Static Configuration Research Articles

Stable relativistic polytropic objects with cosmological constant

The effects of the cosmological constant on the static equilibrium configurations and stability against small radial perturbations of relativistic polytropic spheres are investigated. This study numerically solves the hydrostatic equilibrium equation and the radial stability equation, both of which are modified from their standard form to introduce the cosmological constant. For the fluid, we consider a pressure p and an energy density $$\rho $$, which are connected through the equation of state $$p=\kappa \delta ^{\varGamma }$$ with $$\delta =\rho -p/(\varGamma -1)$$, where $$\kappa $$, $$\varGamma $$ and $$\delta $$ represent the polytropic constant, adiabatic index and rest mass density of the fluid, respectively. The dependencies of the mass, radius and eigenfrequency of oscillations on both the cosmological constant and the adiabatic index are analyzed. For ranges of both the central rest mass density $$\delta _c$$ and the adiabatic index $$\varGamma $$, we show that the stars have a larger (lower) mass and radius and a diminished (enhanced) stability when the cosmological constant $$\varLambda >0$$ ($$\varLambda 0$$ and $$\mathrm{d}M/\mathrm{d}\delta _c<0$$, respectively.

Primary and super-harmonic resonances of Timoshenko pipes conveying high-speed fluid

High-speed flow pipes suffer from severe vibration problems. When the fluid velocity is higher than the critical value, the straight equilibrium configuration of the pipe will lose stability. What follows is the supercritical vibration of the pipe near the non-trivial static equilibrium configuration. This paper attempts to reveal multiple resonance responses of forced vibration of pipes in the supercritical regime. Based on Timoshenko beam theory, the nonlinear coupled partial differential equations are deduced. The non-trivial static equilibrium configuration causes the parameters to vary with space variable. The approximate responses of the pipe are obtained and verified numerically. The results show that the flow velocity near the critical value is more prone to cause severe vibration. Unlike in the subcritical regime, there are third-order super-harmonic resonance and second-order super-harmonic resonance in the supercritical regime. So there are more resonance areas in the supercritical regime. High flow velocity or large external excitation can aggravate the difference between the Euler-Bernoulli model and the Timoshenko model, and the relative error of two models varies non-monotonically. Even for slender pipes, the difference between the two models is still very clear. Therefore, the Timoshenko model is more necessary to analyze the vibration of the high-speed pipe.

A novel method for dynamic analysis of complex multi-segment cable systems

In actual engineering, the cable structure is divided into several natural cable segments due to the existence of lateral supports. During the vibration, each cable segment will have different static and dynamic configurations, which greatly increases the difficulty of structural dynamic analysis. To accurately analyze the dynamic characteristics of this type of systems, this paper proposes an exact dynamic analysis method which can consider the influence of multiple factors such as bending stiffness, sag, and additional cable force based on dynamic stiffness theory, and the analytical expression of the additional cable forces of multi-segment cable systems is obtained. Besides, to consider the geometric nonlinear characteristics of small sag cables, an improved Wittrick-Williams algorithm is proposed, which is suitable for solving the frequency equation of multi-segment cable system. The accuracy of the method is verified by experiment and the finite element method.

Relativistic geoid: Gravity potential and relativistic effects

The Earth's geoid is one of the most essential and fundamental concepts to provide a gravity field-related height reference in geodesy and associated sciences. To keep up with the ever-increasing experimental capabilities and to consistently interpret high-precision measurements without any doubt, a relativistic treatment of geodetic notions (including the geoid) within Einstein's theory of General Relativity is inevitable. Building on the theoretical construction of isochronometric surfaces and the so-called redshift potential for clock comparison, we define a relativistic gravity potential as a generalization of (post-)Newtonian notions. This potential exists in any stationary configuration with rigidly co-rotating observers, and it is the same as realized by local plumb lines. In a second step, we employ the gravity potential to define the relativistic geoid in direct analogy to the Newtonian understanding. In the respective limit, the framework allows to recover well-known (post-) Newtonian results. For a better illustration and proper interpretation of the general relativistic gravity potential and geoid, some particular examples are considered. Explicit results are derived for exact vacuum solutions to Einstein's field equation as well as a parametrized post-Newtonian model. Comparing the Earth's Newtonian geoid to its relativistic generalization is a very subtle problem, but of high interest. An isometric embedding into Euclidean three-dimensional space is an appropriate solution and allows a genuinely intrinsic comparison. With this method, the leading-order differences are determined, which are at the mm-level.

