Framework Of Numerical Simulations Research Articles

Self-similarity of rogue wave generation in gyrotrons: Beyond the Peregrine breather

Within the framework of numerical simulations, we study the gyrotron dynamics under conditions of a significant excess of the operating current over the starting value, when the generation of electromagnetic pulses with anomalously large amplitudes (“rogue waves”) can be realized. The averaged shape of high-power pulses is shown to be very close to the celebrated Peregrine breather. At the same time, we demonstrate that the relation between peak power and duration of rogue waves is self-similar, but does not reproduce the one characteristic for Peregrine breathers. Remarkably, the discovered self-similar relation corresponds to the exponential nonlinearity of an equivalent Schrödinger-like evolution equation. This interpretation can be used as a theoretical basis for explaining the giant amplitudes of gyrotron rogue waves.

Comparative Study of Two Materials Combining a Standard Building Material with a PCM

Phase change materials (PCMs) have the ability to store thermal energy and make it available at a later stage to keep indoor temperature within a specific range and achieve better thermal comfort in buildings. This study focuses on the performances of materials obtained by combining a standard building material with a PCM. In particular, two different materials mixed with the same PCM are considered under the same climatic conditions. The related thermal behavior is assessed in the framework of numerical simulations conducted with ANSYS Fluent assuming parameters representative of a city located in Europe. The results show that the addition of PCM to concrete and bricks can improve the thermal inertia of the resulting material.

Extraction Of Temperature Fields From PIV Data Of Turbulent Rayleigh-Bénard Convection Using DNS

To gain insights into the dynamical processes that govern the problem of turbulent Rayleigh-Bénard convection, information about the temperature field---next to the velocity field---is required. In the present study, we exploit the governing flow equations to extract the temperature field from a velocity field measured via tomographic particle image velocimetry. The methods used within this study are derived from algorithms originally used in the framework of direct numerical simulations. First, we enforce the divergence-free condition on the measured velocity field using a fractional-step method based on a Poisson equation. Second, we extract the temperature field from the velocity field via the momentum equation. Third, we extract the temperature field from the velocity field via the solution of the stationary convection-diffusion equation. The extracted velocity field---together with the corresponding divergence-free velocity field---can serve as an initial condition for a direct numerical simulation of the problem. Validating the extraction method based on the stationary convection-diffusion equation with data from direct numerical simulations shows a good correlation coefficient between the extracted and the simulated temperature field (0.86). Although the third method incorporates the ambitious assumption of stationarity, temperature fields are successfully extracted from the measured velocity fields. However, with a correlation coefficient of 0.57, the agreement between the average extracted temperature field and the average temperature field measured in a subvolume of the flow domain is somewhat less than the one found in the simulated data. The latter is presumably attributed not only to the extraction method, but also to the underlying measurement data. Thus, an increase in measurement accuracy in future applications is likely to improve the accuracy of the extraction methods as well.

Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

Ocean circulation, upwelling phenomena and chlorophyll-a concentrations were investigated within the framework of numerical model simulations with 1/12° nested horizontal grid-size, in the tropical western Indian Ocean, along the coasts of Tanzania and Kenya. Ekman driven upwelling exhibited high levels of spatial and temporal variability in the region, characterized by a more vigorous occurrence/intensification during the Northeast than the Southwest Monsoon season. A similar trend was observed for chlorophyll-a distribution, but with an additional strong contribution during the inter-monsoon period from March to April. Trend analysis of a SST-derived coastal upwelling index (CUI) computed over the Pemba Channel and offshore of the East African Coastal Current (EACC), for 24 years (1990 - 2013), revealed a general linear relation of the form CUI(yr) = 2.4x10-7yr – 285, with a steady small annual increase of the upwelling phenomena by 0.0024/year ≃ 4% during the whole period of the simulation, which could be attributed to documented increasing trends of wind intensity and water volume transport in the region. The CUI exhibited the two most dominant peaks of variabilities on the range of annual and semi-annual timescales. The wind-stress southward component and the easting/westing veering of the northward EACC at 6°S revealed that these parameters were moderate and significantly correlated with the CUI (r = - 0.53 and 0.52, p<0.05) respectively, further suggesting its intensification during the Northeast Monsoon season.

