Meccanica最新文献

The aim of the paper is to clarify the semantic and thus the difference between tyre rolling resistance and tyre rolling loss. Reference is exclusively made to longitudinal forces. The focus is on which are the actual forces and moments that cause a dissipated power during tyre rolling. The equilibrium equation for rotation of the wheel is unable to describe which are such actual forces and moments. In accordance with the existing literature, the power balance of rolling tyre is more suitable to identify the actual forces and moments causing dissipated power. We propose to refer to “rolling resistance” when the longitudinal slip is vanishing or low and to refer to “rolling loss” when the longitudinal slip is relatively high. A new index defining the rolling loss is proposed, namely the tyre rolling dissipation index (TRDI). The theoretical investigation is substantiated by an experimental activity performed via a new trailer for tyre testing. The rolling resistance torque, the external applied torque, the wheel axle height, the longitudinal force at the hub and the kinematics of the rim have been measured and combined to describe the TRDI. Results shows that tyre rolling loss increases as a braking torque is applied to the wheel and the effect of the TRDI is shown as function of longitudinal slip. A practical rule is proposed, stating that the ratio of tyre dissipated power to the total dissipated power is nearly equal to the longitudinal slip.

The reduced multibody system transfer matrix method is a multibody system dynamics method adopting tree topology description and utilizing joint coordinates as the generalized coordinates of the system. For a multibody system containing closed loops, it is necessary to “cut” a joint in each inner loop, which will be replaced by geometric constraint equations confined on the generalized coordinates of the spanning tree system, as well as a pair of internal forces in equilibrium implied on the two connected body elements. The reduced multibody system transfer matrix method employs relatively lower order coefficient matrices that are independent of the degrees of freedom of the system, resulting in faster computational speed compared to those methods that establish and solve the global dynamics equations of the system characterized by the global inertial matrix, whose dimension is no less than the degrees of freedom of the system, no matter utilizing joint coordinates or not. However, in the process of multibody system dynamics modeling, the redundancy in constraints, which causes rank deficiency of the constraint equations’ Jacobian matrix, is an unavoidable problem. In this paper, an algorithm based on the singular value decomposition is proposed to resolve the singularity problem in the context of reduced multibody system transfer matrix method when handling redundant constraints. The singular value decomposition is utilized to compute the Moore-Penrose generalized inverse, resulting in the minimum norm solution for the constraint forces at the cut-off joints within closed-loop constraints. The proposed method is validated by two numerical examples.

Sealing technology is critical for the reliability and efficiency of mechanical systems, especially in rotating shaft applications. Traditional ferrofluid (FF) seals, while effective in narrow gaps (0.1–0.3 mm), face significant limitations in maintaining effective sealing under large gap conditions (more than 0.3 mm). To address this challenge, a magnetorheological fluid (MRF) seal optimized for high-speed dynamic applications was proposed. Firstly, a sealing structure was designed, and the rheological properties of MRF were characterized. Then, theoretical models for both FF and MRF seals were derived to analyze their operating principles and performance differences. A custom test bench was constructed to evaluate static sealing performance at 0.1 mm and 0.4 mm gaps and dynamic sealing performance at shear velocities of 0.2, 0.4, 0.6, 0.8, and 1.0 m/s. Experimental results demonstrated that MRF seals achieve higher pressure differentials compared to FF seals, particularly in large gap scenarios. These findings suggest that MRF seals offer a promising alternative for advanced sealing applications in mechanical systems.

Tuned Mass Dampers (TMDs) and Tuned Liquid Column Dampers (TLCDs) are widely recognized passive vibration control devices used to reduce structural vibrations. While TMDs have been extensively studied for mitigating the seismic responses of multi-story buildings considering Soil-Structure Interaction (SSI), the efficiency of TLCDs in these conditions remains largely unexplored. Furthermore, a direct comparison of these devices under similar conditions has not been conducted. Then, to address these gaps, this study investigates the efficiency of TLCDs and compares them to TMDs in reducing seismic-induced vibrations, focusing on the influence of SSI. The control performance of both devices depends on various parameters, primarily the frequency and damping ratios. Therefore, the Mouth Brooding Fish (MBF) metaheuristic algorithm is applied to optimize these parameters, accounting for SSI effects. To evaluate the different efficiency between TMDs and TLCDs under SSI conditions, three types of shear buildings are considered: an eight-story, a sixteen-story and a forty-story structure. The seismic responses of the uncontrolled, TMD-controlled, and TLCD-controlled buildings are examined under twenty-two far-field and fourteen near-field earthquakes, considering both fixed-base and flexible-base scenarios. Results indicate that while both devices significantly reduce seismic responses, TMDs generally outperform TLCDs, particularly in taller buildings where the impact of SSI is more significant. Further, this study highlights that neglecting SSI in the design of these devices may lead to an overestimation of their effectiveness, especially in softer soils, emphasizing the importance of considering SSI in the optimization process for accurate and reliable outcomes.

The problem considered can in general be described as the problem of proving rigorously that as the Reynolds number of a fluid flow tends to infinity, the drag coefficient remains bounded. It has been proven for flows in domains of certain, rather restrictive, set of geometries, but not in general. The drag is the sum of the pressure drag and friction drag. In essence, this paper gives a proof that the friction drag coefficient does remain bounded. Rather nontrivial implications of this result are discussed.

