Issue 4

JTAM, Sofia, vol. 54 Issue 4 (2024)

Advances in Theoretical and Applied Mechanics: Selected Contributions from
THE 14th NATIONAL CONGRESS ON THEORETICAL AND APPLIED MECHANICS


Mustafa Arda1, Svetoslav Nikolov2
1Department of Mechanical Engineering, Trakya University, Edirne, Türkiye
2Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria


The 14th Congress on Theoretical and Applied Mechanics (NCTAM2024) was held from 2 to 4 September 2024 at the Hilton Hotel, Sofia.
This Special Issue, featuring a selection of papers presented at the Congress, aims to further advance research in theoretical and applied mechanics and to sustain interest and engagement leading up to the next NCTAM.

JTAM, Sofia, vol. 54 Issue 4 pp. 373-374 (2024), [Full Article]


GREEN'S FUNCTIONS FOR THE VISCOELASTIC HALFSPACE: A CONVOLUTIONAL NEURAL NETWORK APPROACH

George D. Manolis1, Georgios I. Dadoulis1, Petia S. Dineva2, Tsviatko V. Rangelov3
1Laboratory of Experimental Strength of Materials and Structures, Department of Civil Engineering, Aristotle University, Thessaloniki, GR-54124, Greece
2Institute of Mechanics, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria
3Institute of Mathematics and Informatics, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

An identification problem in elastodynamics is addressed through the use of machine learning (ML) to recognize the presence of a free surface in a 3D viscoelastic continuum by measuring the dynamic response (displacements) at a receiver node due to an impulse at a source node. Specifically, a convolutional neural network (CNN) is constructed based on numerical solutions in the form of spectrograms, which are furnished through an implementation of the fundamental solutions of elastodynamics for both the full- and the half-spaces, while both frequency and time domains are considered. Following training of the CNN and its subsequent validation, data streams are evaluated and the results are given in the form `confusion' matrices, which quantify the probability that a given displacement signal moves unimpeded or is being scattered. This in turn indicates the presence or absence of a free surface in the continuum. Furthermore, it is possible to re-train the CNN to estimate of the depth of the source point from the free surface, if such a surface exists. Finally, applications of this work are in areas ranging from geophysics to material science.

JTAM, Sofia, vol. 54 Issue 4 pp. 375-390 (2024), [Full Article]


OPTIMAL VERSUS SUBOPTIMAL TUNED MASS DAMPERS: A COMPARISON STUDY BASED ON ENERGY TRANSFER BETWEEN PRIMARY AND SECONDARY SYSTEMS

Georgios I. Dadoulis, George D. Manolis, Konstantinos V. Katakalos
Laboratory of Experimental Strength of Materials and Structures, Department of Civil Engineering, Aristotle University, Thessaloniki, GR-54124, Greece

The passage of heavy traffic in bridges potentially causes damage, which can be limited by passive device configurations such as the widely used tuned mass damper (TMD). In here, we investigate the vibratory motion caused by a heavy mass sliding on a simply supported bridge deck with a TMD attached at center span. The TMD, modelled as a single-degree-of-freedom (SDOF) secondary system comprising a mass and spring, is attached to the bottom flange to act as an energy absorber of vibrations coming from the primary system (the bridge). This sub-optimal TMD without a damper is analyzed using a Lagrangian energy balance formulation to derive the governing equations of motion, followed by a time-stepping numerical solution. Results are given in terms of time histories and power spectral densities, which allows for an investigation of the transmission of vibratory energy between primary and secondary systems. These numerical simulations help gauge the performance of a number of TMD configurations with optimized material parameters, using as benchmark the sub-optimal TMD where damping is provided by the spring element only.