A minimal model for household effects in epidemics

Shelter-in-place and other confinement strategies implemented in the current COVID-19 pandemic have created stratified patterns of contacts between people: close contacts within households and more distant contacts between the households. The epidemic transmission dynamics is significantly modified as a consequence. We introduce a minimal model that incorporates these household effects in the framework of mean-field theory and numerical simulations. We show that the reproduction number R 0 depends on the household size in a surprising way: linearly for relatively small households, and as a square root of size for larger households. We discuss the implications of the findings for the lockdown, test, tracing, and isolation policies.

Circulation of the European northwest shelf: a Lagrangian perspective

Abstract. The dynamics of the European northwest shelf (ENWS), the surrounding deep ocean, and the continental slope between them are analysed in a framework of numerical simulations using Lagrangian methods. Several sensitivity experiments are carried out in which (1) the tides are switched off, (2) the wind forcing is low-pass filtered, and (3) the wind forcing is switched off. To measure accumulation of neutrally buoyant particles, a quantity named the “normalised cumulative particle density (NCPD)” is introduced. Yearly averages of monthly results in the deep ocean show no permanent particle accumulation areas at the surface. On the shelf, elongated accumulation patterns persist in yearly averages, often occurring along the thermohaline fronts. In contrast, monthly accumulation patterns are highly variable in both regimes. Tides substantially affect the particle dynamics on the shelf and thus the positions of fronts. The contribution of wind variability to particle accumulation in specific regions is comparable to that of tides. The role of vertical velocities in the dynamics of Lagrangian particles is quantified for both the eddy-dominated deep ocean and for the shallow shelf. In the latter area, winds normal to coasts result in upwelling and downwelling, illustrating the importance of vertical dynamics in shelf seas. Clear patterns characterising the accumulation of Lagrangian particles are associated with the vertical circulations.

On the selection of perturbations for thermal boundary layer control

The convective instability of the natural convection boundary layers of air (Pr = 0.7) in the laminar-to-turbulent transition regime (Ra = 8.7 × 107–1.1 × 109) is investigated by stability analysis in the framework of direct numerical simulations. To understand the spatial and temporal evolution of the convective instability of the thermal boundary layers, small-amplitude random-mode numerical perturbations are first introduced into the boundary condition of the boundary layer flow. The prescribed full spectral perturbations (i.e., white noise) are mostly damped out immediately by a limited upstream boundary layer. A low-frequency band is initially distinct in the upstream near the leading edge but decays spatially as the instability propagates downstream. In contrast, a high-frequency band emerges to finally become the most dominant frequency band in the thermal boundary layer transition regime. To obtain further insights into the nature of the established high-frequency band, single-mode perturbations of various frequencies are then introduced into the boundary layer near the leading edge. It is found that a single-mode perturbation at the peak frequency within the high-frequency band excites the maximum response of the thermal boundary layer, suggesting that the peak frequency is in fact the characteristic frequency or resonance frequency of the thermal boundary layer. The dimensionless form of the dependence of the characteristic frequency on Ra is then found to be fc = 0.07Ra2/3. The single-mode perturbation numerical experiments also revealed the propagation speed of convective instability waves, which was significantly greater than the convection speed of the thermal boundary layer. The smaller the Ra, the larger the difference between the two propagation speeds. A semi-analytical scaling of the wave propagation speed in the form csc ∼ Ra1/2y1/2Pr was derived (y denoting the streamwise location of the boundary layer), providing a predictive correlation that can be used for thermal boundary layer control.

