基于FFT的自适应频率估计算法

为了解决FFT算法中因能量泄露和栅栏效应而导致的算法估计性能下降的问题,提出了一种基于FFT的自适应频率估计算法.分析了Rife算法,指出当信号频率位于量化频率附近时,由于插值方向错误,会导致频率估计性能下降;对于分段FFT相位差频率估计算法,当2段信号最大谱线处对应的相位相差比较大时,容易产生相位模糊,从而增大估计误差.基于FFT的自适应频率估计算法将2种算法进行了综合,既保留了2种算法的优点,又对算法性能有所改进.仿真结果表明:该算法的估计精度、稳定性和抗噪能力都有显著提高.     

A lost sales (S — 1, S) perishable inventory system with renewal demand

This article analyzes a one-to-one ordering perishable inventory model with renewal demands and exponential lifetimes. The leadtimes are independently and exponentially distributed and the demands that occur during stock out periods are lost. Although the items are assumed to decay at a constant rate, the output process is not renewal and the Markov renewal techniques are successfully employed to obtain the operating characteristics. The problem of minimizing the long run expected cost rate is discussed and numerical values of optimal stock level are also provided. © 1996 John Wiley & Sons, Inc.     

Multivariate age-dependent imperfect repair

In this article we consider models of systems whose components have dependent life lengths with specific multivariate distributions. Upon failure, components are repaired. Two types of repair are distinguished. After perfect repair, a unit has the same life distribution as a new item. After imperfect repair, a unit has the life distribution of an item which is of the same age but has never failed. We study a model in which the mechanism for determining the nature of the repair is age dependent.     

Damage modeling of ballistic penetration and impact behavior of concrete panel under low and high velocities

This work presents a numerical simulation of ballistic penetration and high velocity impact behavior of plain and reinforced concrete panels. This paper is divided into two parts. The first part consists of numerical modeling of reinforced concrete panel penetrated with a spherical projectile using concrete damage plasticity (CDP) model, while the second part focuses on the comparison of CDP model and Johnson-Holmquist-2 (JH-2) damage model and their ability to describe the behavior of concrete panel under impact loads. The first and second concrete panels have dimensions of 1500 mm × 1500 mm × 150 mm and 675 mm × 675 mm × 200 mm, respectively, and are meshed using 8-node hexahedron solid elements. The impact object used in the first part is a spherical projectile of 150 mm diameter, while in the second part steel projectile of a length of 152 mm is modeled as rigid element. Failure and scabbing characteristics are studied in the first part. In the second part, the com-parison results are presented as damage contours, kinetic energy of projectile and internal energy of the concrete. The results revealed a severe fracture of the panel and high kinetic energy of the projectile using CDP model comparing to the JH-2 model. In addition, the internal energy of concrete using CDP model was found to be less comparing to the JH-2 model.     

双线性表面建模电大目标RCS仿真

目标的电磁散射特性研究对于实现雷达探测、识别、跟踪目标起着至关重要的作用,而雷达散射截面(Radar Cross Section,RCS)又是体现目标电磁散射特性的一个重要方面.参数曲面能精确模拟目标的几何外形,提高计算精度.采用参数曲面——双线性表面建模,运用物理光学(Physical Optics,PO)法计算电大...     

Just How Important Are ‘Hearts and Minds’ Anyway? Counterinsurgency Goes to the Polls

Despite all the talk of ‘hearts and minds’ being the key to counterinsurgency, local public opinion is rarely studied and when it is, it often yields surprising conclusions. Through analyzing polling data from Vietnam, Iraq and Afghanistan, this article shows that public opinion is less malleable, more of an effect rather than a cause of tactical success, and a poor predictor of strategic victory. As a result, modern counterinsurgency doctrine’s focus on winning popular support may need to be rethought.     

高速下的外压式空气轴颈轴承研究

本文基于有限元方法,针对不同的速度参数和离心率,采用一种直接的数字方法来计算多孔外压空气轴颈轴承的性能指标,并把模拟结果同参考资料上一种空气心轴轴颈轴承的计算进行了比较。