Showing papers by "Polytechnic University of Turin published in 1989"

The effect of execution policies on the semantics and analysis of stochastic Petri nets

Abstract: Petri nets in which random delays are associated with atomic transitions are defined in a comprehensive framework that contains most of the models already proposed in the literature. To include generally distributed firing times into the model one must specify the way in which the next transition to fire is chosen, and how the model keeps track of its past history; this set of specifications is called an execution policy. A discussion is presented of the impact that different execution policies have on semantics of the mode, as well as the characteristics of the stochastic process associated with each of these policies. When the execution policy is completely specified by the transition with the minimum delay (race policy) and the firing distributions are of the phase type, an algorithm is provided that automatically converts the stochastic process into a continuous time homogeneous Markov chain. An execution policy based on the choice of the next transition to fire independently of the associated delay (preselection policy) is introduced, and its semantics is discussed together with possible implementation strategies. >

Cusp catastrophe interpretation of fracture instability

Abstract: A cohesive crack model is applied to analyse slow crack growth in elastic-softening materials. The shape of the structural load-displacement response is changed substantially by varying the size-scale while keeping the geometrical shape of the structure unchanged. The softening branch becomes steeper when the sizescale increases. A critical size-scale exists for which the softening slope is infinite. In such a case the load carrying capacity drastically decreases for relatively small displacement increments. Then, for size-scales larger than the critical one, the softening slope becomes positive and part of the load-displacement path becomes virtual if the loading process is displacement-controlled. In such a case, the loading capacity will present a discontinuity with a negative jump. The size-scale transition from ductile to brittle behaviour is governed by a nondimensional brittleness number SE which is a function of material properties and structure size-scale. A truly brittle failure occurs only with relatively low fracture toughnesses G ic, high tensile strengths σu, and/or large structure size-scales b, i.e. when S E = G ic σ u b → 0. On the other hand, if the loading process is controlled by a monotonically increasing function of time (e.g. the crack mouth opening displacement), the snap-back instability in the load-displacement curve can be captured experimentally. When the post-peak behaviour is kept under control up to the complete structure separation, the area delimited by the load-displacement curve and the displacement-axis represents the product of G ic and the initial ligament area. Finally, it is verified that. for SE → 0, the maximum load for catastrophic failure is provided by the simple LEFM condition : K 1 = K IC = G ic E (plane stress), and that there is no slow crack growth prior to instability.

The Dean equations extended to a helical pipe flow

Abstract: In this paper the Dean (1928) equations are extended to the case of a helical pipe flow, and it is shown that they depend not only on the Dean number K but also on a new parameter λ/[Rscr ] where λ is the ratio of the torsion τ to the curvature κ of the pipe axis and [Rscr ] the Reynolds number referred in the usual way to the pipe radius a and to the equivalent maximum speed in a straight pipe under the same axial pressure gradient. The fact that the torsion has no first-order effect on the flow is confirmed, but it is shown that this is peculiar to a circular cross-section. In the case of an elliptical cross-section there is a first-order effect of the torsion on the secondary flow, and in the limit λ/[Rscr ] → ∞ (twisted pipes, provided only with torsion), the first-order ‘displacement’ effect of the walls on the secondary flow, analysed in detail by Choi (1988), is recovered.Different systems of coordinates and different orders of approximations have recently been adopted in the study of the flow in a helical pipe. Thus comparisons between the equations and the results presented in different reports are in some cases difficult and uneasy. In this paper the extended Dean equations for a helical pipe flow recently derived by Kao (1987) are converted to a simpler form by introducing an appropriate modified stream function, and their equivalence with the present set of equations is recovered. Finally, the first-order equivalence of this set of equations with the equations obtained by Murata et al. (1981) is discussed.

