Showing papers by "National Technical University of Athens published in 1993"

Cyclic transfer algorithms for multivehicle routing and scheduling problems

Abstract: This paper investigates the application of a new class of neighborhood search algorithms—cyclic transfers—to multivehicle routing and scheduling problems. These algorithms exploit the two-faceted decision structure inherent to this problem class: First, assigning demands to vehicles and, second, routing each vehicle through its assigned demand stops. We describe the application of cyclic transfers to vehicle routing and scheduling problems. Then we determine the worst-case performance of these algorithms for several classes of vehicle routing and scheduling problems. Next, we develop computationally efficient methods for finding negative cost cyclic transfers. Finally, we present computational results for three diverse vehicle routing and scheduling problems, which collectively incorporate a variety of constraint and objective function structures. Our results show that cyclic transfer methods are either comparable to or better than the best published heuristic algorithms for several complex and important ...

Parallel computations on reconfigurable meshes

Abstract: The mesh with reconfigurable bus is presented as a model of computation. The reconfigurable mesh captures salient features from a variety of sources, including the CAAPP, CHiP, polymorphic-torus network, and bus automation. It consists of an array of processors interconnected by a reconfigurable bus system that can be used to dynamically obtain various interconnection patterns between the processors. A variety of fundamental data-movement operations for the reconfigurable mesh are introduced. Based on these operations, algorithms that are efficient for solving a variety of problems involving graphs and digitized images are also introduced. The algorithms are asymptotically superior to those previously obtained for the aforementioned reconfigurable architectures, as well as to those previously obtained for the mesh, the mesh with multiple broadcasting, the mesh with multiple buses, the mesh-of-trees, and the pyramid computer. The power of reconfigurability is illustrated by solving some problems, such as the exclusive OR, more efficiently on the reconfigurable mesh than is possible on the programmable random-access memory (PRAM). >

Oregano flavonoids as lipid antioxidants

Abstract: Oregano (Origanum vulgare L.) leaves were successively extracted with hexane, ethyl ether, ethyl acetate and ethanol. The ethanol extract was reextracted in a separatory funnel with petroleum ether, ethyl ether, ethyl acetate and butanol. The ethyl ether layer was the most effective in stabilizing lard against oxidation, with activity equal to butylated hydroxytoluene. It also showed antioxidant activity when tested on vegetable oils under storage or frying conditions. The main antioxidant factors isolated from the ethyl ether layer consisted of flavonoids. Chromatographic and spectrophotometric analysis demonstrated the presence of the flavone apigenin, the flavanone, eriodictyol and the dihydroflavonols, dihydrokaempferol and dihydroquercetin.

Advanced Modeling of Fluid Catalytic Cracking Riser-Type Reactors

Abstract: A mathematical model is developed that predicts three-dimensional, two-phaseflow, heat transfer and reaction inside catalytic cracking riser-type reactors. The model consists of the full set of partial-differential equations that describe the conservation of mass, momentum, energy and chemical species for both phases in the reactor, coupled with empirical correlations concerning interphase friction, interphase heat-transfer and fluid-to- wall frictional forces. The cracking reaction is simulated by a simple three-lump kinetic model, but more realistic kinetic models can be easily included. The model can predict the most important engineering aspects of a riser reactor including pressure drop, catalyst holdup, interphase slip velocity, catalyst acceleration zone, choking behavior and temperature distribution in both phases, and yields distribution all over the reactor. It can also predict other complex engineering three-dimensional, two-phase problems realistically using computational fluid dynamics techniques.

Flow simulation of herschel‐bulkley fluids through extrusion dies

Abstract: The flow of viscoplastic materials through extrusion dies has been studied numerically using the finite element method. Rheological data for viscoplastic doiighs have been fitted by the Herschel-Bulkley model, which incorporates a yield stress into the power-law model. Non-isothermal simulations show the extent and shape of yieldedlunyielded regions and the development of temperature field assuming different modes of heat transfer at the boundaries. The results reveal that viscous dissipation causes appreciable temperature rises in the extrudate in agreement with measured values at the extruded material surface. The extrudate swell results show a maximum for a certain range of apparent shear rates also observed experimentally. However, the inelastic simulations based on the Herschel-Bulkley model always underpredict the experimental swelling values. A heuristic approach is also used to determine the Ievel of elasticity required to produce the experimental values. On a CtudiC numtriquement 1’Ccoulement de matCriaux viscoplastiques dans des filDres d’extrusion par une mCthode d’CICments finis. Les donnCes rhCologiques des pltes viscoplastiques ont CtC calCes ii I’aide du modtle Herschel-Bulkley , qui introduit une contrainte de cisaillement dans le modMe de loi de puissance. Les simulations non isothermes montrent 1’Ctendue et la forme des regions cisaillies et non cisaillCes ainsi que le dCveloppement du champ de temp6rature en supposant differents modes de transfert de chaleur aux limites. Les rCsultats rtvtlent que la dissipation visqueuse entraine des ClCvations de tempkratures apprCciables dans I’extrudat en accord avec les valeurs mesurkes ii la surface des matCriaux extrudts. Les rksultats sur le gonflement de I’extrudat indiquent un maximum pour une certaine gamme de vitesses de cisaillement apparentes Cgalement observtes expkrimentalement . Cependant, les simulations non Clastiques s’appuyant sur le modMe Herschel-Bulkley s’avtrent toujours insuffisantes pour prkdire les valeurs de gonflement expkrimentales. On utilise Cgalement une approche heuristique afin de determiner le degrC d’ClasticitC requis pour produire des valeurs exPCrimentales.

