Showing papers by "Polytechnic University of Turin published in 1990"

A dynamic subgrid-scale eddy viscosity model

Abstract: One major drawback of the eddy viscosity subgrid-scale stress models used in large-eddy simulations is their inability to represent correctly with a single universal constant different turbulent field in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work, a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity (Germano 1990) between the subgrid-scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid-scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near-wall region of a turbulent boundary layer. The results of large-eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

Inference of motor unit recruitment order in voluntary and electrically elicited contractions

Abstract: The relationship between surface myoelectric signal parameters and the level of voluntary or electrically elicited contractions was studied in 32 experiments on the tibialis anterior muscle of 22 healthy human subjects. Contractions were performed at 20 and 80% of the maximum voluntary contraction torque. Two levels of stimulation current were used, yielding, respectively, a maximum M wave and an M wave approximately 30% of the maximum. A four-bar electrode probe was used to detect single- and double-differential signals from which mean and median frequency of the power spectrum and average muscle fiber conduction velocity were estimated. Measurements obtained from voluntary contractions showed a positive correlation between contraction levels and both conduction velocity and spectral parameters. Conduction velocity increased by 21.2 +/- 10.9% when voluntary contraction level increased from 20 to 80% of the maximal value. Spectral parameters increased by similar amounts. Tetanic electrical stimulation was applied to a muscle motor point for 20 s via surface electrodes. Rectangular current pulses with 0.1-ms width and frequencies of 20, 25, 30, 35, and 40 Hz were used. Four types of behavior were observed with increasing stimulation level: 1) the two spectral parameters and conduction velocity both increased with stimulation in 15 experiments, 2) the two spectral parameters decreased and conduction velocity increased in 8 experiments, 3) the two spectral parameters and conduction velocity both decreased in 6 experiments, and 4) the two spectral parameters increased and conduction velocity decreased in 3 experiments. Conduction velocity increased with increasing stimulation current in 72% of the experiments, indicating a recruitment order similar to that of voluntary contractions, although it decreased in the other 28% of the cases, indicating a reverse order of recruitment. Contrary to what is observed in direct stimulation of nerves, motor units are not in general recruited in reverse order of size during electrical stimulation of a muscle motor point. This discrepancy may be the result of geometric factors or a lack of correlation between axonal branch diameter and the diameter of the parent motoneuron axon. Changes of conduction velocity and spectral parameters in opposite directions may be the result of the combined effect of the motor unit recruitment order and of the different tissue filtering function associated with the geometric location of the recruited motor units within the muscle.

Instabilities and spatial complexity in a laser

Abstract: We discuss the emergence of spatial structures in a ring laser model with transverse effects. The emphasis of this study is on the development of a description that can capture the essential features of transverse dynamics without the need for large-scale numerical efforts. We introduce an extension of the uniform field limit and derive a set of modal equations that we solve with conventional numerical methods. Our solutions show evidence of transverse mode competition in the laser dynamics, leading to both time-dependent and multimode stationary (cooperative frequency-locking) behaviors. In the time-dependent regime we analyze the resulting spatial structures and suggest a scheme for the investigation and characterization of spatial complexity.

Parallel CRC generation

Abstract: Theoretical aspects of encoding cyclic redundant codes (CRCs) are reviewed. A method of designing hardware parallel encoders for CRCs that is based on digital system theory and z-transforms is presented. It allows designers to derive the logic equations of the parallel encoder circuit for any generator polynomial. A few interesting application areas for hardware parallel encoders are pointed out. >

A diagnostic test pattern generation algorithm

Abstract: The authors present a novel ATPG (automatic test pattern generation) algorithm, based on PODEM, that makes diagnostic test pattern generation feasible for medium-sized combinational circuits described at the gate level with the single-stuck-at-fault assumption. The input to the ATPG is a couple of faults, and either the output is a test pattern that distinguishes them or they are tagged as indistinguishable. The need to consider the fault-free circuit and the two faulty circuits at the same time required the extension of the algebra to encompass two additional values, Delta and delta . A Delta appears on the nodes of the circuit whenever a difference between the two faulty circuits exists. The presence of a delta marks the locations where a difference might exist if the X values on one or both faulty circuits were suitably set. The algorithm excites and propagates Delta s onto the primary outputs and is thus called the Delta -algorithm. Preliminary results on a set of benchmark circuits are reported. >

A catastrophe theory approach to fracture mechanics

Abstract: A cohesive crack model is applied to analyze the crack stability in elastic-softening materials. The shape of the global load-displacement response changes substantially by varying size-scale and keeping the geometrical shape of the structure unchanged. The softening branch becomes steeper and when the size-scale increases. A critical size-scale does exist for which the softening slope is infinite. In such a case, the load carrying capacity drastically decreases for relatively small displacement increments. Then, for larger size-scales, the softening slope becomes positive and a cusp catastrophe appears. It is proved that such a bifurcation point can be revealed by the simple LEFM condition.

