Computation of three-dimensional parabolic laminar flows

A simple yet accurate method for computing three-dimensional parabolic laminar flow of an incompressible, viscous fluid in a constant area duct is presented. The boundary-layer type nonlinear equations of motion are solved numerically using finite differences and the variable mesh technique. Results of the present calculations are compared with experimental and previous theoretical data for flow through square and rectangular ducts; the agreement is satisfactory.     

Numerical solution of slender channel laminar flows

The slender channel approximate equations are used to describe flows in curved channels of varying height. The governing equations are transformed in order that similar solutions can be obtained and used as initial profiles. A second-order accurate Crank-Nicolson finite difference scheme is developed with the pressure treated directly as one of the dependent variables. The rechnique is applied to several channel geometries, and solutions are obtained at various Reynolds numbers.     

A calculation procedure for three-dimensional steady recirculating flows using multigrid methods

An efficient finite difference calculation procedure for three-dimensional recirculating flows is presented. The algorithm is based on a coupled solution of the three-dimensional momentum and continuity equations in primitive variables by the multigrid technique. A symmetrical coupled Gauss-Seidel technique is used for iterations and is observed to provide good rates of smoothing. Calculations have been made of the fluid motion in a three-dimensional cubic cavity with a moving top wall. The efficiency of the method is demonstrated by performing calculations at different Reynolds numbers with finite difference grids as large as 66 × 66 × 66 nodes. The CPU times and storage requirements for these calculations are observed to be very modest. The algorithm has the potential to be the basis for an efficient general-purpose calculation procedure for practical fluid flows.     

Calculation of steady confined jets for two-dimensional plane laminar flows

The numerical method of Cebeci, based on the solution of boundary-layer equations and the Mechul approach, has been developed further and applied to the problem of two-dimensional, laminar, coflowing, confined jets. For high ratios of the velocity of the jet to the surrounding stream, a region of recirculation forms on the confining wall and the method has been used to calculate the wall pressure and velocity distributions for ratios of 1, 3, 5, 8 and 10 and for values of the ratio of duct to jet width of 1, 2 and 8. The results indicate a smooth transition through separation and reattachement to fully-developed flow and, as a consequence, confirm that the method forms a basis for extension to turbulent coflowing confined jets, with small regions of separated flows, and to laminar and turbulent flows in similar configurations with imposed fluctuations.All calculations were performed with an Apple II microprocessor and demonstrate that practical boundary-layer calculations can be performed for low cost.     

Block-implicit multigrid calculation of two-dimensional recirculating flows

The performance of a recently developed calculation procedure for steady multidimensional fluid flows is assessed in four laminar two-dimensional recirculating flows representative of those in industrial equipment. The calculation procedure is based on a coupled solution of the momentum and continuity equations by the multigrid technique. The rate of convergence of the algorithm is critically assessed by varying the flow Reynolds number, the number of finite difference nodes, and the numerical underrelaxation factor. The procedure is observed to converge rapidly to an acceptable accuracy in all the flow situations.     

Numerical calculation of a laminar two dimensional straight cascade flow

The laminar and incompressible flow in a straight cascade is investigated. Numerical solutions of the full Navier-Stokes equations are obtained using the vorticity-stream function formulation and body fitted coordinate system. The numerical method includes a special force balance for the determination of the downstream boundary condition and a double sweep deferred correction which allows a second order accuracy but with the stability properties of an upwind first order scheme. Results for cylindrical, elliptical and NACA 0012 airfoils are presented including separated flow regions. Good agreement with experiments and previous computations is obtained.     

一些代理签名方案的进一步改进

对两个不需要安全信道的代理签名方案进行了分析,发现它们并不满足不可伪造性。在[4]中不能抵抗原始签名人改变攻击,在[5]中原始签名人可以伪造一个有效的代理签名。提出一个改进的强代理签名方案,新方案在不增加计算复杂性的前提下,保证了签名的安全性,同时代理授权过程中仍然保持不需要安全信道。     

几个签密方案的密码学分析与改进

对六个签密方案进行了安全性分析,指出它们都存在保密性的安全问题,其中两个方案还存在不可伪造性的安全问题。对每一个安全问题都给出了具体的攻击方法,并使用加密部分绑定发送者、签名部分绑定接收者、验证等式不含明文信息和部分私钥生成时绑定公钥的方法对每一个方案给出了改进措施。对改进后的方案给出了随机预言机模型下的安全性证明。安全分析表明,改进方案是安全的。最后提出了设计签密方案时必须注意的几个问题。     

Improvements of authenticated encryption schemes with message linkages for message flows

Recently, Tseng et al. proposed two authenticated encryption schemes (basic scheme and generalized scheme) with message linkages, which are efficient in terms of the communication and computation costs in comparison with all the previously proposed schemes. The basic authenticated encryption scheme suited for only after receiving the entire signature blocks, the recipient can then recover the message blocks. In order to allow the receiver to perform the receiving and the recovering processes simultaneously according to application requirements and the transmission efficiency of the network, the generalized authenticated encryption scheme was then proposed. In this paper, we show that both Tseng et al.’s authenticated encryption schemes do not achieve integrity and authentication. Improvements are then proposed to repair the weaknesses.     