Esterification reactions catalyzed by lipases in microemulsions: The role of enzyme localization in relation to its selectivity

Abstract: The activity of lipases from Rhizopus delemar, Rhizopus arrhizus, and Penicillium simplicissimum entrapped in microemulsions formulated by bis-(2-ethylhexyl)sulfo-succinate sodium salt (AOT) in isooctane has been studied in esterification reactions of various aliphatic alcohols with fatty acids. The effect of the nature of the fatty acids (chain length) and of the alcohols (primary, secondary, or tertiary; chain length; cyclic structures) on the lipase activities was investigated in relation to the reverse micellar structure. The lipases tested showed a selectivity regarding the structure of the substrates used when hosted in the AOT/isooctane microemulsion systems. Penicillium simplicissimum lipase showed higher reaction rates in the esterification of long chain alcohols as well as secondary alcohols. Primary alcohols had a low reaction rate and tertiary a very slow rate of esterification. Long chain fatty acids were better catalyzed as compared to the shorter ones. Rhizopus delemar and R. arrhizus lipases showed a preference for the esterification of short chain primary alcohols, while the secondary alcohols had a low rate of esterification and the tertiary ones could not be converted. The reaction of medium chain length fatty acids was also better catalyzed than in the case of the long ones. The observed lipase selectivity appeared to be related to the localization of the enzyme molecule within the micellar microstructure due to the hydrophobic/hydrophilic character of the protein. The reverse micellar structural characteristics, as well as the localization of the enzyme, were examined by fluorescence quenching measurements and spectroscopical studies.

Design of Analog-Digital VLSI Circuits for Telecommunications and Signal Processing

Abstract: 1. Circuit-Level Methods for VLSI Components, F. M. Klaassen. 2. CMOS Operational Amplifiers, Daniel Senderowicz. 3. Micropower Techniques, Eric A. Vittoz. 4. Dynamic Analog Techniques, Eric A. Vittoz. 5. Optical Receivers, David J. T. Heatley. 6. Continuous-Time Filters, Yannis Tsividis and Venugopal Gopinathan. 7. Switched-Capacitor Filter Synthesis, Adel S. Sedra and Martin Snelgrove. 8. Multirate Switched-Capacitor Filters, Jose E. Franca and R. P. Martins. 9. Analog-Digital Conversion Techniques for Telecommunications Applications, Paul Gray and Robert R. Nepp. 10. Delta-Sigma Data Converters, Gabor C. Temes. 11. Layout of Analog and Mixed Analog-Digital Circuits, Franco Maloberti. 12. System Architectures and VLSI Circuits for Telecommunications, Guenter Weinberger. 13. Integrated Circuits for the ISDN, Peter Adams. 14. VLSI Architectures and Circuits for Visual Communications, P. Pirsch and T. Wehberg. 15. Digital Television, A. Rothermel. 16. VLSI Architectures and Circuits for Digital Coding of High Definition Television, P. Pirsch and K. Gruger. 17. IC Solutions for Mobile Telephones, Juha Rapeli. Index.

