Quadratic stabilizability of switched linear systems with polytopic uncertainties

In this paper, we consider quadratic stabilizability via state feedback for both continuous-time and discrete-time switched linear systems that are composed of polytopic uncertain subsystems. By state feedback, we mean that the switchings among subsystems are dependent on system states. For continuous-time switched linear systems, we show that if there exists a common positive definite matrix for stability of all convex combinations of the extreme points which belong to different subsystem matrices, then the switched system is quadratically stabilizable via state feedback. For discrete-time switched linear systems, we derive a quadratic stabilizability condition expressed as matrix inequalities with respect to a family of non-negative scalars and a common positive definite matrix. For both continuous-time and discrete-time switched systems, we propose the switching rules by using the obtained common positive definite matrix.     

Clutter suppression method for heterogeneous environment

Clutter suppression is a key process in ground moving target indication (GMTI). Adaptive filtering is an effective method for clutter suppression but the performance degrades severely in heterogeneous clutter environment. To solve this problem, a new clutter suppression method is proposed. The approach uses subspace tracking technique to update clutter subspace of different clutter patches which may mitigating the heterogeneous effects. Simulation results illustrate that the method performs well when lacking of the independent and identically distributed (i.i.d.) samples.     

洞庭湖盆地线性影象特征分析

本文从基底时代、基底结构、沉积盖层、构造格局和水系分布等特征,论述了洞庭湖盆地北部与南部的差异性及与江汉盆地的相似性,从遥感图象上提取大量信息,经处理、解译认为,可把洞庭湖盆地北部和江汉盆地看作为一个整体,这对石油天然气勘探将是有益的。     

Effects of shell thickness of polystyrene‐encapsulated Mg(OH)2 on flammability and rheological properties of high‐impact polystyrene composites

Polystyrene (PS)‐encapsulated magnesium hydroxide (Mg(OH)₂) particles with various shell thicknesses were successfully prepared using a method of in situ polymerization of styrene in a high‐speed mixer. High‐impact polystyrene (HIPS)/Mg(OH)₂ composites were prepared by melt blending. They were characterized using cone calorimetry, horizontal burning rate, rheology and scanning electron microscopy in order to investigate the effects of the shell thickness of the PS‐encapsulated Mg(OH)₂ on the flame retardancy and rheological properties of the resulting composites. Rheological tests showed that the composites containing encapsulated Mg(OH)₂ had a stronger solid‐like response at low frequency than that of the sample containing untreated Mg(OH)₂. However, with PS/Mg(OH)₂ ratio increasing up to 6.0 wt% and above, the dynamic viscosity, loss modulus and storage modulus of HIPS/Mg(OH)₂ composites decreased. The optimum PS/Mg(OH)₂ ratio, 4.5 wt%, was determined using a new ‘crossover point’ rheological method. The combustion tests showed that compared to the composites containing untreated Mg(OH)₂, the fire retardancy of the composites containing PS‐encapsulated Mg(OH)₂ was improved significantly. Also, there appeared to be a critical PS/Mg(OH)₂ ratio, namely 6.0 wt%, for optimum flame‐retarding properties. However with the continuous increase of PS/Mg(OH)₂ ratio, the fire resistance of the composites declined somewhat, which can be explained by acceleration of combustion of the composites due to the introduction of free PS chains of low molecular weight on the surface of Mg(OH)₂. Copyright © 2007 Society of Chemical Industry     

基于温度场的冻土路基变形数值分析及对比

考虑相变和全球气温升高的影响,利用数值分析方法,对青藏铁路典型路基的变形问题进行了分析和数值模拟。通过建立路基有限元模型,选取适当的边界条件、初始条件及热学计算参数,在计算了典型路基断面温度场变化的前提下,对三年的路基变形场进行了数值分析并与实际测试资料进行了对比分析。分析表明,融化沉降在一年内已基本完成,并逐渐开始回冻,两三年后,除路堤填料和地基的进一步压密外,蠕变效应开始显现。因此,对高温、高含冰量冻土而言,蠕变产生的沉降变形是不可忽略的。     

Crystallization,melting behavior,and wettability of poly(ε‐caprolactone) and poly(ε‐caprolactone)/ poly(N‐vinylpyrrolidone) blends

