Stratified Geometry Research Articles

Sensitivity analysis of simulated premixed layer and vapour explosion in stratified configuration

Experiments of fuel–coolant interaction in stratified geometry at the PULiMS and SES (KTH, Sweden) test facilities resulted in spontaneous steam explosions. Prior to the explosion, a premixed layer of ejected melt drops in the water layer was observed in the experiments. Based on the experimental and analytical knowledge, we have recently developed a model for premixed layer formation in the Fuel-Coolant Interaction code MC3D and applied it to estimate steam explosion energetics.In the paper, a sensitivity study of this model is performed on the three main uncertain parameters of the premixed layer formation model, which define the melt fragmentation rate, the size of the ejected melt drops and the ejected melt drop velocity. The analysis is performed against the SES S1 and the PULiMS E6 experimental results. The chosen tests were selected as in both of them the same material was used, the geometry was similar, and both of them resulted in a spontaneous steam explosion. The effects can be observed in all the performed analyses and they are consistent in simulations of both experiments, affecting the premixed layer height as well as the explosion strength and duration. Some uncertainty of the experimental results is assessed, with the main limitation related to the visual observations. The combination of both analyses provides us with the assessment of future work necessity and prioritization.The presented sensitivity analysis of the premixed layer formation model enables a more reliable assessment of the stratified vapour explosion risk and its uncertainty in nuclear power plants and other industries.

Collapse and dynamics of a bubble near a rigid boundary enveloped by a vapor film

One of the possible consequences of a severe accident at a nuclear power plant with a pressurized water reactor is a steam explosion that occurs as a result of the interaction of a high-temperature melt of core materials with water and can lead to the failure of the containment. The paper considers the initial stage of the steam explosion (premixing of the melt with water) under the stratified geometry. The key phenomenon leading to premixing is the dynamic effect of a collapsing vapor bubble in a liquid on the melt surface. The influence of a vapor film located near a melt surface on the dynamics of a vapor bubble is considered. The Kelvin impulse is used as the main criterion characterizing the dynamic effect of a collapsing bubble on the liquid-vapor interface and on the melt surface. The influence of all the main parameters on the Kelvin impulse was numerically studied. Based on the calculations carried out on the plane of the parameters “film thickness - vapor density”, the region of the dynamic impact of the collapsing bubble on the surface is determined. The greatest impacts are observed for thin films of vapor having a high density.

Channelling mobilities: migrant-owned businesses as mobility infrastructures

ABSTRACT Migration infrastructures have usually been identified with stable socio-material arrangements controlling migration (e.g. airports and detention camps), stressing highly stratified power geometries and hierarchies. Recent debates about arrival infrastructures, however, have highlighted the informal, ephemeral and improvisational character of ‘bottom-up’ infrastructures. Departing from a widened understanding of infrastructure, this paper looks at migrants’ businesses as urban infrastructures assembling various kinds of mobilities. In particular, we address small businesses established by Senegalese migrants in Brazil, and Brazilian-owned cafés in Portugal. We approach these businesses as urban infrastructures where different forms of mobilities overlap and interact, exposing various trajectories and scales of circulation. While the businesses in Brazil cater mainly for Senegalese and other migrants’ needs (money transfer, ICTs, and job offers), the Brazilian-owned coffee shops in Portugal function as sites of co-working and sociality of tourists, digital nomads, and other urban creatives. Building on ethnographic fieldwork in the cities of São Paulo and Caxias do Sul (Brazil) and in Lisbon (Portugal), this paper makes innovative connections between migration research, mobility studies and urban theory. We discuss the infrastructural production of transnational and local mobilities and how these businesses both result from and facilitate the existence of mobile lifestyles.

Identification of a multi-stack structure of graphene electrodes doped layer-by-layer with benzimidazole and its implication for the design of optoelectronic devices.

Optical properties of benzimidazole (BI)-doped layer-by-layer graphene differ significantly from those of intrinsic graphene. Our study based on transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling reveals that such a difference stems from its peculiar stratified geometry formed in situ during the doping process. This work presents an effective thickness and optical constants that can treat these multi-stacked BI-doped graphene electrodes as a single equivalent medium. For verification, the efficiency and angular emission spectra of organic light-emitting diodes with the BI-doped graphene electrode are modeled with the proposed method, and we demonstrate that the calculation matches experimental results in a much narrower margin than that based on the optical properties of undoped graphene.