On the Effects of Package on the PMUTs Performances-Multiphysics Model and Frequency Analyses.

This paper deals with a multiphysics numerical modelling via finite element method (FEM) of an air-coupled array piezoelectric micromachined ultrasonic transducers (PMUTs). The proposed numerical model is fully 3D with the following features: the presence of the fabrication induced residual stresses, which determine a geometrically non-linear initial deformed configuration of the diaphragms and a remarkable shift of the fundamental frequency; the multiple coupling between different physics, namely electro-mechanical-coupling for the piezo-electric model, acoustic-structure interaction at the acoustic-structure interface and pressure acoustics in the surrounding air. The model takes into account the complete set of PMUTs belonging to the silicon die in a 4 × 4 array configuration and the protective package, as well. The results have been validated by experimental data, in terms of initial static pre-deflected configuration of the diaphragms and frequency response function of the PMUT. The numerical procedure was applied, to analyze different package configurations of the device, to study the influence of the holes on the acoustic transmission in terms of SPL and propagation pattern and consequently extract a set of design guidelines.

Intelligent Traffic Flow Assist: Optimized Highway Driving Using Conditional Behavior Prediction

Many conventional systems for driver assistance on highways determine vehicle motion based on static situation configurations only. Adaptive Cruise Control systems for example react to vehicles cutting-in from neighboring lanes only after these vehicles have entered the ego-lane. Some more recent systems include prediction of future behaviors, but often neglect mutual influences between these and the best behavior for the ego vehicle. This paper introduces ?intelligent Traffic Flow Assist?, an assistance system that analyzes the current traffic situation and predicts candidate future behaviors conditioned on different options for the ego behavior. It computes vehicle trajectories optimized for safety, comfort and utility for each of the possible future situations and selects the best option for vehicle control. We describe the system architecture and implementation on a prototype vehicle with close-toproduction sensors and report on tests performed in different situations on proving ground and public highways.

Helicity of magnetic vortices and skyrmions in soft ferromagnetic nanodots and films biased by stray radial fields

Static magnetization configurations of thin soft ferromagnetic films and nanodots, coupled to a hard antidot matrix with out-of-plane magnetization, are studied by micromagnetic simulations and analytical calculations. When the antidot matrix produces sufficient stray fields, having radial symmetry, these nanostructures support the formation of topologically nontrivial magnetic configurations---vortices and skyrmions in nanodots and films, respectively. It is demonstrated that the studied nanostructure reveals an additional degree of freedom---the helicity of the vortex or skyrmion---which can be tuned on demand by a variation of the material parameters and geometry. The variation of helicity $\ensuremath{\gamma}$ is not abrupt. In addition to Neel-like (radial) vortices and skyrmions $(\ensuremath{\gamma}=0,\ensuremath{\pi})$, it is possible to achieve unconventional configurations with an intermediate helicity $\ensuremath{\gamma}\ensuremath{\ne}0,\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2,\ensuremath{\pi}$, which transform to common Bloch-like configurations $(\ensuremath{\gamma}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2)$ in the limit of negligible stray fields from the matrix. We present an analytical model, which allows us to calculate the stability region of pure Neel-like states, outside which unconventional magnetization states with intermediate helicity are realized.

