Contents of the Journal of Mechanical Engineering 48, 5 (1997)

DANEK, O.: Identification methods in the dynamics of machines           297
(Review paper) (in Czech)


ORAVSKY, V., SLIZEK, O.: Dynamic efficiency of a symmetric 
hydrostatic transmission at harmonic load. Part I. Determination        
(in Slovak)                                                             315


ALTENBACH, H., ALTENBACH, J., NAUMENKO, K.: On the long-time 
behaviour of creeping thin plates                                       328


FRANGI, A.: Regularized BE approaches for axisymmetric problems         338


HORACEK, J., MAZUCH, T., TRNKA, J: Natural vibration of a cylindrical 
shell containing water in a coaxial annular gap                         351


MEURIC, O. F. J.,  CHIU, T. W., WAKEMAN, R. J., FISHER, K. A.: 
A numerical study of helical flow of generalised Newtonian fluids 
in eccentric annuli (in English)                                        363

The aim of this paper is to give a brief overview of the basic identification methods in a frequency and time domain which were developed just about in the last forty years. The frequency-domain methods evaluate frequency dependent signals (forced amplitudes) excited mainly by harmonic excitations. The time-domain methods evaluate time dependent signals excited by impulse excitations evoked by a hammer impact. Three methods from the frequency domain and five methods from the time domain are presented. In the frequency domain the direct method, which yields coefficient matrices of stiffness, damping, and mass, and two indirect universal methods evaluating spectral and corresponding modal matrices are considered. Four of the mentioned methods in the time domain are aimed at the modal analysis developed from Hankel's matrix created from all time samples of a dynamic experiment. The last method, so-called "ARMA", evaluates coefficients of a differential equation. All the discussed identification methods lead to a strongly overdetermined system of algebraic equations solved by the pseudo-inversion. This solution minimizes influences of random errors from measurements and Gauss distributed noise.

We start from a linear dynamic model of a symmetric hydrostatic transmission (HsT) and its steady solution. Then, mechanical powers of HsT at input (hydrogenerator, HG) and at output (hydromotor, HM) are derived. By means of them transfer of powers, dynamic efficiency (dEf), and quasi-static efficiency (qsEf at constant angular velocities and load) are determined. These are mutually compared and analysed in the space of 8 dimensionless parameters of the HsT model in a consecutive article.

Many materials show irreversible creep deformation behaviour under moderate loads at high temperature levels. In addition, material deterioration occurs which leads to an acceleration of creep strains. Such material behaviour can be described by a phenomenological model based on the von Mises creep potential and the introduction of a scalar damage parameter. Assuming isotropy, inelastic incompressibility, and independence of the secondary creep on type of loading, the proposed model is applied to the analysis of long-time-behaviour of rectangular plates. The initial-boundary-value problem is solved using the implicit Euler method for the time discretisation and special series expansions.

The topic of the regularization of axisymmetric Boundary Element formulations for potential and elasticity fully axisymmetric problems is addressed. Traditionally, the strongly singular contributions in the governing integral equations are evaluated by means of the "inflation mode technique". Two alternative procedures are proposed here: a "derivative transfer" (DT) method exploiting integration by parts via Stokes theorem, and a "singularity subtraction" (SS) procedure.

An experimental and numerical investigation of the frequency-modal characteristics of a vertical thin-walled cylindrical shell containing water in a coaxial annular region is presented. Particularly, the effect of the thickness of the liquid layer in the annular region between the vibrating shell and another coaxial rigid cylinder is studied. Experimental results were obtained using the time average holographic method. Parallel computations of natural frequencies and mode shapes of the coupled fluid-structural vibrations were carried out by means of the finite element method. The comparison between experimental results and finite element results is found to be reasonable in most of the studied cases.

This paper presents numerical solutions for the laminar flow of generalised Newtonian (Herschel-Bulkley) fluids in vertical annuli which is of some interest in oil drilling operation. A three-dimensional Finite Element Method (FEM) was applied to solve the momentum and continuity equations. A Galerkin weighted residual approach with incomplete quadratic elements (hexahedron, 20 nodes, isoparametric) was used to discretise the governing equations. The resulting nonlinear system of equations in the nodal velocities (bilinear) and pressure (linear) is solved in an iterative way either by the direct method or by the Newton-Raphson method. Results have been obtained for various annular eccentricities, radius ratios, fluid properties, and flow parameters.