A stratified homotopy hypothesis

We show that conically smooth stratified spaces embed fully faithfully into \infty -categories. This articulates a stratified generalization of the homotopy hypothesis proposed by Grothendieck. Hence, each \infty -category defines a stack on conically smooth stratified spaces, and we identify the descent conditions it satisfies. These include \mathbb R^1 -invariance and descent for open covers and blow-ups, analogous to sheaves for the h-topology in \mathbb A^1 -homotopy theory. In this way, we identify \infty -categories as striation sheaves, which are those sheaves on conically smooth stratified spaces satisfying the indicated descent. We use this identification to construct by hand two remarkable examples of \infty -categories: \mathcal B\mathsf{un} , an \infty -category classifying constructible bundles; and \mathcal E\mathsf {xit} , the absolute exit-path \infty -category. These constructions are deeply premised on stratified geometry, the key geometric input being a characterization of conically smooth stratified maps between cones and the existence of pullbacks for constructible bundles.

Stratified steam explosion energetics

Vapor explosions can be classified in terms of modes of contact between the hot molten fuel and the coolant, since different contact modes may affect fuel-coolant mixing and subsequent vapor explosion energetics. It is generally accepted that most vapor explosion phenomena fall into three different modes of contact; fuel pouring into coolant, coolant injection into fuel and stratified fuel-coolant layers. In this study, we review previous stratified steam explosion experiments as well as recent experiments performed at the KTH in Sweden. While experiments with prototypic reactor materials are minimal, we do note that generally the energetics is limited for the stratified mode of contact. When the fuel mass involved in a steam explosion in a stratified geometry is compared to a pool geometry based on geometrical aspects, one can conclude that there is a very limited set of conditions (when melt jet diameter is small) under which a steam explosion is more energetic in a stratified geometry. However, under these limited conditions the absolute energetic explosion output would still be small because the total fuel mass involved would be limited.

Interference-exact radiative transfer equation

The Purcell effect, i.e., the modification of the spontaneous emission rate by optical interference, profoundly affects the light-matter coupling in optical resonators. Fully describing the optical absorption, emission, and interference of light hence conventionally requires combining the full Maxwell’s equations with stochastic or quantum optical source terms accounting for the quantum nature of light. We show that both the nonlocal wave and local particle features associated with interference and emission of propagating fields in stratified geometries can be fully captured by local damping and scattering coefficients derived from the recently introduced quantized fluctuational electrodynamics (QFED) framework. In addition to describing the nonlocal optical interference processes as local directionally resolved effects, this allows reformulating the well known and widely used radiative transfer equation (RTE) as a physically transparent interference-exact model that extends the useful range of computationally efficient and quantum optically accurate interference-aware optical models from simple structures to full optical devices.

Asymptotic Analysis of Three-Scale Model of pH-Dependent Flows in 1:1 Clays with Danckwerts’ Boundary Conditions

The electroremediation process is an efficient method for removing pollutants from clayey soils. We model this process in kaolinite clays considering three scales—nano, micro and macro—under the assumption of a stratified geometry in conjunction with more realistic Danckwerts’ boundary conditions imposed at the electrodes. The resulting multiscale model is a coupled system of nonlinear partial differential equations. We derive analytical solutions of the macroscopic equations considering the asymptotic behavior of strongly convective and diffusive regimes. We perform numerical simulations of different scenarios for the electroremediation using the Galerkin finite element method together with a staggered algorithm and the Newton–Raphson method. We validate the accuracy of the proposed algorithm by comparing the discrete solutions to the analytical ones. Finally, we explore and discuss optimal scenarios for the electroremediation process depending on the input values of pH, electrical current, and mass inflow using dimensionless numbers defined from the analytical solutions.

Template Shape Estimation: Correcting an Asymptotic Bias

We use tools from geometric statistics to analyze the usual estimation procedure of a template shape. This applies to shapes from landmarks, curves, surfaces, images, etc. We demonstrate the asymptotic bias of template shape estimation using the stratified geometry of the shape space. We give a Taylor expansion of the bias with respect to a parameter $\sigma$ describing the measurement error on the data. We propose two bootstrap procedures that quantify the bias and correct it if needed; they are applicable for any type of shape data. We give a rule of thumb to provide intuition on whether the bias has to be corrected, which reveals the parameters that control the bias' magnitude. We illustrate our results on simulated and real shape data.