In vibration-assisted polishing (VAP), static, kinetostatic, dynamics, and vibration trajectories are extremely important characteristics for compliant mechanisms to generate the vibrating source, but these behaviors lack a systematic modeling theory. This study presents a new XY compliant mechanism design inspired by the grasshopper. The proposed design can provide a wide amplitude amplification by a new self-amplified stroke based on the grasshopper’s legs. The machining qualities of VAP are crucially dependent on the vibrating amplitude, the output force, and the vibration trajectory of the central table. This research is aimed at providing theoretical modeling of the static, kinetostatic, dynamic, and oscillatory trajectories of the suggested structure. The stroke amplification ratio, workspace, stress, and kinetostatic are theoretically formulated by a graphical method and free body diagram. The resonant working frequency of the mechanism is modeled Lagrange’s II method. The dynamic motion and vibration trajectory of the mechanism in two directions are theoretically derived using D’Alembert’s principle. The force ratio between the input and output of the vibrating table is theoretically calculated. The results obtained from the theory closely align with those from the finite element analysis. The results found that the proposed mechanism can work at a wide workspace of 736.6 × 736.6 μm, a large stroke amplification ratio of 4.53, and a high working frequency of 548 Hz. The experimental tests are matched with the theoretical ones. This novel design facilitates a comprehensive design and modeling synthesis for two-dimensional vibrating mechanism for VAP.

The present research presents potentials and complementary potentials used in the one-dimensional nonlocal integral formulations. The pure stress and the pure strain nonlocal formulations were considered. While the potential used in the strain driven formulation is well known, the complementary potential has not yet been presented in the literature. The same applies to the stress driven formulation. The equivalent formulations are obtained by resorting to the Legendre transformation, and their equivalence is proved. It is also shown that these results can be used to postulate a novel potential, i.e. a kind of mixed stress–strain potential, which is, however, as ill-conditioned as the pure strain-driven formulation. Finally, an example is given that practically confirms that the stress-driven formulations resulting from the potential and the complementary potential are equivalent.

Evaluating the state of historical masonry structures, particularly those built with irregular masonry, presents challenges in determining their load-bearing capacity. Most current approaches use macroscopic numerical models that treat the material as a homogeneous continuum, where defining an appropriate constitutive law is essential. To this end, homogenization has proven useful in bridging the gap between meso and macro-scales, yet using homogenized macromodels may prevent to capture specific failure modes. Alternatively, mesoscopic methodologies represent stacking modes explicitly. However, full structural analysis at this scale has mainly been applied to regular masonry due to the complexities of explicitly representing the geometry of the irregular stones. The present contribution aims to propose a method that leverages image acquisition techniques, such as orthophotos, to address these challenges. Mortar joints are lumped onto zero-thickness interfaces, determined through a distance field-based morphing procedure and medial axis principles. The load-bearing capacity is assessed using a Kinematic Limit Analysis problem, formulated and solved as a linear programming problem. A Mohr-Coulomb frictional behavior, modified with a tensile cut-off and a linearized compression cap, is assigned to the mortar joints. The blocks are considered infinitely rigid and strong. It is shown that the proposed methodology can efficiently model structures of large sizes, by illustrating its use on two 2D problems.

Cableways have been used intensively for transportation of goods and people since the early nineteenth century. Aerial cableways are primarily modelled based on a continuous representation, well-suited for traditional analytical models. However, due to the increasing complexity of these systems, cableway modeling is evolving to discretised approaches like Finite Element (FE) analysis. The Finite Element Method (FEM) is an alternative to the modal approach for studying the dynamics of nonlinear systems, and it is more suitable for the representation of large displacements needed in the view of the modeling of an entire plant; it is also flexible and computationally accurate. Dynamic models of aerial cableways based on finite element computation with significant geometry changes are currently lacking in the literature. The main purpose of this study is to fill this gap, through the development of a finite element model which is able to describe the dynamic behavior of aerial cableway, keeping into account its time-varying geometry. The system modelling is based on two cables (carrying and hauling), describing a back-and-forth aerial cableway to which a concentrated mass representing the cabin is added. After the introduction of the modelling approach, the finite element model is applied to investigate typical conditions such facilities are exposed to, i.e. overlapping between the hauling and the carrying cables or the large vibrations the hauling cable is subjected to, when the cabin is approaching the station. Versatility and ease of implementation are the main strength of the proposed model, making it suitable to further extensions.

In this study, we investigate the static and dynamic characteristics of electrostatically actuated nanobeams with elliptical cross-sections, focusing on their potential applications in nano-electromechanical systems (NEMS). Using finite element analysis (FEA) in COMSOL Multiphysics, we explore the effects of key parameters on the nanobeams’ performance. Our analysis includes a detailed examination of the static characteristic curves and natural frequencies under varying conditions. The study reveals significant insights into the behaviour of elliptical nanobeams, demonstrating how alterations in eccentricity and tilt angle influence the operational efficiency of NEMS devices. The impact of geometric variations on vibrational modes provide critical guidelines for designing robust and precise NEMS sensors and actuators.