Mode field diameter measurements in single-mode optical fibers

Abstract: The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments. >

Moment method with isoparametric elements for three-dimensional anisotropic scatterers

Abstract: A new method for computing the frequency-domain electromagnetic fields scattered from, and penetrating into, arbitrarily shaped, three-dimensional, lossy, inhomogeneous anisotropic scatterers is presented. The method is based on a general volume integro-differential formulation of the scattering problem, and consists of the numerical solution of the coupled integral equations by the moment method and point matching. A particularly powerful feature of this method is that the numerical model of the scatterer is obtained by parametric volume elements and the basis functions used to represent the field within each element are the same used in the finite-element method. Element integration problems due to the singular kernel of the integral equations are treated in some detail. Numerical results for both the isotropic and the anisotropic spherical scatterer are presented, including comparisons with results obtained by different numerical methods for the isotropic cases considered. The capability of the numerical code presented here to deal with cases where the material parameters of the scatterer are given by singular matrices is discussed for two particular examples. >

Softening and snap-back instability in cohesive solids

Abstract: The nature of the crack and the structure behaviour can range from ductile to brittle, depending on material properties, structure geometry, loading condition and external constraints. The influence of variation in fracture toughness, tensile strength and geometrical size scale is investigated on the basis of the π-theorem of dimensional analysis. Strength and toughness present in fact different physical dimensions and any consistent fracture criterion must describe energy dissipation per unit of volume and per unit of crack area respectively. A cohesive crack model is proposed aiming at describing the size effects of fracture mechanics, i.e. the transition from ductile to brittle structure behaviour by increasing the size scale and keeping the geometrical shape unchanged. For extremely brittle cases (e.g. initially uncracked specimens, large and/or slender structures, low fracture toughness, high tensile strength, etc.) a snap-back instability in the equilibrium path occurs and the load–deflection softening branch assumes a positive slope. Both load and deflection must decrease to obtain a slow and controlled crack propagation (whereas in normal softening only the load must decrease). If the loading process is deflection-controlled, the loading capacity presents a discontinuity with a negative jump. It is proved that such a catastrophic event tends to reproduce the classical LEFM-instability (KI = KIC) for small fracture toughnesses and/or for large structure sizes. In these cases, neither the plastic zone develops nor slow crack growth occurs before unstable crack propagation.

On the evolution of AC machines for spindle drive applications

Abstract: In the field of AC spindle drives, the induction motor is widely used. Synchronous solutions (reluctance, interior permanent magnets) are often suggested to overcome some drawbacks of the induction motor. The different options are compared by considering the machine torque-density and the inverter power size needed for a given constant-power speed range. It is shown that an axially laminated reluctance motor gives more torque density than the induction motor but requires nearly the same inverter size. By adding a proper quantity of permanent magnets, the inverter size can be greatly reduced. A comprehensive discussion of this subject is given. The goal is to find a design solution that is optimal for the whole drive. >

Dynamical superalgebra of the "dressed" Jaynes-Cummings model.

Abstract: We show how, by resorting to a one-fermion realization of the Pauli operators, the Hamiltonian of the Jaynes-Cummings (JC) model can be identified as an element of the superalgebra u(1|1), which plays the role of a dynamical algebra. The extension of this notion to osp(2|2) allows adding both virtual and real two-photon processes to the JC Hamiltonian. The exact diagonalization problem is tackled here in the special case when the coupling constants of the fermionic terms of the "dressed" JC Hamiltonian are assumed to nilpotent Grassman-Banach numbers.