Dispersion in Porous Media with Heterogeneous Nonlinear Reactions

The upscaling of mass transport in porous media with a heterogeneous reaction at the fluid–solid interface, typical of dissolution problems, is carried out with the method of volume averaging, starting from a pore-scale transport problem involving thermodynamic equilibrium or nonlinear reactive boundary conditions. A general expression to describe the macro-scale mass transport is obtained involving several effective parameters which are given by specific closure problems. For representative unit cell with a simple stratified geometry, the effective parameters are obtained analytically and numerically, while for those with complicated geometries, the effective parameters are only obtained numerically by solving the corresponding closure problems. The impact on the effective parameters of the fluid properties, in terms of pore-scale Peclet number Pe, and the process chemical properties, in terms of pore-scale Damkohler number Da and reaction order (n), is studied for periodic stratified and 3D unit cells. It is found that the tortuosity effects play an important role on the longitudinal dispersion coefficient in the 3D case, while it is negligible for the stratified geometry. When Da is very small, the effective reaction rate coefficient is nearly identical to the pore-scale one, while when Da is very large, the reactive condition turns out to be equivalent to pore-scale thermodynamic equilibrium, and the macro-scale mass exchange term is consequently given in a different form from the reactive case. An example of the application of the macro-scale model is presented with the emphasis on the potential impact of additional, non-traditional effective parameters appearing in the theoretical development on the improvement of the accuracy of the macro-scale model.

Semiclassical theory of multisubband plasmons: Nonlocal electrodynamics and radiative effects

Coherent multisubband plasmons in doped semiconductor quantum wells have recently attracted large interest as they allow us to strongly enhance light-matter interaction via collective Coulomb coupling among different intersubband transitions. In this work, we develop a semiclassical theory of intersubband plasmons in quantum wells, on the basis of nonlocal electrodynamics. The nonlocal treatment provides a proper description of collective effects in the electromagnetic response of the system and, in the long-wavelength approximation, it predicts the same resonance frequencies as the quantum mechanical description. The nonlocal formalism is applied to the study of the radiative decay rate of multisubband plasmons and plasmon polaritons, both in the case of an isolated quantum well and of a planar microcavity. We show that subpicosecond radiative lifetimes are to be expected for intersubband plasmons in semiconductor quantum wells, similarly to quantum well excitons. The theory is formulated in the context of the transfer-matrix method and it can be applied in a straightforward way to stratified geometries of any degree of complexity.

Stochastic Methods for PEMFC Parameter Identification

Fuel cells are complex multiphysical systems characterized by a stratified geometry consisting of thin layers of different materials. Their full characterization involves a number of physical parameters, which are important for quantifying and comparing the performance of the materials. The physical parameters are also used in fuel cell models, which allow the fast exploration of different operating scenarios and can be implemented in optimization procedures aimed at achieving the best structural and operating conditions. The system of equations involved in these models, e.g. the Nernst equation, the Butler-Volmer equation and the diffusion equation, are strongly non-linear; thus, the model itself is extremely sensitive to parameter variations. Careful ex-situ measurements of such parameters can be performed by means of a number of diagnostic techniques, whose results are however not completely consistent with fuel cell operation due to several effects, e.g. variable interactions and boundary effects in the thin layers of the fuel cell. Conversely, “in situ” measurements can provide meaningful operational values, but a very few techniques are available to determine a limited number of parameters.A solution can consist in multiple parameter identification from multiple fundamental measurements, such as polarization curve and electrochemical impedance spectroscopy, performed in different conditions, e.g. at different temperature, pressure, concentration, and humidification.If there is only one unknown parameter, the solution is easy, requiring just a statistical interpolating technique. In the case of multiple unknown parameters, which is the most frequent condition, the problem becomes increasingly difficult as the number of parameters increases as duplicity problem emerges, i.e., several groups of parameters may lead to the same performance (e.g. polarization curve).Recently, a number of numerical tools are under development to face this kind of problems, some based on stochastic mathematical models, each presenting specific features in terms of accuracy, convergence, stability, robustness, and calculation speed [1]. Among the most promising methods, some of which have already been applied to multiple fuel cell parameter identification, are the Ishibashi–Kimura method [2], the Lambert W-function method [3], the Levenberg-Marquardt algorithm (LMA), the Gauss-Newton algorithm (GNA), the hybrid artificial bee colony algorithm [4], the trust region method, the current switching method [5], the simulated annealing method [6], and the particle swarm optimization (PSO) [7], and the differential evolution method [8].The presentation will focus on the application of a particular class of stochastic optimization methods to the problem of multiple parameters identification. These methods have already been widely tested and applied successfully to the simultaneous search of sets of optimal parameters with different physical dimensions, making them ideal tools to cope with the present problem. Their recent relevant literature is vast. They are especially suited for multimodal and noisy functions over large search spaces, which makes them particularly appealing for the problem at hand. The main drawback of such methods is the high number of objective function evaluations needed for convergence; however, thanks to a particular formulation of the PEMFC model, such drawback can be of minor impact, i.e. the overall computational times are on the order of a few tens of seconds.We are going to present the application of two of the most well-known metaheuristic methods, namely Particle Swarm Optimization (PSO) and Differential Evolution (DE), developed in previous work and suitably adapted. It should be noted that PSO has already been applied to parameter identification for FCs, while the application of DE is novel in this context. This is somewhat surprising since, as results will show, DE provides better results and offers higher robustness. We will present the implementation details regarding PSO and DE. The optimization algorithms have not been tuned to the specific problem and algorithm control parameters have been chosen from reasonable values suggested in literature.