Kinetic study of lipase catalyzed esterification reactions in water-in-oil microemulsions

Abstract: The kinetics of the esterification of lauric acid by (-)menthol, catalyzed by Penicillium simplicissimum lipase, was studied in water/bis-(2-ethylhexyl)sulfosuccinate sodium salt (AOT)/isooctane microemulsions. Due to their low water content, microemulsions assist in reversing the direction of lipase activity, favoring synthetic reactions. The kinetics of this synthesis follows a Ping-Pong Bi--Bi mechanism. The values of all apparent kinetic parameters were determined. The theoretical model for the expression of enzymic activity in reverse micelles, proposed by Verhaert et al. (Verhaert, R., Hilhorst, R., Vermue, M., Schaafsma, T. J., Veeger, C. 1990. Eur. J. Biochem. 187: 59-72) was extended to express the lipase activity in an esterification reaction involving two hydrophobic substrates in microemulsion systems. The model takes into account the partitioning of the substrates between the various phases and allows the calculation of the intrinsic kinetic constants. The experimental results showing the dependence of the initial velocity on the hydration ratio, W(o) = [H(2)O]/[AOT], of the reverse micelles, were in accordance with the theoretically predicted pattern.

Factors affecting the lipase catalyzed transesterification reactions of 3-hydroxy esters in organic solvents.

Abstract: Chiral resolutions of racemic 3-hydroxy esters were performed in organic phases with lipases from Pseudomonas cepacia , Chromobacterium viscosum and Porcine pancreas. The reaction conditions have been optimized with 3-hydroxy octanoic acid methyl ester. Different organic solvents have been tested showing a tendentious correlation with the hydrophobicity of the solvents expressed as log P. The reaction time was shortened six fold by using irreversible acylating agents. We have found solvent type, lipase type and acylating agent acting as tools for changing the enantioselectivity. Lipase from Pseudomonas cepacia was lyophilized at different pH and the influence of the amount of water added was investigated, resulting in the highest activity at the pH optimum and a denaturation of the lipase above 1 % water (w/w lipase ). The water activity was measured on-line with a humidity sensor. Water activities greater than 0.4 led to a decrease in enantioselectivity and reaction rate. In the optimized system the resolutions of other 3-hydroxy esters were tested. Aliphatic compounds reacted with lower enantioselectivity, only the substrates could be isolated in high enantiomeric purity. In contrast, aromatic 3-hydroxy esters were acylated by lipases with high stereoselectivity. A model of the active site of lipase from Pseudomonas sp . explained these experimental observations.

Metamorphic evolution of metapelites in the high-pressure terrane of the Rhodope Zone, northern Greece

Abstract: The Rhodope zone of northern Greece has been involved in an Alpine metamorphic cycle that consists of a high-pressure (eclogite-facies) extensively overprinted under medium-pressure conditions in Eocene time. Abrupt differences in the grade of metamorphism within the Rhodope zone allows its subdivision into a lower and an upper tectonic unit. The pelitic rocks of the lower tectonic unit are typically gamet - chloritoid t staurolite schists. These rocks preserve the high-pressure paragenesis garnet+ chloritoid +chlorite +phengite + quartz+rutile. Minerals formedduringexhumation are staurolite, muscovite, Fe-rich chlorite and, rarely, biotite and andalusite. In the upper tectonic uni! the metapelites are generally gamet - biotite 1 kyanite gneisses and schists; the high-pressure paragenesis has been partly obliterated during extensive retrograde overprinting. Peak (minimum) conditions of pressure determined for the lower tectonic unit by use of the phengite geobarometer (for T = 550' to 600'C) are I 3-l 3.5 kbar. P-T estimates for peak of metamorphism of the upper tectonic unit are uncertain. Textural features and mineral chemical data provide information on metamorphic conditions for various stages of decompression of both units. The exhumation path of the lower tectonic unit was nearly isothermal to a depth of - I 2 km from the surface. ln the upper tectonic unit" exhumation proceeded along a P-T path characterized by cooling. These differences are probably attributed to ttmsting oftle deeper lying, upper tectonic unit over the lower one at depth during unloading. ln this case, the upper unit acted as a warm "shield" and prevented the rocks of the undedying lower unit from losing heat. Since the P-T path of Rhodope shows important similarities in form with that of other high-pressure terranes (1a., the Westem Alps, the Seward Peninsula in Alaska, the Cyclades in the Aegean Sea), analogous constraints, such as rapid umoofing tectonics or continuous underthrusting ofcold material, can be invoked to explain the cooling that accompanied exhumation.