Discretization limits of lattice‐Boltzmann methods for studying immiscible two‐phase flow in porous media

SummaryDigital images of porous media often include features approaching the image resolution length scale. The behavior of numerical methods at low resolution is therefore important even for well‐resolved systems. We study the behavior of the Shan‐Chen (SC) and Rothman‐Keller (RK) multicomponent lattice‐Boltzmann models in situations where the fluid‐fluid interfacial radius of curvature and/or the feature size of the medium approaches the discrete unit size of the computational grid. Various simple, small‐scale test geometries are considered, and a drainage test is also performed in a Bentheimer sandstone sample. We find that both RK and SC models show very high ultimate limits: in ideal conditions the models can simulate static fluid configuration with acceptable accuracy in tubes as small as three lattice units across for RK model (six lattice units for SC model) and with an interfacial radius of curvature of two lattice units for RK and SC models. However, the stability of the models is affected when operating in these extreme discrete limits: in certain circumstances the models exhibit behaviors ranging from loss of accuracy to numerical instability. We discuss the circumstances where these behaviors occur and the ramifications for larger‐scale fluid displacement simulations in porous media, along with strategies to mitigate the most severe effects. Overall we find that the RK model, with modern enhancements, exhibits fewer instabilities and is more suitable for systems of low fluid‐fluid miscibility. The shortcomings of the SC model seem to arise predominantly from the high, strongly pressure‐dependent miscibility of the two fluid components.

New class of naked singularities and their observational signatures

By imposing suitable junction conditions on a spacelike hypersurface, we obtain a two-parameter family of possible static configurations from gravitational collapse. These exemplify a new class of naked singularities. We show that these admit a consistent description via a two-fluid model, one of which might be dust. We then study lensing and accretion disk properties of our solution and point out possible differences with black hole scenarios. The distinctive features of our solution compared to the existing naked singularity solutions in the literature are discussed.

Proximal Femoral Nail Unlocked versus Locked (ProFNUL): a protocol for a multicentre, parallel-armed randomised controlled trial for the effect of femoral nail mode of lag screw locking and screw configuration in the treatment of intertrochanteric femur fractures

IntroductionIntertrochanteric fractures are common fragility injuries in the elderly. Surgical fixation using intramedullary devices are one of the widely used management options. To date, evidence demonstrating the effects of lag...

Transient Regimes of the Screw Dynamo

The screw dynamo is a critical phenomenon—a self-excitation that occurs only when the magnetic Reynolds number (Rm) reaches some threshold value. Temporal magnetic field growth is shown to occur at Rm slightly smaller than this critical value. The appearance of temporal growth is also limited from below by the second critical value of Rm. If, however, a seed magnetic field is present permanently, and not just at the initial time of evolution, temporal subcritical magnetic field growth gives rise to a stationary magnetic configuration. This stationary magnetic field turns out to be particularly strong if the seed field in the form of a traveling wave coincides in phase velocity with the magnetic field that emerges in the presence of a self-excitation. The possibilities for detecting such a resonance in the subcritical regime of operation of an experimental setup based on the screw dynamo are considered.

Dynamical (in)stability analysis of a radiating star model, cast from an initial static configuration

A static model, which has been extended to a dynamical, heat dissipating model via a perturbative scheme, is investigated for dynamical (in)stability. The static model is based on a linear equation of state of the form $$p_r = \gamma \rho - \delta $$, suitable for describing strange stars. Our model allows for pressure anisotropy, and we monitor the change in anisotropy as a function of both time and radial coordinate to gain further physical insight into the collapse process. The stability of the collapsing star is studied in both the Newtonian and the post-Newtonian approximations.

Automated Model-Based Optimization of Data-Adaptable Embedded Systems

Dynamic data-driven applications such as object tracking, surveillance, and other sensing and decision applications are largely dependent on the characteristics of the data streams on which they operate. The underlying models and algorithms of data-driven applications must continually adapt at runtime to changes in data quality and availability to meet both functional and designer-specified performance requirements. Given the dynamic nature of these applications, point solutions produced by traditional design tools cannot be expected to perform adequately across varying execution scenarios. Additionally, the increasing diversity and interdependence of application requirements complicates the design and optimization process. To assist designers of data-driven applications, we present a modeling and optimization framework that enables developers to model an application's data sources, tasks, and exchanged data tokens; specify application requirements through high-level design metrics and fuzzy logic--based optimization rules; and define an estimation framework to automatically optimize the application at runtime. We demonstrate the modeling and optimization process via an example application for video-based vehicle tracking and collision avoidance. We analyze the benefits of runtime optimization by comparing the performance of static point solutions to dynamic solutions over five distinct execution scenarios, showing improvements of up to 74% for dynamic over static configurations. Further, we show the benefits of using fuzzy logic--based rules over traditional weighted functions for the specification and evaluation of competing high-level metrics in optimization.

