基于随机共振原理的低频弱信号检测方法研究

介绍了双稳系统,讨论了产生随机共振的条件以及输出频谱的特性。针对原双稳系统不能检测中低频率信号的问题,利用参数变换原理,推导出了一种新的模型,阐述与分析了该模型的参数变化对输出混合信号的影响。模型相对于经典双稳系统模型降低了对输入的要求。     

一种多层圆管纵向导波频散特性分析方法研究

针对导波频散特性对圆管纵向导波探伤效果影响的问题,提出一种基于频散特性的外推分析方法,通过优化选取导波检测模式和频率来减少频散特性对圆管纵向导波探伤结果的不良影响。首先应用全局矩阵法推导出多层圆管纵向导波的Pochhammer-Chree频散特性方程,并提出一种能够反映纵向导波传播强弱的截面功率流模型。然后结合外推预测和双尺度迭代逼近过程,给出了导波频散特性以及截面功率流的数值计算方法,并利用计算结果进行分析,提出一种导波检测模式和频率的选择方法。实验表明其结果与理论分析基本一致。上述方法可为圆管损伤检测中导波参数的合理选择提供参考。     

Vegetable waste as substrate and source of suitable microflora for bio-hydrogen production

Self-fermentation of cellulosic substrates to produce biohydrogen without inoculum addition nor pretreatments was investigated. Dark fermentation of two different substrates made of leaf-shaped vegetable refuses (V) and leaf-shaped vegetable refuses plus potato peels (VP), was taken in consideration. Batch experiments were carried out, under two mesophilic anaerobic conditions (28 and 37 °C), in order to isolate and to identify potential H₂-producing bacterial strains contained in the vegetable extracts. The effect of initial glucose concentration (at 1, 5 and 10 g/L) on fermentative H₂ production by the isolates was also evaluated.H₂ production from self-fermentation of both biomasses was found to be feasible, without methane evolution, showing the highest yield for V biomass at 28 °C (24 L/kg VS). The pH control of the culture medium proved to be a critical parameter. The isolates had sequence similarities ≥98% with already known strains, belonging to the family Enterobacteriaceae (γ-proteobacteria) and Streptococcaceae (Firmicutes). Four genera found in the samples, namely Pectobacterium, Raoultella, Rahnella and Lactococcus have not been previously described for H₂ production from glucose. The isolates showed higher yield (1.6–2.2 mol H₂/mol glucose_added) at low glucose concentration (1 g/L), while the maximum H₂ production ranged from 410 to 1016 mL/L and was obtained at a substrate concentration of 10 g/L. The results suggested that vegetable waste can be effectively used as both, substrate and source of suitable microflora for bio-hydrogen production.     

Pulse-modulation eddy current probes for imaging of external corrosion in nonmagnetic pipes

Since the nonmagnetic pipe is normally utilized in corrosive and hostile environment, it is prone to the external corrosion which occurs on the outer surface of the pipe and severely undermines the structural integrity and safety. Although Pulsed Eddy Current technique (PEC) is currently preferred for detection and evaluation of subsurface defects in tubular conductors, it is subject to technical drawbacks. In light of this, Pulse-modulation Eddy Current technique (PMEC) is intensively investigated in the paper for enhancement of the evaluation sensitivity to external corrosion and accuracy of corrosion imaging. Closed-form expressions of the PMEC response and its sensitivity to external corrosion in tubular conductors are formulated via the Extended Truncated Region Eigenfunction Expansion (ETREE) modeling. Following simulations for analysis and comparison of field signals and evaluation sensitivities of PMEC and PEC, experiments of PMEC for evaluation and imaging of external corrosion are carried out. Through theoretical and experimental investigation, it has been found that regarding the evaluation and imaging of external corrosion in nonmagnetic pipes, the PMEC-based probe have higher sensitivity and imaging accuracy than that based on PEC. The superiority of PMEC to PEC in inspection of tubular conductors is further identified.     

Isogeometric computation reuse method for complex objects with topology-consistent volumetric parameterization

Volumetric spline parameterization and computational efficiency are two main challenges in isogeometric analysis (IGA). To tackle this problem, we propose a framework of computation reuse in IGA on a set of three-dimensional models with similar semantic features. Given a template domain, B-spline based consistent volumetric parameterization is first constructed for a set of models with similar semantic features. An efficient quadrature-free method is investigated in our framework to compute the entries of stiffness matrix by Bézier extraction and polynomial approximation. In our approach, evaluation on the stiffness matrix and imposition of the boundary conditions can be pre-computed and reused during IGA on a set of CAD models. Examples with complex geometry are presented to show the effectiveness of our methods, and efficiency similar to the computation in linear finite element analysis can be achieved for IGA taken on a set of models.     

Combustion characteristics and radiation performance of premixed hydrogen/air combustion in a mesoscale divergent porous media combustor

To improve flammability and radiation efficiency, a divergent porous media combustor is proposed and numerically studied. The local thermal non-equilibrium model is used to consider the temperature difference between gas and solid matrix. Effects of equivalence ratio, the wall thermal conductivity, solid matrix thermal conductivity, and divergent ratio on combustion characteristics, radiation efficiency, and flammability limits are studied. The results show that the divergent channel extends the blowout limit by 186% and obtains a maximum radiation efficiency of 29.3%, increased by 70% compared with the straight channel. A smaller wall thermal conductivity is recommended considering the flammability range and radiation efficiency. A careful choice of solid matrix thermal conductivity and the divergent ratio is suggested to balance their opposing effects on the radiation efficiency and the flammability.     

Robust motion tracking control of partially nonholonomic mechanical systems

This paper proposes a stable motion tracking control law for mechanical systems subject to both nonholonomic and holonomic constraints. The control law is developed at the dynamic level and can deal with model uncertainties. The proposed control law ensures the desired trajectory tracking of the configuration state of the closed-loop system. A global asymptotic stability result is obtained in the Lyapunov sense. A detailed example is presented to illustrate the proposed method.