Showing papers by "University of Twente published in 1997"

Large-eddy simulation of the turbulent mixing layer

Abstract: Six subgrid models for the turbulent stress tensor are tested by conducting large-eddy simulations (LES) of the weakly compressible temporal mixing layer: the Smagorinsky, similarity, gradient, dynamic eddy-viscosity, dynamic mixed and dynamic Clark models. The last three models are variations of the first three models using the dynamic approach. Two sets of simulations are performed in order to assess the quality of the six models. The LES results corresponding to the first set are compared with filtered results obtained from a direct numerical simulation (DNS). It appears that the dynamic models lead to more accurate results than the non-dynamic models tested. An adequate mechanism to dissipate energy from resolved to subgrid scales is essential. The dynamic models have this property, but the Smagorinsky model is too dissipative during transition, whereas the similarity and gradient models are not sufficiently dissipative for the smallest resolved scales. In this set of simulations, at moderate Reynolds number, the dynamic mixed and Clark models are found to be slightly more accurate than the dynamic eddy-viscosity model. The second set of LES concerns the mixing layer at a considerably higher Reynolds number and in a larger computational domain. An accurate DNS for this mixing layer can currently not be performed, thus in this case the LES are tested by investigating whether they resemble a self-similar turbulent flow. It is found that the dynamic models generate better results than the non-dynamic models. The closest approximation to a self-similar state was obtained using the dynamic eddy-viscosity model.

Kinetics of the low-temperature pyrolysis of polyethene, polypropene and polystyrene modeling, experimental determination and comparison with literature models and data

Abstract: The pyrolysis kinetics of low-density polyethylene, high-density polyethylene, polypropylene, and polystyrene has been studied at temperatures below 450 C. In addition, a literature review on the low-temperature pyrolysis of these polymers has been conducted and has revealed that the scatter in the reported kinetic data is significant, which is most probably due to the use of simple first-order kinetic models to interpret the experimental data. This model type is only applicable in a small conversion range, but was used by many authors over a much wider conversion range. In this investigation the pyrolysis kinetics of the forementioned polymers and a mixture of polymers has been studied at temperatures below 450 C by performing isothermal thermogravimetric analysis (TGA) experiments. The TGA experimental data was used to determine the kinetic parameters on the basis of a simple first-order model for high conversions (70-90%) and a model developed in the present study, termed the random chain dissociation (RCD) model, for the entire conversion range. The influence of important parameters, such as molecular weight, extent of branching and -scission on the pyrolysis kinetics was studied with the RCD model. This model was also used to calculate the primary product spectrum of the pyrolysis process. The effect of the extent of branching and the initial molecular weight on the pyrolysis process was also studied experimentally. The effect of the extent of branching was found to be quite significant, but the effect of the initial molecular weight was minor. These results were found to agree quite well with the predictions obtained from the RCD model. Finally, the behavior of mixtures of the aforementioned polymers was studied and it was found that the pyrolysis kinetics of the polymers in the mixture remains unaltered in comparison with the pyrolysis kinetics of the pure polymers.

Linear motor motion control using a learning feedforward controller

Abstract: The design and realization of an online learning motion controller for a linear motor is presented, and its usefulness is evaluated. The controller consists of two components: (1) a model-based feedback component, and (2) a learning feedforward component. The feedback component is designed on the basis of a simple second-order linear model, which is known to have structural errors. In the design, an emphasis is placed on robustness. The learning feedforward component is a neural-network-based controller, comprised of a one-hidden-layer structure with second-order B-spline basis functions. Simulations and experimental evaluations show that, with little effort, a high-performance motion system can be obtained with this approach.

Oxygen Exchange and Diffusion Coefficients of Strontium‐Doped Lanthanum Ferrites by Electrical Conductivity Relaxation

Abstract: Electrical conductivity relaxation experiments were performed on thin specimens of La1–xSrxFeO3–delta (x = 0.1, 0.4) at oxygen partial pressures pO2 = 10–5 – 1 bar in the temperature range 923 to 1223 K. The transient response of the electrical conductivity after a sudden change of the ambient oxygen partial pressure was analyzed in the frequency domain. The latter procedure allowed diffusion-limited and surface exchange-limited kinetics of re-equilibration to be distinguished. The response of specimens with thicknesses of 350 to 460 µm indicated diffusion-controlled kinetics at pO2 > 0.03 bar. The chemical diffusion coefficients, D-tilde , were found invariant with oxygen pressure. At 1073 K the absolute values were D-tilde = 6.5 × 10–6 cm2 s–1 for x = 0.1 and D-tilde = 1.1 × 10–5 cm2 s–1 for x = 0.4, with activation energies of about 80 kJ/mol. The equilibration process was governed by surface exchange at pO2 O[sub 2] n , where n = 0.65 to 0.85. This pressure dependency was interpreted in terms of a slow surface process involving an oxygen molecule and a surface oxygen vacancy, and causes the observed sharp transition from diffusion- to exchange-controlled kinetics. The activation energy of kO was estimated at 110 to 135 kJ/mol.

