Behavior Of Stacks Research Articles

Formalization of Android Activity-Fragment Multitasking Mechanism and Static Analysis of Mobile Apps

The multitasking mechanism between activities and fragments plays a fundamental role in the Android operating system, which involves a wide range of features, including launch modes, intent flags, task affinities, and structured activities containing fragments. All of them are being widely used in Android apps, both open-source and commercial ones. In this paper, we present a formal semantics of the Android multitasking mechanism between activities and fragments, which accommodates all the important features and gives insofar the most comprehensive and accurate formalization. In particular, our semantics is formulated based on multi-stack systems, and fully captures the behavior of task stacks and activity stacks regarding fragments. Based on the semantics, we provide new static analysis algorithms, which are both multi-stack-aware and fragment-sensitive, thus achieve more precise static analysis for Android apps. We validate our approach by extensive experiments on both open-source and commercial Android apps. The results highlight the benefits of the considering the semantics of the multitasking mechanism between activities and fragments in static analysis, and confirm the efficacy of our approach.

Utilizing Neural Networks for Optimizing the Performance of Solid Oxide Cell Stacks

Solid oxide cells (SOCs) are one of the promising energy conversion devices due to their high efficiency and relevance to many commercial and industrial applications. Much development progress has been made in the last 10-years improving the scale-up, degradation, and commercialization outlook globally. Further commercialization of the SOCs necessitates a deeper understanding of their complex internal processes. Experimental methods, albeit valuable, are constrained by cost and time and often cannot capture intricate details like distributions of species, current density, and temperature. Therefore, numerical analyses are essential as they offer a deeper understanding of the complex phenomena within SOCs. Yet, high-fidelity modeling SOCs is computationally expensive as it includes a multitude of physical and chemical phenomena spanning various layers and length scales. Even at the single-cell and repeating unit levels, 3D simulations are computationally costly due to the intricate interplay of mass, momentum, species transport, charges, and heat. Cell-level models, while useful, cannot capture all of the thermophysical behavior of SOC stacks accurately, particularly along the height of the stack. Full-stack models, while more comprehensive, come with significant computational costs. A breakthrough in addressing this challenge comes through a multi-scale modeling approach, leveraging homogenization techniques to transform the layered complexity of SOC stacks into computationally tractable representations [1,2]. By replacing the layered domains of the stacks with equivalent mediums and calculating effective modeling variables, this approach makes the stack-scale simulations feasible on standard computational computers/workstations.Even with homogenized models, the optimization at the stack scale is not feasible due to prolonged run times and high computational demands. To address this challenge, this study employs parametric studies on stack-scale models to train a neural network. The trained neural network correlates stack outputs such as voltage, outflow temperatures, and fuel utilization with operational parameters like load current, inlet temperatures, flow rates, and inlet fuel compositions. The neural network is utilized to optimize the stack performance under various conditions while ensuring adherence to manufacturer-defined limits. For instance, the neural network can be used to optimize stack output power under fuel cell mode while avoiding overheating or exceeding maximum current densities. In essence, this work combines the detailed physics of multi-scale stack models with the efficiency of neural networks, paving the way for faster and more comprehensive optimization of SOC stacks.

Dynamic and quasi static stiffness characterization of a lamination stack of an electric motor

Dynamic and quasi static stiffness characterization of a lamination stack of an electric motor

The impact of boundary conditions on the acoustical behavior of lightweight granular particle stacks

The acoustical behavior of air-saturated granular stacks can differ significantly from that of more traditional sound absorbing materials such as fibrous webs and foams when tested in a standing wave tube. The latter materials can often be modeled as equivalent fluids by using one-dimensional theory, with the further assumption that their behavior does not depend on the input signal level. In contrast, level dependence of the response of lightweight granular materials such as glass bubbles has been consistently observed in previous studies. At low input levels, the absorption coefficient of glass bubble stacks shows solid-like behavior with multiple peaks associated with radial modes of the solid phase of the edge-constrained stack: i.e., the response is two-dimensional. In contrast, at high input levels, glass bubble stacks show one-dimensional fluid-like behavior, with the quarter wave depth resonance dominating the response. It is hypothesized here that at high input levels the particles slip axially at the circumferential boundary, thus showing a one-dimensional response, while at low levels, the particles are constrained to have zero axial velocity, thus creating a two-dimensional response. It is shown here that a two-dimensional finite difference implementation of the poro-elastic Biot theory can accurately reproduce the observed level-dependent behavior.