THE ROLE OF ACTIVE REGION LOOP GEOMETRY. I. HOW CAN IT AFFECT CORONAL SEISMOLOGY?

We present numerical results of coronal loop oscillation excitation using a three-dimensional (3D) MHD model of an idealized active region (AR) field. The AR is initialized as a potential dipole magnetic configuration with gravitationally stratified density and contains a loop with a higher density than its surroundings. We study different ways of excitation of vertical kink oscillations of this loop by velocity: as an initial condition, and as an impulsive excitation with a pulse of a given position, duration, and amplitude. We vary the geometry of the loop in the 3D MHD model and find that it affects both the period of oscillations and the synthetic observations (difference images) that we get from oscillations. Due to the overestimated effective length of the loop in the case of loops which have maximum separation between their legs above the footpoints (>50% of observed loops), the magnetic field obtained from coronal seismology can also be overestimated. The 3D MHD model shows how the accuracy of magnetic field strength determined from coronal seismology can be improved. We study the damping mechanism of the oscillations and find that vertical kink waves in 3D stratified geometry are damped mainly due to wave leakage in the horizontal direction.

Functional testing of a tissue-engineered vocal fold cover replacement

Objectives Tissue engineering may provide a treatment for severe vocal fold scars. This study quantifies mechanical properties and demonstrates vibration of a tissue-engineered vocal fold cover replacement. Methods Tissue-engineered constructs were produced from fibrin and adipose-derived stem cells. Optimized bilayered constructs contained epithelial and mesenchymal cell phenotypes in a stratified geometry. For comparison, homogeneous constructs did not have epithelial differentiation. Elastic modulus was determined using indentation. Immunohistochemical labeling for type I collagen was performed. A bilayered construct was also tested in phonation in an excised larynx model. Results Bilayered vocal fold cover replacements had indentation moduli similar to human vocal fold covers (mean construct modulus 6.8 kPa). Collagen deposition occurred in the middle of the construct. Homogeneous constructs had a mean modulus of 8.3 kPa, and collagen was concentrated at the surface. An excised larynx with unilateral vocal fold cover replacement phonated and exhibited mucosal waves at physiologic airflow. Conclusion Bilayered tissue-engineered constructs were produced that exhibited indentation modulus, microstructure, and vibration similar to that exhibited by human vocal fold covers.

Crustal P‐Wave Velocity Structure and Layering Beneath Zhujiangkou–Qiongdongnan Basins in the Northern Continental Margin of South China Sea

AbstractBased on the stacking velocity spectrum data of 14 near‐vertical deep reflection profiles in the deep waters of Zhujiangkou–Qiongdongnan basins along the northern continental margin of South China Sea, we use the Dix formula to transform the stacking velocities into corresponding crust interval velocities (in TWT domain) and construct the crustal velocity structure in depth domain by time‐depth transformation. Integrating all the velocity profiles we analyze the spatial variation of P wave velocity at different depths and the stratified geometry of crust in the Zhujiangkou and Qiongdongnan basins in the continental margin of northern South China Sea. The result indicates that the crust of Qiongdongnan basin can be divided into a 4~8 km thick sediment layer (Vp is 1.7~4.7 km/s), a 4~10 km thick upper crust layer (Vp is 5.2~6.3 km/s), a lower‐crust layer about 5 km thick (Vp is 6.4~7.0 km/s), and a high‐velocity lower‐crust bottom layer about 2~6 km thick (Vp > 7.0 km/s). The existence of the high‐velocity lower crust layer can be considered as the seismological signature of underplating at the lower crust bottom induced by lithosphere extension in South China and its continental margin, or the residual mafic layer of the original Cathaysia lower crust. Synthesizing the available geophysical sounding results we construct the images of Moho and the lithosphere bottom across the continent of South China and the continental slope of South China Sea, which reveals the lithosphere thinning in continental South China and the northern continental margin of South China Sea.

