No. 4, 1999


Frantisek Chvala: On linearization of the Boltzmann equation with an external force and a source term, 321–331

An initial value problem for the nonlinear Boltzmann equation with an external force and a source term is linearized around a non-Maxwellian distribution. Around the solution of the linearized problem, which is taken as a new approximation to the solution of the original problem, new linearization is then performed. Repeating this process successively, an iteration scheme delivering a sequence of distribution functions is constituted. A sufficient condition for convergence of this sequence to a solution of the original nonlinear problem is found and the rate of convergence is established.

Vratislav Kafka: Cumulative damage and its localization in concrete: Mesomechanical model, 333–348

The general author's concept, describing cumulative damage and localization as processes, in which one substructure of the material that ensures its stability looses progressively its continuity, is applied to concrete. The grains of the aggregates are described as elastic inclusions, the matrix as inelastic, loosing successively its continuity due to a substructure of cracks, the continuity of which increases. The cases of deviatoric loading and of uniaxial compression are analyzed individually, comparison with experimental data is shown for one kind of concrete in uniaxial compression.

Ales Tondl: The effect of dynamic absorbers for a parametrically excited pendulum type system, 349–363

Two types of dynamic absorbers are applied to a parametrically excited basic system which is considered as an elastically mounted pendulum. The first absorber can move perpendicularly to the pendulum axis the other one moves along the pendulum axis. The analysis shows that the first absorber type is able to reduce the basic system amplitude in the parametric resonance but this effect is not high for a relatively small mass of the absorber. The other absorber type has practically no effect.

Daniel Mayer, Svatopluk Zacharias: Mutual inductance of a couple of twisted pair lines in a general position, 365–376

A method of calculating the mutual inductance between two cables is proposed with special reference to the electromagnetic compatibility of computer networks. The two cables are assumed to be straight, their mutual position is general and the wires in the cables may be twisted in a different way. The calculation follows the Neumann formula which is appropriate for cables with the wires in the form of a helix. Numerical results of various configurations are presented.

Jaroslav Feda: Classification of collapse modes of soils, 377–392

Some soils under specific conditions display a collapsible behaviour. Various types of collapses are classified (branching-off, garlandlike, diffusive, stepwise, hydrocollapse). Some of them are familiar (e.g. hydrocollapse), others are classified as collapses for the first time (branching-off, garlandlike). Some are well known in other field of behaviour but it is shown that their occurrence is much wider (diffusion). Mathematical modelling of collapses represents a fundamental difficulty: it is necessary to cope with discontinuities. From the physical point of view, soil, owing to the collapse, changes and more or less different constitutive relations should be applied to the soil behaviour before and after collapse.

Collapsible behaviour is often difficult to be predicted due to its chaotic nature.

Jiri Klima: Analytical model of induction motor fed from four-switch space vector PWM inverter. Time domain analysis, 393–410

This paper presents an analytical model and analysis of the response of the motor currents and electromagnetic torque for a four-switch inverter driving a three-phase induction machine. As the phase voltages are in principle asymmetrical, first the asymmetrical space vectors are expressed in the symmetrical forms. Next the mathematical model uses mixed description in the p and z domains. The operation region of SV PWM control of a four-switch inverter is investigated theoretically in time domain, compared to SVPWM control of a conventional six-switch inverter.

D. Vokoun, V. Kafka, C.T. Hu: Return point memory in NiTi shape memory alloys, 411–419

The constitutive model for hysteresis phase transition proposed by Ivshin and Pence is shown to be inadequate for the description of "return point memory", a property of shape memory alloys that has been experimentally verified by a number of authors, as well as by our own experiments. A modification of this model is proposed that makes it consistent with these experimental findings. An illustrative example of this agreement is presented.

Dimitrios V. Bandekas: Robust excitation control method applied to a gasturbine synchronous generator in a multimachine power system, 421–433

A robust excitation controller is designed using Control-Canonical-Form (CCF) method with state feedback and is applied to a gas-turbine synchronous generator in a multimachine power system, while an unsymmetrical fault or a three-phase symmetrical fault being applied. The simulation results of the study shows that the robust excitation controller performs satisfactorily in a multimachine environment, when operating in conjunction with the conventional regulators of the other system generators.