Effect of disk-to-disk variations on the nonlinear static characteristics and stability regimes of coned disk spring stacks: Experimental and computational studies of quasi-zero-stiffness isolators

Effect of disk-to-disk variations on the nonlinear static characteristics and stability regimes of coned disk spring stacks: Experimental and computational studies of quasi-zero-stiffness isolators

Electrochemical Analysis of an Electrolyte-Supported Solid Oxide Cell-Based MK35x Stack during Long-Term Electrolysis Operation

The currently ongoing scale-up of high-temperature solid oxide electrolysis (SOEL) requires an understanding of the underlying dominant degradation mechanisms to enable continuous progress in increasing stack durability. In the present study, the degradation behavior of SOEL stacks of the type “MK35x” with chromium-iron-yttrium (CFY) interconnects and electrolyte-supported cells (ESC) developed at Fraunhofer IKTS was investigated. For this purpose, the initial electrochemical performance of a 10-cell stack was characterized in various operating conditions in both fuel cell and electrolysis mode. Degradation was evaluated during galvanostatic steady-state steam electrolysis operation for more than 3000 h at an oxygen side outlet temperature of 816 °C and a current density of −0.6 A cm−2 and showed an average voltage evolution rate of −0.3%/kh demonstrating high stability. Initial and final characterization at the part load operating point at −0.39 A cm−2 and 800 °C led to the determination of a positive overall degradation rate of 0.4%/kh showing a considerable impact of the operating conditions on the degradation rate. By means of electrochemical impedance spectroscopy analysis it was shown that the stack’s ohmic resistance increased whereas the polarization resistance decreased most likely due to an enhancement in LSMM’/ScSZ oxygen electrode performance.

SOFC stack modeling: a hybrid RBF-ANN and flexible Al-Biruni Earth radius optimization approach

Abstract This study introduces a novel hybrid methodology for model identification of solid oxide fuel cell (SOFC) stacks by integrating a radial basis function-based artificial neural network (RBF-ANN) with a flexible Al-Biruni Earth radius optimizer (FA-BERO). The primary objective of the proposed method is to augment the precision and efficiency of SOFC stack modeling by considering the advantages of both RBF-ANN and FA-BERO algorithms. The main purpose of using these two methods is to optimize the structure of the RBF-ANN based on the suggested FA-BERO algorithm. The other contribution of this study is improving the efficiency of the Al-Biruni Earth radius optimizer (A-BERO) by applying two improvements on it, including constriction factor and elimination phase to increase the exploration and exploitation strength of the basic A-BERO. To validate the effectiveness of the proposed model, it is compared with some state-of-the-art models in the field, such as the artificial neural network and multi-armed bandit algorithm (ANN/MABA) and rotor Hopfield neural network and grey wolf optimization (RHNN/GWO). Furthermore, the model is validated by experimental data, and the final results demonstrate the efficacy of the hybrid approach in accurately representing the intricate behavior of SOFC stacks. The proposed model achieves lower error rates (ERs) and root mean squared errors (RMSEs) than the comparative methods across different network arrangements and temperature conditions. The results show that, for instance, for the 2/12/1 network arrangement at 900°C, the proposed model attains an ER of 6.69% and an RMSE of 2.13, while the ANN/MABA and RHNN/GWO methods obtain ERs of 9.67% and 8.54%, as well as RMSE values of 24.48 and 9.23, respectively. The proposed model also exhibits superior accuracy and convergence speed compared to the comparative methods, as shown by the current–voltage curves and the convergence analysis. Consequently, this novel hybrid methodology offers a valuable tool for researchers and engineers working in the domain of fuel cell technology, enabling them to better understand and optimize SOFC stack performance.

