Meccanica最新文献

In this paper, we analyze the local linear stability of plane Poiseuille flow of an upper convected Maxwell (UCM) fluid through a periodic channel under two flow regimes, i.e., inertial (Re (ne) 0) and purely elastic (Re (equiv) 0). The analysis is conducted with respect to the dimensionless control parameters: Reynolds number (Re), elasticity number (E), and Weissenberg number (We). We focus on the stability of two-dimensional perturbations, using spectral methods and Chebyshev collocation to discretize the dispersion equations. For creeping flow, we perform a numerical study to explore the combined effects of periodic modulation ((epsilon)), section (x), and control parameters (E, We) on the stability of UCM fluid flow, and to examine the elasto-inertial interplay in flow stability. Our results reveal two key findings: first, the existence of a critical position ((x_{c})=(frac{pi }{2n})) and ((x_{c})=(frac{3pi }{2n})) for small wavenumbers (n); and second, insights into the structure of the full elasto-inertial eigenspectrum, consisting of multiple discrete modes influenced by the section (x) and channel amplitude ((epsilon)).

Jet dispensing is a foundational technology in microelectronics packaging. Piezoelectric-driven jet dispensing involves short-distance jet processes, typically ranging between 0.7 and 1.2 mm. During this process, jet breakup occurs after the droplet contacts the substrate, leading to substrate contamination, irregular adhesive dot shapes, and various other challenges. The underlying mechanism of jet breakup is complex, and the formation mechanism for satellite droplets remains unclear. To address this issue, in this paper, a certain epoxy resin adhesive is investigated to analyze the mechanism of satellite droplet formation during short-distance jet dispensing through experiments, theoretical model, and numerical simulations. The results indicate that the second jet breakup above satellite droplets is most likely to form scattered spots, and the probability of satellite droplet generation is negatively correlated with jet breakup time. Reducing jet velocity, lowering jetting height, and adjusting nozzle temperature can effectively increase jet breakup time and reduce the generation of satellite droplets. Additionally, decreasing jet velocity by modifying parameters such as the length-to-diameter ratio of the micro-jet orifice and the nozzle cone angle can help suppress satellite droplet formation.

Free vibrations of third-order nanobeams are investigated by exploiting an effective nonlocal methodology. Notably, the stress-driven nonlocal theory, which is a consistent tool for modeling nanostructures, is here generalized and combined with a refined shear deformation beam theory. The need for shear correction factors, which is a crucial point in nonlocal continuum mechanics, is thus bypassed by adopting a third-order nanobeam theory. The governing elastodynamic problem is mathematically described by an integro-differential formulation. An equivalent purely differential problem is derived to reduce computational burdens. Parametric analyses are carried out to investigate size dependent free vibration responses. The relevant eigenproblem is thus solved for exemplar structural schemes assessing the relative nonlocal natural frequencies and eigenfunctions. The obtained numerical outcomes can be conveniently exploited in modeling and design of ultrasmall components of smart devices.

The introduction of micro-texture on gear tooth surface can potentially enhance the contact fatigue life and operational reliability of the gear pair, and is attracting increasing attention in an effort to provide improved tribological properties and contact performances. In this work, a new tribological model of micro-textured gear tooth in elastohydrodynamic lubrication (EHL) contact considering the coupled effect of rough surface topography is developed. The combined effect of micro-texture, surface roughness topography, elastic deformation of the tooth surface and lubrication on the contact characteristics of the meshing interface are included to obtain a revised oil film thickness equation. The rough surface contact of gear pair is characterized by the real rough morphology of gear tooth. The coefficient of friction at the transient meshing point of the micro-textured tooth surface is derived with the effect of flash temperature included. The sub-surface stress–strain distributions of the micro-textured gear are determined and the contact fatigue life is evaluated based on the Brown-Miller-Morrow multiaxial fatigue life criterion. Effects of surface roughness and micro-texture parameters on the lubrication behavior, friction coefficient and fatigue life are investigated and discussed. Experimental validation is performed and good agreement is observed between the model predictions and experimental results.

