Issue 1

JTAM, Sofia, vol. 18 Issue 1 (1987)

A Class of Periodic Vibroshock Regimes of a Rotating Roller in Bearings with Radial Slackening

I. Ivanov, I. Kochev
High. Inst. Food Tobacco Ind., Plovdiv

What is studied in the paper is the roller neck motion of a static non-balanced bearing with radial slackening. The shock occurrence influence has been taken into consideration in the roller-bearing system. The existence of periodic vibroshock regimes has been proved. Results show that a suitable system parameter choice imply the elimination of vibroshock occurrence.

JTAM, Sofia, vol. 18 Issue 1 pp. 011-016 (1987)

A Stationary Motion Stability of Lagrange Gyroscopic System with a Tree-like Structure

N. Vassileva, L. Lilov
Inst. Mech. Biomech., Bulg. Acad. Sci., Bl. 8

What is examined is the stationary motion stability of Lagrange gyroscopes that have been connected each other by ideal ball-and-socket joints with a tree-like structure. One of the bodies is a stationary point. The bodies themselves have been located either by Euler or by Krilov angles with respect to the basis that rotates round a vertical axis at a constant angular velocity. The angles of rotation round the axes of symmetry are chosen to be cyclic coordinates. It is shown that stability is possible with respect to all position coordinates in case of vertical axes only. Stability conditions are obtained in the general case with respect to maximum number of position coordinates.

JTAM, Sofia, vol. 18 Issue 1 pp. 017-026 (1987)

A Method to Analyze Linear Systems with Variable Parameters

I. Iliev
High Inst. Mech. Electr. Eng., Sofia

A procedure is proposed to obtain approximate solution of a homogeneous linear differential equation with periodic parameters. The differential equation is reduced to nonlinear one with a periodic perturbation function by means of a particular solution that has been proposed using the physical idea of the process. Efficient approximate methods exist to study the equation itself and they can be put into practice successfully. The procedure provides a determination of the parametric system stability when obtaining the approximate solution. Corresponding criteria have been listed for the purpose.

JTAM, Sofia, vol. 18 Issue 1 pp. 027-030 (1987)

A Characteristic of Autonomous Mobile Robot Working Space

S. Grozdev, T. Mitkova
Inst. Mech. Biomech., Bulg. Acad. Sci., Bl. 8

What is proposed is a division of the autonomous mobile robot working space into parts that are named regions of lighting. Such a region is characterized by the property of all its points to be seen from working space fixed vertices.

JTAM, Sofia, vol. 18 Issue 1 pp. 031-034 (1987)

On Nonlinear Statistical Examination of One-Dimensional Deformable Plane Systems

S. Simeonov
High. Inst. Arch. Civ. Eng., 1 Hr. Smirnenski blvd., Sofia

A general method is proposed in the paper to solve a large class of nonlinear deformable one-dimensional systems. The nonlinearity itself is either of physical or of geometric kind. Thus, the rotation influence of the elements over their length changes is not accounted for. It is possible to construct a computation algorithm which develops the general deformation method for one-dimensional linear systems. The matrix terminology provides a fitting automation of the computation process. The geometric nonlinearity account is quite exact under step loading and large displacements. A fast approximation method is proposed to solve nonlinear equations.

JTAM, Sofia, vol. 18 Issue 1 pp. 035-042 (1987)

A Numerical Examination of the Crystallization Problem for Ingot Steel. One-Dimensional Approximation

T. Tchernogorova
Inst. Math. with Comp. Cent.. Bulg. Acad. Sci., Bl. 8

The ingot quality depends essentially on the smelt crystallization process. Producers need a technology to guarantee for high quality ingot under slow stilt wear and tear. Various technological means have been used for the purpose including optimization of the ingot and stilt gauge, choice and control of the cooling regime and the regime of metal worming up by exothermic mixture, etc. A detailed quantitative analysis of the means is not possible without mathematical modelling and computer utilization. The process of ingot steel crystallization is examined in the paper numerically under different technological conditions. What is used to solve the non-stationary problem of Stephan type is a difference scheme when the phase transition boundary has not been determined explicitly.