Challenges in the Virtual Geometry Assurance of Proton Exchange Membrane Fuel Cell Stacks

Challenges in the Virtual Geometry Assurance of Proton Exchange Membrane Fuel Cell Stacks

Degradation Behavior of MK35x Stacks with Chromium-Based Interconnects in Steam Electrolysis Operation

The currently ongoing commercialization of high-temperature solid oxide electrolysis (SOEL) requires the understanding of the underlying dominant degradation mechanisms to enable continuous progress in reducing stack degradation rates.In the present study, the degradation behavior of stacks with chromium-based interconnects (MK35x) and electrolyte-supported cells (ESC) developed at Fraunhofer IKTS is investigated. For this purpose, the initial electrochemical performance of 10-layer stacks was extensively characterized in various operating conditions in both fuel cell and electrolysis mode by electrochemical impedance spectroscopy (EIS). Degradation was evaluated during galvanostatic steady-state steam electrolysis operation over >3000 h in the temperature range of 800-825°C at a current density of -0.6 A cm-2, corresponding to a steam conversion rate of 75%. EIS analysis was used to separate the different loss contributions in the stack and showed that the ohmic resistance increase was the dominating performance degradation phenomenon. Moreover, electrochemical degradation rates in steady-state SOEL mode are compared to the ones in reversible mode with a daily cycling between electrolysis and fuel cell operation.Post-mortem analysis was carried out with scanning electron microscopy (SEM) to correlate the observed degradation phenomena with microstructural and morphological changes. The observed low degradation rates demonstrate that MK35x are ready for scale-up, and the present work gives guidance for operation strategies and stack design to extend their lifetime even further.

Compressive Stress-Strain Behavior of REBCO Coated Conductors and Cables

Rare earth barium copper oxide (REBCO) coated conductor tapes and cables are promising material candidates for constructing high field magnets. In both the magnet constructing stage and operation stage, transverse pressure is exerted on the tapes/cables. Therefore, understanding the mechanical behavior of tapes/cable under transverse pressure and figuring out certain properties, such as modulus and stress limits, will be helpful for magnet design and magnet simulation study. We accomplished these goals by performing compression tests on several commonly used tapes/cables in high field magnets. We found that the mechanical behavior of tape stacks can be characterized in three distinct regimes. In the first regime (<20 MPa), tape stacks underwent flattening and self-relocation. Then in the second regime (20 - 60 MPa), the REBCO tape stack showed a modulus of 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 7% GPa, and the Ni-plated REBCO tape stack showed a modulus of 1.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 6% GPa. In the third regime (>60 MPa), the REBCO tape stack and its Ni-plated derivative had a modulus of 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 11% GPa and 4.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 9% GPa respectively. Roebel cable had similar mechanical behavior to the tape stack with three regimes, and the second and third regimes showed a modulus of 3.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 5% GPa and 9.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm $</tex-math></inline-formula> 11% GPa respectively. The conductor on round core (CORC) cable was more vulnerable to stress, and its fracture stress was around 27 MPa.

Adsorbing Volatile Organic Compounds within Bitumen Improves Colloidal Stability and Air Quality

The mass loss of bitumen due to volatilization is one of the deterioration mechanisms through which the chemical composition of bitumen deviates from its original condition, leading to change in the physical and rheological properties of bitumen. The emissions from bitumen not only accelerate its aging but also negatively impact air quality and consequently human health. Here, we characterized the molecular compositions of the volatiles emitted from bitumen in an isothermal aging process (150 °C for 24 h). We also investigated the impact of losing these compounds on the association behavior of asphaltene stacks. Based on the GC-MS results, the main mass loss of SARA (saturates, aromatics, resins, asphaltenes) fractions is attributed to the emission of lightweight nonpolar aromatic and aliphatic compounds. The most dominant volatiles are single-ring aromatics carrying short side chains, followed by saturated and unsaturated aliphatic compounds including short-chain n-alkanes, iso-alkanes, cyclic alkenes, alkenes, and dienes. Using molecular modeling in the framework of density functional theory (DFT), we showed how the reduction in aromatics and saturates can impact the colloidal stability of asphaltenes in the matrix of bitumen. Based on the DFT results, the surface adsorption of aromatics and saturates on asphaltene planes provides the steric repulsion required to maintain the asphaltene planes at a distance sufficient to inhibit their agglomeration. In addition, aromatic molecules with the least hindrance repulsion can disturb the π–π interfacial interactions between asphaltene planes, facilitating their deagglomeration. Therefore, a direct consequence of the mass loss of bitumen due to volatilization is disturbing the asphaltene distribution within the bitumen and stimulating the aggregation of asphaltenes. We also studied the merits of incorporating biochar into the bitumen surface to reduce bitumen’s mass loss. Modification of bitumen separately with each of six biochars derived from biomass feedstocks (walnut shell, peanut shell, Douglas fir, pine bark, birch, and algae) showed the efficacy of each biochar to reduce the volatiles emitted from bitumen, thereby reducing the changes in the composition of maltene with consequent colloidal stability losses. The study outcomes highlight the importance of reducing emissions from bitumen to delay its aging and minimize negative impacts on air quality and human health.

