Acta Mechanica Sinica最新文献

To thoroughly examine the complex relationships between tire and pavement vibrations, a sophisticated vehicle-pavement coupled system is proposed, incorporating a non-uniform dynamic friction force between the tire and the pavement. According to the Timoshenko beam theory, a dynamic model of pavement structure with a finite length beam was formulated on a nonlinear Pasternak foundation. To more accurately describe the coupling relationship between the tire and the pavement, and to take into account the vibration state under vehicle-pavement interaction, the load distribution between the tire and the pavement is modeled as a dynamic non-uniform contact. Combined with the classic LuGre tire model, the adhesion between the tire and the pavement is calculated. The Galerkin truncation method is employed to transform the pavement vibration partial differential equation into a finite ordinary differential equation, and the integral expression of the nonlinear foundation beam term is derived using the product to sum formula. By using the Runge-Kutta method, the tire-road coupled system can be numerically calculated, thus determining tire adhesion. This research demonstrates that compared with tire force under the traditional static load distribution, load distribution has a significant influence on adhesion. This study offers valuable insights for pavement structure design and vehicle performance control.

This paper presents a novel modelling method to study the thrust generation mechanism of biplane flapping wings made of thin and highly deformable membrane. Based on the principle of strain energy equivalence, the membrane structures were modelled by mass-spring systems. The aerodynamic loads were calculated by a simplified quasi-steady aerodynamic model with consideration of the clap-and-fling mechanism. The impact force was introduced into the system when two wing surfaces were in contact. For wing-dynamics simulation problems, convergence analyses were conducted to obtain suitable mesh resolution. To validate the present modelling method, the predicted thrust and required power of a biplane flapping-wing air vehicle were compared with the experimental data. The effect of the forward speed was also analyzed in this paper. It was shown that as the forward speed increases the thrust production efficiency becomes lower together with smaller wing deformation.

Both the thickness effect and surface effect should be important in nano-indentation behavior of coatings due to the finite thickness and small indentation size. As a basic solution, the two-dimensional Boussinesq problem of a finite elastic layer bonded to a rigid substrate is studied in this paper, employing the surface-energy-density-based elastic theory. The Airy stress function and Fourier integral transform methods are adopted to solve the problem. A nalytical solutions of both the stress and displacement fields are well achieved for a finite elastic layer under a concentrated force and a uniform pressure. Unlike the classical solutions, it is discovered that both the thickness effect and surface effect will show significant influences on the Boussinesq elastic behaviors. The surface effect would harden the finite elastic layer and induce a more uniformly distributing displacements and stresses. Only when the thickness is sufficiently large, the Boussinesq solution of an elastic half space may represent that of a finite elastic layer case. A generalized hardness is further defined to include the coupling effects of thickness and surface for the Boussinesq problem of a finite elastic layer. Such a study would assist in the design and property evaluation of coatings and micro-devices with layer-substrate structures.

The task of achieving high-accuracy full-field reconstruction in the realm of water waves is widely acknowledged as a challenge, primarily due to the sparsity and incompleteness of data measurement in both temporal and spatial dimensions. We develop a full-field velocity and pressure reconstruction approach for non-linear water waves based on physics-informed neural networks from the free surface measurement. The fully non-linear highly dispersive Boussinesq model is integrated to reduce the training cost by representing the three dimensional water wave problems in the horizontal two-dimensional plane with the inherent velocity distribution along water depth. A series of test cases, including the solitary waves, fifth-order Stokes waves, standing waves, and superimposed waves, are employed to evaluate the performance of the algorithm. The proposed novel neural networks are capable of accurately reconstructing the flow fields even when assimilating the limited and sparse free surface deformation data, which facilitates the development of detecting the flow characteristics in real ocean waves.

Bends contribute to a flexible layout of pipeline system, but also lead to intensive energy costs due to the complex flow characteristic. This experimental study is conducted to investigate the impact of a single coarse particle on the flow field in a bend. The velocity profiles of fluid on the axial symmetry plane of the bend are measured using time-resolved particle image velocimetry. The flow structures are extracted using the proper orthogonal decomposition method. The results reveal that there is a shear-layer flow in the bend during the transportation. With the increase in particle size, the particle has a dominant influence on the flow energy distribution of the overall flow. The impact of particles on the first few energetic flows is mainly in the latter part of the transportation, both temporally and spatially. As the particle size decreases, the most energetic unsteady flow within the bend changes from the convective flow to the flow of the shear layer.

