Volume 30 Number 2  2000

R. Tenchev, G. Kolarov.

Department of Strength of Mathetials
Technical University of Sofia,
8, St. Kl. Ohridski Str., 1756 Sofia, Bulgaria,

Finite Element Analyses of Stress Concentration Problems: Is It Possible to Avoid Local Mesh Refinement

Abstract. A 2D stress concentration problem ``elliptic hole in a plate'' and its solution by FEM is considered. The accuracy of
the maximum stress computed by three stress recovery schemes, applied to course meshes of 8-node isoparametric elements, is
investigated. The three schemes are: (a) stress recovery directly at the nodes; (b) extrapolation from the 2×2 Gauss points; (c)
empirical formula using the stresses from schemes (a) and (b). Grounds for establishing scheme (c) are presented. It is shown
numerically that scheme (b), used in most commercial FEM programs, is least accurate. Analytical grounds for the better
accuracy of scheme (a) are given. It is shown that scheme (c) is most accurate, up to the extent that the problem may be solved
without local mesh refinement.

Key words: finite element analysis, mesh refinement, stress concentration.

T. Tanev, Iv. Chavdarov.

Central Laboratory of Mechatronics and Instrumentation,
Bulgarian Academy of Sciences,
Acad. G. Bonchev Str., Bl. 1,  1113 Sofia, Bulgaria,

Performance Evaluation of Manipulation Systems and Graphical Representation of the Characteristics

Abstract. The paper considers some performance characteristics for robots such as service index, manipulability, condition
number, minimum singular value and accuracy. All indices are applied to a SCARA type of robot and results are graphically
represented. It is of interest to present kinematic capabilities of robots as graphs over the workspace. Some known indices are
considered and new ones are introduced. The presented graphs give a clear idea of the performance of the manipulator in
different areas of the workspace.

Key words: manipulation system, service index, manipulability, condition number.  

P. Genova, S. Karapetkov, T. Dimitrov, I. Moneva.

Technical University of Sofia,
Faculty of Engineering and Pedagogy -- Sliven,
59, Bourgasko Shaussee Blvd, 8800 Sliven, Bulgaria

Dynamics of Lifting Transport Crane with Rope Suspended Load Taking into Account the Wheels Sliding

Abstract. In this paper we investigate the lifting transport crane with rope suspended load at simultaneous action of the driving
and lifting mechanisms as a space problem is solved. The mechanical system with five or six degrees of freedom according to
the sliding presence in the crane carriage drive wheels is modelled.

The differential equations of the system movement are derived. The structural schemes of the model are designed by means of
program product MATLAB, toolbox SIMULINK. The results from the numerical simulation are represented. On the basis of
these results, conclusions about the character of the transition and stationary regimes at the load transportation and lifting are

Key words: dynamics of lifting transport systems, wheels sliding.

Z. Tcherneva-Popova, D. Vassilev.

Technical University of Sofia,
8, St. Kl. Ohridski Str., 1000 Sofia, Bulgaria
D. Vassilev
Higher Military School of Transport ``Todor Kableschkov'',
158, Geo Milev Str., 1574 Sofia, Bulgaria,

Movement Control of a Robocrane with a Load Variable Mass in a Vertical Plane

Abstract. The vibrations of the automated guided crane (robocrane) with a variable mass of the load in the vertical plane are
examined. The dynamic model is a two-mass system. The catch of the load is modelled as a flexible unstretchable thread with a
length depending on the velocity of the lifting. The problem is solved by modelling and numerical analysis of the differential
equations of the movement. The solution is made for the case of a simultaneous lifting and a horizontal transporting of the load
taking into account linear and turbulent resistant forces. The results of the examination are a base for synthesizing of optimal
equation of the movement of the automated guided crane by different criteria for declination of the load and the vibration
parameters of the system.

Key words: robocrane, movement control, variable mass.

S. Tabakova.

Technical University of Sofia, Branch Plovdiv,
61, Saint Petersburg Blvd., 4000 Plovdiv, Bulgaria,

Solidification of Free Thin Films

Abstract. A free film of a molten material attached on a frame solidifying in an ambient gas is modelled as a liquid/solid shell-like
body with variable thickness symmetric to its middle plane. After averaging theirs 3D counterparts along the film thickness, the
mass, momentum and energy balance laws are given as in the ordinary shell theory. With the film thickness taken as a small
parameter, two evolutionary systems in an invariant tensorial form for the liquid and solid phases are obtained.

Key words: thin film, solidification, mathematical model of free film, liquid/solid shell-like body.

G. H. Vatkova, M. D. Kartalev.

Institute of Mechanics, Bulgarian Academy of Sciences,
Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria,

Numerical Simulation of the Non-Linear Instabilities in the Magnetopause Layer

Abstract. A numerical scheme is developed for solving time-dependent magnetohydrodynamic (MHD) equations for
incompressible viscous and conductive flow. The scheme is utilized in simulating a coupled Kelvin - Helmholtz and tearing mode
instability in a simplified model of magnetopause boundary layer. Some selected numerical results are presented, demonstrating
that this mechanism could be responsible for induced in the polar ionosphere transient events with multiple current vortices.
Some signatures of such vortices, obtained by interpretation of the Greenland chain magnetometer data, have been presented in
the literature. It seems that some characteristic features of these phenomena could be explained in terms of so modelled coupled
magnetopause instability.

Key words: magnetopause, magnetohydrodynamic instability, tearing mode instability.

P. Stankov.

8, St. Kl. Ohridski Str., 1000 Sofia, Bulgaria,

Factors Influencing the Aerodynamic Interaction in a Combustion Chamber of a Hot Water Boiler

Abstract. The paper presents an analysis of factors influencing the aerodynamic interaction in the boiler furnace. The analysis
concerns the influence of important factors such as furnace geometry, burners configuration, swirl of the flow issuing from
burners, outlet flow conditions, etc. The investigation is carried out by computer simulation of the combustion process in a hot
water boiler with six burners located, in the basic case, in two rows on different levels. The location of the burners was varied
within the computer simulation so that different burners configurations were investigated. At the same time the swirl direction of
the flow issuing from the burners was varied in order to influence the flow interaction. The numerical results for the flow
characteristics, i.e. velocity field and turbulent kinetic energy field show significant influence of the investigated factors on the
flow interaction. An assessment is made with respect to the possibility the effectiveness of the combustion process to be
improved through the main factors influencing the process. In particular, the possibility to find the respective combination of
these factors through computer simulation is also assessed.

Key words: turbulent combustion, aerodynamic interaction, industrial furnace, computer simulation.

R. Iankov,  N. Bontcheva.

Institute of Mechanics, Bulgarian Academy of Sciences,
Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria,

Finite Element Simulation of Multi-Level Powder Metal Compaction Processes

Abstract. The rigid-plastic finite element model is used to simulate multi-level powder metal compaction in powder metallurgy.
The finite elemnt code PORO based on flow approach is applied to predict the relative density destribution, efective strain,
strain rates, velocities and etc. in axisymmetric preforms loaded with two punches. The powder material model proposed by
Shima is applied. During the deformation process the boundary conditions are changed by means of an appropriate remeshing

Key words: powder forming, numerical simulation, finite element method.

Prof. Dr. Techn. Sc. Ventzeslav Alexandrov Dzhupanov, C.E. (On the occasion of his 65 birth day)

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