Full 3D Numerical Simulations Research Articles

Engineering of the Fano resonance spectral response with non-Hermitian metasurfaces by navigating between exceptional point and bound states in the continuum conditions.

We address the engineering of Fano resonances and metasurfaces, by placing it in the general context of open non-Hermitian systems composed of coupled antenna-type resonators. We show that eigenfrequency solutions obtained for a particular case of scattering matrix are general and valid for arbitrary antenna radiative rates, thanks to an appropriate transformation of parametric space by simple linear expansion and rotation. We provide evidence that Parity-Time symmetry phase transition path and bound states in continuum (BIC) path represent the natural axis of universal scattering matrix solutions in this parametric coupling-detuning plane and determine the main characteristics of Fano resonance. Specifically, we demonstrate the control of asymmetry and sharpness of Fano resonance through navigation between BIC and PT-symmetric phase transition exceptional point. In particular, we demonstrate a fully symmetric Fano resonance in a system of two coupled bright and dark mode resonators. This result goes beyond current wisdom on this topic and demonstrates the universality of scattering matrix eigenfrequency solutions highlighted in our study. The validity of our approach is corroborated through comparison with experimental and full 3D numerical simulations results published in the literature making it thus possible to grasp a large body of experimental work carried out in this field. The detrimental impact of absorption losses on the contrast of the Fano resonance, which must be two orders of magnitude lower than the radiative losses, is also evidenced.

A novel multi‐slice electromagnetic field‐circuit coupling method for transient computation of long‐distance gas‐insulated transmission lines

AbstractAccurate calculation of short‐circuit electromagnetic force is crucial for both mechanical strength check and the optimal design of gas‐insulated transmission lines (GIL). Since the full 3D numerical simulation method is highly time‐consuming, a novel lightweight 2D multi‐slice electromagnetic field‐circuit coupled method for computing transient electromagnetic force is proposed, where appropriate port voltage degrees of freedom (DoFs) are introduced for the solid GIL conductor terminals. When the transient magnetic field equations are combined with the constraint equations of circuit part, including nodal voltage and loop current DoFs, a direct field‐circuit coupling scheme is thus derived. The proposed method can simultaneously consider the effect of interphase‐shunts and ground wires, as well as the skin effect and proximity effect. It can accurately capture the transient electromagnetic characteristics of GIL spanning from several to tens of kilometers under different short‐circuit conditions. The transient electromagnetic forces, as well as the induced voltages and currents of the enclosure, are analysed by the proposed method for both single‐phase and three‐phase enclosed GIL under various short‐circuit conditions. The proposed method has the advantages of high accuracy and lightweight computational cost, and thus it is also suitable for conducting important simulation tasks such as mechanical strength checks during the design optimisation phase of long‐distance GIL.

Coupled waveguide model for computing phase and transmission through nanopillar-based metasurfaces.

Dielectric metasurfaces are important in modern photonics due to their unique beam shaping capabilities. However, the standard tools for the computation of the phase and transmission through a nanopillar-based metasurface are either simple, approximating the properties of the surface by that of a single cylinder, or use full 3D numerical simulations. Here we introduce a new analytical model for computing metasurface properties which explicitly takes into account the effect of the lattice geometry. As an example we investigate silicon nanopillar-based metasurfaces, examining how the transmission properties depend on the presence of different modes in the unit cell of the metasurface array. We find that the new model outperforms the isolated cylinder model in predicting the phase, and gives excellent agreement with full numerical simulations when the fill fraction is moderate. Our model offers a waveguide perspective for comprehending metasurface properties, linking it to fiber optics and serving as a practical tool for future metasurface design.

Finite element modeling of partially-confined concrete and RC columns with embedded hexagonal-FRP strips under axial and horizontal loading

Recently, GFRP-strips partial confinement has demonstrated significant revelations as an effective and economical method of structural rehabilitation and strengthening. In this connection, this paper firstly presents a finite element modeling (FEM) on the behavior of a novel confinement configuration of concrete cylinders with embedded hexagonally designed GFRP strips under axial loading using ABAQUS. The simulation outcomes in terms of stress–strain response and damages were experimentally and analytically validated. The second objective of this paper is to develop a FEM to study a new reinforcement method of large scale partially-confined RC columns subjected to a monotonic horizontal loading. For this purpose, full 3D numerical simulations were carried out taking into account the nonlinear behavior of all used materials. The accuracy of the numerical results was emphasized through a comparison with ones of previous experimental tests. In addition, the effects of various parameters on the failure mode and the ultimate lateral carrying load capacity of the columns were studied on twenty-four numerical models. As main conclusion, the predicted results indicated that the proposed strengthening design with a greater encased-GFRP strip thickness combined with smaller spacing significantly improved the performances of RC columns.

