Showing papers by "Technical University of Madrid published in 1994"

An overview of robust Bayesian analysis

Abstract: Robust Bayesian analysis is the study of the sensitivity of Bayesian answers to uncertain inputs. This paper seeks to provide an overview of the subject, one that is accessible to statisticians outside the field. Recent developments in the area are also reviewed, though with very uneven emphasis.

Accurate simulation of GaAs MESFET's intermodulation distortion using a new drain-source current model

Abstract: An accurate characterization of the nonlinear distortion caused by the Ids(Vgs,Vds) current in a MESFET, does not allow the common approach of splitting this nonlinear equivalent circuit element in two voltage dependent nonlinear current sources, Gm(Vgs) and Gds(Vds). By an improved laboratory characterization procedure, it was possible to extract the cross terms of the Ids(Vgs,Vds) Taylor series expansion. Measurements and Volterra series simulations, made at 2 GHz, have shown that they can give an important contribution to the prediction and understanding of MESFET's intermodulation load-pull behavior. >

Self-consistent analytical model of the Rayleigh-Taylor instability in inertial confinement fusion.

Abstract: The model presented overcomes past inconsistencies by applying asymptotic techniques. The obtained growth rate, [gamma][approx][alpha]([ital k]) [radical][ital kg] [minus]2[ital kv][sub [ital a]] (where [ital v][sub [ital a]]=ablation velocity, [alpha]([ital k])[equivalent to] [radical]1[minus]([ital k]/[ital k][sub [ital c]])[sup [ital r]] represents the stabilization heat conduction effect, and the cutoff wave number [ital k][sub [ital c]] is much smaller than the inverse of the density scale length at the ablation front), reproduces numerical simulations and experiments in a more complete way than the so-called Takabe formula, [gamma]=0.9 [radical][ital kg] [minus]3[ital kv][sub [ital a]].

Downstream effects of a new hydropower impoundment on macrophyte, macroinvertebrate and fish communities

Abstract: The downstream effects of Valparaiso Reservoir (Rio Tera, north-western Spain) on macrophyte, macroinvertebrate and fish communities are examined by comparing their respective structures before (1986) and after (1990-1991) the creation of this hydropower impoundment. A single sampling station was selected 2.4 km below the dam. Macrophytes significantly (p < 0.05) decreased their total biomass, although the species composition remained dominated by two species (Myriophyllum verticillatum and Ranunculus fluitans). The macroinvertebrate community, on the other hand, exhibited a significant (p < 0.05) decrease in taxonomic richness, total density and total biomass. In general, planarians, ephemer-opterans, coleopterans, plecopterans and trichopterans disappeared or decreased their abundances. Scrapers (as relative biomass) were the functional feeding group most adversely affected by the new flow regulation. With regard to the fish community, the most significant change was the absence of all resident coarse fishes (cyprinids, primarily) at the sampling site during the 1990 and 1991 sampling surveys. Conversely, salmonids (Salmo trutta) persisted; the trout population was dominated by individuals of the 0+ age group (⩽ 1 year old) both before and after the construction of Valparaiso Dam. It is concluded that short-term flow fluctuations induced by hydroelectric power generation were the main factor causing the observed adverse effects. In this respect, some management measures to minimize these effects are recommended.

Purification of Rhizobium leguminosarum HypB, a nickel-binding protein required for hydrogenase synthesis.

Abstract: The products of the Rhizobium leguminosarum hyp gene cluster are necessary for synthesis of a functional uptake [NiFe] hydrogenase system in symbiosis with pea plants, and at least for HypB and HypF, a role in hydrogenase-specific nickel metabolism has been postulated (L. Rey, J. Murillo, Y. Hernando, E. Hidalgo, E. Cabrera, J. Imperial, and T. Ruiz-Argueso, Mol. Microbiol. 8:471-481, 1993). The R. leguminosarum hypB gene product has been overexpressed in Escherichia coli and purified by immobilized nickel chelate affinity chromatography in a single step. The purified recombinant HypB protein was able to bind 3.9 +/- 0.1 Ni2+ ions per HypB monomer in solution. Co2+, Cu2+, and Zn2+ ions competed with Ni2+ with increasing efficiency. Monospecific HypB antibodies were raised and used to show that HypB is synthesized in R. leguminosarum microaerobic vegetative cells and pea bacteroids but not in R. leguminosarum aerobic cells. HypB protein synthesized by R. leguminosarum microaerobic vegetative cells could also be isolated by immobilized nickel chelate affinity chromatography. A histidine-rich region at the amino terminus of the protein (23-HGHHHH DGHHDHDHDHDHHRGDHEHDDHHH-54) is proposed to play a role in nickel binding, both in solution and in chelated form.

