Structural optimization: A new dual method using mixed variables

A new and powerful mathematical programming method is described, which is capable of solving a broad class of structural optimization problems. The method employs mixed direct/reciprocal design variables in order to get conservative, first-order approximations to the objective function and to the constraints. By this approach the primary optimization problem is replaced with a sequence of explicit subproblems. Each subproblem being convex and separable, it can be efficiently solved by using a dual formulation. An attractive feature of the new method lies in its inherent tendency to generate a sequence of steadily improving feasible designs. Examples of application to real-life aerospace structures are offered to demonstrate the power and generality of the approach presented.     

Continuous-valued probabilistic behavior in a VLSI generative model

This paper presents the VLSI implementation of the continuous restricted Boltzmann machine (CRBM), a probabilistic generative model that is able to model continuous-valued data with a simple and hardware-amenable training algorithm. The full CRBM system consists of stochastic neurons whose continuous-valued probabilistic behavior is mediated by injected noise. Integrating on-chip training circuits, the full CRBM system provides a platform for exploring computation with continuous-valued probabilistic behavior in VLSI. The VLSI CRBM's ability both to model and to regenerate continuous-valued data distributions is examined and limitations on its performance are highlighted and discussed.     

Multi-parameter performance analysis for decentralized cognitive radio networks

In this paper, we investigate the impact of primary user activity, secondary user activity, interface switching, channel fading and finite-length queuing on the performance of decentralized cognitive radio networks. The individual processes of these service-disruptive effects are modeled as Markov chains based on cross-layer information locally available at the network nodes. A queuing analysis is conducted and various performance measures are derived regarding the packet loss, throughput, spectral efficiency, and packet delay distribution. Numerical results demonstrate the impact of various system parameters on the system performance, providing insights for cross-layer design and autonomous decision making in decentralized cognitive radio networks.     

Dispersion characteristics of asymmetric coupled anisotropic dielectric waveguides using FDFD

The formulation is developed in the frequency domain and the finite difference method is used for the numerical solution of the scalar wave equation, written in terms of the transverse components of the magnetic field. As a result a conventional eigenvalue problem is obtained without the presence of spurious modes due to the implicit inclusion of the divergence of the magnetic field equal to zero. The formulation is developed to include biaxial anisotropic dielectrics with an index profile varying arbitrarily in the cross section of the waveguide under analysis. This formulation is then applied to the analysis of the influence on the dispersion characteristics of the dimensions of asymmetric coupled rectangular uniaxial anisotropic dielectric waveguides. As expected, the reduction of the height or the width of one of the rectangular dielectric waveguides causes the dispersion curves to move towards higher frequencies.     

Properties of squeeze cast Mg-6Zn-3Cu alloy and its saffil alumina short fibre reinforced composites

In the present work, Mg-Zn-Cu alloy (ZC63) and its saffil alumina short fibre reinforced composites produced using the squeeze casting technique were evaluated for their properties. The unreinforced base alloys and their composites were characterized for their microstructure, hardness, yield strength, impact strength, wear resistance and corrosion resistance. The dependence of the properties of composites was studied as a function of fibre volume fraction. Results showed that the composites exhibited improved hardness, yield strength at elevated temperature and wear resistance in comparison to the monolithic alloy. However, ductility, impact strength and corrosion resistance of the composites were inferior to that of the base alloy. The nature of the base alloy matrix in determining the properties of the composites was discussed based on fractographic analysis.     

Globulization mechanism of the primary Al of Al−15Cu alloy during slurry preparation for rheoforming

Semisolid slurries of Al-15Cu alloy were produced for rheoforming by a low temperature pouring technique. To investigate the morphological change of the slurry in terms of the particle mean diameter and the roundness factor, samples were extracted during the continuous cooling and the isothermal holding stage of the slurry by a simple technique of interrupt quenching. Results demonstrated that the fine-grained equiaxed dendritic structure, which formed during low temperature pouring, is changed to a globular structure when held at a semisolid temperature for sufficiently long holding time. With regard to the globulization mechanism of the primary α-phase, local melting is considered to take place at the neck of equiaxed dendrites, leading to the separation of small new particles during continuous cooling. These newly formed particles eventually grow during isothermal holding in the semisolid temperature by obeying theD ³=Kt kinetic law, which suggests coarsening by Ostwald ripening.     

An uncertainty relation for WSS processes and its application toWSSUS systems

An uncertainty relation of the same kind as the one by Heisenberg is established between the coherence interval and the spectral spread of zero-mean wide-sense-stationary mean-square-continuous processes whose spectrum has a finite second moment. Pairs of estimates of the coherence bandwidth and of the corresponding delay spread which have been obtained from measurements in an indoor radio environment are confined as stated by this uncertainty relation     

Shape optimal design using B-splines

Shape optimal design of an elastic structure is formulated using a design element technique. It is shown that Bezier and B-spline curves, typical of the CAD philosophy, are well suited to the definition of design elements. Complex geometries can be described in a very compact way by a small set of design variables and a few design elements. Because of the B-splines flexibility, it is no longer necessary to piece design elements together in order to agree with the shape complexity, nor to restrict the shape variations. Moreover, the additional optimization constraints that are most often needed to avoid unrealistic designs when the shape variables are the nodal coordinates of a finite element mesh, are automatically taken into account in the new formulation. An analytical derivation of the sensitivity analysis will be established, giving rise to numerical efficiency. It will be seen that the resulting optimization problem does not involve highly nonlinear functions with respect to the shape variables, so that simple mathematical programming algorithms can be applied to solve it. Some numerical examples are offered to demonstrate the power and generality of the new approach presented in this paper.