Prediction of acoustical behavior of granular material stacks as measured in a standing wave tube by using a Biot theory-based model

The acoustical behavior of granular materials, such as activated carbon and silica gel, has drawn attention in recent studies, due to their favorable properties such as good low frequency sound absorption. Like other more traditional porous materials, granular materials can also be tested in a standing wave tube for a convenient assessment of their acoustical properties. However, the behavior of granular materials stacked in a standing tube is more complex than that of traditional materials. For example, the response of lightweight glass bubbles reveals a clear dependence on the sound pressure level of the input signal. Also, when tested in standing wave tubes of different diameters, the same type of granular materials displays differences in their behavior. The apparent stiffness of granule pack is also related to the depth of the stack. In the present work, a model based on Biot theory is proposed, together with a consideration of the effect of the change of boundary conditions and the granule stack stiffness in different test configurations. The model is realized by using a finite difference method, and the simulation results are compared with measurements of different types of granular materials.

Experimental study and modeling of the level-dependent acoustical behavior of granular particle stacks

Researchers have previously observed elastic modulus softening and increased damping when granular particle stacks are exposed to progressively increasing acoustical excitation levels. However, the level-dependent behavior of granular particle stacks is not well understood, and there are no comprehensive approaches to modeling those effects. Earlier, the authors measured the absorption coefficient of a stack of one type of granular activated carbon stack by using signals having different bandwidths and levels. In the present work, five more types of granular particle stacks were studied to validate and generalize the previous conclusions: i.e., both the modulus softening, and increased damping can be characterized by the total RMS fluid displacement at the sample surface. Therefore, a strain-dependent modulus and damping formula from the literature (based on cyclic loading tests on sand particles) was converted into a total RMS fluid displacement-dependent formula (based on acoustic measurements). In addition, a multi-layered model based on this displacement-dependent formula has been developed to iteratively update each layer’s modulus, damping, and total RMS fluid displacement to solve for the particle stack’s acoustic properties. This approach allows modeling of the particle stack’s acoustical behavior by using a single set of parameters, even for different level and bandwidth test signals.

A finite difference approach to study the impact of boundary conditions on the acoustical behavior of particle stacks

The responses of particle stacks to incident sound waves show interesting features that are very different from those of a homogeneous continuum. Further studies of the acoustical performance of particle stacks can help both to discover potential noise control applications of these types of materials, and to help provide better insight into the internal status of the particle stacks. In the current study, a finite difference (FD) model for a particle stack was built based on the Biot poro-elastic theory. The intention in developing this model is to describe the acoustical behavior of particle stacks with consideration of not only the finite stiffness of the particles, but also the influence of gravity and friction between the particles and the inner wall of their enclosure: i.e., the cylindrical sample holder of a standing wave tube, in this work. The derivation of governing equations and boundary conditions is introduced, together with acoustic measurement results of particles stacks consisting of micron-scale glass bubbles, including absorption coefficients and surface impedances that are compared with the theoretical predictions. The possible application scenarios of such materials, and potential developments that will further improve the FD model will also be discussed.

