Machinery - Repository of the Faculty of Mechanical Engineering, University of Belgrade
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Experimental Study of a Hybrid Dolphin Airfoil Aerodynamics
No full textcontract number 451-03-136/2025-03/200213 dated February 4, 2025
Structural integrity of designed sandwich panels under low temperature conditions: blast effect assessment using finite element simulation
No full textThis research employs a detailed finite element analysis to investigate the blast resilience of metallic sandwich panels under various thermal and structural conditions. The investigation considers a range of parameters, including core shapes (tetragonal, hexagonal, and octagonal), core heights (0.021 m, 0.051 m, and 0.081 m), levels of pre-existing damage, and TNT charge masses (0.5 kg, 1.5 kg, and 2.5 kg), all tested under three distinct temperatures: ambient (293 K), sub-zero (193 K), and cryogenic (113 K). Blast loading is simulated using the ConWep method with a consistent stand-off distance of 0.1 m. The findings reveal that panels with octagonal cores significantly outperform other geometries, reducing front-face deflection by up to 46%. Similarly, increasing the core height leads to a more than 60% reduction in back-face deformation. Cryogenic conditions further enhance the structural response, with simulations showing faster kinetic energy decay and a smoother stress distribution across both damaged and intact models. Cryogenic panels absorbed up to 25% more internal energy while maintaining lower overall strain levels, indicating a more efficient mechanism for dissipating blast energy. Overall, the study highlights how geometric configuration, thermal environment, and damage state collectively govern the blast performance of sandwich panels, offering valuable guidance for designing high-performance protective structures in sectors such as aerospace, defense, and cryogenic systems
Developing the measurement scale for organizational resilience assessment based on IMF SWARA method
No full textThe purpose of this research is to develop a novel conceptual framework for organizational resilience measurement, based on the employees’ assessment on different managerial levels. The methodology of research is based on the questionnaire survey that is enabling the technological, human, organizational and sustainability factors assessment in industrial organizations, operating in Serbia and Montenegro. Obtained survey results were further processed with the reliability analysis and discriminant validity check of proposed measurement scales and analyzed by applying the IMF SWARA method from the perspective of four identified resilience corners: anticipate, monitor, react and learn. Organizational resilience was calculated separately based on the survey collected among the middle and top-level managers in investigated organizations. The highest resilience coefficients, based on the middle level managers’ assessment, are obtained regarding their observation of the organizational performance (4.8), motivation of employees regarding safety and health at work (4.7) and organizational growth and business profitability (4.7) while the lowest value was observed regarding working environment conditions (3.9). In case of the top-level managers’ assessment, the highest resilience coefficients are also calculated regarding motivation of employees regarding safety and health at work (4.7), followed by knowledge of occupational safety (4.6), associate support (4.6), support from superiors (4.6) and management commitment (4.6). The lowest value, for this group of respondents, was also observed related to the working environment conditions (4.1), which appears to be the weakest influencing factors, from the point of view of both management structures. When it comes to observed resilience corners, the “monitor” corner should be substantially improved, being with the lowest resilience indicator, assessed by the both groups of management structures (4.29 and 3.05, for the middle and top level managers, respectively). The highest level among the resilience corners, for the case of middle level managers was observed for the “anticipate” (4.43) and “react” (4.43), while in the case of the top-level managers, again it was the “react” corner (4.61). The originality of this research is in systematical approach to organizational resilience measurement, using the novel measuring scale proposed here in combination with different MCDA methods and techniques
Transient simulation on internal flow characteristics and pressure pulsation of variable speed Francis turbines during acceleration process
No full textVariable-speed Francis turbines offer the advantages of a broader operational range and enhanced stability. To investigate the internal flow dynamics and pressure pulsation characteristics during the variable-speed operation of the Francis-99 turbine, this paper employs numerical
simulations to examine the internal pressure, flow behavior, and energy loss variations over time during the acceleration process. The Hilbert–Huang Transform method is utilized to generate the corresponding time-frequency diagram of the pressure pulsation signal, thereby
enhancing the accuracy and reliability of feature extraction. The pressure pulsation signal was decomposed into an intrinsic mode function (IMF) using variational modal decomposition, and fast Fourier transform was applied to the IMF to identify the components contributing
most significantly to the pressure fluctuations. The final results demonstrate that the accelerated operation of the Francis-99 turbine positively affects the reduction of hydraulic losses and enhances operational stability. Regarding hydraulic stability, the rotor stator interference effects
must be considered. Following a change in rotor speed, the flow within the runner becomes more complex. However, total entropy production within the Francis-99 turbine decreased by 9.1% following accelerated operation. During acceleration, the dominant frequencies of pressure
pulsations at the guide vane outlet and impeller inlet remained at 30 fn and 28 fn, respectively, as speed increased. Furthermore, the peak pressure pulsations at these dominant frequencies were reduced by 83% and 76%, respectively, compared to those observed during fixed speed
operation. This study offers a valuable reference for assessing the instability characteristics of the turbine during variable-speed operation and enhancing operational efficiency
THE TECHNICAL DRAWING TOOLS IN THE LEGACY OF NIKOLA TESLA