Effect of FFT based algorithms on estimation of myoelectric signal spectral parameters

Abstract: The effects of epoch duration, time windowing, and an algorithm upon the estimates of spectral parameters of surface myoelectric signals are discussed. Simulation with computer-generated signals shows that: (a) rectangular window and linear interpolation between contiguous spectral values are acceptable for epoch durations as short as 0.25 s; (b) for the raw periodogram and epoch durations of 1-2 s, coefficients of variation near 3-4% for f/sub mean/ and 4-6% for f/sub med/ should be expected; and (c) for epochs of 0.25 s these values double, and for shorter epochs the estimates become meaningless because of high variance. These conclusions should be taken into account when selecting windows and epoch durations for myoelectric signal analysis. >

SU(1,1) Squeezed States as Damped Oscillators

Abstract: The conventional squeezed states of quantum optics, which can be thought of as generalized coherent states for the algebra SU(1,1), are dynamically generated by single-mode hamiltonians characterized by two-photon process interactions. By the explicit construction of a (highly non-linear) faithful realization of the group $\mathscr G$ of automorphisms of SU(1,1), such hamiltonians are shown to be equivalent — up just to elements of $\mathscr G$ — to that describing quantum mechanically a damped oscillator.

Simulation of an external gear pump and experimental verification

Abstract: A detailed simulation program is presented for investigating the performances of an external gear pump. The geometrical aspects and the differential equations representing the quasi-steady state mathematical model of the pump are accounted for and discussed. Initial conditions are determined accordingly to a new proposed method and numerical aspects bound to the solution of the stiff ODE system are addressed. Experimental data generated on a commercially available medium size external gear pump were used to validate the model predictions. Agreement between experimental and predicted values of pump performances was satisfactory. Being the model unable to simulate pressure ripples generated by the unsteady nature of the flow within volumes and along the lines, several experimental results of inlet pressure fluctuations are reported. Predicted and unpredicted fluid borne noise sources are hence accounted. Finally, the influence exerted on the operation of the pump by pressure forces and forces exchanged between meshing teeth is investigated and a predictive method foreseen (useful in condition monitoring) based on the observation of outlet pressure waveforms.

Extraction Techniques and Analysis of Turbulence Quantities From In-Cylinder Velocity Data

Abstract: In addition to the frequently used statistical ensemble-average, non-Reynolds filtering operators have long been proposed for nonstationary turbulent quantities. Several techniques for the reduction of velocity data acquired in the cylinder of internal combustion reciprocating engines have been developed by various researchers in order to separate the mean flow from the fluctuating motion, cycle by cycle, and to analyze small-scale engine turbulence by statistical methods. Therefore a thorough examination of these techniques and a detailed comparison between them would seem to be a preliminary step in attempting a general study of unconventional averaging procedures for reciprocating engine flow application. To that end, in the present work, five different cycle-resolved data reduction methods and the conventional ensemble-average were applied to the same in-cylinder velocity data, so as to review and compare them. One of the methods was developed by the authors. The data were acquired in the cylinder of a direct-injection automotive diesel engine, during induction and compression strokes, using an advanced hot-wire anemometry technique. Correlation and spectral analysis of the engine turbulence, as determined from the data with the different procedures, were also performed.

Second order elasticity in nematics: A new anchoring source

Abstract: By considering, in the expression of the nematic free energy density, an additional term in the square of the director second derivatives, an unexpected anchoring source results, due only to surface and bulk elastic constants. As an example, the case of a planar homogeneous and of a homeotropic nematic cell, equally anchored on both walls, is discussed. In both situations the new anchoring source has a destabilizing effect.

Fracture mechanics of brick masonry: size effects and snap-back analysis

Abstract: Fracture energy,GF, and the critical value of stress-intensity factor,KIC, are determined for brick masonry specimens tested in bending with different notch depths. The experimental results are compared with numerical simulations, obtained through a cohesive crack model developed originally for concrete. Theoretical and experimental load-deflection curves present very similar softening branches. In some cases, a snap-back instability is predicted by the model and confirmed by the experimental data. A size-scale transition appears evident from an ultimate strength collapse at the ligament to a brittle fracture due to stress-intensification. Such a transition demonstrates that LEFM is a very suitable model for brick masonry structures at the usual size-scale. A non-dimensional brittleness number is introduced as a measure of the LEFM applicability.