Particle Deposition Pattern on an Automotive Diesel Filter Using an Eulerian Probability Density Function Method

A full 3D numerical simulation of the two-phase flow made up of (bio)diesel and particles, has been carried out to reproduce the deposition pattern of particles in a BOSCH automotive filter. From a probability density function (PDF), a simple Eulerian-Eulerian two-phase model is proposed for diesel and particles. The proposed formulation allows for a detailed description of the relationship between the velocity and size of the particles. A Brinkman-Darcy approximation has been considered for the flow through the filtering paper and is proved to be sufficient for the typical filter working conditions. The new tool is able to reproduce the deposition pattern shown by the used filters.

Pseudo 3D topology optimization of microchannel heat sink with an auxiliary objective

This paper investigated the application of pseudo 3D (P3D) variable-density topology optimization to the design of microchannel heat sink (MHS). To mitigate the grey-element issue and improve the precision of P3D model, outside surface viscous dissipation, adaptive equivalent heat transfer coefficient and auxiliary objective was introduced. The effects of adaptive equivalent heat transfer coefficient, auxiliary objective and its corresponding weight factor on the optimal design were analyzed. The results showed that the new proposed topological model can effectively mitigate intermediate density, improved accuracy and produced a well-defined structure. Furthermore, to validate the design of topology optimization, full 3D numerical simulations were adopted to investigate the fluid flow and heat transfer performance of the topological model and the results were compared with the conventional straight MHS. It is found that the increase of average inlet velocity and temperature uniformity of the topological MHS can be up to 305% and 69%, respectively, under the same inlet pressure. While under the same pumping power consumption, the average heat source temperature of the topological microchannel heat sink can be 9.3°C lower than the conventional one.

Analysis of the symmetry of 3D silicon melts flow generated by a bitter traveling magnetic field in a cylindrical crucible

Full 3D numerical simulation and experimental characterization of the topology and sensitivity of a magnetohydrodynamic flow pattern driven by a travelling magnetic field generated by a specifically designed cylindrical Bitter coil have been carried out. This Bitter coil has six windings composed of multiple circular copper plates and insulator separators disposed in a specific way for an optimum round circulation of electric currents, which are three-phase currents at a network frequency. For experimental characterization, experimental measurement data of radial components of the induced magnetic field (Br) at different z-positions were used without any liquid metal for different applied electric currents, and ultrasound Doppler velocimetry was employed to acquire the vertical velocity component along the beam axis in the centre and at different azimuthal positions for a fixed radial coordinate in the liquid GaInSn alloy. On the other hand, for numerical characterization, a numerical model has been developed using the Comsol MultiphysicsTM software to resolve the set of magnetohydrodynamic equations, fundamentally based on low frequency and low magnetic induction, and on an averaged expression of the Lorentz force. The results clearly show that the choice of 3D modelling and the Bitter coil design, which generates instabilities, can change significantly the usually expected axisymmetric torus-type magnetohydrodynamic flow topology and result in a distinct 3D flow configuration. Figs 8, Refs 16.

Optimization Design and Numerical Analysis of Flow Passage Converters in LWD Tools

Improper design of flow passage converter section in LWD ( logging while drilling) tools can cause local flow-field turbulence and result in serious local erosion of the tool, thus reducing its service life. It can also lead to large pressure loss of the tool and affect its applicability. For this reason, CFD (computational fluid dynamics) method was applied for the optimization design of a certain type of flow passage converter in an LWD tool. According to the full 3D numerical simulation and comparison of four design schemes, the main factors affecting the flow-field performance of the flow passage converter were thought to be the expansion angle and the continuity of the cross-sectional area of the internal flow passage. The optimal design has a smaller expansion angle, a more continuous cross-sectional area of the internal flow passage, a gentler decline in axial velocity, a minimum total pressure loss, and a more uniform flow velocity distribution in the flow field. The research results showed that the expansion angle and the discontinuity of the cross-sectional area of the internal flow passage were negatively associated with the uniformity of the flow field distribution in the flow passage converter and positively with the pressure loss. The difference between theoretical and experimental total pressure loss coefficients was not more than 0.076%, and the change trend was the same as the result of theoretical analysis. The research results can effectively serve as a theoretical basis for the optimization design of flow passage converters.