Re-encoding sequential circuits to reduce power dissipation

Abstract: We present a fully implicit encoding algorithm for minimization of average power dissipation in sequential circuits, based on the reduction of the average number of bit changes per state transition.We have studied two novel schemes for this purpose, one based on recursive weighted non-bipartite matching, and one on recursive minicut bi-partitioning. We employ ADDs (Algebraic Decision Diagrams) to computate the transition probabilities, to measure the potential area saving, and in the encoding algorithms themselves.Our experiments show the effectiveness of our method in reducing power dissipation for large sequential designs.

An optimized DC-to-DC converter topology for high-voltage pulse-load applications

Abstract: This paper presents the study of a high-voltage DC/DC resonant converter which has excellent behaviour to be used in applications with strong variations in output voltage and in output current, because it maintains high efficiency in these types of applications. To do so, the switching losses have been minimized by integrating all parasitic elements (leakage inductance, secondary side capacitance of the high-voltage transformer and parasitic capacitances of the power switches and diodes) into the power topology, and the conduction losses have been minimized by operating the converter in a special mode (optimum switching line) in which no reactive energy is returned from the resonant elements to the input voltage source. The type of control used (PWM phase-shifted) allows one to maintains these desirable characteristics even when the operating point suffers a very strong variation. The final power topology might be called a "full-bridge, clamped mode, LCC-type parallel resonant converter with capacitive output filter". >

Nickel availability to pea (Pisum sativum L.) plants limits hydrogenase activity of Rhizobium leguminosarum bv. viciae bacteroids by affecting the processing of the hydrogenase structural subunits.

Abstract: Rhizobium leguminosarum bv. viciae UPM791 induces the synthesis of an [NiFe] hydrogenase in pea (Pisum sativum L.) bacteroids which oxidizes the H2 generated by the nitrogenase complex inside the root nodules. The synthesis of this hydrogenase requires the genes for the small and large hydrogenase subunits (hupS and hupL, respectively) and 15 accessory genes clustered in a complex locus in the symbiotic plasmid. We show here that the bacteroid hydrogenase activity is limited by the availability of nickel to pea plants. Addition of Ni2+ to plant nutrient solutions (up to 10 mg/liter) resulted in sharp increases (up to 15-fold) in hydrogenase activity. This effect was not detected when other divalent cations (Zn2+, Co2+, Fe2+, and Mn2+) were added at the same concentrations. Determinations of the steady-state levels of hupSL-specific mRNA indicated that this increase in hydrogenase activity was not due to stimulation of transcription of structural genes. Immunoblot analysis with antibodies raised against the large and small subunits of the hydrogenase enzyme demonstrated that in the low-nickel situation, both subunits are mainly present in slow-migrating, unprocessed forms. Supplementation of the plant nutrient solution with increasing nickel concentrations caused the conversion of the slow-migrating forms of both subunits into fast-moving, mature forms. This nickel-dependent maturation process of the hydrogenase subunits is mediated by accessory gene products, since bacteroids from H2 uptake-deficient mutants carrying Tn5 insertions in hupG and hupK and in hypB and hypE accumulated the immature forms of both hydrogenase subunits even in the presence of high nickel levels.

Scars in Groups of Eigenstates in a Classically Chaotic System

Abstract: A general method to construct wave functions highly localized on a given periodic orbit, using the information contained in the short term quantum dynamics of the system, is presented. The relationship with the Husimi's quasiprobability distribution in phase space is also discussed.