Magnetoelectrostatics of axionically active systems: Induced field restructuring in magnetic stars

In the framework of the Einstein-Maxwell-axion theory, we consider a static field configuration in the outer zone of a magnetic star. We assume that this field configuration is formed by the following interacting quartet: a spherically symmetric gravitational field of the Reissner-Nordstr\"om type, pseudoscalar field, associated with an axionic dark matter, strong intrinsic magnetic field, and axionically induced electric field. Based on the analysis of the solutions to the master equations of the model, we show that the structure of the formed field configuration reveals the following triple effect. First, if the strong magnetic field of the star has the dipole component, the axionic halo around the magnetic star with spherically symmetric gravitational field is no longer spheroidal, and the distortion is induced by the axion-photon coupling. Second, due to the distortion of the axionic halo, the magnetic field is no longer pure dipolar, and the induced quadruple, octupole, etc., components appear, thus redistributing the magnetic energy between the components with more steep radial profiles than the dipolar one. Third, the interaction of the axionic and magnetic fields produces the electric field, which inherits their multipole structure. We attract attention to possible applications of the predicted field restructuring to the theory of axionic rotating magnetars.

Role of Intermediate Dynamics in Controlling Hydrogenation Selectivity by Heterogeneous Catalysis.

Selectivity in heterogeneous catalysis is essential while being the most difficult parameters to obtain from theoretical simulations. Although theory holds the key for catalytic design, in the search of complex reaction networks, intermediates are considered static and different configurations for addition reactions are not considered. Here, we present the crucial role of intermediate dynamics to understand reaction selectivity of heterogeneous catalysts by studying the dynamic properties. The hydrogenation of two different intermediates hydroperoxide (OOH) and vinyl (HCCH2) is crucial to in the direct synthesis of water peroxide and alkyne semi-hydrogenation are taken as examples on the bare and alloyed surfaces. For them, the reaction network is studied by density functional theory coupled to molecular dynamics to present the role of rotations and how they affect paths for the addition of atomic hydrogen to lead to the products. In summary, the thermodynamic selectivity can be mapped to the dynamic control in the kinetics of the process.

Impact Analysis of Demand Management on Runway Configuration in Metroplex Airports

Congestion in airports and metroplexes is increasingly becoming a key bottleneck in the global air transport system. It is largely due to inefficient utilization of runway resources and its consequences of imbalance between demand and capacity. Existing studies mainly focus on runway configuration in a single airport system, and little consideration is given to the impact of demand management on runway configuration in metroplexes. Therefore, this paper proposes a methodology and assessment framework for runway configuration with a focus on the exploitation of multiple active runways in metroplex airports. The primary innovation is an integrated runway configuration management formulation with air traffic demand management options. The first is static and dynamic runway configuration management models featuring three optimization objectives, four cases of demand-capacity imbalance, three air traffic demand management options and three assessment scenarios, for eleven priority settings. The second is an efficient multi-objective evolutionary algorithm with a mechanism of objective-guided individual selection, which can obtain close-to optimal solutions with a very low computational cost within 6 seconds. Computational experiments for the real-world case of the Shanghai metroplex airports show that, the inducing strategy of minimizing the total number of adjusted flights is the best mechanism for making satisfactory tradeoffs among multiple objectives in dynamic runway configuration. The proposed model reduces the total number of adjusted flights by an average of 36% compared with the baseline static runway configuration management. Furthermore, unlike the conventional approach of giving priority to arriving aircraft, a higher priority for departures is more effective in enhancing the performance of runway systems and reducing the number of adjusted flights. The proposed framework can be applied at pre-tactical (e.g., one-day planning) as well as tactical (e.g., several-hours rolling horizon) levels.

