

Modeling mass transport of propylene polymerization on Ziegler–Natta catalyst

In the present article, a comprehensive mathematical model for single particle propylene polymerization mainly extended from polymeric multigrain model (PMGM) and multigrain model (MGM) has been developed to describe kinetic behavior, molecular weight distribution, monomer concentration, degree of polymerization and polydispersity index (PDI) for slurry-phase propylene polymerization using heterogeneous Ziegler–Natta catalysts. The modified model gives a more valid mathematical description by accounting for the monomer diffusion phenomena at two levels, namely, taking the effect of monomer diffusion at both the macro- and microparticle levels into account, and the latter is aside from the subject under consideration by PMGM. It has been observed that the present model can predict higher values of polydispersity index (PDI about 6–25) with obtaining some results which are more applicable to the conditions existing in most polymerizations of industrial interest such as the reasonable monomer concentration at the center of particles throughout polymerization process and the effect come nearer to the actual physical process of the initial radius of macro- and microparticles as well when using single-site, non-deactivating catalyst. Further, special attention is also paid in this article to discuss the computational rate, which is the most disadvantage of MGM. It has been shown that the significant computational time saving is also acquired by employing the novel solution methodology.

A molecule wired: Electrostatic investigation

We present calculations of ground state and excitation energies for ethene between metal electrodes. The model determines the effects of the electrodes by incorporating a heterogeneous and structured solvation model with the quantum mechanical description of the molecular system. The effects of the electrodes are of electrostatic origin and an external bias can be applied enabling a close connection to the actual physical process related to the measurements of charge transport in molecules located between electrodes.

A STUDY ON THE RANDOMNESS OF THE DIGITS OF π

We apply a newly-developed computational method, Geometric Random Inner Products (GRIP), to quantify the randomness of number sequences obtained from the decimal digits of π. Several members from the GRIP family of tests are used, and the results from π are compared to those calculated from other random number generators. These include a recent hardware generator based on an actual physical process, turbulent electroconvection. We find that the decimal digits of π are in fact good candidates for random number generators and can be used for practical scientific and engineering computations.

Simulation of Asymmetric Lamb Waves for Sensing and Actuation in Plates

Two approaches used for monitoring the health of thin aerospace structures are active interrogation and passive monitoring. The active interrogation approach generates and receives diagnostic Lamb waves to detect damage, while the passive monitoring technique listens for acoustic waves caused by damage growth. For the application of both methods, it is necessary to understand how Lamb waves propagate through a structure. In this paper, a Physics‐Based Model (PBM) using classical plate theory is developed to provide a basic understanding of the actual physical process of asymmetric Lamb mode wave generation and propagation in a plate. The closed‐form model uses modal superposition to simulate waves generated by piezoceramic patches and by simulated acoustic emissions. The generation, propagation, reflection, interference, and the sensing of the waves are represented in the model, but damage is not explicitly modeled. The developed model is expected to be a useful tool for the Structural Health Monitoring (SHM) community, particularly for studying high frequency acoustic wave generation and propagation in lieu of Finite Element models and other numerical models that require significant computational resources. The PBM is capable of simulating many possible scenarios including a variety of test cases, whereas experimental measurements of all of the cases can be costly and time consuming. The model also incorporates the sensor measurement effect, which is an important aspect in damage detection. Continuous and array sensors are modeled, which are efficient for measuring waves because of their distributed nature.

Some improvements in visco-plastic model considering dynamic recrystallization

Some improvements in Jin's thermal visco-plastic constitutive model considering dynamic recrystallization is presented in this paper. By introducing the influence of the strain rate on the mobility of dynamic recovery, the improved model can be more smoothly applied to numerical simulation of material flow behaviour and microstructure prediction during hot working. Another improvement is to consider the accumulated dislocation energy in the newly recrystallized grains as a resistance to the driving force of dynamic recrystallization volume. This improvement makes the predicted results of dynamic recrystallization progress agree better with the actual physical process. Finally, some numerical examples are given to show the advantages of the improved model and the ability to predict the dynamic recrystallization.

Systematic studies of pulse propagation in ducted oceanic waveguides in normal mode representation

Acoustic pulse propagations in typical environments of the Gulf of Mexico and Yellow Sea are modeled by a new normal mode code, SWAMP, and compared to the experimental data. The presence of the seasonal surface duct in the ocean is confirmed to be responsible for the generation of a unique set of modes forming the so-called “Precursor Phenomenon” first discovered by DeFerrari and Monjo. It is shown that the conventional theory of group velocity fails to predict precursor's arrival time. Two different approaches, the mean rate of energy transport and Airy-wave analysis, are proposed for the description of the actual physical process of energy transport by precursors. The influence of the range-dependent bathymetry on a precursor structure is addressed. The presence of precursors appears to be sensitive to range dependence and may be diminished due to rapidly changing bathymetry. Based on numerical results, inversion procedures of extracting different environmental and source parameters from a received pulse structure are discussed.

Coronal Mass Ejection (CME)-induced shock formation, propagation and some temporally and spatially developing shock parameters relevant to particle energization

Interplanetary shocks accelerate solar energetic particles (SEPs) from the point of shock formation in the lower corona, and continuously as the shock propagates outward to 1 AU and beyond. In this study, the formation properties of a CME-induced shock and propagation characteristics are studied from the inner corona to 1 AU. We use a 2D, three-component (i.e., 2.5D), time-dependent MHD code in our model. A well-studied CME event (the 1997 January 6−12 Sun-Earth Connection Event) is used as a baseline for this study. The solar wind conditions measured at 1 AU (WIND data) are used to motivate our effort to model the CME driven shock. It is found that the fast forward shock forms originally at ∼3.2 Rs (solar radii) from the solar surface in the ecliptic plane for the assumed CME and background solar wind parameters. In our model, this occurs ∼2 hrs after CME initiation. The shock formation at higher (∼30 ◦ ) latitudes measured from the ecliptic plane is further from the Sun (∼3.6 Rs) because of higher local magnetosonic speeds that must be exceeded by the original disturbance for shock formation. Finally, the shock becomes symmetric at 16 Rs. In the ecliptic plane at 16 Rs the fast shock Mach number (Mf) is ∼3.5, and at 30 ◦ latitude, Mf ∼ 1.7, considerably weaker. A maximum in the fast shock Mach number of 4 is reached at 130 Rs in the ecliptic plane. The Mf decreases to 3.5 by 1 AU. Other properties of the shock, as well as its relationship to the local interplanetary properties through which it passes, are discussed. The interplanetary counterpart, ICME, of the coronal CME, is also discussed. These shock properties, we believe, are relevant to the shock's ability to accelerate particles to energies as high as 100 MeV. The actual physical process, however, is not discussed in this paper.