JTAM, Sofia, vol. 18 Issue 1 pp. 043-050 (1987)

A Method to Solve the Two-Dimensional Thermo-Physical Problem in Regions with Complex Geometry

J. Popov, S. Bushev
Inst. Met. Res. Techn., Bulg. Acad. Sci., Bl. 1

A method is proposed to define and to solve heat and mass transfer problems in regions with complex boundary. The essential part of the method is the construction of a Riemann space where the continuity equation has been subjected to geometrization. The heat conductivity equations are deduced and examined. What are investigated in addition are the complicated boundary value conditions that arise from the metric and coordinate system type. The crystallization itself is described by the equivalent heat capacity method when recooling temperature has been accounted for. Some model and practical problems of cooling and crystallization are solved.

JTAM, Sofia, vol. 18 Issue 1 pp. 051-059 (1987), [Full Article]

Heat Transfer between Two Spherical Particles in Oscillating Flow

S. Tabakova1, Z. Zapryanov2
1Inst. Math. with Comp. Cent., Plovdiv
2Inst. Mech. Biomech., Bulg. Acad. Sci., Bl. 8

What is considered is the heat transfer problem between two spherical particles that have been immersed in a viscous fluid. The assumptions include constant temperature of the spheres, small oscillation amplitude in comparison with the characteristic length, high oscillation frequency, small steady flow Reynolds number and unity order of the Prandtl number. The matching asymptotic expansion method has been used to solve the problem. The time average Nusselt number is is compared to the corresponding Nusselt number of heat transfer that is caused by a spherical unit particle in unbounded oscillating flow. A part of the first number is connected with the secondary steady flow while another one is due to diffusion. Both of them are with identical signs when the radius of the considered sphere is not greater than the radius of the other one only.

JTAM, Sofia, vol. 18 Issue 1 pp. 060-070 (1987), [Full Article]

Thermo-Conductivity Influence on Raly-Taylor Instability of Two Superposed Fluids

B. Sharma
College of Agriculture, 303 329 Jobner, India

What is examined is the stability of plane interface that separates two non-viscous superposed fluids with different density in horizontal magnetic field. It is established that the stability criterion depends on the orientation and the magnetic field intensity but does not depend on heat-conductivity. Further, the magnetic field stabilizes a definite wave inclusion of the instability configuration. If thermo-conductivity increases, then the instability pass velocity increases too. Thus, thermo-conductivity destabilizes configuration.

JTAM, Sofia, vol. 18 Issue 1 pp. 071-075 (1987), [Full Article]

Some Features of Rectangular Finite Element Applications by Small Computing Machines

G. Kalchev
6, Lenin Sq., Sofia

An approach is described to form the structure stiffness matrix line by line. Effectiveness is due to explicit formulae that have been derived for two classic rectangular elements. The first one is connected with the plate while the other is with eight degrees of freedom and is connected with the plane problem. The approach fits small computing machines because of certain advantages in organizing machine memory operation.

JTAM, Sofia, vol. 18 Issue 1 pp. 076-080 (1987)

Acoustic Emission Application to Crack Process Investigations

G. Kortenski, K. Minchev
Inst. Met. Res. Techn., Bulg. Acad. Sci., Bl. 1

The paper reviews dependences between acoustic emission parameters and macrocrack process character. The advantages of the acoustic emission method are shown then investigating the generation and the defect development in the material volume. The state of the examined subjects is estimated by detected signal parameters. The basic schemes of acoustic emission have been considered. Using the acoustic emission one can follow various changes of the material including plastic deformation generation and crack development. Thus, the acoustic emission method is applicable to material quality control.

JTAM, Sofia, vol. 18 Issue 1 pp. 081-084 (1987)

Describtion of Concrete by the Micro-Fracture Kinematic Theory

P. Henryk

A micro-fracture theory of concrete under small deformations has been developed (geometric linearity). What is taken into consideration is the possibility of macrocrack propagation in the concrete during deformation. A general formula of the structure de pendences is derived and some experimental examples are listed.

JTAM, Sofia, vol. 18 Issue 1 pp. 085-089 (1987), [Full Article]