Meccanica最新文献

The stability analysis of a linear system with the multiple delays as parameters in given intervals is not a new but hard topic in general, for which a key step is to find out all the critical stability curves/surfaces in the parameter space. In this paper, the critical stability condition is regarded as a complex equations depending nonlinearly on the delays, and it is solved in three parts: (1) The solvability of the nonlinear equation; (2) The representation of the solutions; 3) Numerical algorithms for finding the solutions. For the solvability, a necessary and sufficient condition in terms of a delay-independent inequality with clear geometrical meaning has been derived from the critical stability condition in the form of vector equation. For the representation, the critical delays in nested form are expressed explicitly in terms of a number of hypersurfaces, all the quantities have clear geometrical meaning. Based on the nested representation, two effective algorithms are proposed for finding the solutions, and illustrated with simple examples. The main results not only generalize the previous ones for systems with two delays and three delays of the nondegenerate cases, but also add new findings for the degenerated cases which have important impact on the stability of the time-delay systems.

The numerical solution of dynamic problems often results spurious oscillations. In order to eliminate them, a damping effect must be included in the numerical scheme. However, the concrete shape of the damping characteristics has a great importance in the efficiency of oscillation reduction. In this article, a novel approach has been introduced with adjustable damping character. The damping effect is exerted as viscous damping according to the formulation of Caughey damping. Using the proposed method, a wide range of damping curves can be approximated with high accuracy. The newly developed method is mainly useful for contact-impact and wave propagation problems.

The contact region is actually an ellipse during point-contact, as stated by Hertzian contact theory. The load in the contact ellipse (CE) gradually decays rather than uniformly distributes due to the varying curvature of contact surfaces. Spur-face gear drive (SFGD) is highly valued in modern machines due to its point-contact characteristics. An improved theoretical model of time-varying mesh stiffness (TVMS) for SFGD has been constructed by coupling tooth, contact and fillet-foundation stiffness. A novel thought named deformation suppression which there is a fixed effect on the CE by the area outside the CE due to the absence of force is proposed according to the local contact characteristics of point-contact. The coupling effect between the meshing teeth pairs and the fillet-foundation stiffness are both considered. The position, size and incompleteness of the CE are determined by the movement of the meshing tracks which are derived from the explicit equation and multi-state meshing regions of SFGD and the load distribution ratio (LDR) calculated based on them. The characteristics of point contact are qualitatively verified experimentally. The results show that LDR mutations of gear switching are reduced considering incomplete CE. TVMS of point-contact considering deformation suppression is improved by an order of magnitude compared to the one of line-contact which may be the cause for the high carrying capacity of point-contact. It can be the basis to investigate the characteristics of SFGD and the point-contact further. This method is applicable to the study of point-contact in other fields.

Aeration irrigation has garnered significant attention due to its effectiveness in enhancing crop yield and water use efficiency. Understanding the two-phase flow mechanism of water–gas in the aeration pipeline is of great significance for promoting the development of the aeration irrigation system. In this study, a formula for calculating Reynolds number of water–gas flow in aeration pipeline was established by combining theoretical analysis with experimental verification. In addition, based on the YOLO v5 image classification model, a classification and detection model of water–gas flow pattern was proposed. The results show that increasing aeration rate can increase pipeline pressure. The overall accuracy of the developed flow pattern detection software was 86.3%, which can meet the requirements of practical application. Finally, based on image classification and theoretical analysis, the formulas of the corresponding flow patterns were obtained. This study provides a solution for determining the two-phase flow pattern of water–gas in aerated irrigation system, which was of great significance for improving the hydraulic calculation of aerated irrigation system and promoting the wide application of aerated irrigation technology.

This study investigates the ditching dynamics of the ARJ21 regional airliner under both calm and wave-influenced water conditions using the Smoothed Particle Hydrodynamics (SPH) method. The DualSPHysics platform is adopted to simulate water entry events, with the modeling framework validated against experimental data from canonical wedge and cylinder impact tests. A numerical wave tank incorporating an Active Wave Absorption System (AWAS) is constructed to suppress boundary reflections and produce a stable wave environment. Comparative analyses of the aircraft's hydrodynamic responses reveal that wave conditions significantly intensify vertical acceleration and pressure loads, while amplifying the secondary rise effect. The SPH method demonstrates strong agreement with experimental results, particularly in early-stage impact behavior. These findings support the feasibility of using meshless SPH-based methods for simulating complex aircraft–wave interactions in ditching scenarios.

In self-reconfigurable structures, the mechanical design of the joints is one of the most challenging tasks. Within this context, flexural pivots are widely adopted as compliant mechanisms due to their ideal design for achieving low rotational stiffness and high off-axis stiffness. To maximize performance, they are often optimized for specific application requirements. However, designing flexural pivots for self-reconfigurable structures with an arbitrary center of rotation remains a significant challenge. To address this, we propose an approach for optimizing the topology of beam-based flexural pivots undergoing large deflections, aiming to achieve an optimal configuration with an arbitrary center of rotation. To this end, both the stiffness-based objective function and the strain energy-based objective function are introduced. For the implementation, a geometrically exact beam element is utilized to establish a dual-layer ground structure for optimization. A genetic algorithm is employed to identify optimal configurations for flexural pivots, including traditional notch hinges and cross-spring pivots. Additionally, the influence of different objective functions and their corresponding parameters on the optimized topology is examined and verified. Ultimately, this approach yields optimal topologies in three representative examples with different centers of rotation, establishing a foundation for the design of compliant mechanisms with user-defined rotational behavior.

This paper proposes a novel theoretical study on the onset of failure in finitely deformed periodic nonlinear composite materials because of microscopic instability and bifurcation mechanisms in conjunction with decohesion and contact effects at interfaces between different constituents. Original analytical investigations are firstly carried out on an introductory 2-DOF example highlighting the main features of the examined problem and using a structural mechanics approach. The theoretical setting of the problem is then developed within a finite strain continuum mechanics framework and a nonlinear homogenization formulation is adopted to drive the system along macro-deformation loading paths. The formulation includes a continuum contact mechanics model in conjunction with a class of irreversible cohesive traction–separation laws for treating both unilateral contact constraint and progressive decohesion at discontinuity interfaces. The main equations governing the equilibrium problem of the microstructure in both finite and rate forms are developed, and the relevant issues associated with loss of uniqueness in the rate equilibrium solution together with the instabilities onset are also investigated by developing an exact second-order analysis. The introductory example is then re-examined by using the proposed continuum mechanics formulation and comparisons with simplified cohesive-contact models frequently adopted in the literature are performed. The obtained results show the role played by contact and cohesive mechanisms and the significance of an appropriate modelling of their deformation sensitivity and conditionality nature to perform accurate stability and bifurcation analyses. Strategies to circumvent the complications arising both from cohesive behavior and contact mechanics nonlinearities arising at the interface are also discussed.

In this paper, an analytical iterative method is presented to determine the optimal tension distribution for redundant cable-driven robots. In this method, a solution domain is defined based on the lower and upper tension limits of the cables. Then an objective function with a variable objective point is introduced. The continuous adjustment of the optimization objective point provides flexibility to increase or decrease the stiffness and energy consumption of the robot along a specified path. Furthermore, the convergence of the method to the optimal solution and the continuity of the resulting tension distribution are assured by the algorithm outlined in this study. Finally, two examples are included to compare the proposed method with some notable approaches in the literature. The first example demonstrates that other methods may fail to identify any acceptable tension distribution, while the second example illustrates the advantages of utilizing a variable objective point in the objective function.