Spectral approximations of the Stokes problem by divergence-free functions

The method of Moser, Moin, and Leonard (1983) for the approximation of the three-dimensional Navier-Stokes equations using divergence-free subspaces is revisited and analyzed. It is shown that the computed velocity field converges to the physical one with spectral accuracy. Moreover, a method for recovering the pressure field is proposed. This method is stable and provides a pressure that converges to the physical one with spectral accuracy.     

Analogue implementation of a neural network controller for UPSinverter applications

An analogue neural-network controller for UPS inverter applications is presented. The proposed neural-network controller is trained off-line using patterns obtained from a simulated controller, which had an idealized load-current-reference. Simulation results show that the proposed neural-network controller can achieve low total harmonic distortion under nonlinear loading condition and good dynamic responses under transient loading condition. To verify the performance of the proposed NN controller, a hardware inverter with an analogue neural network (NN) controller (using mainly operational amplifiers and resistors) is built. Additionally, for comparison purposes, a PI controller with optimized parameters is built. Experimental results confirm the simulation results and show the superior performance of the NN controller especially under rectifier-type loading condition. Implementing the analogue neural-network controller using programmable integrated circuits is also discussed     

Effects of Sc2O3 and MgO passivation layerson the output power of AlGaN/GaN HEMTs

The low temperature (100°C) deposition of Sc₂O₃ or MgO layers is found to significantly increase the output power of AlGaN/GaN HEMTs. At 4 GHz, there was a better than 3 dB increase in output power of 0.5×100 μm² HEMTs for both types of oxide passivation layers. Both Sc₂ O₃ and MgO produced larger output power increases at 4 GHz than conventional plasma-enhanced chemical vapor deposited (PECVD) SiN_x passivation which typically showed ⩽2 dB increase on the same types of devices. The HEMT gain also in general remained linear over a wider input power range with the Sc₂O₃ or MgO passivation. These films appear promising for reducing the effects of surface states on the DC and RF performance of AlGaN/GaN HEMTs     

Shape optimization of an elastic thin shell under various criteria

The aim of this study is to propose a methodology for optimizing the shape (middle surface and thickness) of an elastic general thin shell (i) under different criteria: minimization of the weight, of the stresses, of the strain energy; (ii) with or without constraints: if any, these can be bounds on some displacements, on some stresses.This is a common problem in real life, the engineer has to construct structures which have the best mechanical behaviour and the best price; the price is often proportional to the weight of the structure.In this paper only the general continuous formulation of the problems is considered. From this strong basis a corresponding discrete formulation and some industrial applications will subsequently be developed.     

Change in physicomechanical properties of steel D1604AT15 during fatigue at different loading frequencies

Results are presented in a statistical arrangement for studying the effect of loading frequency on the fatigue life in bending of plates with a stress concentrator. The nature of change in microhardness, electrical conductivity, dislocation density, and block dimensions during fatigue at frequencies of 2.8, 8.8, and 18 kHz has been demonstrated. It has been established that in spite of some qualitative difference the main indicators of fatigue at these frequencies are retained.S. M. Kirov Belorussian Technology Institute, Minsk. Translated from Problemy Prochnosti, No. 10, pp. 31–34, October, 1989.     

Dewatering of liwuid radioactive wastes in thin-film rotary evaporators

Translated from Atomnaya Énergiya, Vol. 67, No. 1, pp. 11–15, July, 1989.