Issue 1
JTAM, Sofia, vol. 45 Issue 1 (2015)

Analytical and Numerical Investigation on the Duffing Oscilator Subjected to a Polyharmonic Force Excitation

Svetlin Stoyanov
Department of Technical Mechanics, University of Ruse, 8 Studentska Str, POB 7017, Ruse, Bulgaria


An analytical solution for a specific case of the forced Duffing oscillator is proposed. The excitation force contains two harmonics with significant difference frequencies. This case corresponds to a presence of a defect in the machinery and is in the art of the machinery vibration diagnostics. The results obtained show an amplitude modulation. Therefore, the presence of an amplitude modulation in the vibration signal may be used as an indicator for a malfunction. Analytical solution derived clarifies how the amplitude modulation occurs. Also, a numerical solution is realized and compared with the analytical one. For this, the Duffing equation is solved numerically and then, the spectrograms of vibrations are obtained through a Discrete-time Fourier Transform.

JTAM, Sofia, vol. 45 Issue 1 pp. 03-16 (2015), [Full Article]

Electric Vehicles Mileage Extender Kinetic Energy Storage

Venelin Jivkov, Vutko Draganov, Yana Stoyanova
Department of Theory of Mechanisms and Machines, Technical University of Sofia, 8, Kl. Ohridski Blvd., 1797 Sofia, Bulgaria

The proposed paper considers small urban vehicles with electric hybrid propulsion systems. Energy demands are examined on the basis of European drive cycle (NEUDC) and on an energy recuperation coefficient and are formulated for description of cycle energy transfers. Numerical simulation results show real possibilities for increasing in achievable vehicle mileage at the same energy levels of a main energy source – the electric battery. Kinetic energy storage (KES), as proposed to be used as an energy buffer and different structural schemes of the hybrid propulsion system are commented. Minimum energy levels for primary (the electric battery) and secondary (KES) sources are evaluated. A strategy for reduced power flows control is examined, and its impact on achievable vehicle mileage is investigated. Results show an additional increase in simulated mileage at the same initial energy levels.

JTAM, Sofia, vol. 45 Issue 1 pp. 17-38 (2015), [Full Article]

On the Modulation of Oscillation in Thermohaline Convection Problems Using Temperature Dependent Viscosity

Joginder Singh Dhiman1, Vijay Kumar2
1Department of Mathematics, Himachal Pradesh University, Summerhill, Shimla (H.P.)-171005
2Centre of Excellence, Govt. College, Sanjauli, Shimla (H.P.)-171006

The present paper mathematically investigates the effect of temperature dependent viscosity on the onset of instability in thermohaline convection problems of Veronis and Stern type configurations, using linear stability theory. A sufficient condition for the stability of oscillatory modes for thermohaline configuration is derived. When the compliment of this sufficient condition is true, the oscillatory motions of neutral or growing amplitude may exist, and hence the bounds for the complex growth rate of these neutral or unstable modes are derived, when viscosity of the fluid is an arbitrary function of temperature. Some general conclusions for the cases of linear and exponential variations of viscosity are worked out. The present analysis thus shows that the oscillations in thermohaline convection problems can be modulated or arrested by considering the temperature dependent viscosity of the fluid.

JTAM, Sofia, vol. 45 Issue 1 pp. 39-52 (2015), [Full Article]

Building and Programming a Smart Robotic System for Distinguishing Objects Based on Their Shape and Colour

T. M. Sharari
Institute of Engg & Tech, Dep. of Ele.eeng, Control Lab.& Aut., Po. Box. 81. Jordan

This paper presents a robotic system designed for holding and placing objects based on their colour and shape. The presented robot is given a complete set of instructions of positions and orientation angles for each manipulation motion. The main feature in this paper is that the developed robot used a combination of vision and motion systems for holding and placing the work-objects, mounted on the flat work-plane, based on their shapes and colours . This combination improves the flexibility of manipulation which may help eliminate the use of some expensive manipulation tasks in a variety of industrial applications. The robotic system presented in this paper is designed as an educational robot that possesses the ability for holding-and-placing operations with limited load. To process the various instructions for holding and placing the work objects, a main control unit - Manipulation Control Unit (MCU) is used as well as a slave unit that performed the actual instructions from the MCU.

