Cover Image

The cover image is based on the Research Article Pressure boundary condition in a multi-phase lattice Boltzmann method and its applications on simulations of two-phase flows by Lei Wang and Ze-Rui Peng, https://doi.org/10.1002/fld.4800 .

     

多层非晶薄膜爆炸焊接原理

 从冲击动力学原理出发，建立了多层薄膜爆炸焊接的动力学模型。多层薄膜组合从某种意义上讲可以看成是一种特殊的多孔材料，其在冲击波作用下的行为完全可用多孔材料状态方程来描述。文中还针对非晶态材料的特点，建立了多层焊接的传热模型。计算结果表明，多层爆炸焊接完全可以保证非晶态材料不发生相变。     

盾构作业区域湿热环境的研究

通过对掘进中的盾构及隧道内的温度和湿度的实地测量,本文针对盾构施工过程中在其作业区域存在的高温高湿等恶劣的工作环境问题,就其形成的原因和热湿传递的特性进行了分析,还对其中的流场进行了初步的数值模拟,提出了改善盾构作业区域内工作环境的措施。     

A multiclass network with non-linear, non-convex, non-monotonic stability conditions

We consider a stochastic queueing network with fixed routes and class priorities. The vector of class sizes forms a homogeneous Markov process of countable state space Z⁶ ₊. The network is said “stable” (resp.“unstable”) if this Markov process is ergodic (resp. transient). The parameters are the traffic intensities of the different classes. An unusual condition of stability is obtained thanks to a new argument based on the characterization of the “essential states”. The exact stability conditions are then detected thanks to an associated fluid network: we identify a zone of the parameter space in which diverging, fluid paths appear. In order to show that this is a zone of instability (and that the network is stable outside this zone), we resort to the criteria of ergodicity and transience proved by Malyshev and Menshikov for reflected random walks in Z⁶ ₊ (Malyshev and Menshikov, 1981). Their approach allows us to neglect some “pathological” fluid paths that perturb the dynamics of the fluid model. The stability conditions thus determined have especially unusual characteristics: they have a quadratic part, the stability domain is not convex, and increasing all the service rates may provoke instability (Theorem 1.1 and section 7). This revised version was published online in June 2006 with corrections to the Cover Date.     

Computing centre conditions for certain cubic systems

We present necessary and sufficient conditions for a critical point of certain two-dimensional cubic differential systems to be a centre. Extensive use of the computer algebra system REDUCE is involved. The search for necessary and sufficient conditions for a centre has long been of considerable interest in the theory of nonlinear differential equations. It has proved to be a difficult problem, and full conditions are known for very few classes of systems. Such conditions are also required in the investigation of Hilbert's sixteenth problem concerning the number of limit cycles of polynomial systems.     

On optimality conditions for maximizations with respect to cones

For a Pareto maximization problem defined in infinite dimensions in terms of cones, relationships among several types of maximal elements are noted and optimality conditions are developed in terms of tangent cones.     

Are Field Derivatives Needed in Linear Constitutive Relations?

General linear constitutive relations need not contain any dependence on field derivatives, and the standard boundary conditions need not be modified. We show that the answer to the question asked as the title of this communication is therefore in the negative.     

Optimal Control of a Chemical Vapor Deposition Reactor

We study a simple model of chemical vapor deposition on a silicon wafer. The control is the flux of chemical species, and the objective is to grow the semiconductor film so that its surface attains a prescribed profile as nearly as possible. The surface is spatially fast oscillating due to the small feature scale, and therefore the problem is formulated in terms of its homogenized approximation. We prove that the optimal control is bang-bang, and we use this information to develop a numerical scheme for computing the optimal control.