Analytical framework for minimizing freeway-incident response time

Abstract: Freeway‐incident‐management (FIM) programs are receiving growing attention, since they have the potential of generating substantial savings for highway users The goal of any incident‐management program is the minimization of incident delay through the rapid restoration of the freeway capacity The most important aspect of freeway‐incident management is the servicing and the removal of the incidents Currently a number of FIM programs use a fleet of trucks for the quick restoration of freeway capacity Although the deployment of these traffic‐flow restoration units (TFRUs) is an important component of freeway‐management operations, the literature lacks analytical models that can be used to rationalize the deployment of the TFRUs The present paper proposes an integrated methodological framework for the minimization of freeway‐incident delays through the optimum deployment of TFRUs The proposed model consists of three basic modules The first module determines the number of required TFRUs and their servic

Availability accumulation and destruction in a DI diesel engine with special reference to the limited cooled case

Abstract: This work develops a method for the calculation of both the irreversibility produced during combustion and the working medium availability at the end of the expansion process in a high speed, direct injection (DI), naturally aspirated, four-stroke diesel engine, on which experiments were conducted at the authors' laboratory. The experimental data were processed for the determination of fuel reaction rates; the combustion irreversibility production rate was then computed from the fuel reaction rates via an analytical mathematical expression which was developed by the present research group, based on the combined resolution of the first and second laws of thermodynamics. This expression is coupled with standard first law calculations and then is integrated to give the accumulated combustion irreversibility, while the working medium availability variation is also computed throughout the engine closed cycle. These calculations are applied for a wide range of measured loads, injection timings and engine rotational speeds; they are also expanded in the direction of the intensity of the rate of heat transfer loss (to the engine cooling medium) for every combination of the experimentally determined engine variables. Therefore, apart from investigating the effect of various operating parameters on the availability balance, it is possible to evaluate the effect of the engine heat transfer reduction (limited cooled engine), from the second law analysis point of view, on the potential for efficiency improvements made by using the increased exhaust heat in recovery devices (e.g. the exhaust turbine or Rankine bottoming cycle compounding). With the present second law analysis, which forms the spearhead of this work, the exhaust gas availability offers more useful information than its enthalpy counterpart (first law analysis) for the operation of such compounding devices. The irreversibility calculation also provides useful information for the combustion loss, which cannot be isolated and evaluated at all by a first law analysis.

Elements of Statistical Mechanics

Abstract: Thermodynamics, as we have studied it so far, is referred to as Classical Thermodynamics and is built upon its empirical laws. They are called empirical because they are the results of observations through the years (Chapter 4) and their validity lies with the fact that they have never failed.

Textural modifications in nickel electrodeposition under pulse reserved current

Abstract: Nickel electrodeposits consists of crystallites whose orientation, microstructure and size depend on the electrolytic conditions. Study of the mechanism of nickel electrocrystallization has indicated that the oriented growth observed is determined by the specific inhibition exerted by the chemical species which exist or are formed on the metal-electrolyte interface (catholyte) and which are selectively absorbed on the continually renewed metal surface. The pulse reversed current (PRC) technique was used to perturb the electrocrystallization process and hence to change the texture and the structural characteristics of nickel electrodeposits. The strucrtural changes that occur in deposit orientation under PRC conditions were studied. Nickel electrodeposition was realized from a Watts-type bath on a rotated cathode, and the electrolysis parameters (current density, rotation velocity, electrolyte pH and temperature) were chosen in such a way that using direct current they led to the preparation of perfectly [100], [211], [210], [110] A and [110] B textured deposits. The deposit texture was determined by X-ray diffractometry and the results are summarized in texture diagrams given as a function of PRC parameters: pulse frequency and duty cycle. It was proved that the texture is directly related to PRC parameters, which provoke strong perturbation in the crystal growth process. This phenomenon is reinforced when both the pulse frequency and the duty cycle are small. The PRC technique affects the nickel electrocrystallization process according to a mechanism analogous to the action of organic additives under direct current conditions by intervening in the catholyte composition and hence in the adsorption-desorption phenomena occuring at the metal-solution interface during the dissolution and the deposition time.

Evaluation of schmidt rebound hammer testing: A critical approach

Abstract: In the present paper, a critical approach to the evaluation of Schmidt hammer testing is attempted. Through a statistical analysis of field measurements on sandstone blocks from the archaeological site of Delphi (Greece), it was shown that there is an inconsistency in the correction method proposed by the manufacturer, in relation to the direction of rebound. The horizontally obtained rebound values, when corrected, yielded lower values than the vertically obtained ones. When examining the various factors potentially responsible for possible differences between the vertical and horizontal values, we conclude that the proposed correction method is not applicable to all cases and that there are many factors that may influence the end results.

Effect of alkali delignification on wheat straw saccharification by fusarium oxysporum cellulases

Abstract: The effect of alkaline delignification of wheat straw on the chemical composition and the subsequent enzymic hydrolysis of the pretreated straw are reported. Both hot (120°°C) and cold (20–36°°C) delignification were investigated, using either aqueous or organic alkaline solutions. The treated lignocellulosic materials were hydrolyzed by the cellulases of Fusarium oxysporum strain F3. Both delignification and saccharification yield showed linear relationships with the level of alkali used. Under the chosen experimental conditions 70–100% hydrolysis was achieved either by hot or cold delignification. Delignification to at least 50% appeared crucial for total polysaccharide conversion.