JTAM, Sofia, vol. 45 Issue 1 pp. 53-68 (2015), [Full Article]

Propagation of Surface Waves in a Homogeneous Layer of Finite Thickness over an Initially Stressed Functionally Graded Magnetic-Electric-Elastic Half-Space

Li Li1, P. J. Wei2
1College of Science, Qiqihar University, Qiqihar , 161006, China
2Department of Applied Mechanics, University of Sciences and Technology Beijing, Beijing, 100083, China

The propagation behaviour of Love wave in an initially stressed functionally graded magnetic-electric-elastic half-space carrying a homogeneous layer is investigated. The material parameters in the substrate are assumed to vary exponentially along the thickness direction only. The velocity equations of Love wave are derived on the electrically or magnetically open circuit and short circuit boundary conditions, based on the equations of motion of the graded magnetic-electric-elastic material with the initial stresses and the free traction boundary conditions of surface and the continuous boundary conditions of interface. The dispersive curves are obtained numerically and the influences of the initial stresses and the material gradient index on the dispersive curves are discussed. The investigation provides a basis for the development of new functionally graded magneto-electro-elastic surface wave devices.

JTAM, Sofia, vol. 45 Issue 1 pp. 69-86 (2015), [Full Article]

Wave Propagation Due to an Embedded Seismic Source in a Graded Half-Plane with Relief Peculiarities Part I: Mechanical Model and Computational Technique

I.-K. Fontara1, F. Wuttke1, S. Parvanova2, P. Dineva3
1Institute of Applied Geo-science, Christian-Albrechts-University of Kiel, Germany
2University of Architecture, Civil Engineering and Geodesy Faculty of Structural Engineering, Structural Mechanics, 1, Hr. Smirnenski Blvd, 1046 Sofia, Bulgaria
3Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria

This work addresses the evaluation of the seismic wave field in a graded half-plane with free-surface and/or sub-surface relief subjected to shear horizontally (SH)-polarized wave, radiating from an embedded seismic source. The considered boundary value problem is transformed into a system of boundary integral equations (BIEs) along the boundaries of the free-surface and of any sub-surface relief, using an analytically derived frequency-dependent Green’s function for a quadratically inhomogeneous in depth half-plane. The numerical solution yields synthetic seismic signals at any point of the half-plane in both frequency and time domain following application of Fast Fourier Transform (FFT). Finally, in the companion paper, the verification and numerical simulation studies demonstrate the accuracy and efficiency of the present computational approach. The proposed BIE tool possesses the potential to reveal the sensitivity of the seismic signal to the type and properties of the seismic source, to the existence and type of the material gradient and to the lateral inhomogeneity, due to the free-surface and/or sub-surface relief peculiarities.

JTAM, Sofia, vol. 45 Issue 1 pp. 87-98 (2015), [Full Article]

Deterministically – Probabilistic Approach for Determining the Steels Elasticity Modules

Alexander Popov1, Galina Klitcheva2
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, 1113 Sofia, Bulgaria
2Institute of Metal Science, Equipment and Technologies with Hydro- and Aerodynamics Centre, Bulgarian Academy of Sciences, 67, Shipchenski Prohod Blvd, 1574 Sofia, Bulgaria

The known deterministic relationships to estimate the elastic characteristics of materials are not well accounted for significant variability of these parameters in solids. Therefore, it is given a probabilistic approach to determine the modules of elasticity, adopted to random values, which increases the accuracy of the obtained results. By an ultrasonic testing, a non-destructive evaluation of the investigated steels structure and properties has been made.

JTAM, Sofia, vol. 45 Issue 1 pp. 99-106 (2015), [Full Article]