A Fast Analytical Method for the Dynamic Energy Simulation of Energy Piles With Short Time Resolution

Abstract This paper proposes an analytical method for the dynamic thermal simulation of energy piles with a short time resolution (e.g., tens of minutes) as an alternative to numerical approaches, which require relevant computational resources. The discussion is tailored to the implementation of analytical models in dynamic energy simulation software for buildings and HVAC systems. The main modeling challenges consist of accounting for the pile thermal capacity, configuration of pipes, and time-varying inlet temperature and flow rate values. The heat transfer process occurs in three characteristic periods, each of them characterized by a 2D or 3D geometry. The first period concerns the evolution of the fluid temperature and heat transfer over the length of the pipes, the second period concerns the thermal diffusion within the heat capacity of the foundation, and the third period is driven by pile geometry and ground source characteristics. For short time resolution analyses, we proposed a general linear set of equations based on the ε-NTU theory for heat exchangers, the infinite composite-medium line source solution, and the finite line source for the ground source. The proposed method is compared with a full transient 3D numerical simulation. The maximum deviation in terms of return temperature to the heat pump is 0.2 K. The general dimensionless form, the short time resolution, and the limited computational time makes the method suitable for building simulation software and optimization codes for thermal analysis and energy pile design.

Directional Water Wicking on a Metal Surface Patterned by Microchannels.

This work focuses on the simulation and experimental study of directional wicking of water on a surface structured by open microchannels. Stainless steel was chosen as the material for the structure motivated by industrial applications as fuel cells. Inspired by nature and literature, we designed a fin type structure. Using Selective Laser Melting (SLM) the fin type structure was manufactured additively with a resolution down to about 30 μm. The geometry was manufactured with three different scalings and both the experiments and the simulation show that the efficiency of the water transport depends on dimensionless numbers such as Reynolds and Capillary numbers. Full 3D numerical simulations of the multiphase Navier-Stokes equations using Volume of Fluid (VOF) and Lattice-Boltzmann (LBM) methods reproduce qualitatively the experimental results and provide new insight into the details of dynamics at small space and time scales. The influence of the static contact angle on the directional wicking was also studied. The simulation enabled estimation of the contact angle threshold beyond which transport vanishes in addition to the optimal contact angle for transport.

Mitigation of seismic waves: Metabarriers and metafoundations bench tested

The article analyses two potential metamaterial designs, the metafoundation and the metabarrier, capable to attenuate seismic waves impact on buildings or structural components in a frequency band between 3.5 and 8 Hz. The metafoundation serves the dual purpose of reducing the seismic response and supporting the superstructure. Conversely the metabarrier surrounds and shields the structure from incoming waves. The two solutions are based on a cell layout of local resonators whose dynamic properties are tuned using finite element simulations combined with Bloch periodicity boundary conditions. To enlarge the attenuation band, a graded design where the resonant frequency of each cell varies spatially is employed. If appropriately enlarged or reduced, the metamaterial designs could attenuate lower frequency seismic waves or groundborne vibrations respectively. A sensitivity analysis over various design parameters including size, number of resonators, soil type and source directivity, carried out by computing full 3D numerical simulations in time domain for horizontal shear waves is proposed. Overall, the metamaterial solutions discussed here can reduce the spectral amplification of the superstructure by approx. 15–70% depending on several parameters such as the metastructure size and the properties of the soil. Pitfalls and advantages of each configuration are discussed in detail. The role of damping, crucial to avoid multiple resonant coupling, and the analogies between graded metamaterials and tuned mass dampers is also investigated.

Astrophysical jets from boosted compact objects

We perform full 3D numerical simulations of compact objects, such as black holes or neutron stars, boosted through an ambient force-free plasma that posses a uniform magnetization. We study jet formation and energy extraction from the resulting stationary late time solutions.The implementation of appropriate boundary conditions has allowed us to explore a wide range of boost velocities, finding the jet power scales as $\gamma v^2$ (being $\gamma$ the Lorentz factor). We also explore other parameters of the problem like the orientation of the motion respect to the asymptotic magnetic field or the inclusion of black hole spin. Additionally, by comparing a black hole with a perfectly conducting sphere in flat spacetime, we manage to disentangle curvature effects from those produced by the perfect conducting surface. It is shown that when the stellar compactness is increased these two effects act in combination, further enhancing the luminosity produced by the neutron star.