The Intelligent Offense and Defense Mechanism of Internet of Vehicles Based on the Differential Game-IP Hopping

The Road Side Unit (RSU) is an important part of the Internet of Vehicles (IoV), which collects a large number of user's privacy information and connects vehicles to the Internet. The current RSU is running with static configurations, and the adversary can get enough time to analyze the vulnerability of the RSU and launch attacks, which poses a huge threat to the privacy security of the IoV. Under this asymmetrical condition, the attackers occupy a predominant position, and it is difficult to fully defend. To solve the above asymmetry offense-defense problem, we adopt the moving target defense (MTD) to dynamically change the IP address of the RSU, which increases the difficulty of the attacker's attack. However, due to the unreasonable selection of the RSU's IP hopping frequency, it is difficult for the RSU's IP hopping to exert its defense effect or it may introduce excessive defense overheads. Given the above issue, we propose an intelligent offense and defense mechanism based on a differential game approach, which can adaptively adjust the RSU's IP hopping frequency and maximize its defense benefits. First, based on the offense and defense relationship, we model the RSU's IP hopping frequency intelligent arrangement as a differential game (i.e., IP hopping game) in which the attacker attempts to maximize his potential benefits, and the RSU manages to maximize its defense benefits. On this basis, we calculate a potential Nash equilibrium of the IP hopping game. Finally, simulation results show our proposed mechanism can adaptively adjust the RSU's IP hopping frequency according to the attack frequency and effectively defend attacks. Compared with the mechanisms with a fixed IP hopping frequency, our proposed mechanism can make up for the ineffectiveness of the fixed low-frequency IP hopping defense and avoid the excessive defense overheads of the fixed high-frequency IP hopping defense.

Web Proxy Auto Discovery for Dynamically Created Web Proxies

The WLCG Web Proxy Auto Discovery (WPAD) service provides a convenient mechanism for jobs running anywhere on the WLCG to dynamically discover web proxy cache servers that are nearby. The web proxy caches are general purpose for a number of different http applications, but different applications have different usage characteristics and not all proxy caches are engineered to work with the heaviest loads. For this reason, the initial sources of information for WLCG WPAD were the static configurations that ATLAS and CMS maintain for the Conditions data that they read through the Frontier Distributed Database system, which is the most demanding popular WLCG application for web proxy caches. That works well for use at traditional statically defined WLCG sites, but now that usage of commercial clouds is increasing, there is also a need for web proxy caches to dynamically register themselves as they are created. A package called Shoal had already been created to manage dynamically created web proxy caches. This paper describes the integration of the Shoal package into the WLCG WPAD system, such that both statically and dynamically created web proxy caches can be located from a single source. It also describes other improvements to the WLCG WPAD system since the last CHEP publication.

Transformation of an Aviation Turboshaft Engine into a Gas Turbine Experimental Facilities for Laboratory Testing

The article deals with the use of a small aviation turboshaft engine for laboratory purposes. This study describes its transformation into an experimental device for research and education. Various constructional, technological and controlling modifications and settings of the gas turbine test stand were carried out and tested on a stationary configuration. The stationary system can be used as a small backup power generator or as a drive unit for a compressor, pump, etc. The laboratory equipment can also be used for research into biofuels (fatty acid methyl esters and bioethanol) for use in aviation and energetics. New control systems, electronic elements and methods of measuring rotations, pressure and temperature are tested for educational and research purposes. The study includes a schematic description of modelling measurements and subsequent numerical evaluation of the thermodynamic characteristics of the cycle in an experimental gas turbine. The laboratory device presented here is, thanks to technological, material and thermodynamic research, suitable for educating and testing the knowledge of future aviation and mechanical engineers.

Chance-Constrained MPC for Voronoi-based Multi-Agent System Deployment

This paper proposes a new chance-constrained model predictive control (CCMPC) algorithm with state estimation applied to the two-dimensional deployment of a multi-vehicle system where each agent is subject to process noise and measurement noise. The bounded convex area of deployment is partitioned into time-varying Voronoi cells defined by the position of each agent. Due to the presence of noise in the system model, stochastic constraints appear in the model predictive control problem. The proposed decentralized robust CCMPC algorithm drives the multi-agent system into a static Chebyshev configuration where each agent lies on the Chebyshev center of its Voronoi cell. Simulation results show the effectiveness of the proposed control strategy on a fleet of quadrotors subject to wind perturbations and measurement noise.