Effect of rock discontinuities on certain rock strength and fracture energy parameters under uniaxial compression

Abstract: Five series of test blocks of Pendeli marble with artificially created discontinuities of different crack densities (simulating three mutually orthogonal joint sets) were tested in uniaxial compression in order to study the effect of discontinuities on: (a) the compressive strength and the modulus of elasticity, and (b) certain fracture energy parameters expressed by the ratio WA/WV, where WA is the surface energy and WV the volume elastic strain energy. Mathematical relationships are derived similar to those suggested by other authors relating strength parameters to crack densities. Such relationships clearly show a reduction in strength with increased crack density. The experimental results obtained permit the extension of Persson's relation (which refers to ideal intact rock) to the more realistic case of discontinuous rock mass by introducing the appropriate term that takes into consideration the effect of rock mass discontinuities on the energy ratio WA/WV. A comparison between laboratory results and field observations was subsequently carried out assuming the rock mass to behave as a linearly elastic material, obeying the Hoek and Brown failure criterion. This comparison showed that laboratory results can be extended to larger scale. Furthermore, in order to predict the in situ strength and stability of a rock mass in uniaxial compression (which is of major importance in underground excavations) certain concepts are proposed based on laboratory tests, in situ investigations and first principles of linear elastic fracture mechanics.

Coordination of joint tasks in organizational processes

Abstract: Organizational productivity can be maximized by creating, using and maintaining structural and dynamic configurations of multi-participant interaction. The paper highlights a number of areas for consideration that arise when studying coordination within an organizational setting. The focus of the analysis is on two types of tasks: decision-making tasks and routine office processes. The paper argues that a number of (conflicting) options exist when developing the coordination aspects of group systems; they are classified across the following axes: specification and implementation of coordination; use of synchronous and asynchronous working phases; information exchange and information sharing; support of sequential and concurrent processing; support of negotiation and conflict resolution; support of analytical modelling; and description of the organizational environment.

Runge-Kutta pairs for periodic initial value problems

Abstract: We study the relative merits of the phase-lag property of Runge-Kutta pairs and we propose new explicit embedded pairs for the numerical solution of first order differential systems with periodical solution. We analyze two families of 5(4) paris and one family of 6(5) pairs with respect to the attainable phase-lag order. From each family we choose a pair with the highest achievable phase-lag order, optimized with respect to a measure of the magnitude of its truncation error coefficients. The new 5(4) algebraic order pairs are of phase-lag order 8(4) and 8(6) and they are both non-dissipative, while the 6(5) pair is dissipative and of phase-lag order 10(6). The new pairs exhibit an improved performance, in comparison with other currently known general and special purpose methods, when they are applied to semidiscretized hyperbolic equations and problems describing free and weakly forced oscillations.

Enantiomeric selectivity of a lipase from Penicillium simplicissimum in the esterification of menthol in microemulsions

Abstract: Lipase from Penicillium simplicissimum catalyzes the stereospecific esterification of menthol with fatty acids. The studies on the specificity of this new lipase were carried out using (+), (-) and racemic menthol with water soluble enzyme entrapped in microemulsion systems stabilized with sodium(bis-2-ethylhexyl)sulfosuccinate (AOT) as surfactant, in isooctane. Microemulsions appear to be an effective and fast system for racemic resolution of alcohols.

Mathematical Modelling of Single- and Two-Phase Flow Problems in the Process Industries

Abstract: Many key issuesin design for the process industries are related to the behaviour of fluids in turbulent flow, often involving more than one phase, chemical reaction or heat transfer. Computational-Fluid-Dynamics (CFD) techniques have great potential for analysing these processes and can be of great help to the designer, by reducing the need to resort to cut and try : approaches to the design of complex equipment. The paper presents the fundamental principles of CFD within the context of the so-called finite-domain technique. The procedure can handle one-, two-, and three-dimensional distributions of the variables in space, steady or transient processes, multi-phase processes, and effects such as turbulence, compressibility of phases, buoyancy, phase-change, chemical reactions, gravity stratification, etc. Demonstrations are made of the application of the procedure to the numerical computation of some process industry situations, such as those occurring in adsorbers/regenerators, combustors, cement kilns, and heat - exchangers. It is concluded that :- The finite - domain versions of the differential equations are soluble, with modest computer costs;- The solutions are always physically plausible; and,- There is a need for extensive evaluation and validation of CFD physical and chemical sub-models, particularly those concerning turbulence, chemical kinetics and interphase-transport processes.