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981.
为了提高小长径比弹丸射击质量,设计了一种大展弦比张开式尾翼,采用AUSM+格式、SST(shearstress transport)湍流模型和隐式算法(lower-upper symmetric Gauss-Seidel implicit method, LU-SGS),求解三维RANS 方程,对前体形状完全相同,不同展弦比的3 种尾翼弹进行了数值模拟,得到了三者在马赫数1.5~3.5 下的气动力特性的差异,分析其原因,并给出了不同展弦比张开式尾翼的适用范围. 计算结果表明:C型弹的升阻比较B 型弹在1.5 马赫数区域附近增加了7% 以上,当马赫数达到2.5 以上时,A 型弹的升阻比大于B 型弹和C 型弹,在3.5 马赫数区域附近A 型弹的升阻比较B 型弹增加了5.4% 以上. 3 种弹丸的俯仰力矩系数随着马赫数的增大而负向减少,且减少的趋势随着展弦比的增加而增大. A 型弹、B 型弹、C 型弹的静稳定裕度的变化范围分别为4%~20.3%,8.5%~23.2%,11.4%~25.6%.  相似文献   
982.
Experimental study of the ultrahigh-frequency chaotic dynamics generated in an improved Colpitts oscillator is performed. Reliable and reproducible chaos can be generated at the fundamental frequency up to 1.5 GHz using the microwave BFG520 type transistors with the threshold frequency of 9 GHz. By the tuning of the supply voltages, we observe complex nonlinear dynamics like period-one oscillation, period-two oscillation, multiple-period oscillation, and chaotic oscillation. Typical time series, autocorrelation, and broadband continuous power spectrum are presented. Furthermore, compared with the corresponding classical Colpitts oscillator, the main advantage of the improved circuit is in the fact that by operating in a chaotic mode it exhibits higher fundamental frequencies and a lower peak side-lobe level.  相似文献   
983.
结构的响应实质上是材料的响应,宏观结构损伤至断裂的发展过程也是材料性质不断演化的结果.构元组集模型从材料的微观物理变形机制出发,基于对泛函势理论和Cauchy-Born准则,抽象出两种构元:弹簧束构元和体积构元.在微观层次上,结构损伤和断裂的实质都是原子间键合力减弱和丧失的结果,而弹簧束构元是同一方向上的原子键的抽象,因此损伤可以通过弹簧束构元的响应曲线来反映.组集两种构元的响应,建立了材料的弹性损伤本构关系,从而能一致描述材料从弹性到损伤、破坏的发展过程.将构元组集模型的本构关系嵌入ABAQUS的用户材料单元子程序UMAT,实现对结构响应的数值模拟.论文模拟了包含中心预制裂纹三点弯曲梁的裂纹扩展过程,并与内聚区模型比较,给出了内聚区模型所假设的应力-位移关系曲线,并从材料损伤演化的角度对材料裂纹扩展过程做出了物理解释.  相似文献   
984.
Polymer nanoparticles with dendrimer-Ag shell were prepared and their application in catalytic reduction of 4-nitrophenol (4-NP) was investigated. Cross-linked polystyrene (PS) microspheres were prepared through dispersion copolymerization of styrene, acrylic acid and crosslinking monomer 1, 2-divinylbenzene. PS microspheres with average size of 450 nm and narrow size distribution were used as support for the immobilization of dendrimer-Ag shell. The polyamidoamine (PAMAM) dendrimer shell was successively grafted onto the surface of PS microspheres through repetitive Michael addition reaction of methyl acrylate (MA) and amidation of the obtained esters with large excess of ethylenediamine (EDA). Silver nanoparticles were formed directly inside the PAMAM shell through reduction with NaBH4. The resulting PS@PAMAM-Ag nanoparticles were packed in a stainless steel column and used successfully for catalytic reduction of 4-NP. This technique for packing catalytic polymer particles in a column could improve the efficiency of using the metal catalyst and the tedious separation in catalytic reaction.  相似文献   
985.
We study conjugate points on a type of Khler manifolds, which are submanifolds of Grassmannian manifolds. And then we give the applications to the study of the index of geodesics and homotopy groups.  相似文献   
986.
In this paper, we construct the function u in L2(Bn, dA) which is unbounded on any neighborhood of each boundary point of Bn such that Tu is the Schatten p-class (0 〈 p 〈 ∞) operator on pluriharmonic Bergman space h2(Bn, dA) for several complex variables. In addition, we also discuss the compactness of Toeplitz operators with L1 symbols.  相似文献   
987.
Dirac operator with eigenvalue-dependent boundary and jump conditions is studied. Uniqueness theorems of the inverse problems from either Weyl function or the spectral data (the sets of eigenvalues and norming constants except for one eigenvalue and corresponding norming constant; two sets of different eigenvalues except for two eigenvalues) are proved. Finally, we investigate two applications of these theorems and obtain analogues of a theorem of Hochstadt-Lieberman and a theorem of Mochizuki-Trooshin.  相似文献   
988.
The marginal distributions of the number of rises and the number of falls have been used successfully in various areas of statistics, especially in non-parametric statistical inference. Carlitz (1972, Duke Math. J.39, 268–269) showed that the generating function of the joint distribution for the numbers of rises and falls satisfies certain complex combinatorial equations, and pointed out that he had been unable to derive the explicit formula for the joint distribution from these equations. After more than two decades, this latter problem remains unsolved. In this article, the joint distribution is obtained via the probabilistic method of finite Markov chain imbedding for random permutations. A numerical example is provided to illustrate the theoretical results and the corresponding computational procedures.  相似文献   
989.
In this paper, by using fractional power of operators and Sadovskii fixed point theorem, we study the controllability of abstract neutral functional integrodifferential systems with infinite delay. As application, an example is provided to illustrate the obtained results.  相似文献   
990.
4,4-dibromobiphenyl nanocrystals with different sizes in the range from 20 nm to 300 nm were prepared by the reprecipitation method. It was found that their absorption peaks experienced a red shift while the size of the nanocrystals increased. Through analyzing these spectra of the nanocrystals with different sizes, it is suggested that this size-dependent optical property is contributed by two factors, the size effect and the J-aggregate formation. Received: 28 August 2000 / Accepted: 2 March 2001 / Published online: 23 May 2001  相似文献   
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