Showing papers by "École Polytechnique de Montréal published in 1990"

A New Optimization Algorithm for the Vehicle Routing Problem with Time Windows

Abstract: The vehicle routing problem with time windows VRPTW is a generalization of the vehicle routing problem where the service of a customer can begin within the time window defined by the earliest and the latest times when the customer will permit the start of service. In this paper, we present the development of a new optimization algorithm for its solution. The LP relaxation of the set partitioning formulation of the VRPTW is solved by column generation. Feasible columns are added as needed by solving a shortest path problem with time windows and capacity constraints using dynamic programming. The LP solution obtained generally provides an excellent lower bound that is used in a branch-and-bound algorithm to solve the integer set partitioning formulation. Our results indicate that this algorithm proved to be successful on a variety of practical sized benchmark VRPTW test problems. The algorithm was capable of optimally solving 100-customer problems. This problem size is six times larger than any reported to date by other published research.

Algorithms for the maximum satisfiability problem

Abstract: Old and new algorithms for the Maximum Satisfiability problem are studied We first summarize the different heuristics previously proposed, ie, the approximation algorithms of Johnson and of Lieberherr for the general Maximum Satisfiability problem, and the heuristics of Lieberherr and Specker, Poljak and Turzik for the Maximum 2-Satisfiability problem We then consider two recent local search algorithmic schemes, the Simulated Annealing method of Kirkpatrick, Gelatt and Vecchi and the Steepest Ascent Mildest Descent method, and adapt them to the Maximum Satisfiability problem The resulting algorithms, which avoid being blocked as soon as a local optimum has been found, are shown empirically to be more efficient than the heuristics previously proposed in the literature

Production of poly-(beta-hydroxybutyric-co-beta-hydroxyvaleric) acids.

Abstract: Alcaligenes latus, Alcaligenes eutrophus, Bacillus cereus, Pseudomonas pseudoflava, Pseudomonas cepacia, and Micrococcus halodenitrificans were found to accumulate poly-(beta-hydroxybutyric-co-beta-hydroxyvaleric) acid [P(HB-co-HV)] copolymer when supplied with glucose (or sucrose in the case of A. latus) and propionic acid under nitrogen-limited conditions. A fed-batch culture of A. eutrophus produced 24 g of poly-beta-hydroxybutyric acid (PHB) liter-1 under ammonium limitation conditions. When the glucose feed was replaced with glucose and propionic acid during the polymer accumulation phase, 17 g of P(HB-co-HV) liter-1 was produced. The P(HB-co-HV) contained 5.0 mol% beta-hydroxyvaleric acid (HV). Varying the carbon-to-nitrogen ratio at a dilution rate of 0.15 h-1 in a chemostat culture of A. eutrophus resulted in a maximum value of 33% (wt/wt) PHB in the biomass. In comparison, A. latus accumulated about 40% (wt/wt) PHB in chemostat culture under nitrogen-limited conditions at the same dilution rate. When propionic acid was added to the first stage of a two-stage chemostat, A. latus produced 43% (wt/wt) P(HB-co-HV) containing 18.5 mol% HV. In the second stage, the P(HB-co-HV) increased to 58% (wt/wt) with an HV content of 11 mol% without further addition of carbon substrate. The HV composition in P(HB-co-HV) was controlled by regulating the concentration of propionic acid in the feed. Poly-beta-hydroxyalkanoates containing a higher percentage of HV were produced when pentanoic acid replaced propionic acid.

The Steepest Descent Direction for the Nonlinear Bilevel Programming Problem

Abstract: In this paper, we give necessary optimality conditions for the nonlinear bilevel programming problem. Furthermore, at each feasible point, we show that the steepest descent direction is obtained by solving a quadratic bilevel programming problem. We give indication that this direction can be used to develop a descent algorithm for the nonlinear bilevel problem.

Friction properties of the interface between porous-surfaced metals and tibial cancellous bone.