Extreme Point Results for Robust Stabilization of Interval Plants with First Order Compensators

Abstract: It has recently been shown that a first order compensator robustly stabilizes an interval plant family if and only if it stabilizes all of the extreme plants. That is, if the plant is described by m-th order numerator and monic n-th order denominator with coefficients lying in prescribed intervals, it is necessary and sufficient to stabilize the set of 2m+n+l extreme plants. These extreme plants are obtained by considering all possible combinations for the extreme values of the numerator and denominator coefficients. In this paper, we prove a stronger result. Namely, it is necessary and sufficient to stabilize only sixteen of the extreme plants. These sixteen plants are generated using the Kharitonov polynomials associated with the numerator and denominator. Furthermore, when additional apriori information about the compensator is specified (sign of the gain and signs and relative magnitudes of the pole and zero), then in some cases, it is necessary and sufficient to stabilize eight critical plants while in other cases, it is necessary and sufficient to stabilize twelve critical plants.

Diagnosis oriented test pattern generation

Abstract: This paper addresses the generation of test patterns having diagnostic properties. The authors goal is to produce patterns able not only to detect, but also to distinguish faults in combinational circuits. A general formalization of the problem is first given; a new technique is then introduced to improve the diagnostic capabilities of a traditional automatic test pattern generation (ATPG); the experimental results showing its effectiveness are finally presented. >

Survey of Square Rooting Algorithms

Abstract: The paper reviews the algorithms for the computation of square roots for binary machines. After an initial classification, the algorithms are analysed in detail by considering their specific peculiarities and properties. Finally, some comments are made regarding their ideal and effective implementation in hardware, software or microcode

Finite difference methods for the analysis of integrated optical waveguides

Abstract: A finite difference method based on a Crank–Nicolson discretisation procedure has been implemented and applied to the analysis of propagation of optical beams in dielectric waveguides. It is based on a new formula which takes into account the correct boundary conditions. Comparisons with the well known BPM technique show a definite improvement in computer time saving, accuracy and ease of use.

A dual result to Kharitonov's theorem

Abstract: A dual problem to Kharitonov's problem, involving a diamond instead of a rectangle, is considered. The results show that a family of polynomials with coefficients varying in the diamond is strictly Hurwitz if and only if eight one-dimensional exposed edges of the diamond are strictly Hurwitz. >

Power losses under sinusoidal, trapezoidal and distorted induction waveform

Abstract: A setup has been developed by which one can measure magnetic power losses under arbitrary induction waveforms in the frequency range 1 Hz >

Static buckling of moderately thick, anisotropic, laminated and sandwich cylindrical shell panels

Abstract: This paper deals with the development of the linearized equations governing the buckling behavior of cylindrical shell panels faced with fiber-reinforced plastic under uniform shear and normal membrane forces. The formulation takes into account the transverse shear and normal membrane forces. The formulation takes into account the transverse shear flexibility. The first approach is an analytical approach of the Galerkin type and rests upon the principle of virtual work. The second is a finite-element displacement approach

System Modelling with Petri Nets

Abstract: Petri Nets (PN) are a graphical formalism which is gaining popularity in recent years as a tool for the representation of complex logical interactions (like synchronization, sequentiality, concurrency and conflict) among physical components or activities in a system. This notes are devoted to introduce the formalism of Petri nets with particular emphasis on the application of the methodology in the area of the performance and reliability modelling and analysis of systems. The quantitative analysis of the behaviour of systems in time requires the superposition of a stochastic timing mechanism to the classical representation of PN. Timed Petri nets and, in particular, Stochastic Petri nets (SPN) are the object of the second part of the notes. Finally, some fully developed examples enlighten peculiar aspects which differentiate PNs from other modelling techniques usual in reliability analysis. In few words, the goal of these notes is to show that the proposed methodology based on the PN formalism can be conveniently used as a user-friendly language to represent and evaluate complex stochastic systems.