Efficient evaluation of spatial-domain MoM matrix entries in the analysis of planar stratified geometries

An efficient hybrid method for evaluation of spatial-domain method-of-moments (MoM) matrix entries is presented in this paper. It has already been demonstrated that the introduction of the closed-form Green's functions into the MoM formulation results in a significant computational improvement in filling up MoM matrices and, consequently, in the analysis of planar geometries. To achieve further improvement in the computational efficiency of the MoM matrix entries, a hybrid method is proposed in this paper and, through some examples, it is demonstrated that it provides significant acceleration in filling up MoM matrices while preserving the accuracy of the results.

Phenomenological studies on melt-coolant interactions in the ALPHA program

Two series of experiments to investigate melt-coolant interactions have been performed as part of the ALPHA program at JAERI. In the melt drop steam explosion experiments, melt simulating a molten core was dropped into a pool of water. Volume fractions of the melt, water and steam in the mixing region prior to the occurrence of spontaneous steam explosions were quantified. Other characteristics of melt-coolant interactions were evaluated for settling velocity of the melt in water, propagation and expansion velocities, energy conversion ratio and debris size distribution. It was found that the probability of the occurrence of spontaneous steam explosions could be reduced by using a melt dispersion device. Measurement of void fraction in the mixing region clearly showed that the melt dispersion device enhanced steam generation. However, one experiment indicated that the use of the dispersion device could possibly result in a more energetic steam explosion. It was found that the mixing of non-condensable gas in the steam phase of the mixing region during melt dispersion played an important role for the suppression of the spontaneous steam explosion. Knowledge of the parametric effects of melt mass, ambient pressure and water temperature was extended. In the melt coolability experiments, water was poured onto the melt to investigate melt-coolant interactions in a stratified geometry where water overlies on a melt layer. Melt eruptions which could induce an explosive interaction were observed when the subcooled water was poured through a pipe nozzle. The eruption was not observed when the water was near the saturation temperature or supplied through a spray nozzle. The explosive interaction in the stratified geometry was found to be much smaller in magnitude than the steam explosion in the melt drop configuration.

NUMERICAL CALCULATION OF THE ELECTROMAGNETIC FIELD IN STRATIFIED MEDIA OF PLANAR OR CYLINDRICAL GEOMETRY

A boundary element technique is applied to an integral formulation of the electromagnetic field quantities, of the electric vector and magnetic scalar potentials for the evaluation of the propagation phenomenon in stratified media, of planar or cylindrical geometry, due to electric dipole sources. The use of the above described potentials simplify the solution procedure, because of the degrees of freedom, which can de introduced in choosing relations between the field quantities. The main goal of these relations is to make the numerical solution easier, while the Maxwell equations and the boundary conditions remain unaffected. Besides a three‐dimensional problem is treated as a two dimensional one. Results and plots of the field, are given for both cases of the geometry. The method can be used for determining the field inside the earth in geological studies, propagation in stratified media, cable technology, shielding problems e.t.c.

Modelling loop antennas for HF plasma heating in the ion cyclotron frequency range

The technique of impedance and admittance matrices allows a simple and elegant solution of the problem of radiation from antennas of arbitrary shape and orientation in plane stratified geometry. The medium towards which the antenna radiates is completely characterized by the values of these matrices at its surface, which for a hot plasma in the IC frequency range the author obtains numerically in the finite Larmor radius approximation. In vacuum, the wave admittance and impedance matrices obey simple transformation rules, which are used to solve the matching problem for arbitrary source currents. Combined with the variational approach originally developed for the case of fast wave launching by Teilhaber and Jacquinot (1984), this method allows the determination of the self-consistent current distribution in the antenna, its radiation resistance and its reactive impedance. The flexibility of the approach is illustrated by several examples, both without and with plasma loading.

Vapor Explosions in a Stratified Geometry

n experimental investigation of stratified vapor explosions and scaling is conducted by constructing test sections of two different sizes and using two different fluid pairs. The horizontal lengths of the interaction vessels are 20 and 50 cm (geometric scale ratio: 2:5). The two liquid pairs are water and liquid nitrogen, and water and Freon-12, with water being the hot liquid. The interactions are either triggered by an external trigger or allowed to occur spontaneously depending on the liquid pair and initial conditions. The major experimental variables are initial water temperature, liquid layer depths, and magnitude of the external trigger pressure. Interaction pressures, mechanical work release, and depth of intermixing are measured. The water/Freon-12 pair produces more violent interactions than the water/liquid nitrogen. In both cases, the explosion propagation speeds are supersonic, ranging from 40 to 250 m/s. The small depths (≤1 cm) of liquid-liquid mixing during the explosion propagation are observed in both liquid pairs. A simple model for the depth of intermixing is derived, and the result shows reasonable agreement with the experimental observations. The length of the liquid layer interface and the depth of the top liquid appear to be the key geometric parameters in stratified vapor explosions.