Abstract: Friction tests between cancellous bone cubes and porous-surfaced metal plates were conducted in order to determine the mechanical properties of the interface in a knee porous-surfaced metal implant. Bone specimens were obtained from fresh frozen amputated tibiae and three metal plates were chosen: titanium bead porous-surfaced, titanium fiber mesh porous-surfaced, and smooth stainless steel. Results show that the friction curve is highly nonlinear. Friction coefficients measured vary between 0.3 and 1.3. The friction coefficient of the interface is independent of the excision site of the bone cubes and of the magnitude of the rate of relative displacement at the interface. The friction coefficient appears to vary slightly with the normal contact pressure for all the metal surfaces. Both porous surfaces have statistically a higher friction coefficient than the smooth surface. This is likely due to the presence of surface asperities whereby the metal ploughs the bone surface. However, no significant differences is observed between bead and fiber mesh types.

Recovery of poly-3-hydroxyalkanoic acid granules by a surfactant-hypochlorite treatment

Abstract: When Alcaligenes eutrophus biomass was treated with a surfactant and then washed with hypochlorite, the recovered poly-3-hydroxyalkanoic acid (PHA) granules were 97 to 98% pure with a molecular weight (MW) between 730,000 and 790,000, depending on the surfactant used. When treated with only surfactant, the MW was slightly higher than that obtained with the surfactant-hypochlorite treatment but the purity was 10% lower. PHA of higher purity but lower MW was obtained with just a hypochlorite treatment.

Microscopic examination of the interface region in 6061-Al/SiC composites reinforced with as-received and oxidized SiC particles

Abstract: Aluminum (6061) matrix composites reinforced with different SiC particles were processed. Black SiC particles were used in their as-received form or after artificial oxidation, leading to a 50-nm-thick SiO2 layer surrounding the SiC particles. The manufacturing route used was the compocasting technique, which allows maintenance of the semisolid slurry at relatively low temperatures (<650 °C) during the incorporation of the reinforcement. Before squeeze casting, the liquid alloy is held at 700 °C for 5 to 10 minutes. The interface between the aluminum matrix and SiC was characterized by transmission electron microscopy (TEM). Results show that no reaction takes place during compocasting between the as-received SiC particles and the molten aluminum. This is a consequence of the low temperatures and short holding times in the liquid state of the 6061 alloy, possible with this process. Prolonged holding at 800 °C of this material leads to extensive formation of A14C3. In the case of artificially oxidized SiC particles, the SiO2 layer surrounding the SiC particles reacts with the molten Al-Mg-Si alloy to produce MgAl2O4. However, the amount of Mg from the base alloy lost to form this spinel phase is not sufficient to prevent age hardening of the material.

Biosynthesis of Poly-β-Hydroxyalkanoates from Pentoses by Pseudomonas pseudoflava

Abstract: The potential of Pseudomonas pseudoflava to produce poly-β-hydroxyalkanoates (PHAs) from pentoses was studied. This organism was able to use a hydrolysate from the hemicellulosic fraction of poplar wood as a carbon and energy source for its growth. However, in batch cultures, growth was inhibited completely at hydrolysate concentrations higher than 30% (vol/vol). When P. pseudoflava was grown on the major sugars present in hemicelluloses in batch cultures, poly-β-hydroxybutyric acid (PHB) accumulated when glucose, xylose, or arabinose was the sole carbon source, with the final PHB content varying from 17% (wt/wt) of the biomass dry weight on arabinose to 22% (wt/wt) of the biomass dry weight on glucose and xylose. Specific growth rates were 0.58 h−1 on glucose, 0.13 h−1 on xylose, and 0.10 h−1 on arabinose, while the specific PHB production rates based on total biomass ranged from 0.02 g g−1 h−1 on arabinose to 0.11 g g−1 h−1 on glucose. PHB weight-average molecular weights were 640,000 on arabinose and 1,100,000 on glucose and xylose. The absolute amount of PHB in the cells decreased markedly when nitrogen limitation was relaxed by feeding ammonium sulfate at the end of the PHB accumulation stage of the arabinose and xylose fermentations. Copolymers of β-hydroxybutyric and β-hydroxyvaleric acids were produced when propionic acid was added to shake flasks containing 10 g of glucose liter−1. The β-hydroxyvaleric acid monomer content attained a maximum of 45 mol% when the initial propionic acid concentration was 2 g liter−1.