Bronsted acid and pore controlled siting of alkanes sorption in acidic molecular sieves

Abstract: The sorption of alkanes in zeolites was investigated using calorimetry, gravimetry, and in situ infrared spectroscopy. At temperatures below 373 K alkanes are found to sorb localized on the strong Bronsted acid sites (bridging Si−OH−Al groups) of H−MFI, H−MOR, and H−FAU via hydrogen bonding of the dipole induced in the alkane and the hydroxyl group. Two n-alkanes, but only one isoalkane, simultaneously interact with the Bronsted acid sites of H−MFI. With the other zeolites only one alkane molecule is sorbed locally per strong Bronsted acid site. About one-third of these sites are accessible to alkanes in H−MOR and about 70% in H−FAU. The heat of sorption decreases in the sequence H−MFI > H−MOR > H−FAU. With increasing size of the hydrocarbon the heat of adsorption increases primarily due to an increase in the dispersion forces. The linear relationships between the enthalpies and the entropies of sorption suggest that the interaction is of similar nature for all hydrocarbons and characteristic for a partic...

Effect of magnetic and nonmagnetic impurities on highly anisotropic superconductivity

Abstract: We generalize Abrikosov-Gor'kov solution of the problem of weakly coupled superconductors with impurities to the case of a multiband superconductor with arbitrary interband order parameter anisotropy, including interband sign reversal of the order parameter. The solution is given in terms of the effective (renormalized) coupling matrix and describes not only ${\mathrm{T}}_{\mathrm{c}}$ suppression but also renormalization of the superconducting gap basically at all temperatures. In many limiting cases we find analytical solutions for the critical temperature suppression. We illustrate our results by numerical calculations for two-band model systems.

Design principles for the development of measurement systems for research and development processes

Abstract: Based on a comprehensive literature review and the activities of numerous case study companies, it is argued in this paper that performance measurement in R&D is a fundamental aspect to quality in R&D and to overall business performance. However, it is apparent from the case companies that many companies still struggle with the issue of R&D performance measurement. Excuses for not measuring are easily found, but there are also empirical examples and literature available with suggestions how it can be done. In this article this literature is reviewed and placed within the context of general performance control and contingency theory. Furthermore, the main measurement system design parameters are discussed and some basic system requirements are described as well as several design principles that can be useful for those who accept the challenge of establishing a meaningful measurement system.

Melt block copolymerization of ε-caprolactone and L-lactide

Abstract: AB block copolymers of e-caprolactone and (L)-lactide could be prepared by ring-opening polymerization in the melt at 110°C using stannous octoate as a catalyst and ethanol as an initiator provided e-caprolactone was polymerized first. Ethanol initiated the polymerization of e-caprolactone producing a polymer with e-caprolactone derived hydroxyl end groups which after addition of L-lactide in the second step of the polymerization initiated the ring-opening copolymerization of L-lactide. The number-average molecular weights of the poly(e-caprolactone) blocks varied from 1.5 to 5.2 × 103, while those of the poly(L-lactide) blocks ranged from 17.4 to 49.7 × 103. The polydispersities of the block copolymers varied from 1.16 to 1.27. The number-average molecular weights of the polymers were controlled by the monomer/hydroxyl group ratio, and were independent on the monomer/stannous octoate ratio within the range of experimental conditions studied. When L-lactide was polymerized first, followed by copolymerization of e-caprolactone, random copolymers were obtained. The formation of random copolymers was attributed to the occurrence of transesterification reactions. These side reactions were caused by the e-caprolactone derived hydroxyl end groups generated during the copolymerization of e-caprolactone with pre-polymers of L-lactide. The polymerization proceeds through an ester alcoholysis reaction mechanism, in which the stannous octoate activated ester groups of the monomers react with hydroxyl groups.

Electrode Properties of Sr‐Doped LaMnO3 on Yttria‐Stabilized Zirconia I. Three‐Phase Boundary Area

Abstract: The interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrxLa1–xMnO3 (SLM), was investigated with respect to its electrochemical performance. The interface microstructure was characterized by grain size and coverage of SLM on the electrolyte surface. Variation of the grain size was obtained by using three different sintering temperatures, whereas variation of the coverage was obtained by using two powders with a different morphology. This resulted in a set of six cathode/electrolyte samples with different combinations of grain size and SLM coverage at the interface. The cathode overpotential, as a measure for the electrochemical performance, could not be related to the length of the three-phase boundary. Based on the constriction resistance occurring in the electrolyte a model was developed which provides an estimate for the width of the active three-phase boundary zone.This zone is most likely to extend outside the cathode particle across the zirconia surface. The width calculated in this way was found to vary in the range of 0.03 to 0.07 µm for the different electrode microstructures. It is argued that the actual values may be smaller by one or two orders of magnitude.