Nano-sized islands growth control in plasma-condensate devices

In this article, we provide a theoretical study of nanostructured thin films growth in a prototype model of plasma-condensate systems. By taking into account anisotropy in the transfer of adatoms between neighbor layers induced by electric field near substrate, we derive the generalized model, describing adsorbate evolution on an intermediate layer of the multi-layer system. In the framework of stability analysis and numerical simulations, we define ranges of the control parameters, responsible for pattern formation. We will discuss the influence of the adsorption coefficient, interaction strength and strength of the anisotropy in transference of adatoms between layers on the dynamics of the pattern formation and statistical properties of the surface structures. We will show that the derived 1-layer model allows one to reproduce the multi-layer pyramidal-like surface structures. This study provides insight into details of self-organization of adatoms into nano-sized adsorbate islands in plasma-condensate systems.

A 3D printed melt-compounded antibiotic loaded thermoplastic polyurethane heart valve ring design: an integrated framework of experimental material tests and numerical simulations

Heart diseases are one of the commonest causes of death worldwide. These include valve pathologies such as valve stenosis, regurgitation, failure and similar, for which usually a valve substitution procedure is required. Different prosthesis alternatives (both biological and artificial) are nowadays available in the heart surgery panorama, but there are still issues and aspects to improve. The principal requirements for heart valve prostheses are an efficient fluid dynamic function and long-term durability without the need for anticoagulation therapy, coupled with the possibility of patient-specific customization. Given this scenario, the presented tasks might be fulfilled by the recent advances of additive manufacturing technology (AM), which offers versatility of shapes and materials to be printed.In this work, the full flexibility of AM technique has been exploited to demonstrate the feasibility of a custom drug-loaded polymeric heart valve ring for crimpable prostheses. Two different medical-grade polymeric filaments for AM have been extruded: an aromatic Polyester-Based Thermoplastic Polyurethane (TPU-E) and a Thermoplastic Silicone-Polycarbonate-urethane compound filament (TSPCU). Both materials find different applications in medical fields, thanks to their mechanical and biocompatibility features.A drug-loading procedure has been set to obtain an antibiotic-filled polymer material and the relative biocompatibility has been consequently investigated. Specimens have been printed with a Fused Deposition Method and uniaxial traction tests have been performed at different printing temperatures and infill orientation angles. To evaluate the damaging risk given by the HVR crimping process, a Finite Element (FE) simulation of the crimping load has been set.The TSPCU has appeared to be the best material to realize the prosthesis in terms of tensile stress values. The ultimate mechanical characteristics of TSPCU have resulted to be higher if compared with TPU-E, regardless of raster orientation angle and temperature. The qualitative characterization of the drug loading process of TSPCU has been successful: standard disc diffusion method has revealed a well-defined inhibition zone on the bacterial culture. The effectiveness of the antibiotic has been maintained even after the extrusion and the printing process. The simulated crimping procedure on the HVR has revealed that the maximum Von Mises stress value is below the ultimate stress taken from experimental tests. The reported results demonstrate the feasibility of a crimpable antibiotic loaded HVR realized through TSPCU 3D printing.

Numerical Simulation of Temperature Fields in a Direct-Current Plasma Torch

Thermal characteristics of a direct-current plasma torch have been studied in the framework of numerical simulations of temperature fields in a plasma channel, cathode, and anode. It is established that the temperature of the operating cathode surface exceeds the melting temperature of the cathode material.

Численное исследование температурных полей в плазмотроне постоянного тока

AbstractThermal characteristics of a direct-current plasma torch have been studied in the framework of numerical simulations of temperature fields in a plasma channel, cathode, and anode. It is established that the temperature of the operating cathode surface exceeds the melting temperature of the cathode material.