Numerical Computation of the Natural Convection Flow about a Horizontal Cylinder Using Splines

Abstract: The present work is devoted to the numerical study of laminar natural convection flow from a heated horizontal cylinder under diverse surface boundary conditions using the spline fractional step method. A general formulation to treat mixed boundary conditions using the spline approximation has been presented. Numerical solutions have been obtained by solving the Navier-Stokes and energy equations. The results for the isothermal boundary condition as well as for the uniform heat flux are in good agreement with published experimental data and with other solutions presently available in the literature. Some new computations at very high Rayleigh numbers indicate the existence of attached separation vortices in the downstream plume region, the appearance of these vortices being dependent on the values of the Biot number. All results were computed on a personal computer using unequally spaced grids that provided good results with a minimum number of computational points. The numerical scheme presented here app...

Dicumyl peroxide-modified cellulose/LLDPE composites

Abstract: The present study reports results on the processing and mechanical properties of composites modified by the addition of dicumyl peroxide (DCP). The addition of minute amounts of peroxide to the cellulose/polymer system during processing has been shown to significantly improve the physical properties of composites. SEM micrographs of fracture surfaces of the peroxide modified composites reveal direct grafting of polyethylene onto cellulose fibers. The existence of a critical peroxide concentration indicates that the grafting reactions terminate when cellulose surfaces are no longer accessible. Possible mechanisms involved are discussed.

The unified interaction parameter formalism: Thermodynamic consistency and applications

Abstract: In a recent article, we proposed modifications to the standard interaction parameter formalism. The modified formalism, known as the “Unified Interaction Parameter Formalism,” is discussed in the present article with respect to thermodynamic consistency at finite concentrations in binary, ternary, and multicomponent systems. A new method, which is independent of integration paths, is proposed to derive the equations of the formalism by differentiation of the integral Gibbs energy expression. It is shown that the formalism is thermodynamically exact in both dilute and nondilute composition regions. It is also shown that the formalism reduces to Wagner’s formalism at infinite dilution and to Darken’s quadratic formalism in dilute solutions. Examples are presented and methods are discussed for determining the parameters of the formalism from thermodynamic data.

Generalized constitutive equation for polymeric liquid crystals Part 1. Model formulation using the Hamiltonian (poisson bracket) formulation

Abstract: The Hamiltonian formulation of equations in continuum mechanics through Poisson brackets, developed in Z.R. Iwinski and L.A. Turski, Lett. Appl. Eng. Sci., 4 (1976), 179–191, P.J. Morrison and J.M. Greene, Phys. Rev. Lett., 45 (1980) 790–794, I.E. Dzyaloshinskii and G.E. Volovick, Ann. Phys., 125 (1980) 67–97, D.D. Holm, J.E. Marsden, T. Ratiu and A. Weinstein, Phys. Rep., 123 (1985) 1–116, M. Grmela, Phys. Lett. A, 130 (1988) 81–86, and A.N. Beris and B.J. Edwards, J. Rheol., 34 (1990) 55–78, for a class of incompressible fluids, is used here in order to generate a constitutive equation for the stress and the order parametr tensor for a polymeric liquid crystal. A free energy expression, of the type used by Doi in his theory for concentrated solutions of rigid rods, is used in addition to the Frank elasticity expression employed in the Leslie—Ericksen—Parodi (LEP) theory to model the effect of spatial gradients in the liquid crystalline microstructure. For homogeneous systems, the analysis leads to a model which is equivalent to a generalization of Doi theory out to fourth-order terms in S . Truncating this model at second-order terms gives the Doi equations exactly. To evaluate the expanded model, results for steady simple shear and extensional flows are compared against the Doi model predictions. The constitutive equation resulting from the expanded model is compared against the LEP constitutive equation and the parameters between the two are correlated. The additional stress terms for non-homogeneous systems reduce to a recently presented (B.J. Edwards and A.N. Beris, J. Rheol., 33 (1989) 1189–1193; M. Grmela, Phys. Lett. A, 137 (1989) 342–348) generalization of the Ericksen stress expression in terms of the second-order parameter tensor. The model presented is a generalization and extension of the order-parameter-based theory of Doi which allows a greater flexibility in describing the rheological properties of polymeric liquid crystalline systems

