模拟突放水体问题的SPH方法

采用SPH方法数值模拟了二维方形水箱突放水体各时刻流动的情况,计算结果表明:模拟过程中,整个水体的密度和速度值变化不大,而压强值变化比较大。数值模拟粒子分布图可以显示出各时刻整个水体的流态、各时刻闸门开启情况及闸门对周围水体流态的影响等,闸门开启的1.460 3 s时刻,闸门左侧部分水体有相对下降缓慢的特殊流态。     

MOTION OF TRACER PARTICLES IN A CENTRIFUGAL PUMP AND ITS TRACKING CHARACTERISTICS

The Basset-Boussinesq-Oseen (BBO) equation can be used for most flows to trace the motion of a particle, but in a centrifugal pump, among the forces that act on the particles, one should also include those due to the impeller rotation, as additional effects. This paper firstly reviews various approximations of the BBO equation for the motion of dispersion particles in a viscous fluid. Then based on the motion equation for particles in low Reynolds number centrifugal pumps, a formula for calculating the tracking characteristics of tracer particles is deduced through the Fourier integral transformation. After that the deviations of the particle motion from the fluid motion, as predicted by the various approximations, are discussed and compared. At last, with an emphasis on the Particle Image Velocimetry (PIV) results, the tracking characteristics of particles are estimated. Also, advantages and disadvantages of different tracer particles are discussed and suitable tracer particles for application in PIV studies for flow fields in centrifugal pumps are suggested.     

状态方程计算高分子共混物的比体积

基于一个简单的非均核链流体分子热力学模型,采用不同形式的微扰项,可构筑不同形式的高分子共混物系统的状态方程。方程已被应用于关联聚苯乙烯/聚2,6-二甲基苯醚(PS/PPO)、聚苯乙烯/聚乙烯甲醚(PS/PVME)、聚苯乙烯/聚丁二烯(PS/PBD)、聚醚砜/聚氧乙烯(PESP/PEO)等高分子共混物的比体积。结果显示,用一个与温度无关的二元相互作用可调参数可满意关联所选体系的比体积,并可预测所选体系的比体积。     

Simultaneous Estimation of Legal and Illegal Supply and Demand: The Case of Motion Pictures

Prior research into illegal goods has typically looked at one-way effects, such as illegal demand on legal demand. This research investigates a previously unexamined component of the market, illegal supply. The authors examine the supply and demand of legal goods and their illegal counterparts as a market system of four interdependent components. This research makes theoretical and empirical contributions by evaluating illegal supply in this system. Simultaneous equations estimate each market component on the others using data from the motion picture industry. The results find illegal supply has no effect on legal supply (movie screens), positive effects on illegal demand (piracy downloads), and some effect on legal demand (box office revenues). Timing effects highlight this: illegal supply has a positive effect on legal demand during a film’s opening week, but no effect post-launch. The other market components have positive effects on illegal supply (except legal supply, which is negative in the opening week). Additionally, illegal demand has a negative effect on legal demand during the opening week of release, but not in the subsequent weeks. This finding alleviates prior research tension as to whether piracy helps or hurts legal sales, as omitting illegal supply could result in biased estimates.     

Solvability of perturbation solutions in DSGE models

We prove that the undetermined Taylor series coefficients of local approximations to the policy function of arbitrary order in a wide class of discrete time dynamic stochastic general equilibrium (DSGE) models are solvable by standard DSGE perturbation methods under regularity and saddle point stability assumptions on first order approximations. Extending the approach to nonstationary models, we provide necessary and sufficient conditions for solvability, as well as an example in the neoclassical growth model where solvability fails. Finally, we eliminate the assumption of solvability needed for the local existence theorem of perturbation solutions, complete the proof that the policy function is invariant to first order changes in risk, and attribute the loss of numerical accuracy in progressively higher order terms to the compounding of errors from the first order transition matrix.     

Board size,corporate regulations and firm valuation in an emerging market: a simultaneous equation approach

We investigate the association between board size and firm valuation for a sample of 169 firms from 2002 to 2011 in South Africa (SA). The SA corporate context is interestingly and uniquely characterised by an urgency to meet affirmative action regulations, such as black empowerment in board appointments, limited qualified and experienced directors, especially black directors, concentrated ownership, weak enforcement of corporate regulations and greater government ownership. These features make SA corporate boards perform a weaker agency (advisory, monitoring and disciplining) role than Western European and US boards, but a stronger resource dependence role, by providing access to resources, such as business contacts and contracts. This suggests that any positive impact of board size on firm valuation is likely to depend on the effective execution of the resource dependence role more than the agency role. Our results suggest that board size has a positive association with firm valuation, consistent with larger boards providing better access to resources. Overall, our results support the resource dependence role of boards more than their agency role. The results are robust across a raft of econometric models that control for different types of endogeneity, as well as different types of accounting and market-based firm valuation measures.     

Independence in bank governance structure: Empirical evidence of effects on bank risk and performance

We investigate how different governance arrangements affect risk and return in banks. Using a new data set for UK banks over the period 2003–2012, we employ a simultaneous equations framework to control for the reciprocal relationship between risk and return. We show that separation of the roles of CEO and Chairman increases bank risk without causing a concurrent increase in return. We also find that oversight by a Remuneration Committee and Non-Executive Directors (NEDs) lowers the probability of bank failure, indicating that empowering an independent Chairman has different effects from empowering independent NEDs. Overall, our results underline the importance of accounting for the heterogeneity in corporate governance arrangements within banks.     

Stability and determinacy conditions for mixed-type functional differential equations

This paper analyzes the solution of linear mixed-type functional differential equations with either predetermined or non-predetermined variables. Conditions characterizing the existence and uniqueness of a solution are given and related to the local stability and determinacy properties of the steady state. In particular, it is shown that the relationship between the uniqueness of the solution and the stability of the steady-state is more subtle than the one that holds for ordinary differential equations, and gives rise to new dynamic configurations.     

PRICING ASIAN OPTIONS FOR JUMP DIFFUSION

We construct a sequence of functions that uniformly converge (on compact sets) to the price of an Asian option, which is written on a stock whose dynamics follow a jump diffusion. The convergence is exponentially fast. We show that each element in this sequence is the unique classical solution of a parabolic partial differential equation (not an integro‐differential equation). As a result we obtain a fast numerical approximation scheme whose accuracy versus speed characteristics can be controlled. We analyze the performance of our numerical algorithm on several examples.