Meccanica最新文献

In this paper, an isotropic environment with a static crack under thermal shock is studied using generalized thermoelasticity equations of Chandrasekharaiah–Tzou. The discretization of the governing equations in dimensionless space is done using the extended finite element method and the matrix form of the equations is extracted. Newmark’s method is used to solve the resulting system of nonlinear equations in the time domain. Some examples are solved using the numerical method and the temperature, displacements and stress fields along the length of the homogeneous layer are obtained. Also, the stress intensity factors under thermal shock are calculated using the interaction integral method and compared with the values obtained from the classical thermoelasticity theory. The results show that the speed of temperature, displacement and stress waves in the Chandrasekharaiah–Tzou theory are limited, unlike the classical thermoelasticity theory. Also, the maximum values of displacement and stress according to the theory of Chandrasekharaiah–Tzou occur at a smaller distance from the edge to which the thermal shock is applied.

Recent investigations on active materials have introduced a new paradigm for soft robotics by showing that a complex response can be obtained from simple stimuli by harnessing dynamic instabilities. In particular, polyelectrolyte hydrogel filaments actuated by a constant electric field have been shown to exhibit self-sustained oscillations as a consequence of flutter instability. Owing to the nonreciprocal nature of the emerging oscillations, these artificial cilia are able to generate flows along the stimulus. Building upon these findings, in this paper we propose a design strategy to break the left-right symmetry in the generated flows, by endowing the filament with a natural curvature at the fabrication stage. We develop a mathematical model based on morphoelastic rod theory to characterize the stability of the equilibrium configurations of the filament, proving the persistence of flutter instability. We show that the emerging oscillations are nonreciprocal and generate thrust at an angle with the stimulus. The results we find at the level of the single cilium open new perspectives on the possible applications of artificial ciliary arrays in soft robotics and microfluidics.

This paper proposes and analyzes some simplified dynamic modeling methods for parallel robots. The proposed methods are based on a Lagrangian approach where the key concept consists in simplifying the expression of the energy for robots, especially for the distal links. First, the slender link method is discussed, where the link energy is calculated from the endpoints’ velocities. Then, the commonly used equivalent point mass (EPM) method is analyzed. Since the EPM method uses point masses to replace links, the main problem is how to assign the values of point masses. According to the energy equivalence principle, some methods are proposed to solve the value-assigning problem. The derivations of the proposed methods are given, and the errors caused by each method are analyzed. Based on the error analyses, some simple approaches are introduced to improve the accuracy of the EPM methods. Inverse dynamic models of a spatial robot are established based on the proposed methods. Then, detailed comparisons and analyses of the different dynamic models are given to show that the proposed methods are simple and have relatively high accuracy.

In order to adapt to the high speed of a high-speed permanent magnet synchronous motor (HSPMSM) used in hydrogen fuel cells, air foil bearings are applied to support the motor rotor. However, the stability of the motor rotor system supported by air foil bearings is relatively complex. In this article, the Castigliano’s second theorem and the theory of small deflection elastic thin plates are used to establish a deformation model of air foil bearings. Based on the generalized Reynolds equation, the stiffness and damping of the air foil bearings are calculated using perturbation methods and finite difference methods as the support model for the motor rotor. Subsequently, the transfer matrix of a typical disc-shaft unit is analyzed and obtained by using the transfer matrix method. On this basis, combined with Riccati transform, the natural frequencies and critical speeds of the motor rotor system in the spatial domain are calculated using MATLAB. And the unbalanced magnetic pull (UMP) is calculated by Maxwell stress tensor method, which is equivalent to the unbalanced mass and is applied to the disc it acts on to ascertain the unbalanced response, forming a theoretical model for calculating the dynamic performance of high-speed motor rotors. Then, the accuracy of the constructed model is verified by comparing the experimental results with a literature. Based on the estalished model, the effects of structural parameters and operating conditions on motor rotordynamics are studied, and the responses of the motor rotor system to unbalanced mass and unbalanced magnetic tension are compared. It is found that the critical speeds of the motor rotor system increase with the increase of the dynamic viscosity of air, and the motor can operate stably under the influence of the maximum UMP.

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To address the issues concerning energy conversion efficiency and total energy in the propulsion process of wave energy conversion devices in deep water environments, this study examines the array layout of the existing dual-buoy point-absorption wave energy converter. Two array layouts, namely linear and triangular, are proposed. The coupling hydrodynamic coefficient between the dual-buoy wave energy converter array and the wave action is determined using the linear wave hydrodynamic theory. The optimal PTO damping coefficient of the dual-buoy wave energy converter array can be determined by combining the multi-degree-of-freedom vibration theory and optimization method. In this study, we introduce the array impact factor to conduct a comprehensive investigation into the energy conversion characteristics of dual-buoy wave energy converter arrays with linear and triangular configurations. We examine the effects of various factors such as wave angles, float spacing, and array layout on the wave energy conversion of dual-buoy wave energy converter arrays, as well as the causes of interference. Furthermore, the optimal wave energy capture impact factor of the linear array initially increases and then decreases with the wave direction angle and the distance between the buoys. This suggests that there is an ideal wave angle and spacing arrangement that can enhance the efficiency of the array system. The peak values of the optimal wave energy capture interference factors of the triangular array are all greater than 1, confirming that the array arrangement effectively improves the energy capture efficiency of the device. These research findings provide a theoretical basis for the engineering application of deep-water wave energy utilization and serve as a foundation for optimizing and designing the layout of oscillatory buoy wave energy generation converters.

Exothermic autocatalytic reaction fronts propagating between two conductive walls shows an increase of speed and change in shape due to buoyancy driven convection. We modeled the system using reaction-diffusion-advection equations for chemical concentration and temperature. In these equations, a cubic autocatalytic exothermic reaction leads to a propagating front. The fluid flow is determined by the Stokes equation allowing for density changes due to thermal expansion. The front propagates in a liquid confined in a narrow rectangular domain resembling a two dimensional tube. Fluid motion enhances the front speed and modifies its curvature. In vertical domains, a transition to a nonaxisymmetric front takes place as the width increases. Heat conductivity across the walls delays the transition to larger critical widths. We find regions of bistability between nonaxisymmetric fronts and lower speed fronts at different values of conductivity. Heat losses diminish convection in horizontal tubes, resulting in a decrease of front speed.