No. 3, 1998


Ivo Dolezel, Daniel Mayer: Operation parameters and characteristics of a superconducting DC electromagnet for magnetic separation, 217–231

The paper deals with optimization and evaluation of operation parameters and characteristics of a magnetic separator, whose DC electromagnet is excited by a superconducting winding. The optimization process is based on minimum volume of ferromagnetics of its magnetic circuit for prescribed values of the average magnetic flux density in its canister. Steady state and transient magnetic field analyses then provide a complete information for determining inductances, additional losses in the walls of the cryostat and ferromagnetics and also forces acting on the field winding and its fixing construction with respect to possible axial shifting the coil due to mechanical and thermal reasons.

Jaroslav Feda: The phenomenon of percolation in the soil structure generation, 233–247

Binary systems of sand and clay were tested with the intention to study the percolation phenomenon. For both shear strength and compression the percolation threshold was defined (sand:clay = 5:1). Bilinear shear envelopes are typical for binary systems while the shear envelopes of pure sand and (reconstituted) clay are linear.

Other clay-content sensitive features were discovered (collapse by wetting, time-dependent structural collapse, overconsolidation at low stress levels etc.). They are typical for the (saturated) "clayey" interval of behaviour. Comparison with the behaviour of a natural loess showed the analysis of binary systems corresponding with the behaviour of real soils.

Viktor Valouch: Induction motor voltage-based calculation of magnetic flux, 249–258

The alternative method of the calculation of the induction motor flux based on its stator voltages, instead of the common numerical integration, has been developed. The method is insensitive to the presence of parasitic dc signals in the stator voltages, while harmonic signals are "integrated" without significant errors. The developed method has been verified by a lot of simulations.

Bernard Baron: Substitute impedance of a single-phase current circuit with cylindric conductors, 259–276

An algorithm has been constructed by application of an integral equation method. A discretication of the integral operator of a vector magnetic potential type has been carried out for a current density in the cylindrical conductors, in the form of a matrix with elements expressed by convergent functional series. It allows to express the operator inverse to the integral equation operator in a numerical way. Thus, it is possible to express current density in particular conductors, and also circuit substitute impedance as a function of a cylindrical conductor pack geometry.

Frantisek Kroupa, Martin Vysohlid: Quenching residual stresses in continuously thermal sprayed tubes, 277–300

An analytical theory of quenching macroscopic residual stresses in thermally sprayed free-standing tubes is presented. The theory is based on a simplified model of continuous spraying. Layers of infinitesimal thickness are gradually deposited on a temporary substrate and immediately cooled down to the constant bulk deposit temperature. The temporary substrate is then removed. The effect of the removed substrate on the quenching stresses in the free-standing tube is shown for two types of the substrate. The hoop and axial quenching stresses are tensile at the external surface and compressive at the internal surface of the tube.

Ales Tondl: Vibration quenching of an externally excited system by means of dynamic absorber, 301–309

The action of the dynamic absorber for an externally excited basic system is analysed. This basic system is modelled as an elastically mounted pendulum where the gravitation represents the negative component of the restoring moment due to the position of the suspension point below the centre of gravity. This contribution supplements the previous paper published in this journal (see [1]). The efficiency of the dynamic absorber, his position and the effect of damping is analysed.

B.K. Chakraborty, H.P. Mazumdar, S. Bandyopadhyay, P. Chatterjee: On the formulation of local skin friction coefficient in a turbulent boundary layer, 311–317

In this paper, we construct some formulae for the local skin friction coefficient in a two-dimensional boundary layer from the laws of velocity profiles due to Spalding (1961) and Byggstøyl and Saetran (1981). Numerical values of the skin friction coefficient obtained from such formulae for different values of the Reynolds number and the shape factor are then compared with the corresponding data from Ludwieg and Tillmann (1950), Fraser (1987) and White (1974). Finally, the results are discussed.

Pankaj K. Chatterjee, G.C. Layek, H.P. Mazumdar: A simple model for the sediment concentration distribution in a turbulent flow with suspended sediment, 319–324

A simple model is developed for the distribution of sediment concentration in a turbulent flow with suspended sediment over a fixed bed. The model is based on a form of velocity profile law for the wall region, suggested by Byggstøyl and Saetran (1982). Such a law is constructed from the idea that the mean velocity field adjusts itself near the wall so that mean kinetic energy may be transported down to the viscous sublayer where it dissipates. Combining this law for the velocity profile with the equation for the vertical concentration distribution of suspended sediment, a working formula for the sediment concentration is derived in terms of the friction velocity at the wall, fall velocity of the particle, a proportionality factor relating mixing coefficient for sediment to a coefficient of turbulence mixing and the distance from the wall. The predicted values of sediment concentration are compared with the corresponding measured data. The results are finally discussed.