European energy crops: A synthesis

Abstract: The European Energy Crops Overview (EECO) project was carried out with 20 partners from fourteen EU countries during 1996. The EECO-project provides the state-of-the-art on energy crops activities in Europe. More than 30 potential energy crop species have been investigated in Europe, but only a few have achieved commercial status so far. The introduction of energy crops in agriculture is relatively easy in the case of well-known agricultural crops such as rape and grain crops, but new crops are hampered by both technical and non-technical barriers. Production, pre-treatment and use of woody and herbaceous energy crops for power and heat generation is still mainly in the pilot to demonstration phase, while use of sugar and oil rich crops for transport purposes has been developed at a commercial scale already. Production, pre-treatment and use of SRC is fully developed in Sweden and in the pilot to demonstration phase in the north-west European countries. Herbaceous crops are tested up to a large scale in the Scandinavian countries, Germany, Austria and the Netherlands. Biodiesel is produced on a commercial scale in France, Germany, Austria and Italy, while bioethanol is produced at a commercial scale in France. In southern Europe, emphasis is on the production aspects of energy crops; only a limited number of efforts on use of energy crops have been realized so far.

On the Role of the Pore Size and Tortuosity for Sorption of Alkanes in Molecular Sieves

Abstract: Adsorption of light alkanes on the acidic zeolites theta-1 (H-TON), ferrierite (H-FER), and ZK-5 (H-KFI) was studied by gravimetry, calorimetry, and infrared spectroscopy. Sorption of alkanes on the Bronsted acid sites is energetically favored, and the uptake at low alkane partial pressures is directly correlated to the concentration of acid sites. With H-FER, the accessibility of the two intersecting channel systems depends on the chain length of the alkanes. Although propane and n-butane sorb in both channels, the sorption of n-pentane is hindered in the 8-ring channels and sorption of n-hexane only occurs in the 10-ring channels. In the presence of cavities (such as found in H-KFI) pronounced intermolecular interactions between sorbed alkanes were monitored. In general, the heat of adsorption of alkanes increases with decreasing pore size. At pore sizes smaller than 0.5 nm repulsion forces start to be important, their impact being most prominent for larger isoalkanes.

Copoly(ester-amides) and copoly(ester-urethanes)

Abstract: The invention provide biodegradable copolymers of the copoly(ester-amide) or copoly(ester-urethane) type, which copolymers consist of building blocks with the general structure (I): [CB-VB] wherein CB and VB of different building blocks may be the same or different, and CB represents a block with a constant length and VB represents a block with a variable length.

A principal-agent model of corruption

Abstract: One of the new avenues in the study of political corruption is that of neo-institutional economics, of which the principal-agent theory is a part. In this article a principal-agent model of corruption is presented, in which there are two principals (one of which is corrupting), and one agent (who is corrupted). The behaviour of these principals and agent is analysed in terms of the costs and benefits associated with different actions. The model is applied to political corruption in representative democracies, showing that, contrary to common belief, the use of principal-agent models is not limited to bureaucratic corruption.

Information technology and gender equality: a contradiction in terminis?

Abstract: Using the data source of the Computers in Education (Comped) study, carried out under the auspices of the International Association for the Evaluation of Educational Achievement (IEA), the status of gender and computer use in education in a number of countries has been investigated. The findings in this study indicate that the concern about gender equity expressed by many educational practitioners are right. Females know less about information technology, enjoy using the computer less than male students, and perceive more problems with software. Possible causes of this are differences in parental support, access to computers, amount of female role models and activities carried out with computers in school. Gender differences are being found both outside and inside schools. This means that both teachers and parents have to be made aware of this as a starting point for proper action. Schools rarely have a policy concerning gender issues; and when it exists, it is not directed to parents as well. The U.S.A. is the most ?gender equal? country of the countries examined.

Children and Videogames: Leisure Activities, Aggression, Social Integration, and School Performance

Abstract: A survey was conducted among 346 children from the 7th and 8th grade of 7 elementary schools to examine possible positive and negative effects of playing videogames. Analyses revealed that playing videogames did not appear to take place at the expense of children's other leisure activities, social integration, and school performance. A gender difference arose: Boys spent more time playing videogames than did girls. There was no significant relationship between the amount of time children spent on videogames and aggressive behavior. A negative relationship between time spent playing videogames and prosocial behavior was found; however, this relationship did not appear in separate analyses for boys and girls. Furthermore, a positive relationship was found between time spent on videogames and a child's intelligence.