3D modeling of acoustofluidics in a liquid-filled cavity including streaming, viscous boundary layers, surrounding solids, and a piezoelectric transducer

We present a full 3D numerical simulation of the acoustic streaming observed in full-image micro-particle velocimetry by Hagsater et al., Lab Chip 7, 1336 (2007) in a 2 mm by 2 mm by 0.2 mm microcavity embedded in a 49 mm by 15 mm by 2 mm chip excited by 2-MHz ultrasound. The model takes into account the piezo-electric transducer, the silicon base with the water-filled cavity, the viscous boundary layers in the water, and the Pyrex lid. The model predicts well the experimental results.

Full 3D numerical relativity simulations of neutron star–boson star collisions with BAM

With the first direct detections of gravitational waves (GWs) from the coalescence of compact binaries observed by the advanced LIGO and VIRGO interferometers, the era of GW astronomy has begun. Whilst there is strong evidence that the observed GWs are connected to the merger of two black holes (BH) or two neutron stars (NS), future detections may present a less consistent picture. Indeed, the possibility that the observed GW signal was created by a merger of exotic compact objects (ECOs) such as boson stars (BS) or axion stars (AS) has not yet been fully excluded. For a detailed understanding of the late stages of the coalescence full 3D numerical relativity simulations are essential. In this paper, we extend the infrastructure of the numerical relativity code BAM, to permit the simultaneous simulation of baryonic matter with bosonic scalar fields, thus enabling the study of BS–BS, BS–NS, and BS–BH mergers. We present a large number of single star evolutions to test the newly implemented routines, and to quantify the numerical challenges of such simulations, which we find to partially differ from the default NS case. We also compare head-on BS–BS simulations with independent numerical relativity codes, namely the SpEC and the GRChombo codes, and find good general agreement. Finally, we present what are, to the best of our knowledge, the first full NR simulations of BS–NS mergers, a first step towards identifying the hallmarks of BS–NS interactions in the strong gravity regime, as well as possible GW and electromagnetic observables.

Neutron star–axion star collisions in the light of multimessenger astronomy

Axions are increasingly favoured as a candidate particle for the dark matter in galaxies, since they satisfy the observational requirements for cold dark matter and are theoretically well motivated. Fluctuations in the axion field give rise to stable localised overdensities known as axion stars, which, for the most massive, compact cases, are potential neutron star mimickers. In principle, there are no fundamental arguments against the multi-messenger observations of GW170817/GRB170817A/AT2017gfo arising from the merger of a neutron star with a neutron star mimicker, rather than from a binary neutron star. To constrain this possibility and better understand the astrophysical signatures of a neutron star--axion star (NSAS) merger, we present in this work a detailed example case of a NSAS merger based on full 3D numerical relativity simulations, and give an overview of the many potential observables - ranging from gravitational waves, to optical and near-infrared electromagnetic signals, radio flares, fast radio bursts, gamma ray bursts, and neutrino emission. We discuss the individual channels and estimate to which distances current and future observatories might be able to detect such a NSAS merger. Such signals could constrain the unknown axion mass and its couplings to standard baryonic matter,thus enhancing our understanding of the dark matter sector of the Universe.

Numerical analysis of oil injection effects in a single screw expander

The pursuit of higher efficiency for compression and micro-power generation systems has pushed the researchers to an in-depth analysis of positive displacement machines. Single-screw machines, among the others, are gaining attention in the Organic Rankine Cycle (ORC) systems as expanders, thanks to their extended maintenance intervals and compactness. The performances of such devices are strongly affected by the working conditions, and especially the presence of oil has major effects on the operability. The main advantage of adopting an oil-injected device consists in the lube sealing effect, which permits better performance (greater shaft power for assigned boundary conditions) as well as higher reliability. The choice of whether using an oil-free configuration or not is related to the working fluid cleanness, system complexity (oil separator, filters, recovery pump), flow rate and pressure ratio. In this paper, the full 3D numerical simulation of an oil-injected single-screw expander operating with R245fa refrigerant is presented. Oil is injected together with the working fluid at the inlet of the machine. Oily droplets are tracked over the admission duct to show how the oil droplets reach the inlet ports of the screw machine. Different behaviors related to different oil droplet diameters in the range of (0.5 - 50) μm are studied, for the same operating point. The proper distribution of the oil droplets on the screw inlet ports are directly related to the single screw expander performance. In addition, a particular screw position is analyzed for studying the effects of leakages on the oil injection and oil film evolution over the time.

A three-dimensional (3D) two-way coupled fluid-structure interaction (FSI) study of peristaltic flow in obstructed ureters.

