Journal of Theoretical and Applied Mechanics, Sofia, vol. 51 Issue 1 (2021)

Table of contents



EFFECT OF TEMPERATURE DEPENDENT VISCOSITY ON THERMAL CONVECTION IN FERROFLUID LAYER

Joginder Singh Dhiman, Nivedita Sharma
Department of Mathematics, Himachal Pradesh University, Summerhill, Shimla 171005(HP), India


A linear stability analysis of onset of thermal convection in ferromagnetic fluid layer heated from below with temperature-dependent viscosity is investigated for general bounding surfaces. The eigenvalue problem is solved using a single term Galerkin technique and the values of critical Rayleigh numbers for different combinations of stress free and no-slip boundary conditions are computed for linear and exponential temperature dependent viscosity variations. The effects of temperature dependent viscosity parameter, magnetic Rayleigh number and the magnetization parameter on the onset of stationary convection are investigated numerically. It is found that increase in temperature dependent viscosity and decrease in magnetic number delay the onset of ferroconvection, while the nonlinearity of fluid magnetization has marginal influence on the stability of the system.

JTAM, Sofia, vol. 51 Issue 1 pp. 003-021 (2021), [Full Article]




EVALUATION OF THE MACROMECHANICAL PROPERTIES OF PLA-BASED NANOCOMPOSITES BY MEANS OF THREE-POINT BENDING METHOD

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


The synergistic effect of using two carbon nanofillers in PLA die on the nanocomposites mechanical properties was studied in the present work. The polylactic acid is relatively brittle and semi-crystalline polymer selected to be used as matrix because of its biodegradable features. It was modified with graphene nanoplatelets (GNP) and multiwall carbon nanotubes (MWCNT) by means of melt extrusion preparation method. The maximum flexural stress and tangent modulus of elasticity of 3D printed nanocomposite samples were defined by three-point bending experiments using UMT-2 equipment. It was found that some compounds, incorporated with both graphene and carbon nanotubes in the PLA structure, exhibit better mechanical performance comparing to the nanocomposites loaded with single carbon nanofiller. SEM analysis was made to assess the carbon fillers dispersion.

JTAM, Sofia, vol. 51 Issue 1 pp. 022-034 (2021), [Full Article]




MODELING OF CROWD FLOWS FOR FOOTBRIDGE DESIGN

Krumka Kasapova1, Dobromir Dinev2, Peter Van den Broeck3
1Mott MacDonald Ltd., Bulgaria
2University of Architecture, Civil Engineering and Geodesy, Bulgaria
3KU Leuven, Belgium


Pedestrians exert different dynamic loads (with regard to their type, magnitude and frequency) on civil structures. The commonly used practice of designing such structures as lightweight leads to inevitable decrease in the structural mass and stiffness, thus making the structures less inert and less resistant to the loads induced by humans. The currently available codes of practice, adopt simplified procedures for the vibration serviceability assessment of footbridges. Their procedures disregard the human-human interaction, which may have a substantial effect on the maximum acceleration levels of the structure under consideration. In this article, an extensive study of the human-human interaction is fulfilled. To describe this aspect of the human motion, the widely-applied social force model is calibrated such as to simulate normal traffic conditions on footbridges.

JTAM, Sofia, vol. 51 Issue 1 pp. 035-048 (2021), [Full Article]




EXPERIMENTAL, THEORETICAL, AND NUMERICAL INVESTIGATION OF TEMPERATURE EFFECTS ON LOAD-DEFLECTION BEHAVIOR OF TAPE SPRINGS

Ali Khademi1, Kamal Jahani1, Farhad Farhang Laleh2
1Faculty of Mechanical Engineering, University of Tabriz, Tabriz, Iran
2Faculty of Mechanical Engineering, University of Tabriz, Space Thruster Research Institute, Tabriz, Iran


In this paper, folding of carbon steel tape springs under steady-state hot and cold temperature conditionsis studied analytically, numerically, and experimentally. To consider the thermal load effects on deflection of tape springs, an existing model, namely Mansfield's theory is modified. An appropriate test apparatus is developed to test the tape at several temperature levels. Finite element modeling is implemented to do more validations on theoretical results. It is observed that rising in temperature causes a decrease in maximum moments stored in tape, and low temperature makes this value greater. The results also confirm that the extended theory successfully predicts buckling behavior of tape springs in different temperature conditions for the tapes with length to width ratios greater than ten.

JTAM, Sofia, vol. 51 Issue 1 pp. 049-064 (2021), [Full Article]




CRANE TROLLEY START OPTIMIZATION

Viatcheslav Loveikin, Yuriy Romasevych, Anastasia Liashko
Department of Machines and Equipment Design, National University of Life and Environmental Sciences of Ukraine, Kyiv, 03041, Ukraine


This paper presents the statement and the solving of the optimal control problem for a crane trolley start. In the calculation has been used two-mass model of the trolley with a load on the flexible suspension. The problem has been solved with the variation calculus. Some generalization of the obtained solution made it possible to find the solution of the complex optimization problem. All the obtained results have been investigated with the estimation indexes. Their analysis showed the superiority of the complex optimization problem solution. All the optimal laws of the trolley motion provide the load oscillation elimination at the end of the start mode. It causes an increase of the crane operation rate. At the end of the article, a hypothesis has been set. It concerns the influence of the weight coefficient in the complex optimization criterion.

JTAM, Sofia, vol. 51 Issue 1 pp. 065-075 (2021), [Full Article]




SOME FEATURES OF DOUBLE PENDULUM SYSTEM – NUMERICAL AND SIMULATION STUDY

Hussein Shanak1, Rabab Jarrar1, Hadi Khalilia2, Jihad Asad1
1Department of Physics, College of Applied Sciences, P.O. Box 7, Palestine Technical University, Kadoorie, Tulkarm, Palestine
2Department of Applied Computing, College of Applied Science, P.O. Box 7, Palestine Technical University, Kadoorie, Tulkarm, Palestine


An interesting double pendulum system is investigated in this work. The Lagrangian and the Hamiltonian equations were derived. Furthermore, we obtained the equations of motion describing the system. Numerical and simulation solution was obtained for the equations of motion for some specific initial conditions. MATLAB was used to obtain the numerical solution, especially some build in codes like the ode113.

JTAM, Sofia, vol. 51 Issue 1 pp. 076-091 (2021), [Full Article]




VIBRATION OF SKEW PLATE HAVING EXPONENTIAL VARIATION IN YOUNG'S MODULUS AND DENSITY

Reeta Bhardwaj1, Naveen Mani2
1Department of Mathematics, Amity University Haryana, Gurugram, India
2Department of Mathematics, Chandigarh University, Mohali, Panjab, India


Computation of time period of first four modes of vibration of non-homogeneous, clamped and simply supported skew plate, with variable thickness has been presented, using Rayleigh-Ritz method, on CCCC and SSSS boundary conditions, on different variation of parameters. The thickness variation is assumed to be bi-linear. Non-homogeneity arises in the material because of exponential variation in Young's modulus and density. All the results are presented with the help of figures. A comparative analysis of frequency modes of parallelogram and square plate is also presented with the help of tables.

JTAM, Sofia, vol. 51 Issue 1 pp. 092-102 (2021), [Full Article]