This investigation aims to analyze thermal buckling and post-buckling behavior of functionally graded graphene nanoplatelet-reinforced composite (FG-GPLRC) beams. The beams are classified into two types of ideal and non-ideal FG-GPLRC beams in which the ideal beams have smooth profiles of material distributions and another beams have layer-wise distributions of materials. The material profiles of the ideal beams are utilized as the controlling tracks for producing the material distributions of the non-ideal beams via a layer-to-layer integration technique. This technique confirms that the overall weight fraction of the materials is the same for both types of beams. The proposed models can be used to determine the material properties of the beams for further investigation on thermal buckling and post-buckling of the beams. Third-order shear deformation theory is employed to construct the energy equations of the problems, and then they are solved by the implementation of the Jacobi-Ritz method cooperating with the direct iteration procedure and Newton-Raphson technique. From our investigation, it can be disclosed that when non-ideal beams are created using ideal beams parabolic profile, the results differ significantly. However, the differences between the results of ideal and non-ideal beams can be eliminated by adding more layers.

This paper investigates the effect of the angle-of-attack (AOA) on the windward boundary-layer stability over a blunt cone with a nose radius of 5 mm. We consider a free-stream Mach number of 6 and a unit Reynolds number of 4.0 × 10⁷ m⁻¹ and conduct both wind-tunnel experiments and stability analyses for AOAs ranging from 2°–10° at intervals of 2°. The results suggest that, as the AOA increases, the pressure gradient across the spanwise range becomes more pronounced, and the windward-side boundary layer becomes thinner. Using bi-global stability analysis, two unstable three-dimensional modes (varicose and sinuous) are identified in the windward boundary layer at various AOAs. The most unstable mode is the varicose V1 mode, in which the amplitude peak is initially close to the windward centerline and gradually shifts to the centerline downstream. Hence, the primary unstable disturbance exhibits a “V-like” distribution along the streamwise direction, which is likely to cause the V-shaped transition front observed in the wind-tunnel experiments. The eN method based on bi-global analysis is used to predict the transition location along the centerline on the windward region of the cone. The results indicate that, as the AOA increases, the transition location shifts forward, in line with our experimental results. Moreover, linear stability theory accurately predicts the eigenfunction and growth rate of the V1 mode obtained from bi-global analysis. This indicates that linear stability theory can be used to predict transitions in the windward boundary layer of a blunt cone at large AOAs under the conditions studied in this paper.

Buoyancy-driven flows are prevalent in a wide range of geophysical and astrophysical systems. In this review, we focus on three pivotal effects that significantly influence the dynamics and transport properties of buoyancy-driven flows and may have implications for natural systems. These effects pertain to the role of boundary conditions, the impact of rotation, and the effect of finite size. Boundary conditions represent how the fluid flow interacts with different kinds of surfaces. Rotation, as the Earth’s rotation in geophysical systems or the whirling of astrophysical systems, introduces Coriolis and centrifugal forces, leading to the profound vortical structure and distinct transport property. Finite size, representing geometrical constraints, influences the behavior of buoyancy-driven flows across varying geometrical settings. This review aims to provide a holistic understanding of the intricate interplay of these factors, offering insights into the complex natural phenomena from the perspectives of the three effects.

The state estimation of the flexible multibody systems is a vital issue since it is the base of effective control and condition monitoring. The research on the state estimation method of flexible multibody system with large deformation and large rotation remains rare. In this investigation, a state estimator based on multiple nonlinear Kalman filtering algorithms was designed for the flexible multibody systems containing large flexibility components that were discretized by absolute nodal coordinate formulation (ANCF). The state variable vector was constructed based on the independent coordinates which are identified through the constraint Jacobian. Three types of Kalman filters were used to compare their performance in the state estimation for ANCF. Three cases including flexible planar rotating beam, flexible four-bar mechanism, and flexible rotating shaft were employed to verify the proposed state estimator. According to the different performances of the three types of Kalman filter, suggestions were given for the construction of the state estimator for the flexible multibody system.