Optimization-Based Inverse Identification of the Parameters of a Concrete Cap Material Model

Issues concerning the advanced numerical analysis of concrete building structures in sophisticated computing systems currently require the involvement of nonlinear mechanics tools. The efforts to design safer, more durable and mainly more economically efficient concrete structures are supported via the use of advanced nonlinear concrete material models and the geometrically nonlinear approach. The application of nonlinear mechanics tools undoubtedly presents another step towards the approximation of the real behaviour of concrete building structures within the framework of computer numerical simulations. However, the success rate of this application depends on having a perfect understanding of the behaviour of the concrete material models used and having a perfect understanding of the used material model parameters meaning. The effective application of nonlinear concrete material models within computer simulations often becomes very problematic because these material models very often contain parameters (material constants) whose values are difficult to obtain. However, getting of the correct values of material parameters is very important to ensure proper function of a concrete material model used. Today, one possibility, which permits successful solution of the mentioned problem, is the use of optimization algorithms for the purpose of the optimization-based inverse material parameter identification. Parameter identification goes hand in hand with experimental investigation while it trying to find parameter values of the used material model so that the resulting data obtained from the computer simulation will best approximate the experimental data. This paper is focused on the optimization-based inverse identification of the parameters of a concrete cap material model which is known under the name the Continuous Surface Cap Model. Within this paper, material parameters of the model are identified on the basis of interaction between nonlinear computer simulations, gradient based and nature inspired optimization algorithms and experimental data, the latter of which take the form of a load-extension curve obtained from the evaluation of uniaxial tensile test results. The aim of this research was to obtain material model parameters corresponding to the quasi-static tensile loading which may be further used for the research involving dynamic and high-speed tensile loading. Based on the obtained results it can be concluded that the set goal has been reached.

Modeling phase decomposition and patterning in binary alloy systems subjected to neutron irradiation

ABSTRACTWe study processes of phase decomposition and patterning in a model of a binary alloy system subjected to sustained irradiation. We exploit the reaction rate theory and generalize the Darken approach of vacancy diffusion to describe generation, recombination, annihilation and spatial interaction of point defects. It is shown that an increase in the defect production rate phase, decomposition processes are replaced by disordering and patterning with vacancy clusters' formation. At elevated damage rates, both phase separation and patterning are accompanied by pattern selection processes. In the framework of numerical simulations, dynamics of phase decomposition and vacancy clusters formation is studied in detail. A change in the morphology of vacancy clusters during irradiation and their statistical properties are discussed.

Charged hadrons in local finite-volume QED+QCD with C⋆ boundary conditions

In order to calculate QED corrections to hadronic physical quantities by means of lattice simulations, a coherent description of electrically-charged states in finite volume is needed. In the usual periodic setup, Gauss's law and large gauge transformations forbid the propagation of electrically-charged states. A possible solution to this problem, which does not violate the axioms of local quantum field theory, has been proposed by Wiese and Polley, and is based on the use of C* boundary conditions. We present a thorough analysis of the properties and symmetries of QED in isolation and QED coupled to QCD, with C* boundary conditions. In particular we learn that a certain class of electrically-charged states can be constructed in this setup in a fully consistent fashion, without relying on gauge fixing. We argue that this class of states covers most of the interesting phenomenological applications in the framework of numerical simulations. We also calculate finite-volume corrections to the mass of stable charged particles and show that these are much smaller than in non-local formulations of QED.

Intrinsic Performance Variability in Aligned Array CNFETs

The I-V characteristic of carbon nanotube (CNT) transistors is dictated by the Schottky barrier (SB) at metal-nanotube interface. The SB is defined by the diameter of the CNT along with the source/drain metal and is presumed a device constant for single CNT transistors. Given the wide distribution of diameter of CVD-grown nanotubes, the presumption of single SB, however, is inappropriate for transistors with aligned array of CNTs. Indeed, array transistors with similar geometries and contact material can still exhibit considerable variation in threshold voltage (VT), on current (ION), and device resistance (Rd). We use measured diameter distribution within the framework of detailed numerical simulations to demonstrate that the diameter distribution of CNTs (in CNT FETs) plays a dominant role in defining the fluctuation of array transistors. Besides, it is argued that the large-scale integration of these devices within an IC would be feasible only if the distribution of diameter is considerably narrowed.

