Optimization of polymer extrusion with genetic algorithms

In the manufacture of plastics parts by extrusion or injectionmoulding, polymers are usually plasticized in single-screw units.The mechanisms involved are complex and dependent on the material,the geometry, and the type of operation. Usually, process optimizationis based on trial and error—a very inefficient method.A more efficient approach is to solve the inverse problem, i.e.to determine the operating conditions that produce the desiredoutput and/or product characteristics. An alternative strategyconsists in maximizing the value of an objective function, bysolving the direct problem iteratively. The nature of the objectivefunction—with conflicting criteria—and the characteristicsof the search space make an approach based on genetic algorithmsworth investigating. Therefore, a modelling package, an objectivefunction, and a genetic algorithm are interrelated to solvethe industrial extrusion problem. The advantages and disadvantagesof this implementation are discussed.     

Computed and measured thermal diffusion factor for CO-He mixtures: a test of recent interaction potentials

Several anisotropic interaction potentials which describe the He–CO gaseous mixture were tested via the detailed analysis of recently measured thermal diffusion factors, as functions of both temperature and concentration. The calculations were carried out using the quantum treatment of collision dynamics via the infinite order sudden (IOS) coupling scheme between angular momenta, and further using two additional schemes of approximation to second order: the Chapman–Cowling and the Kihara expansions. The results made it possible to test the relative behaviour of the repulsive regions of the various interactions examined in this work, among which those given by the density functional approach for describing van der Waals forces turn out to give fairly realistic values for the anisotropy of the repulsive wall, but have difficulties in generating the well region correctly. These new calculations confirm also the reliability of an earlier multiproperty potential when treating transport properties at temperatures above 100 K, and therefore when examining features which do not depend much on the details of the attractive well.     

Theoretical prediction of a linear isomer for NeBr2(X1Γg + an ab initio approach

A supermolecular approach is used to predict the stationary structures of the NeBr₂(X¹Γ_g ⁺) van der Waals cluster. The intermolecular potential is calculated using the single and double excitation coupled-cluster method with non-iterative perturbation treatment of triple excitations [CCSD(T)]. Relativistic effects are included for the Br atoms using effective core potentials (ECPs) and their efficiency is tested for Br₂ and NeBr₂ molecules. Our results for NeBr₂ show minima for both linear and T-shaped configurations, in accord with previous ab initio calculations for rare gas-Br₂ species. The dependence of the interaction potential, as a function of the Br-Br bond, is also presented. Finally, vibrational analysis is carried out to examine the stability of the two isomers including zero-point vibrational energy effects.     

Stability of plane-parallel flow of magnetic fluids under external magnetic flelds

In this work, we present a theoretical study on the stability of a twodimensional plane Poiseuille flow of magnetic fluids in the presence of externally applied magnetic flelds. The fluids are assumed to be incompressible, and their magnetization is coupled to the flow through a simple phenomenological equation. Dimensionless parameters are deflned, and the equations are perturbed around the base state. The eigenvalues of the linearized system are computed using a flnite difierence scheme and studied with respect to the dimensionless parameters of the problem. We examine the cases of both the horizontal and vertical magnetic flelds. The obtained results indicate that the flow is destabilized in the horizontally applied magnetic fleld, but stabilized in the vertically applied fleld. We characterize the stability of the flow by computing the stability diagrams in terms of the dimensionless parameters and determine the variation in the critical Reynolds number in terms of the magnetic parameters. Furthermore, we show that the superparamagnetic limit, in which the magnetization of the fluids decouples from hydrodynamics, recovers the same purely hydrodynamic critical Reynolds number, regardless of the applied fleld direction and of the values of the other dimensionless magnetic parameters.