Multiple double layers in a glow discharge

Abstract: The formation of visible layered structures that develop at the anodic zone of a constricted glow discharge is reported They are composed of several tridimensional double layers with axial symmetry enclosing different plasma coronas The dependence of the number and size of plasma shells with the conditions of the discharge is discussed, as well as the nature of these double layers The experimental results evidence the effect of the distribution of neutrals in the volume of the discharge for the development of double layers

Optimized synchronous rectification stage for low output voltage (3.3 V) DC/DC conversion

Abstract: A new strategy to obtain low output voltage (3.3 V) is presented in this paper. The output stage is optimized to minimize losses in the self-driven synchronous rectifiers, by means of a fixed frequency, fixed duty cycle (0.5) driving waveform. The output voltage is controlled by a high switching frequency preregulator. This preregulator can be removed if input voltage variation is low. Very high efficiency (93%) has been obtained in an actual prototype (3.3 V and 20 A) of the optimized SR stage. >

Splitting tests : an alternative to determine the dynamic tensile strength of ceramic materials

Abstract: On etudie les possibilites d'utilisation de l'essai de traction indirecte (essai bresilien) en conditions dynamiques pour obtenir les proprietes mecaniques a traction des materiaux ceramiques a hautes vitesses de contrainte. L'etude se fonde sur la realisation d'essais mecaniques sur eprouvettes cylindriques placees dans la barre Hopkinson et observation par camera photographique de haute vitesse de la nucleation et propagation de la fissure ainsi que sur des simulations numeriques de l'essai par moyen de la methode des elements finis. On constate que les materiaux ceramiques ont un comportement elastique jusqu'a la rupture et que la condition d'equilibre est atteinte avant la rupture de l'eprouvette, donc la methode proposee peut etre alternative pour determiner les proprietes dynamiques a traction des materiaux fragiles

Estimating the computational cost of logic programs

Abstract: Information about the computational cost of programs is potentially useful for a variety of purposes, including selecting among different algorithms, guiding program transformations, in granularity control and mapping decisions in parallelizing compilers, and query optimization in deductive databases. Cost analysis of logic programs is complicated by nondeterminism: on the one hand, procedures can return multiple solutions, making it necessary to estimate the number of solutions in order to give nontrivial upper bound cost estimates; on the other hand, the possibility of failure has to be taken into account while estimating lower bounds. Here we discuss techniques to address these problems to some extent.

Efficient refinement/derefinement algorithm of nested meshes to solve evolution problems

Abstract: An adaptive refinement/derefinement algorithm of nested meshes is presented. Some definitions are introduced. The main properties of the derefinement algorithm are remarked upon and its efficiency is shown through two numerical examples: a time-dependent convection-diffusion problems with dominant convection and a quasistationary problem.

Extracting Non-strict independent and-parallelism using sharing and freeness information

Abstract: Logic programming systems which exploit and-parallelism among non-deterministic goals rely on notions of independence among those goals in order to ensure certain efficiency properties. “Non-strict” independence (NSI) is a more relaxed notion than the traditional notion of “strict” independence (SI) which still ensures the relevant efficiency properties and can allow considerable more parallelism than SI. However, all compilation technology developed to date has been based on SI, because of the intrinsic complexity of exploiting NSI. This paper fills this gap by developing a technique for compile-time detection of NSI. It also proposes algorithms for combined compile-time/run-time detection, presenting novel run-time checks for this type of parallelism. The approach is based on the knowledge of certain properties regarding the run-time instantiations of program variables —sharing and freeness— for which compile-time technology is available, with new approaches being currently proposed.

Influence of matrix strength on reinforcement fracture and ductility in AlAl2O3 composites

Abstract: Two Al alloys (2014 and 6061) reinforced with 15 vol.% Al 2 O 3 particulates were tested in the naturally aged and peak-aged conditions to study the influence of matrix strength on the mechanisms controlling the tensile ductility of these composite. Periodic measurements of the degradtion of elastic properties during deformation, as well as quantitative metallography, indicated that the dominant failure mechanism was progressive fracture of the particulates during deformatin. The probability of particulate fracture increased with the reinforcement size, and elongated particulates oriented in the loading direction were more prone to fail than small, equiaed ones. It was also observed that the particulate fracture rate increased with the matrix strength and that the naturally aged composites were able to withstand more damage prior to failure than those arttificially aged at high temperature. When a critical volume fraction of broken reinforcements was reached at a given section of the specimen, matrix failure led to specimen fracture by a ductile mechanism of shear failure or void nucleation and growth from inclusions or precipitates in the matrix.

