Improved Moment Method Research Articles

System reliability assessment of deteriorating structures subjected to time-invariant loads based on improved moment method

The time-variant system reliability analysis is a significant topic in the field of reliability engineering. Although progress has been made, there remain multiple challenges in the existing methods including the explosive number of possible failure modes, requiring correlation information, and excessive computational efforts.In the present work, an improved moment method with high accuracy, efficiency and robustness is proposed for system reliability analysis of deteriorating structures. First, by introducing the complete system failure process, an equivalent time dependent performance function describing a deteriorating structural system is obtained. Second, a point estimate method based on the adaptive trivariate dimensional decomposition is adopted to calculate the first six moments of the performance function. And third, a saddlepoint approximation involving the first six moments is developed to estimate the system failure probability. Several examples are investigated to verify the accuracy, efficiency and stability of the proposed method.

Soliton dynamics in non-uniform fiber tapers: analytical description through an improved moment method

We develop an improved moment method to model soliton propagation in optical fibers. We account for the full Raman gain spectrum of the material and derive a system of coupled differential equations describing the evolution of five moments of the pulse, valid for arbitrary soliton durations. By comparing with the numerical solution of the generalized nonlinear Schrödinger equation, the improved moment method is shown to accurately represent soliton self-frequency shift under complex dispersion, nonlinearity, and Raman gain spectra. Numerical examples are presented for a dispersion-shifted fused silica fiber and a non-uniform ZBLAN fluoride fiber taper. The latter demonstrates an enhanced soliton self-frequency shift through axial dispersion and nonlinearity engineering along the taper length.

A Moment Method for Evaporation Using a Logarithmic Size Distribution with the Smallest Size

A moment method of the log-normal distribution with the smallest size is applied to evaporation by newly introducing correction factors obtained from the error function. In this article, the improved moment method is tested for the evaporation, and is compared with the exact solutions calculated by the CIP semi-Lagrangian (CIP-SL) method. In small particle regime, the size distributions and the time histories of the total number and volume per unit volume are reproduced by the moment method for the evaporation near the smallest size. In large particle regime, however, the differences between the moment and exact solutions are larger with time, after the exact distribution spreads to the smallest size. This new moment solver can reproduce the evaporation near the smallest size even when large time step size is given, and thus is expected to be used for the parameterization of the evaporation of small particles in aerosol-transport model.

Red shift of the optical absorption edge of ferromagnetic semiconductors

We propose an improved moment method, which explicitly takes into account quasiparticle damping, for determining the temperature dependence of the conduction band of a ferromagnetic semiconductor. The theory predicts a red shift of the lower conduction band edge, which strongly depends on the band width W of the semiconductor. The experimental data for the red shifts of the Eu-chalcoginides can then be used to derive the width of the 5 dt 2 g -conduction band. We find W = 2.31 eV for EuO, W = 1.40 eV for EuS, and W = 1.15 eV for EuSe, taken at room temperature.