JTAM, Sofia, vol. 54 Issue 4 pp. 391-404 (2024), [Full Article]


APPLICATION OF DIFFERENT MODELLING TECHNIQUES FOR SOIL-STRUCTURE INTERACTION PROBLEMS IN ANISOTROPIC MEDIA

Sonia Parvanova1, Georgi Vasilev1, Amar Pashov2
1Department of Structural Engineering, University of Architecture, Civil Engineering and Geodesy, Sofia, Bulgaria
2KU Leuven, Department of Civil Engineering, Kasteelpark Arenberg 40, B-3001 Leuven, Belgium

The current paper investigates wave propagation from time-harmonic embedded point source in a semi-infinite anisotropic medium containing underground structure by applying three different computational techniques. Firstly, direct BEM for 2D elastodynamics is applied using the fundamental solution derived by the Radon transform for general anisotropic continua. The second numerical technique is a computationally efficient two-and-a-half dimensional FEM, used to calculate the 3D wave field in the soil. At the boundaries of the mesh perfectly matched layers are instated to prevent spurious wave reflections. The FEM solutions realized by the built-in options in ANSYS are finally utilized with two types of absorbing boundary conditions. The results obtained by the three adopted modelling techniques are properly compared and respective insights regarding their applications are provided.

JTAM, Sofia, vol. 54 Issue 4 pp. 405-428 (2024), [Full Article]


EVALUATION AND ANALYSIS OF MICROSEISMIC RECORDS ON THE TERRITORY OF THE SOFIA BASIN

Emil Oynakov1, Radan Ivanov1, Liliya Dimitrova1, Alexander Radulov2
1National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences, Acad. G. Bonchev St., bl. 3, 1113 Sofia, Bulgaria
2Earthquake Geology Department, Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev St., bl. 24, 1113 Sofia, Bulgaria

The content of the microseismic wave field continuously recorded at any measurement point is closely related to the structure and geology of the Earth's crust. All registered oscillations have individual amplitude-frequency characteristics, temporal mode of existence and variations (permanent, periodic, spontaneous) and, what is particularly important, characteristic spatial distribution in the geological environment. Seismic noise records were analyzed to study the structure of the microseismic wavefield and the spatial variability of soil dynamic characteristics for the SE part Sofia Basin area. The natural frequencies of the soil were determined by two methods: the standard spectral ratio (SSR) method and the horizontal to vertical spectral ratio (HVSR) method. On the basis of the recorded extensive experimental material, the possibility of using the natural oscillations of the soil (microseisms) for differentiating the ground in a seismic sense has been proven.

JTAM, Sofia, vol. 54 Issue 4 pp. 429-445 (2024), [Full Article]


ANALYSIS OF THE COMPLEX GAS PIPELINE EXPLOITATION PROCESS IN VARIOUS OPERATING MODES

Ilgar Aliyev
Operation and Reconstruction of Buildings and Facilities Department, Azerbaijan University Architecture and Construction, Baku, Azerbaijan

Based on the results of theoretical research, a computational scheme has been developed that allows the establishment of normative-technical documents for the organization and technological decision-making in the exploitation of main gas pipelines. The study extensively explores the dynamic behavior of processes for the supply of modern automatic devices for gas pipelines and the use of an efficient automated control system. The analytical determination of the optimal time for transition processes has been widely applied to ensure the high reliability of pipeline transportation and the most favorable operating conditions for the system. Methods for calculating complex transient processes in main gas pipelines, from non-stationary to stationary modes, have been utilized when gas ingress occurs in a non-stationary regime. A comparison of mathematical expressions for calculating transient processes in complex main gas pipelines has been conducted through theoretical sources.

JTAM, Sofia, vol. 54 Issue 4 pp. 446-464 (2024), [Full Article]


REGIONS OF EXISTENCE OF ANALYTICAL SOLUTIONS OF THE (2+1)-DIMENSIONAL SINE-GORDON EQUATION

Pavlina Atanasova1,2, Valentin Georgiev1
1Paisii Hilendarski University of Plovdiv, Faculty of Mathematics and Informatics, 24 Tzar Asen, 4000 Plovdiv, Bulgaria
2Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 4, 1113 Sofia, Bulgaria

The sine-Gordon equation is a well-known equation, finding application in a multitude of physical phenomena. In this paper we consider the two-dimensional sine-Gordon equation and its Jacobi elliptic function solutions. We find and illustrate the regions of existence for the solutions, expanding on equivalences found between different solutions. Applying the regions of existence in the setting of band two-dimensional Josephson junctions, a different kind of relation appears between the amplitude of the wave and the width of the junction. Exploring this relation, we give a concrete example of the influence of the wave amplitude on the magnetic field in the junction.