Both wettability and crystallizability control poly(ε‐caprolactone)'s (PCL) further applications as biomaterial. The wettability is an important property that is governed by both chemical composition and surface structure. In this study, we prepared the PCL/poly(N‐vinylpyrrolidone) (PVP) blends via successive in situ polymerization steps aiming for improving the wettability and decreasing crystallizability of PCL. The isothermal crystallization of PCL/PVP at different PVP concentrations was carried out. The equilibrium melting point (T), crystallization rate, and the melting behavior after isothermal crystallization were investigated using differential scanning calorimetry (DSC). The Avrami equation was used to fit the isothermal crystallization. The DSC results showed that PVP had restraining effect on the crystallizability of PCL, and the crystallization rate of PCL decreased clearly with the increase of PVP content in the blends. The X‐ray diffraction analysis (WAXD) results agreed with that. Water absorptivity and contact angle tests showed that the hydrophilic properties were improved with the increasing content of PVP in blends. The coefficient for the water diffusion into PCL/PVP blends showed to be non‐Fickian in character. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and thermal characterization of random poly(butylene terephthalate‐co‐2‐methyl‐ethylene terephthalate) copolyesters

Poly(butylene terephthalate‐co‐2‐methyl‐ethylene terephthalate) (PBT/MET) was synthesized by incorporating 1,2‐propandiol(1,2‐PDO) into PBT chains. The molar composition and chemical structure of PBT/MET copolyesters were confirmed by means of FT‐IR and ¹H‐NMR. To investigate the effect of 1,2‐PDO on the thermal properties of PBT/MET copolyesters, the copolymerizations were carried out by varying various contents of MET units, and the prepared materials were evaluated by differential scanning calorimetry and thermogravimetric analysis. Results suggested that with the increase of the content of 1,2‐PDO, the amount of crystallinity and the melting temperature decline, while the glass transition temperature increases and the copolyesters become more transparent and brittle with respect to PBT homopolymer. In addition, the T_g‐composition and T_m‐composition data are well subjected to the Wood equation and Flory's equation, respectively. All these copolyesters are found to consist of the general trend displayed by copolymers reported elsewhere. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

四针状氧化锌晶须对聚丙烯力学性能及抗菌性能的影响

采用偶联剂KH-570对四针状氧化锌晶须(T-ZnOw)进行表面改性,制备了聚丙烯(PP)/改性T-ZnOw复合材料。对复合材料进行了X射线衍射分析,并考察了T-ZnOw含量对复合材料力学性能和抗菌性能的影响。结果表明:T-ZnOw在PP基体中结晶良好;适量添加经过表面改性的T-ZnOw能提高复合材料的拉伸强度、断裂伸长率和弯曲强度;在T-ZnOw含量为4%时,复合材料即可达到国家规定的抗菌要求。     

南阳膨胀土试验段现场大剪及室内试验对比分析

南水北调中线总干渠线路长达1 276.6 km,渠线通过不同的地质单元,其中膨胀土是工程要解决的最主要工程地质问题之一。膨胀土力学参数取值是膨胀土渠段渠道边坡设计和稳定计算的重要依据,通过南阳膨胀土现场大剪土工试验成果与室内试验成果进行对比分析,提出了膨胀土渠坡设计参数参考值,对膨胀土渠坡设计提供了可靠的设计依据。     

不同进气氛围耦合废气再循环对双燃料发动机工作过程的影响

基于正庚烷、甲烷、乙烷、丙烷多组分混合物简化动力学机理耦合三维计算流体力学（computational fluid dynamics,CFD）数值模型,模拟研究高替代率时不同进气氛围（H₂、O₂组分）耦合废气再循环（exhaust gas recirculation,EGR）对天然气/柴油双燃料发动机低负荷工作过程的影响机理。研究表明：在不同EGR率下,进气掺氢会使缸内燃烧速率显著加快,OH活性基浓度明显升高,CH₄排放显著降低,但CO排放升高;进气掺氧后,缸压及瞬时放热率峰值、最大压力升高率、最高燃烧温度及OH活性基浓度均升高,碳烟、CO和CH₄后期氧化作用增强使其最终排放降低,但NO_x排放升高。在EGR率小于29%,掺氢比小于2.5%时,在实现较低CO、碳烟排放的同时能显著降低CH₄排放和NO₂/NO_x比例;高EGR率时,进气掺氧能降低CO、碳烟排放,并改善CH₄与NO_x