Obstruction in the ureter flow path is one of the most common problems in urinary-related diseases. As the ureter transports the urine using the expansion bolus created by the peristaltic pulses, an obstruction in its path can cause unwanted backflow and can also result in damage to the wall. But in order to understand this further, and specifically to quantify and parametrize the effect of the obstruction in the ureter, a detailed study investigating various level of obstructions in peristaltic ureter flow is necessary. In the current study, full 3D numerical simulations of peristalsis in an obstructed ureter are carried out using a finite element solver along with a two-way coupling between the fluid and structural domain with the arbitrary Eulerian-Lagrangian method. Analysis of the results shows that the larger the obstruction, the higher the wall shear stress and pressure gradient in the fluid. In addition, the amount of backflow increases with increase in the obstruction.

Numerical modeling of the coupling efficiency of single quantum emitters in photonic-crystal waveguides

Planar photonic nanostructures have recently attracted a great deal of attention for quantum optics applications. In this article, we carry out full 3D numerical simulations to fully account for all radiation channels and thereby quantify the coupling efficiency of a quantum emitter embedded in a photonic-crystal waveguide. We utilize mixed boundary conditions by combining active Dirichlet boundary conditions for the guided mode and perfectly-matched layers for the radiation modes. In this way, the leakage from the quantum emitter to the surrounding environment can be determined and the spectral and spatial dependence of the coupling to the radiation modes can be quantified. The spatial maps of the coupling efficiency, the $\beta$-factor, reveal that even for moderately slow light, near-unity $\beta$ is achievable that is remarkably robust to the position of the emitter in the waveguide. Our results show that photonic-crystal waveguides constitute a suitable platform to achieve deterministic interfacing of a single photon and a single quantum emitter, which has a range of applications for photonic quantum technology.

Parametric study of the impacts of various geological and machine parameters on thrust force requirements for operating a single shield TBM in squeezing ground

Excavation of long tunnels by shielded TBMs is a safe, fast, and efficient method of tunneling that mitigates many risks related to ground conditions. However, TBM applications in difficult conditions such as squeezing ground, which often happens in a combination of weaker rock and/or high depth, may lead to problems such as shield jamming. Insufficient thrust force, which cannot overcome the shield skin friction between the machine and the ground, causes shield entrapment. For successful tunneling in such conditions, it is essential to study the pertinent ground and operational factors and their impact on ground behavior, to assess the possibility of shield entrapment and to develop possible safeguards to mitigate the related risks. In this study, the controlling parameters influencing the risks of tunneling by single shield TBMs in squeezing ground are examined. For this purpose, a parametric study was performed by using a full 3D numerical simulation to quantify the influence of various factors on machine entrapment. Sensitivity analysis of most important parameters and their impacts on entrapment risks has been conducted and the results are discussed in this paper. Furthermore, nonlinear regression analysis was performed to develop a relationship between ground and machine parameters, and the required thrust force needed to overcome contact/frictional forces for propelling machine forward. The result of the modeling was compiled in a database to observe the trends and develop relationship between different parameters and risk of machine entrapment. This allows for assessment of the applicability of shield TBM in weak formations along the tunnel. Furthermore, the results of the analysis can be used to evaluate the operational parameters such as the installation of a higher thrust force to overcome contact friction loads and to make a choice between increasing the overcut or using lubricants between shield and ground. The results of this study show the role of ground conditions and in-situ stresses, as well as machine operational parameters on convergence and shield pressures.

Understanding the impact of point-contact scheme and selective emitter in a c-Si BC-BJ solar cell by full 3D numerical simulations

Abstract This work presents a detailed analysis of the impact of a rear point-contact (PC) scheme and a selective emitter (SE) design on the performance of a crystalline silicon (c-Si) back contact-back junction (BC-BJ) solar cell featuring narrower highly-doped back surface field (BSF) areas and wider lowly-doped emitter areas. The analysis was performed both through light and dark J-V simulations by using a rigorous full three-dimensional (3D) electro-optical modeling approach to accurately include the 3D effects of several competing physical mechanisms occurring in a PC structure. Simulation results show that the adoption of a relatively thick and wide metallization contacting the silicon at the rear side only via small circular-shaped holes gives an efficiency improvement above 0.3%abs with respect to a conventional BC-BJ solar cell featuring a linear-contact (LC) metallization scheme. Moreover, the introduction of an optimized SE design, featuring a local deeper highly-doped (HDOP) profile underneath rear point contacts and a narrower lowly-doped (LDOP) profile at the non-contacted rear interfaces, can lead to a further efficiency enhancement of about 0.2%abs, which increases with the emitter width.