Efficient treatment of rubber friction problems in industrial applications

Friction problems involving rubber components are frequently encountered in industrial applications. Their treatment within the framework of numerical simulations by means of the Finite Element Method (FEM) is the main issue of this paper. Special emphasis is placed on the choice of a suitable material model and the formulation of a contact model specially designed for the particular characteristics of rubber friction. A coupled thermomechanical approach allows for consideration of the influence of temperature on the frictional behavior. The developed tools are implemented in the commercial FE code ABAQUS. They are validated taking the sliding motion of a rubber tread block as example. Such simulations are frequently encountered in tire design and development. The simulations are carried out with different formulations for the material and the frictional behavior. Comparison of the obtained results with experimental observations enables to judge the suitability of the applied formulations on a structural scale.

A parametric study on the impact of satellite attitude errors on GOCE gravity field recovery

In the recent design of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission, the gravity gradients are defined in the gradiometer reference frame (GRF), which deviates from the actual flight direction (local orbit reference frame, LORF) by up to 3–4°. The main objective of this paper is to investigate the effect of uncertainties in the knowledge of the gradiometer orientation due to attitude reconstitution errors on the gravity field solution. In the framework of several numerical simulations, which are based on a realistic mission configuration, different scenarios are investigated, to provide the accuracy requirements of the orientation information. It turns out that orientation errors have to be seriously considered, because they may represent a significant error component of the gravity field solution. While in a realistic mission scenario (colored gradiometer noise) the gravity field solutions are quite insensitive to small orientation biases, random noise applied to the attitude information can have a considerable impact on the accuracy of the resolved gravity field models.

Basic considerations on the impact of the coil array on the performance of Transmit SENSE

"Transmit SENSE" adapts the idea of parallel imaging to RF transmission. Using multiple independent transmit coils, the duration of a spatially selective RF pulse can be reduced. It is known from parallel imaging that a suboptimal coil-array geometry might lead to an ill-conditioned sensitivity matrix and, thus, to a non-homogenous noise amplification in the resulting image. The current paper investigates the consequences of suboptimal coil arrays for Transmit SENSE. Two possible consequences of a suboptimal coil array are studied in the framework of numerical simulations: the incorrect excitation of the desired spatial pattern and the increase of the specific energy absorption rate (SAR), i.e. the RF power required to excite the desired pattern. Incorrect pattern excitation occurs only in pathologic coil-array scenarios. The increase of the SAR is very moderate for a large range of coil-array geometries. Using spiral excitation k-space trajectories leads to superior results compared to Cartesian trajectories. The problem of an ill-conditioned matrix inversion does not seem to play a major role in Transmit SENSE. Consequently, the freedom in designing coil arrays seems to be much larger in Transmit SENSE than in SENSE in the receive mode.

Higher modes of the mixed buoyant-Marangoni unstable convection originated from a droplet dissolving in a liquid/liquid system with miscibility gap

Some phenomena, never observed before, concerning a system composed by two organic-liquid bicomponent phases with a miscibility gap, used as transparent surrogates for immiscible metal alloys, are discussed and elucidated in the framework of experimental analyses and numerical simulations. It is shown that a single dissolving droplet at the bottom of a test cell behaves as an intriguing pattern-forming dynamical system leading to a wealth of different spatiotemporal modes of convection when the imposed temperature gradient is increased. The last part of the analysis is devoted to comparison with other similar phenomena (the flow instability pertaining to the Marangoni convection around bubbles surrounded by a liquid heated from above, and the case of rising buoyant jets), showing analogies and differences. Such a comparison is also used as a means to focus on the intrinsic nature of the present instability.

Current, induction profiles and hysteretic losses in high-Tc superconducting tapes

Current, induction profiles and hysteretic losses have been calculated in the framework of numerical simulations for high Tc superconducting tapes in transverse magnetic field (self-field effects, anisotropic field dependence of the critical current density are taken into account). It appears that hysteretic losses are widely influenced by both the tape width and the field component perpendicular to the tape width, except for field nearly parallel to the tape.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>