几个无证书签密方案的密码分析与改进

无证书密码体制既能避免基于身份的密码体制中存在的密钥托管问题,又能简化公钥证书的管理,具有巨大的优越性。对四个无证书签密方案进行了密码分析,指出有两个方案存在保密性攻击,有三个方案存在伪造性攻击。使用签名部分绑定接收者、加密部分绑定发送者和增加随机数的方法,分别对它们进行了改进。在随机预言机模型中,对改进方案进行了安全性证明,表明改进方案是安全的。     

几个改进的LKK代理签名方案分析

强不可伪造性和对安全信道依赖性的分析是对LKK代理签名方案安全性分析的重要内容之一。对文献[6-8]中的改进的LKK代理签名方案进行安全性分析,指出文献[6,7]中的方案仍存在缺陷并给出攻击方法,指出文献[8]中方案是一个具有强不可伪造性的不需要安全信道的方案。     

内部安全模型下几个签密方案的密码分析

针对五个文献中的六个签密方案,分析了它们在内部安全模型下的保密性和不可伪造性。分析表明有五个方案存在保密性攻击,有四个方案存在伪造性攻击,提出了一些针对这些方案的攻击方法,并分析了攻击成功的原因。针对这些方案的安全缺陷,使用签名部分绑定接收者和加密部分绑定发送者的方法,分别给出了能有效克服攻击的改进措施。     

A finite element analysis of laminar flows through planar and axisymmetric abrupt expansions

Laminar flow of a Newtonian fluid in planar and axisymmetric abrupt expansions is studied by solving the Navier-Stokes equations using the finite element method. The results consolidate information provided in the literature and provide a broader picture of how the expansion ratio and Reynolds number influence the reattachment length, downstream location of the eddy and the relative eddy intensity in both co-ordinate systems.     

Free-surface flows solved by means of SPH schemes with numerical diffusive terms

A novel system of equations has been defined which contains diffusive terms in both the continuity and energy equations and, at the leading order, coincides with a standard weakly-compressible SPH scheme with artificial viscosity. A proper state equation is used to associate the internal energy variation to the pressure field and to increase the speed of sound when strong deformations/compressions of the fluid occur. The increase of the sound speed is associated to the shortening of the time integration step and, therefore, allows a larger accuracy during both breaking and impact events. Moreover, the diffusive terms allows reducing the high frequency numerical acoustic noise and smoothing the pressure field. Finally, an enhanced formulation for the second-order derivatives has been defined which is consistent and convergent all over the fluid domain and, therefore, permits to correctly model the diffusive terms up to the free surface. The model has been tested using different free surface flows clearly showing to be robust, efficient and accurate. An analysis of the CPU time cost and comparisons with the standard SPH scheme is provided.     

Simulation of viscous flows by highly accurate aeroacoustic schemes on regular grids

Highly accurate finite-difference schemes that are widely used in computational aeroacoustics and referred to as dispersion-relation-preserving (DRP) schemes are adapted for the simulation of viscous flows. The main result consists in the construction and verification of numerical boundary conditions on a solid body, artificial boundaries, and on their interface. Calculations are performed for several test problems, including dissipation of the Lamb–Oseen vortex and decay of the Taylor–Green vortex in two-and three-dimensional cases and in infinite and semi-infinite (open-outlet) plane channels.     

Finite-difference schemes for the computation of viscoplastic medium flows in a channel

Two finite-difference schemes for the computation of incompressible Bingham fluid flows in pipes are proposed. The variational Duvaut-Lions inequality is used as a mathematical model. One of the difference schemes is an analogue of the well-known MAC (marker-and-cell) scheme on staggered grids, the second employs one grid for velocity approximation, and the other for all the components of the strain rate tensor. The convergence of finite-difference schemes and the iteration method are studied. The obtained results are compared with those known from publications. The proposed schemes were used to compute Bingham medium with an inhomogeneous yield stress.     

Evaluation of various inverse docking schemes in multiple targets identification

The lack of accurate and efficient methods for target identification has been the bottleneck in drug discovery. In recent years, inverse docking has been applied as an efficient method in target identification, and several specific inverse docking strategies have been employed in academic and industrial researches. However, the effectiveness of these docking strategies in multiple targets identification is unclear. In this study, five inverse docking schemes were evaluated to find out the most effective approach in multiple targets identification. A target database containing a highly qualified dataset that is composed of 1714 entries from 1594 known drug targets covering 18 biochemical functions was collected as a testing pool for inverse docking. The inverse docking engines including GOLD, FlexX, Tarfisdock and two in-house target search schemes TarSearch-X and TarSearch-M were evaluated by eight multiple target systems in the dataset. The results show that TarSearch-X is the most effective method in multiple targets identification and validation among these five schemes, and the effectiveness of GOLD in multiple targets identification is also acceptable. Moreover, these two inverse docking strategies will also be helpful in predicting the undesirable effects of drugs, such as toxicity.     

若干部分盲签名方案的密码学分析和改进

对最近提出的若干部分盲签名方案进行了安全分析,指出这些方案不像宣称的那样是安全的,存在签名请求者非法篡改事先协商好的公共信息的攻击,并对出现这种篡改攻击的原因进行了分析。分别对这些存在安全缺陷的方案进行了改进,并对改进方案的正确性、安全性和有效性进行了分析。分析表明,改进的方案在不降低原方案效率的情况下是安全的。