Discrete optimization models and methods for management systems of pavement maintenance and rehabilitation

With the rapid development of highway construction and formation of the highway network in China,the man- agement of pavement maintenance and rehabilitation （MR） activities has become important.In this paper,four discrete optimization models are proposed for different parties involved in the management system： government,highway agent,con- tractor and the common users.These four optimal decision models are formulated as linear integer programming problems with binary decision variables.The objective function and constraints are based on the pavement performance and prediction model using the pavement condition index （PCI）.Numerical experiments are carried out with the data from a highway system in Sichuan Province which show the feasibility and effectiveness of the proposed models.

Numerical analysis of plasma arc physical characteristics under additional constraint of keyhole

The physical characteristics of a plasma arc affect the stability of the keyhole and weld pool directly during keyhole plasma arc welding （KPAW）. There will be significant change for these characteristics because of the interaction between the keyhole weld pool and plasma arc after penetration. Therefore, in order to obtain the temperature field, flow field, and arc pressure of a plasma arc under the reaction of the keyhole, the physical model of a plasma arc with a pre-set keyhole was established. In addition, the tungsten and base metal were established into the calculated domain, which can reflect the effect of plasma arc to weld pool further. Based on magneto hydrodynamics and Maxwell equations, a twoSdimensional steady state mathematical model was established. Considering the heat production of anode and cathode, the distribution of temperature field, flow field, welding current density, and plasma arc pressure were solved out by the finite difference method. From the calculated results, it is found that the plasma arc was compressed a second time by the keyhole. This additional constraint results in an obvious rise of the plasma arc pressure and flow velocity at the minimum diameter place of the keyhole, while the temperature field is impacted slightly. Finally, the observational and metallographic experiments are conducted, and the shapes of plasma arc and fusion line agree with the simulated results generally.

ON LINEARIZED FINITE DIFFERENCE SIMULATION FOR THE MODEL OF NUCLEAR REACTOR DYNAMICS

The model of nuclear reactor dynamics is an initial-boundary value problems of a cou- pled nonlinear integrodifferential equation system of one ordinary differential equation and one par-- tial differential equation. In this this,paper,a linearized difference scheme is derived by the method of reduction of order.It is proved that the scheme is uniquely solvable and unconditionally convergent with the convergence rate of order two both in discrete H1norm and in discrete maxinum narm,and one needs only to solve a tridiagonal system of linear algebraic equations at each time lev- el.The method of reduction of order is an indirect constructing-difference-scheme method,which aim is for the analysis of solvablity and convergence of the constructed difference scheme.