The relation between pen force and pen-point kinematics in handwriting

Abstract: This study investigates the spectral coherence and time-domain correlation between pen pressure (axial pen force, APF) and several kinematic variables in drawing simple patterns and in writing cursive script. Two types of theories are prevalent: "biomechanical" and "central" explanations for the force variations during writing. Findings show that overall coherence is low (<0.5) and decreases with pattern complexity, attaining its lowest value in cursive script. Looking at subjects separately, it is found that only in a small minority of writers "biomechanical coupling" between force and displacement takes place in cursive handwriting, as indicated by moderate to high negative overall correlations. The majority of subjects display low coherence and correlation between kinematics and APF. However, APF patterns in cursive script reveal a moderate to high replicatability, giving support to the notion of a "centrally" controlled pen pressure. The sign of the weak residual average correlation between APF and finger displacement, and between APF and wrist displacement is negative. This indictes that small biomechanical effects may be present, a relatively higher APF corresponding to finger flexion and wrist radial abduction. On the whole, however, variance in APF cannot be explained by kinematic variables. A motor task demanding mechanical impedance control, such as handwriting, apparently introduces a complexity that is not easily explained in terms of a passive mass-spring model of skeleto-muscular movement.

Ultrastructure of the human interspinous ligament and ligamentum flavum. A preliminary study.

Abstract: The ultrastructure of ligamenta flava (LF) and interspinous ligaments (ISL) obtained from four patients who underwent surgery for vertebral fracture (control group) and five patients operated for disc herniation was studied. The fine structure of LF was composed of elastic and elaunin fibers. Small diameter collagen fibrils were found between the elastic system fibers. The ISL was constituted predominantly of collagen fibrils. Elastic fibers were seen in the most ventral part of the ligament. In ISL and LF of the control group, the cells were fibroblastic-like cells. Chondrocytes were present only near their attachment sites. The proteoglycans were demonstrated between the collagen fibrils, and they appeared to form a regular interfibrillar linking. In ligaments obtained from patients with disc herniation, several modifications were found. The fibroblasts transformed into chondrocytic cells, which were surrounded by a pericellular matrix rich in proteoglycan filaments. A few cells that had suffered necrosis were found. Alterations in the collagen-proteoglycans arrangement also were evidenced. The proteoglycan filaments were randomly oriented to the collagen fibrils.

An optimal control problem with a random stopping time

Abstract: This paper deals with a stochastic optimal control problem where the randomness is essentially concentrated in the stopping time terminating the process. If the stopping time is characterized by an intensity depending on the state and control variables, one can reformulate the problem equivalently as an infinite-horizon optimal control problem. Applying dynamic programming and minimum principle techniques to this associated deterministic control problem yields specific optimality conditions for the original stochastic control problem. It is also possible to characterize extremal steady states. The model is illustrated by an example related to the economics of technological innovation.

From sol-gel to aerogels and cryogels

Abstract: To preserve the textural properties of wet gels upon drying at least two methods can be used. The first method involves fluid extraction under hypercritical conditions in an autoclave. The dried gels are called aerogels. Silica aerogels may exhibit specific surface areas of up to 900 m2 g−1 and porosities in the range 95 to 98%! Large monoliths or very fine homogeneous powders can easily and rapidly be prepared by this procedure. The second method consists of eliminating the solvent by freeze-drying, i.e., by sublimating the frozen solvent in order to prevent the formation of the meniscii between the solid-liquid and the liquid-vapour interfaces. The freeze-dried gels are called cryogels. Their textural properties are very close to those of aerogels.