Spherical contact under combined normal and cyclic tangential loading is a simple but representative issue in contact mechanics. This work developed a generalized three-phase constitutive model considering elastic, yield plateau, and strain hardening characteristics and contributed an energy conservation model of quasi-static contact under various sliding conditions. The von Mises stress, equivalent plastic strain, contact behaviors, and energy components were examined under different material parameters and loading conditions. The results indicate that the stress concentration zone appears in the heading area of the sliding direction and vanishes in the trailing area; junction growth derives from a new increased zone and an accumulated plastic deformation inside the original contact zone; large normal force decrement, substantial junction growth, and considerable energy dissipation occur in the material more prone to higher plastic deformations; there are more complex responses related to single frictional dissipation and plastic dissipation.

This paper focuses on a novel and computationally efficient large-displacement methodology utilizing a corotational approach for the stability analysis of masonry structural elements. By virtue of the flexibility offered by the Virtual Element Method, each brick is modeled using a single Virtual Element. In contrast, the mortar layer is modeled through multiple cohesive damage-frictional elements. Furthermore, the adopted Virtual Element formulation does not require stabilization. The advantages of the proposed approach are showcased through several examples demonstrating the striking accuracy of the obtained results compared to analytical solutions. The proposed approach is used to assess the sensitivity of the load-bearing capacity and ductility of masonry walls under vertical loading to mortar tensile strength, boundary conditions, load eccentricity, and block irregularity.

To optimize the efficiency and stability of wheat pneumatic conveying systems, this study systematically analyzed the flow characteristics under varying air-to-solid ratios using the CFD-DEM (Computational Fluid Dynamics—Discrete Element Method) approach. The results demonstrate that as the gas velocity increases, the flow regime within the pipe transitions progressively from plug flow to dune flow, and ultimately to stratified flow. The standard deviation of inlet pressure fluctuations was highest under dune flow conditions, followed by plug flow, and lowest during stratified flow, while the system stability showed the inverse trend. Wheat particle velocity is highest under stratified flow conditions, followed by dune flow, and lowest during plug flow. The distribution of wheat particles transitions from a fully filled pipe state to a depositional state at the pipe bottom. Increasing the wheat feed rate reduced both the particle velocity growth rate and system stability. This study demonstrates the critical role of the air-to-solid ratio in regulating flow regimes to influence conveying stability and efficiency, providing direct guidance for optimizing air-to-solid ratio settings in wheat processing lines.

In this paper, an efficient nonlocal higher-order shear deformation beam theory is developed for bending of nanobeams based on the stress-driven model. The theory accounts for several distributions of the transverse shear strains that satisfy the zero traction boundary conditions on the surfaces of the beam. Hence, it is not necessary to use a shear correction factor. Collecting the higher-order shear deformation beam theories and the stress-driven nonlocal model, the equations of nonlocal elastic equilibrium are consistently derived. Hence, it is shown that the stress-driven model is well-posed for higher-order shear deformation theories. The accuracy of the present approach is verified by comparing the obtained results with existing solutions. It can be concluded that the present nonlocal stress-driven approach for higher-order shear deformation theory is not only accurate but also simple in predicting the bending behavior of nanobeams.

Air-core vortices occurring at intakes cause efficiency losses, vibrations, operational difficulties, and erosion at affiliated water-conveying structures. Air-core vortices are in the forms of non-air entraining vortex (air-core vortex in suspension) and air-entraining vortex. The profile of an air-core vortex is considered to be one of the main characteristics of the vortex. There are available semi-empirical formulas for the profile of a vertical non-air entraining vortex (air-core vortex in suspension) occurring at a vertically-flowing downward intake. However, there is no available developed formula relating to the profile of a vertical air-entraining vortex occurring at a vertically-flowing downward intake because the height, radii, and other physical quantities of the imaginary section of the air-entraining vortex downstream of the intake entrance are not measurable. Therefore, the profile of a vertical air-entraining vortex needs to be predicted. In the present study, by modifying the available formula relating to the profile of a non-air-entraining vortex and incorporating available test data, a practical methodology is developed for predicting the profile of a vertical air-entraining vortex. This study provides a practical formula and a chart to determine the necessary parameters to predict the profile of a vertical air-entraining vortex. The validation of the proposed methodology is examined and checked with available test data relating to the profile of a vertical air-entraining vortex occurring at a vertically-flowing downward intake. The results of the present study are in good agreement with available test data relating to the profile of vertical air-entraining vortices (the coefficient of determination is between 0.976 and 0.995).