JTAM, Sofia, vol. 54 Issue 4 pp. 465-479 (2024), [Full Article]


INFLUENCE OF SORET EFFECT, HEAT SOURCE, VISCOUS DISSIPATION, CHEMICAL REACTION, THERMAL RADIATION AND VARIABLE PERMEABILITY ON HEAT AND MASS TRANSFER THREE DIMENSIONAL FREE CONVECTIVE MHD FLOW THROUGH POROUS MEDIUM

Tej Singh1, Hemant Pal2
1Department of Mathematics, Government College Chamba, Himachal Pradesh, India-176314
2Principal, Government College Bharmour, Distt. Chamba, Himachal Pradesh, India-176315

In the present study, the influence of Soret effects, Heat Source, Thermal Radiation, Viscous Dissipation and Chemical Reaction on three-dimensional free convective MHD flow through porous medium with variable permeability was examined. The dimensionless governing equations were sol\-ved analytically by Multiple Regular Perturbation Law. The effects of several flow parameters affecting velocity, temperature and concentration were explained and discussed graphically. The expressions for the skin friction, Nusselt number and Modified Nusselt Number profiles were formulated and discussed with the help of tables. It is observed that the velocity of the fluid initially increases and then decreases with increase of y but it showed reverse behaviour with large increase in radiation parameter (R).

JTAM, Sofia, vol. 54 Issue 4 pp. 480-501 (2024), [Full Article]


BIOMECHANICAL JUMP CHARACTERISTICS OF ADOLESCENT BASKETBALL FEMALE PLAYERS AFTER AN ISOMETRIC STRETCHING PROGRAM

Ivan Ivanov1,2, Blagovest Glavev3, Sergey Ranchev2
1Anatomy and Biomechanics Department, National Sports Academy "Vassil Levski''
2Institute of Mechanics, Bulgarian Academy of Sciences
3Basketball team "Rilski Sportist'', Samokov

The effects of isometric stretching on the muscle-tendon-joint biomechanics in the literature data are contradictory. The aim of the study was to verify the effectiveness of the applied isometric stretching program for the improvement of jump performance of six adolescent basketball female players (17-19 years). The isometric stretching protocol with eleven exercises was used after the last daily regular training program, four times per week for 40 minutes. All exercises were repeated three times with detention duration 15 seconds for 11 weeks. Chronojump DIN-A1 contact platform (Spain), detecting a single jump with one phase of flight, was used to evaluate eight important biomechanical jump characteristics for the participants. The obtained Mann-Witney statistics of the measured parameters for our six participants shows contradictory results and individual stretching body adaptation. The obtained statistical significant decrease only for DJ Power (drop jump power) does not allow to make the important conclusions for the applied isometric stretching program effectiveness. The presented results underline that lower extremity control and performance are strongly related with the motion and synergy work of the upper limbs. In addition, the presented data draw attention to improving coordination between the lower and upper body movements, emphasizing the individual characteristics of each athlete for target increasing of lower limbs biomechanical performance.

JTAM, Sofia, vol. 54 Issue 4 pp. 502-510 (2024), [Full Article]


SPECIFICITY OF INTERMUSCULAR COORDINATION IN THE BASEBALL HIT IN RELATION TO THE ANGULAR VELOCITY OF THE BAT SEGMENT

Liydmil Trenev1, Ognian Tishinov1, Ivan Ivanov1,2
1National Sports Academy
2Institute of Mechanics, Bulgarian Academy of Sciences, Sofia, Bulgaria

The aim of this study was to estimates the relation between the kinematic structure of the baseball strike movement in adolescent baseball players and the applied muscles electromyographic activity. The research was carried out during the movement of the bat in thirteen-year-old baseball players (n = 5). The used video recording with two cameras from different points of view was synchronized with conducted surface electromyography. The obtained results are of essential importance for the coaches in a developing and using correct methods in the training process to improve the technical skills of the athletes. Our results relate the muscle activity with the trajectory and speed of the bat during the stroke, which benefits in improving the efficiency in hitting.

JTAM, Sofia, vol. 54 Issue 4 pp. 511-524 (2024), [Full Article]