Rotary cylinders: Solid transport prediction by dimensional and rheological analysis

Abstract: A semi-experimental model for predicting the axial transport of the granular bed in a rotating cylinder is proposed. It is based on dimensional analysis and on the determination of an apparent viscosity characterizing the flow behaviour of the bed. Unknown constants in the model are determined either by analysis or by tuning with experimental data. An example of such tuning is shown to work well. The model is capable of giving as a function of filling angle and friction angle, the axial velocity that varies along the cylinder axis. This is important for the control of the process taking place inside the cylinder.

Calibration of stereo cameras using a non-linear distortion model

Abstract: The objective of stereo camera calibration is to estimate the internal and external parameters of each camera. Using these parameters, the three-dimensional position of a point in the scene, identified and matched in two stereo images, can be determined by the method of triangulation. We present in this paper a camera model which accounts for major sources of camera distortion: radial, decentering and thin prism distortions. The proposed calibration procedure consists of two steps. In the first step, calibration parameters are estimated using a closed-form solution based on a distortion-free camera model. In the second step, the parameters estimated in the first step are improved iteratively through nonlinear optimization, taking into account camera distortions. According to minimum variance estimation, the objective function to be minimized is the mean-square discrepancy between the observed image points and their inferred image projections computed with the estimated calibration parameters. We introduce a type of measure which can be used to directly evaluate the performance of calibration and compare calibrations among different systems. The validity and the performance of our calibration procedure are tested real images taken by wide angle lenses. Results consistently show significant improvements over less complete camera models.

Surface conductivity of epoxy specimens subjected to partial discharges

Abstract: The surface conductivity of epoxy specimens was found to increase appreciably with the exposure time to partial discharges, the change being most pronounced during the initial stages on the exposure. No definite tendency could be detected in the mean partial discharge pulse amplitude with time. Surface degradation of the epoxy was characterized by the commonly observed occurrence of pitting and deposition of droplets and crystals. >

Microstructure, creep properties, and rejuvenation of service-exposed alloy 713C turbine blades

Abstract: A study was carried out on the microstructure and creep properties of aero engine first-stage turbine blades made from Alloy 713C nickel-base superalloy. Results are reported for new blades, blades in two service-exposed conditions, and service-exposed blades subjected to one of three rejuvenation treatments: a recoating heat treatment, a hot isostatic pressing (HIP) + recoating heat treatment, and a HIP + controlled cooling + recoating heat treatment. The blade microstructure undergoes significant change during service, and this leads to a loss in creep properties exhibited by specimens machined from the blade airfoils. Good correlations were observed between the rupture time and the amount of blade airfoil untwist and between the minimum creep rate and the amount of untwist. The recoating heat treatment and the HIP + controlled cooling + recoating treatment were moderately successful in restoring the microstructure and creep properties of the service-exposed blades. In comparison, the HIP + recoating treatment was very successful in rejuvenating creep properties but only for blades having a chemical composition with a lower propensity to form σ phase. For the blades with an unfavorable composition, σ phase was found to form preferentially near the grain boundaries during creep testing, and this had a detrimental effect on the creep properties. Nonetheless, the degree of rejuvenation for these blades was always at least as good as that obtained through the recoating heat treatment alone.

National, regional or world patterns of entrepreneurial motivation?

Abstract: The international study on which these extensive results are based was undertaken for the purpose of determining to what extent entrepreneurial motivation cuts across cultural and socio-economic lines and what other factors influence entrepreneurship in the various milieus. Statistical analysis of 38 motivation variables for entrepreneurs reveals clearly that motivation transcends national cultures and defines three groups of countries: the Anglo-Saxon, Scandinavian and Mixed groups. By contrast, the career motivation and job satisfaction level of non-entrepreneurs (35 variables) are country-specific. This suggests that entrepreneurial behaviour is much more dependent on a favorable socio-economic and political environment than on cultural values per se. Five basic motives stimulate these entrepreneurs: independence, achievement, recognition, communitarianism and money.