A finite difference approach for predicting acoustic behavior of the poro-elastic particle stacks

The physical properties of particle stacks have been of interest for a long time, and the acoustic properties of such materials have been actively investigated in recent times. Traditional acoustic theories, such as the Biot theory, can serve as a guide for determining the general properties of the stacks, but they do not allow the identification of the differences between the particle stacks and traditional acoustic materials, which are usually modeled as homogeneous continua. Recent research suggests that the Biot theory, combined with depth-dependent stiffness and equivalent density, can be used to model such materials. In this work, a finite difference (FD) scheme based on the Biot theory has been developed based on the idea that the apparent stiffness of the particle stack varies with depth within of the stack. This FD scheme is two-dimensional in cylindrical coordinates with an axisymmetric condition imposed at the axis of the cylinder. This approach is thus suitable for modeling common scenarios in which particle stacks are tested in a cylindrical standing wave tube. The results of several example cases are shown in this paper.

Prospect of modeling industrial scale flow batteries – From experimental data to accurate overpotential identification

Prospect of modeling industrial scale flow batteries – From experimental data to accurate overpotential identification

Mechanical impedance as a tool for electromechanical investigation and equivalent modeling of lithium-ion batteries

Mechanical impedance as a tool for electromechanical investigation and equivalent modeling of lithium-ion batteries

Predicting acoustic performance of high surface area particle stacks with a poro-elastic model

Because of the high sound absorption they offer at low frequencies, there is a growing interest in high surface area particles and how they might be applied in noise control. Therefore, a model that can accurately predict the acoustic behavior of this type of materials will be useful in relevant applications. A poro-elastic model based on a combination of Biot theory and an existing rigid model of granular activated carbon (GAC) is introduced in the current work. The input parameters for this model consist of a certain number of properties that are known by measurement, and a set of values obtained by matching the model prediction with acoustic measurements. Measured absorption coefficients and surface impedance of stacks of several types of different activated carbon particles are shown in this paper. A fitting procedure that determines the unknown parameters is also described. It is shown that the model is able to predict the acoustic behavior of the particle stacks, and especially to capture the frame resonances at low frequencies, thus, validating the proposed model. Beyond the activated carbon used in the present tests, it is reasonable to generalize this model to stacks of other high surface area particles.

Numerical Study on the Plume Behavior of Multiple Stacks of Container Ships

This paper showed different plume behaviors of exhausts from different number of stacks of the container ship, using CFD code PHOENICS version 6.0. The plume behavior was quantitatively analyzed by mass fraction of the pollutant in the exhaust and plume heights. Three simplified typical configurations were constructed by CFD according to the investigation of container ships. The configurations included a single main stack (BL1), one main stack and multiple auxiliary stacks (BL2), and two main stacks and multiple auxiliary stacks (BL3). All the main stacks had the same emission characteristics, and all the auxiliary stacks had the same emission characteristics. The results show that the transmission and diffusion characteristics of the exhaust from multiple stacks are different from those of the exhaust from a single stack. In BL2 and BL3 simulations, the maximum mass fraction of SO2 in the exhaust (C1max) of multiple stack emissions was approximately 329% and 269% higher than that of single stack emissions over the main stack, respectively, and the plume height of multiple stack emissions is higher than that of single stack emissions. In BL2 and BL3 simulations, the plume height of multiple stack emissions was 41% and 75% higher than that of single stack emissions, respectively. The increase of C1max, due to multiple stack emissions, is weakened as the distance of the stacks increase. The difference in plume behavior between multiple stack emissions and single stack emissions is of great significance for air quality management and pollution control in port areas.

Testing of Surface Mounted Superconducting Stacks as Trapped-Flux Magnets in a Synchronous Machine

Stacks of high-temperature superconducting tapes may offer a technically affordable solution for the application of superconducting materials as trapped-flux magnets in the rotor of synchronous electrical machines. Nevertheless, several concerns must be first addressed, among others, the optimal procedure to induce the current vortexes previously to operation (magnetization), the survivability of these vortexes in the electromagnetic environment present in an electrical machine and the accuracy of recently developed numerical models. With the aim of exploring such practicalities, this article presents a modified synchronous machine to test under liquid nitrogen conditions thin stacks of superconducting tapes. The machine is run under realistic conditions: currents in the stacks are induced from the stator, then the shaft is rotated, and finally, the machine is connected to a load, working as a generator. The results confirm previous numerical and experimental studies and establish a procedure for assessing the behavior of stacks in their actual operational environment.