Optimal Periodic Inspection Policy for a Parallel System Subject to Continuous Bi-Levels Degradation

Each system goes through several intermediate states between its initial state and the failure state, which are called degradation states. In addition, knowing these intermediate states allows us the building of an effective maintenance plan for the system under consideration. In this paper, we have considered a two-component parallel system where each component is subjected to two levels of degradation. Our first objective is to identify the mathematical (probabilistic) model associated with this system and this when the hypothesis of periodic inspection of the system is retained. Then, the main idea is to build the economic model encompassing the different average costs (inspection costs, preventive maintenance costs and corrective maintenance costs) spent during a renewal cycle. The latter will allow us to determine the optimal “inspection interval” while minimizing the average global-cost spent. Finally, in order to illustrate and analyze the sensitivity of the optimal inspection policy to the various maintenance costs, numerical examples are presented.

High-order harmonic generation in hexagonal nanoribbons

The generation of high-order harmonics in finite, hexagonal nanoribbons is simulated. Ribbons with armchair and zig-zag edges are investigated by using a tight-binding approach with only nearest-neighbor hopping. By turning an alternating on-site potential off or on, the system describes for example graphene or hexagonal boron nitride, respectively. The incoming laser pulse is linearly polarized along with the ribbons. The emitted light has a polarization component parallel to the polarization of the incoming field. The presence or absence of a polarization component perpendicular to the polarization of the incoming field can be explained by the symmetry of the ribbons. Characteristic features in the harmonic spectra for the finite ribbons are analyzed with the help of the band structure for the corresponding periodic systems.

Likelihood Ratio Test and Non-parametric Test for Load Sharing

In present article, we propose a likelihood ratio test and a non-parametric test for testing the load sharing effect observed in the two component parallel load sharing system. We have modeled the load sharing phenomenon observed in such system by the exponentiated conditional distribution function based load sharing model proposed by Sutar and Naik-Nimbalkar (2016). We have taken component baseline distribution as Weibull distribution and linear failure rate distribution. The simulation study to see the performance of proposed test procedures is reported. We analyze two data sets for illustrative purpose.

Resonant absorption: Transformation of compressive motions into vortical motions

This paper investigates the changes in spatial properties when magnetohydrodynamic (MHD) waves undergo resonant damping in the Alfvén continuum. The analysis is carried out for a 1D cylindrical pressure-less plasma with a straight magnetic field. The effect of the damping on the spatial wave variables is determined by using complex frequencies that arise as a result of the resonant damping. Compression and vorticity are used to characterise the spatial evolution of the MHD wave. The most striking result is the huge spatial variation in the vorticity component parallel to the magnetic field. Parallel vorticity vanishes in the uniform part of the equilibrium. However, when the MHD wave moves into the non-uniform part, parallel vorticity explodes to values that are orders of magnitude higher than those attained by the transverse components in planes normal to the straight magnetic field. In the non-uniform part of the equilibrium plasma, the MHD wave is controlled by parallel vorticity and resembles an Alfvén wave, with the unfamiliar property that it has pressure variations even in the linear regime.

The influence of layer-interface geometry on single-layer folding

Geometrical heterogeneities along layer interfaces play a key role in determining the geometries of folds developed during shortening of competent layers. We present a series of numerical simulations to investigate the influence of initial sinusoidal perturbations on the folding of single layers. Models consist of a competent viscous single layer embedded in a softer matrix, with the layer orientated parallel to the shortening direction. We first generalize the wide spectrum of sinusoidal perturbations accounting for asymmetries along and across a competent single layer, using two parameters: transversal asymmetry (A′) and longitudinal asymmetry (φ). These two parameters allow the transition between classical fold shapes and pinch and swell geometries to be studied. The parameter A′ describes the development of fold hinges with different geometries between the upper and lower layer interfaces, and abnormal curvatures between the outer and inner arcs of fold hinges. The parameter φ induces a strong polarity on the folds, with a systematic preferred orientation of the pinch and swell regions of the layer, even if there is no shear component parallel to the layer. Our results demonstrate the importance of structural inheritance on the resulting fold geometries, and suggest that caution must be taken when using certain types of asymmetrical folds as strain markers and kinematic indicators.

A condition-based opportunistic maintenance policy integrated with energy efficiency for two-component parallel systems

Purpose: In order to improve the energy utilization and achieve sustainable development, this paper integrates energy efficiency into condition-based maintenance(CBM) decision-making for two-component parallel systems. The objective is to obtain the optimal maintenance policy by minimizing total cost.Design/methodology/approach: Based on energy efficiency, the paper considers the economic dependence between the two components to take opportunistic maintenance. Specifically, the objective function consists of traditional maintenance cost and energy cost incurred by energy consumption of components. In order to assess the performance of the proposed new maintenance policy, the paper uses Monte-Carlo method to evaluate the total cost and find the optimal maintenance policy.Findings: Simulation results indicate that the new maintenance policy is superior to the classical condition-based opportunistic maintenance policy in terms of total economic costs.Originality/value: For two-component parallel systems, previous researches usually simply establish a condition-based opportunistic maintenance model based on real deterioration data, but ignore energy consumption, energy efficiency (EE) and their contributions of sustainable development. This paper creatively takes energy efficiency into condition-based maintenance(CBM) decision-making process, and proposes a new condition-based opportunistic maintenance policy by using energy efficiency indicator(EEI).

The shape and motion of gas bubbles in a liquid flowing through a thin annulus

We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.

The effect of roughness and correlation on the Casimir torque between two plates

The effect of roughness and correlation on the Casimir torque is studied. The plates are assumed to be perfect conductors. This is a good approximation when the separation between plates is not too small. The pairwise summation (PWS) method is used, which is a good approximation when the correlation length is much larger than the distance between the plates. Torque components both parallel and perpendicular to the plates are obtained. It is seen that the component parallel to the plates is nonvanishing even if the plates are smooth, but there are contributions due to roughness and correlation as well, and the contribution of the correlation is an increasing function of both the roughness exponent and the correlation length. The component perpendicular to the plates, however, is nonvanishing only if the plates are rough and correlated to each other. As the roughness exponent increases, this component increases, reaches a peak and then decreases.