Generalized constitutive equation for polymeric liquid crystals: Part 2. non-homogeneous systems

Abstract: The Hamiltonian formulation of equations in continuum mechanics through Poisson brackets was used in Ref. 1 to develop a constitutive equation for the stress and the order parameter tensor for a polymeric liquid crystal. These equations were shown to reduce to the homogeneous Doi equations as well as to the Leslie-Ericksen-Parodi (LEP) constitutive equations under small deformations [1]. In this paper, these equations are fitted against the non-homogeneous Doi equations through the simulation of the spinodal decomposition of the isotropic state when it is suddenly brought into a parameter region in which it is thermodynamically unstable. Linear stability analysis reveals the wavelength of the most unstable fluctuation as well as its initial growth rate. Results predicted from this theory compare well with the predictions of Doi for the spinodal decomposition using an extended molecular rigid-rod theory in terms of the distribution function. This completes the development of a generalized constitutive equation for polymeric liquid crystals initiated in Part 1.

Characterization of bonding and crystalline phases in fluxed pellets using peat moss and bentonite as binders

Abstract: Pellets in the basicity range of 0.2 to 1.6 were produced from specular hematite concentrates using bentonite or peat moss as a binder. Specific pellet basicities were achieved through the addition of (1) limestone and (2) a combination of dolomite and limestone. Mineralogical study and microanalysis of the bonding and crystalline phases were done using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). In limestone-fluxed pellets with bentonite as a binder, although formation of calcium ferrite was noted at the basicity of 0.8, its formation in appreciable quantities was delayed until the basicity of 1.6. For the pellets produced with a combination of dolomite and limestone, even at the basicity of 1.6, only minor amounts of this phase were present. In these pellets, some magnesioferrites were formed which did not conform to the stoichiometric composition (MgOFe2O2), but they were quite deficient in MgO. The silicate glass phases, which essentially provided the bonding in the pellets, showed a two-zone structure: (1) magnesian-ferruginous and (2) glass containing high quantities of Si and Ca. However, in pellets with only limestone addition, the zoned structure was less prevalent, probably due to low Mg content. The results indicate that using peat moss in conjunction with limestone, pellet sticking can be minimized at higher basicities. The factors influencing the pellet bonding and the change in the physical and chemical properties of these pellets are discussed.

A preconditioned generalized minimal residual algorithm for the numerical solution of viscoelastic fluid flows

Abstract: The numerical simulation of the flow of a viscoelastic fluid by a generalized minimal residual (GMRES) method is investigated and tested to determine its convergence properties. A method based on the Lesaint—Raviart technique is used as a preconditioner for the GMRES method to enhance the convergence rate and robustness of the algorithm. Numerical results are presented for the stick—slip problem.

Queues with slowly varying arrival and service processes

Abstract: We examine a generalisation of the M/G/1 queue. The arrival and service processes are governed by a Markov chain which determines the rate of arrival and the service time distribution from a finite set. This Markov chain is assumed to vary "slowly", so that we are able to derive analytical results for the stationary distribution of the queue length using an approach based on decomposability. The practical interest of this model stems from the numerous applications where the parameters of queueing systems are time varying, such as inventory models, telephone systems, time-sharing systems, computer networks with bursty traffic, etc. We also show how this approach can be extended to arbitrary networks of queues and in particular to those with product form solution.

Some properties of polyethylene compounds with surface‐modified fillers

Abstract: The dispersion of variously surface-treated CaCo3 fillers in linear low density polyethylene has been studied, along with mechanical properties of the filled compounds. Microwave plasma discharges were used to modify the filler surfaces. Inverse gas chromatographic analyses showed that plasma treatments could change the dispersive and non-dispersive components of filler surface characteristics. A reduction in these surface energies facilitated the dispersion of the filler in the non-polar polyethylene. Mechanical properties, particularly those at high deformation of the filled plastic, also responded to filler surface treatments, the ductility at failure increasing with a decrease in the filler's surface polarity. For any given elongation at failure, it was found that the amount of filler accommodated by the host polymer was raised by the appropriate surface modification procedure. The work illustrates the important contribution made by interfacial phenomena to various performance aspects of complex polymer systems.