Snap-back softening instability in high strength concrete beams

Abstract: Three-point bending tests on pre-cracked slabs of high-strength concrete are interpreted on the basis of a virtual crack propagation model. As theoretically shown by the model, a snap-back softening instability appears only for initial crack lengths smaller than 0.3 times the beam depth. As a limit-case, when the material is sufficiently brittle or the specimen size is sufficiently large, such an instability can be predicted by the LEFM condition K1=KIC.

A complexity analysis of sequential ATPG

Abstract: The problem of automatic test pattern generation (ATPG) for sequential circuits has long been recognized as a very difficult task. An analysis is conducted of the sequential complexity of the benchmarks proposed for ISCAS'89 looking for a correlation between attainable coverage and ATPG approach. To pursue this goal, the authors have performed a topological analysis of the circuits, identifying peculiar structures and extracting numerical quantities useful in summarizing some design characteristics. In order to correlate these measures with the intrinsic testing difficulty, they have implemented an experimental sequential ATPG. The experiments run on the benchmarks enabled the authors to identify some important parameters to estimate circuit resistance to ATPG. The results show that the overall performance is dominated by the structure of the circuit's loops and the number of state-controlling inputs. >

Surface engineering and chemical characterization in ion nitrided titanium and titanium alloys

Abstract: The chemical and physical characteristics of ion-nitrided surface layers, obtained on α-β titanium alloys, are examined and correlated both with the working conditions adopted in the ion-nitriding process and with the alloy chemical composition. Besides the influence of the working parameters on the morphology and on the microstructures of the ion-nitrided surface layers, mainly the alloy element distributions both in surface coatings and in the substrate are analysed for five α-β titanium alloys of industrial use, and for titanium c,p. as reference, ionnitrided at various treatment temperatures. The nitriding process forms, on titanium alloy parts, high-hardness surface layers consisting of TiN (δ phase) and Ti2N (ɛ phase) nitrides and an interstitial solid solution of nitrogen in the close-packed hexagonal lattice of titanium (α phase). The presence and the extent of these phases as well as the ion-nitrided layer morphology are essentially determined by the alloy chemical composition and the working parameters. In particular a low-temperature treatment produces an extended nitrogen diffusion in the matrix beneath a thin continuous nitrided layer, while a high-temperature treatment produces prevalently a continuous nitrided surface layer. The alloy element distribution appears differentiated in the various phases and may be correlated with the different affinity of these elements with nitrogen.

Estimation and prediction in the presence of unknown but bounded uncertainty: a survey

Abstract: Many different problems such as linear and nonlinear regressions, parameter and state estimation of dynamic systems, state space and time series prediction, interpolation, smoothing, function approximation have a common general structure that here is referred to as generalized estimation problem. In all these problems one has to evaluate some unknown variable using available data (often obtained by measurements on a real process). Available data are always associated with some uncertainty and it is necessary to evaluate how this uncertainty affects the estimated variables.

A VLSI systolic adder for digital filtering of delta-modulated signals

Abstract: A fully systolic VLSI architecture allowing addition of N sequentially available input numbers is presented. It consists of a bit-level systolic adder, systolically resettable, which accumulates the partial sums, and of a systolic control network which provides the correct signs of the input data. This architecture has all the advantages of systolic arrays, such as concurrence of calculations and ease of expansion, and it is really simple to design because the complexity of the cells used is comparable to that of a half adder. In addition, the delay introduced by the slowest cell is quite small and, as a consequence, the data throughput can be considered much higher than in other solutions. Furthermore, by introducing a useful mathematical notation, the correctness of the structure is proved. >

PROTOB - A Hierarchical Object-Oriented CASE Tool for Distributed Systems

Abstract: This paper presents PROTOB, an object-oriented CASE system based on high level Petri nets called PROT nets. It consists of several tools supporting specification, modelling and prototyping activities within the framework of the operational software life cycle paradigm. As its major application area it addresses distributed systems, such as real-time embedded systems, communication protocols and manufacturing control systems. The paper illustrates a case study involving the design of a distributed file system.