Hybrid hierarchical scheduling and control systems in manufacturing

Abstract: Some experiments on the integration of algorithmic techniques with knowledge-based ones are discussed. Two case studies are presented: an FMS cell and a press shop. It was found that the algorithmic procedures developed for production scheduling resulted in limiting the ability to cope with the complexity of the real manufacturing world. The scheduling problem, seen as a constraint satisfaction problem, can be approached with rule-based techniques. Nevertheless, algorithmic techniques are found to be valuable for their efficiency and ability to deal with aggregated data. This ability is fundamental for an efficient implementation of hierarchical control systems in general and in the manufacturing context in particular. This suggests that the integration of rule-based techniques with algorithmic ones can increase the efficiency of searching in the space of possible solutions. The ability to deal with aggregated data can have little value when detailed real-time operation scheduling is needed. In this case, simple dispatching rules are often used, and sophisticated operations research methods are not used. In such a dynamic situation, a purely-rule based approach may be more suitable. >

Analysis of second order differential operators on Heisenberg groups. I

Abstract: Given a matrixS∋sp (n, ℝ), one finds a second-order, bi-invariant differential operator □ s on the one-dimensional extensionG s of the Heisenberg groupH n induced byS. We construct solutions to certain Cauchy problems for □ s and fundamental solutions for these operators. This analysis is connected with the discussion of the “oscillator semigroup” by R. Howe.

Analysis and design criteria of three-section DBR tunable lasers

Abstract: Three-section distributed Bragg reflector (DBR) tunable lasers emitting in the 1.55 mu m region are studied through a theoretical model which takes into account thermal effects and exploits an accurate calculation of the optical parameters. Static modal competition is discussed for transverse and longitudinal modes in operation at and above threshold. The static tuning characteristics are presented; a saturation of the wavelength shift is found for control current densities of approximately=20 kA-cm/sup -2/ due to Auger recombination processes and heating effects. The dependence of the tuning ranges on the section lengths, waveguide thickness results, and some design criteria are derived. >

Oscillatory metastability in homogeneous and inhomogeneous flip-flops

Abstract: Models which describe oscillatory metastability for several types of flip-flops are proposed. A constant-slew-rate model is first used to find the bounds on circuit parameters which discriminate between analog and oscillatory metastability. A variable-slew-rate model is then used to evaluate resolving time as a function of input timing. Two classes of flip-flops, homogeneous and inhomogeneous, are defined, and their performances in terms of resolving time are compared. It is shown that, in many cases, inhomogeneous flip-flops provide significant improvements with respect to resolving time when oscillatory metastability is involved. The analytical results are verified by comparison with SPICE simulation results. >

The structure of the distortion free-energy density in nematics: Second-order elasticity and surface terms

Abstract: By means of a phenomenological approach, we demonstrate that the mixed splay-bend elastic constantK 13 in the free energy density of nematic liquid crystals must be considered zero, unless the bulk contributions of the squares of the distortion second-order derivatives are taken into account, together with the squares of the first-order derivatives times the second-order derivatives, and with the fourth powers of the first-order derivatives. Such contributions just reduce to one in the presence of—and close to—a threshold. Furthermore, the saddle-splayK 24-term instead is shown always to play an essential role, as the bulk first-order elasticity, in determining the distortion free energy of nematics with weak anchoring subjected to spatial deformations. Finally, the new surfacelike elastic constants are shown to have a nilpotent character: thus they behave as well asK 24 from the point of view of the variational calculus.

Control of high performance interior permanent magnet synchronous drives

Abstract: Control strategies for an interior permanent magnet (PM) machine suitable for field weakening are considered. Different control approaches are investigated to identify a robust and fast torque control scheme. The use of a flux-oriented reference frame is suggested, mainly for control robustness reasons. A flux observer is proposed which minimizes the sensitivity to errors in the measurements and in the magnetic model. The torque control problem is analyzed in depth, from dynamic and steady-state points of view. An allowable operating area in the state plane is selected, and the related boundaries are implemented in the control scheme. Simulated behaviors are given, together with the experimental ones obtained from a prototype drive. >