No full textThe legacy of Nikola Tesla contains numerous archival documents, monographic and serial publications, newspaper clippings, as well as personal and technical items. It includes thirteen specimens of technical drawing tools and some correspondence with the manufacturers. The tools were acquired and used by the famous scientist, engineer and inventor to produce technical documentation while designing and constructing numerous machines and devices in various areas of his creative efforts. The aim of this work is to analyze the original drawing tools preserved in the Nikola Tesla Museum in Belgrade, to inspect the corresponding catalogs, and to present a brief history of the relevant manufacturers.Contracts No. 451-03-137/2025-03/200105 and No. 451-03-136/2025-03/20013
The Methodology for the Design of Underactuated Adaptive Robotic Finger for Precision Grasping Tasks
No full textRobotic grippers are essential components in modern automation, enabling robots to interact with and manipulate objects in various tasks. While traditional designs excel in the manipulation of identical objects in structured environments, they often lack the adaptability required for handling various objects in unstructured settings. This limitation has led to increased focus on adaptive solutions, with underactuated robotic hands emerging as a promising alternative that combines dexterity, mechanical simplicity, and cost efficiency. Unlike fully actuated hands, which are complex and expensive, underactuated designs use fewer actuators than degrees of freedom and rely on passive elements such as springs and mechanical limits for adaptation to object’s shape. This paper presents the design of a two-phalanges underactuated robotic finger actuated by a four-bar linkage and gear mechanism. The finger is intended for robotic hand primarily used for precision grasping tasks. A methodology for calculating optimal linkage dimensions and mechanism for adaptability are also described, as well as the method for the analysis of grasping force during precision grasping.under the contract No. 451-03-137/2025-03/20010
Novel tabular method for estimation of heat Extraction factor
No full textDesign of industrial buildings significantly depends on the required fire resistance time of
the building structure. In order to determine this parameter, one of the most used procedures is
defined by German standard DIN 18230. In Serbia, calculation procedures given in this standard are mandatory for application according to the current Rulebook of technical standards for fire safety of industrial buildings. In this paper the calculation procedure for determining the heat extraction factor by applying standard DIN 18230 will be analysed. In order to simplify the calculation procedure and reading of input data, the tables will be proposed as an alternative to nomograms given in standard DIN 18230.Grant number 451-03-137/2025-03/ 200105 - 4.2.2025
MATHEMATICAL MODELLING AND SIMULATION OF AN ELECTROMAGNETIC VIBRATORY ACTUATOR
No full textThis paper presents a mathematical model of an electromagnetic vibratory actuator (EVA) used to drive vibratory conveyors. The model captures the fundamental aspects of electromechanical energy conversion by accounting for key energy-storing elements. The system is described by three state variables: the displacement and velocity of the actuator armature, and the current through the excitation coils. Numerical simulations were conducted for a representative vibratory conveyor configuration. To validate the accuracy of the proposed model. experimental measurements were performed using a laboratory prototype. The experimental results show strong agreement with the simulated data, confirming the validity of the nonlinear dynamic model
α-Al2O3 Functionalized with Lithium Ions Especially Useful as Inert Catalyst Bed Supports
No full textThe alumina, in the form of α-Al2O3 tabular balls, considered in this study is a high-purity form of aluminum oxide that has been fired at high temperatures (well above 1900 °C), virtually removing porosity. However, the purity and inertness of the surface of the Al2O3 tabular balls minimize the catalytic activity, which is why lithium doping was tried. Thus, the target of this study was the effect of doping with lithium ions in some tabular balls of Al2O3 (the crystalline structure is corundum) on the improvement of the catalytic properties of alumina. This study examined the impact of a lithium catalyst on the combustion of various fuels within a porous inert medium (PIM) burner. This study specifically compared low calorific gaseous fuel (e.g., biogas) combustion in a PIM burner with and without the lithium catalyst. The experimental setup comprised a gas preparation unit for mixing CNG and CO2 to simulate biogas and a PIM burner. The PIM burner comprised Al2O3 spheres (13 mm diameter, 45% porosity) in a random packing configuration. Three fuels, varying in composition and lower heating value (LHV ranging from 20.771 to 27.695 MJ/m3), were combusted at air ratios ranging from 1.67 to 1.79. The results indicated that the catalyst increased peak combustion temperatures by 23.2 °C to 51.4 °C, depending on the fuel type and air ratio. Significantly higher carbon monoxide (CO) concentrations were observed without the catalyst, particularly with fuel type F1, while nitrous oxide (NOx) levels remained consistently low. Upstream flame propagation was observed in the presence of the catalyst. These findings demonstrate the potential of lithium catalysts to enhance combustion stability and reduce emissions in porous media combustion burners. Following these studies, it can be stated that Li(I) has the role of promoter of the catalytic process.Contract Number: 451-03-66/2024-03/200105, date: 5 February 202
Solar and Wind Power – Sustainability at UB-FME
No full textTo stimulate and facilitate the transition to a more sustainable and cleaner future, several renewable energy possibilities (wind and solar in particular) are considered for the University of Belgrade, Faculty of Mechanical Engineering (UB-FME). The paper presents an initial study of the possible energy contributions that could be achieved in the near future by distributing solar panels or small wind turbines across the rooftop surface, in addition to improved education and raising awareness of the sustainability related issues in general public. The initial estimates indicate that an additional 30-300 MWh could be generated annually, but further socio-techno-economic analyses should be conducted