On the motion of gas bubbles in homogeneous isotropic turbulence

Abstract: This paper is concerned with the motion of small gas bubbles, equivalent diameter about 1.0 mm, in isotropic turbulent flows. Data on the mean velocity of rise and the dispersion of the bubbles have been obtained numerically by simulating the turbulence as a sum of Fourier modes with random phases and amplitudes determined by the Kraichnan and the von Karman–Pao energy-spectrum functions, and by calculating the bubble trajectories from a reasonably well-established equation of motion. The data cover the range β[les ]1, where β is the ratio between the turbulence intensity and the velocity of rise of the bubbles in still fluid. An approximate analysis based on the assumption that β is small yields results that compare favourably with the numerical data, and clarifies the important role played by the lift forces exerted by the fluid.

Optimum electrode geometry for spinal cord stimulation : the narrow bipole and tripole

Abstract: A computer model is used to calculate the optimum geometry of an epidural electrode, consisting of a longitudinal contact array, for spinal cord stimulation in the management of chronic, intractable pain. 3D models of the spinal area are used for the computation of stimulation induced fields, and a cable model of myelinated nerve fibre is used for the calculation of the threshold stimulus to excite large dorsal column and dorsal root fibres. The criteria for the geometry of the longitudinal contact array are: a low threshold for the stimulation of dorsal column fibres compared with dorsal root fibres; and a low stimulation voltage (and current). For both percutaneous and laminectomy electrodes, the contact length should be approximately 1.5 mm, and the optimum contact separation, as determined by the computer model, is 2-2.5 mm. The contacts for a laminectomy electrode should be approximately 4 mm wide. This electrode geometry is applicable to all spinal levels where the dorsal columns can be stimulated (C1-2 down to L1). The stimulating electrode should preferably be used as a tripole with one (central) cathode.

Energy storage and release of prosthetic feet Part 1: Biomechanical analysis related to user benefits

Abstract: The energy storing and releasing behaviour of 2 energy storing feet (ESF) and 2 conventional prosthetic feet (CF) were compared (ESF: Otto Bock Dynamic Pro and Hanger Quantum; CF: Otto Bock Multi Axial and Otto Bock Lager). Ten trans-tibial amputees were selected. The study was designed as a double-blind, randomised trial. For gait analysis a VICON motion analysis system was used with 2 AMTI force platforms. A special measuring device was used for measuring energy storage and release of the foot during a simulated step. The impulses of the anteroposterior component of the ground force showed small, statistically non-significant differences (deceleration phase: 22.7-23.4 Ns; acceleration phase: 17.0-18.4 Ns). The power storage and release phases as well as the net results also showed small differences (maximum difference in net result is 0.03 J kg-1). It was estimated that these differences lead to a maximum saving of 3% of metabolic energy during walking. It was considered unlikely that the subjects would notice this difference. It was concluded that during walking differences in mechanical energy expenditure of this magnitude are probably not of clinical relevance. Ankle power, as an indicator for energy storage and release gave different results to the energy storage and release as measured with the special test device, especially during landing response. In the biomechanical model (based on inverse dynamics) used in the gait analysis the deformation of the material is not taken into consideration and hence this method of gait analysis is probably not suitable for calculation of shock absorption.

Noncovalent Synthesis of Nanostructures: Combining Coordination Chemistry and Hydrogen Bonding

Abstract: Rosettes that are held together by hydrogen bonds (see sketch on the right) were synthesized from metallodendrimers constructed by coordination chemistry. Two orthogonal, noncovalent interactions (metal-ligand and hydrogen bonding) were employed to build these nanosized dendrimers (M 7-28 kDa).

Deflection and maximum load of microfiltration membrane sieves made with silicon micromachining

Abstract: With the use of silicon micromachining, an inorganic membrane sieve for microfiltration has been constructed having a silicon nitride membrane layer with thickness typically 1 /spl mu/m and perforations typically between 0.5 /spl mu/m and 10 /spl mu/m in diameter. As a support a -silicon wafer with openings of 1000 /spl mu/m in diameter has been used. The thin silicon nitride layer is deposited on an initially dense support by means of a suitable chemical vapor deposition method (LPCVD). Perforations in the membrane layer are obtained with use of standard photo lithography and reactive ion etching (RIE). The deflection and maximum load of the membrane sieves are calculated in a first approximation. Experiments to measure the maximum load of silicon-rich silicon nitride membranes have confirmed this approximation.