Some Methodological Issues in the Design of CIAO, a Generic, Parallel Concurrent Constraint Logic Programming System

Abstract: We informally discuss several issues related to the parallel execution of logic programming systems and concurrent logic programming systems, and their generalization to constraint programming. We propose a new view of these systems, based on a particular definition of parallelism. We argue that, under this view, a large number of the actual systems and models can be explained through the application, at different levels of granularity, of only a few basic principles: determinism, non-failure, independence (also referred to as stability), granularity, etc. Also, and based on the convergence of concepts that this view brings, we sketch a model for the implementation of several parallel constraint logic programming source languages and models based on a common, generic abstract machine and an intermediate kernel language.

Impedance, modulation response, and equivalent circuit of ultra-high-speed In/sub 0.35/Ga/sub 0.65/As/GaAs MQW lasers with p-doping

Abstract: On-wafer measurements of the frequency-dependent impedance, modulation response, and RIN power spectra of ultra-high-speed p-doped In/sub 0.35/Ga/sub 0.65/As/GaAs MQW lasers are presented and analyzed. The experimental results are shown to be accurately modeled by an equivalent circuit which accounts for both the carrier transport/capture dynamics and the junction space-charge capacitance. We find that the carrier escape time out of the QW's in our laser structure is much larger. Than the carrier capture time, and therefore the interplay between carrier capture and re-emission is not affecting the high-speed modulation dynamics. On the other hand, the absolute values of both the carrier capture time and the space-charge capacitance still limit the modulation bandwidth. >

A numerical study of the mechanisms of cyclic strain hardening in metal-ceramic composites

Abstract: A numerical study of the mechanical behaviour of discontinously-reinforced metal-matrix composites under uniaxial fully-reversed cyclic deformation is presented. The analyses are carried out within the framework of the finite element analysis of a unit cell. Three reinforcement shapes (whiskers, particulates, and spheres) and two different reinforcement volume fractions (10 and 20%) are studied. The composites exhibit cyclic hardening, which is mainly due to the progressive accumulation of plastic deformations in the matrix upon loading and unloading, and which is very sensitive to the matrix strain hardening capacity. Monotonic and cyclic strain hardening are maxima for the materials containing whiskers and minima in the case of spherical reinforcements, and increase with the volume fraction of reinforcement. However, it was found that the effect of reinforcement shape and volume fraction is more pronounced under monotonic deformation than in fatigue. The differences and similitudes in the strengthening mechanisms during monotonic and cyclic deformation are indicated, and the possible sources of cyclic softening (matrix cavitation, reinforcement fracture and interfacial decohesion) are discussed from the evolution of the plastic strains in the matrix and of the stresses acting on the ceramic reinforcements and at the interfaces.

Stationkeeping at libration points of natural elongated bodies

Abstract: Collinear libration points are proposed as useful locations for the close survey of natural elongated bodies in rotation. The effect of the primary asphericity is introduced qualitatively by means of the mass dipole model, for which the equations of motion and the libration point stability analysis are derived as a generalization of the circular restricted three-body problem. A transformation is provided to obtain a linear mass distribution that is gravitationally equivalent to a body of revolution, and the results for the rotating dumbbell may be applied to a more general class of bodies. A modal control technique is described for stabilization of collinear libration points. This technique is then used to stabilize a simulated probe at LI of the Mars-Phobos system.

Fatigue crack propagation in salt water, air and high vacuum in a spray-formed particulate-reinforced metal matrix composite

Abstract: Fatigue crack in salt water, air and high vacuum was studied in a spray-formed 2618 Al alloy reinforced with 15 vol.% SiC particulates and in the corresponding unreinforced alloy. The composite exhibited better fatigue crack growth resistance than the monolithic alloy in the three environments, although the differences decreased as the crack growth rate increased. The differences in the fatigue crack growth rates between the two materials were attributed to the higher closure levels in the composite, induced by its smaller crack-opening displacements. The presence of SiC particulates did not modify the susceptibility of this Al alloy to corrosion fatigue, the crack growth rates being maxima in salt water and minima in high vacuum.

Ignition and Flame Spread in Laminar Mixing Layers

Abstract: In order to identify some of the mathematical problems encountered in analyzing the ignition and flame spread in mixing layers, we shall describe the structure of the laminar mixing layer between two parallel streams of a fuel and air, initially separated by a splitter plate, undergoing an Arrhenius reaction.