Acta Mechanica Sinica最新文献

Within the context of global energy transitions, many wind turbines have been installed in desert and Gobi regions. Nevertheless, the impact of turbulence characteristics in actual sand-laden atmospheric flows on the aerodynamic performance of wind turbines has not been evaluated. The current study employs the high-quality wind velocity data measured in the Qingtu Lake Observation Array station of Min Qin to reveal the effects of turbulence characteristics in sand-laden atmospheric flows on the power and loads of a small wind turbine. The results demonstrate that turbulent coherent structures under sand-laden conditions occur more frequently and with shorter durations than that under the unladen conditions, leading to frequent and large fluctuations of wind turbine loads, specifically, the power, thrust, and blade root flapwise moment increased by 238%, 167%, and 194%, respectively. The predictions by applying the extreme turbulence model suggested that the maximum extreme thrust, blade root flapwise moment, and blade root edgewise moment of wind turbine under sand-laden conditions are 23%, 19%, and 7% higher than that under unladen conditions. This study is expected to provide a basic supply for wind turbine design and siting decisions in sand-laden environment.

The flow control at low Reynolds numbers is one of the most promising technologies in the field of aerodynamics, and it is also an important source of the innovation for novel aircraft. In this study, a new way of nonlinear flow control by interaction between two flexible flaps is proposed, and their flow control mechanism is studied employing the self-constructed immersed boundary-lattice Boltzmann-finite element method (IB-LB-FEM). The effects of the difference in material properties and flap length between the two flexible flaps on the nonlinear flow control of the airfoil are discussed. It is suggested that the relationship between the deformation of the two flexible flaps and the evolution of the vortex under the fluid-structure interaction (FSI). It is shown that the upstream flexible flap plays a key role in the flow control of the two flexible flaps. The FSI effect of the upstream flexible flap will change the unsteady flow behind it and affect the deformation of the downstream flexible flap. Two flexible flaps with different material properties and different lengths will change their own FSI characteristics by the induced vortex, effectively suppressing the flow separation on the airfoil’s upper surface. The interaction of two flexible flaps plays an extremely important role in improving the autonomy and adjustability of flow control. The numerical results will provide a theoretical basis and technical guidance for the development and application of a new flap passive control technology.

Flexoelectricity refers to the link between electrical polarization and strain gradient fields in piezoelectric materials, particularly at the nano-scale. The present investigation aims to comprehensively focus on the static bending analysis of a piezoelectric sandwich functionally graded porous (FGP) double-curved shallow nanoshell based on the flexoelectric effect and nonlocal strain gradient theory. Two coefficients that reduce or increase the stiffness of the nanoshell, including nonlocal and length-scale parameters, are considered to change along the nanoshell thickness direction, and three different porosity rules are novel points in this study. The nanoshell structure is placed on a Pasternak elastic foundation and is made up of three separate layers of material. The outermost layers consist of piezoelectric smart material with flexoelectric effects, while the core layer is composed of FGP material. Hamilton’s principle was used in conjunction with a unique refined higher-order shear deformation theory to derive general equilibrium equations that provide more precise outcomes. The Navier and Galerkin-Vlasov methodology is used to get the static bending characteristics of nanoshells that have various boundary conditions. The program’s correctness is assessed by comparison with published dependable findings in specific instances of the model described in the article. In addition, the influence of parameters such as flexoelectric effect, nonlocal and length scale parameters, elastic foundation stiffness coefficient, porosity coefficient, and boundary conditions on the static bending response of the nanoshell is detected and comprehensively studied. The findings of this study have practical implications for the efficient design and control of comparable systems, such as micro-electromechanical and nano-electromechanical devices.

In thermal protection structures, controlling and optimizing the surface roughness of carbon/phenolic (C/Ph) composites can effectively improve thermal protection performance and ensure the safe operation of carriers in high-temperature environments. This paper introduces a machine learning (ML) framework to forecast the surface roughness of carbon-phenolic composites under various thermal conditions by employing an ML algorithm derived from historical experimental datasets. Firstly, ablation experiments and collection of surface roughness height data of C/Ph composites under different thermal environments were conducted in an electric arc wind tunnel. Then, an ML model based on Ridge regression is developed for surface roughness prediction. The model involves incorporating feature engineering to choose the most concise and pertinent features, as well as developing an ML model. The ML model considers thermal environment parameters and feature screened by feature engineering as inputs, and predicts the surface height as the output. The results demonstrate that the suggested ML framework effectively anticipates the surface shape and associated surface roughness parameters in various heat flow conditions. Compared with the conventional 3D confocal microscope scanning, the method can obtain the surface topography information of the same area in a much shorter time, thus significantly saving time and cost.

Helmets exacerbate head injuries to some degree under blast load, which has been recently researched and referred to as the underwash effect. Various studies indicate that the underwash effect is attributed to either wave interaction or wave-structure interaction. Despite ongoing investigations, there is no consensus on the explanations and verification of proposed mechanisms. This study conducts experiments and numerical simulations to investigate the underwash effect, resulting from the interaction among blast load, helmets, and head models. The analysis of overpressure in experiments and simulations, with the developed simplified models that ignore unimportant geometric details, reveals that the underwash effect arises from the combined action of wave interaction and wave-structure interaction. Initially reflected in front of the head, the blast load converges at the rear after diffraction, forming a high-pressure zone. Decoupling the helmet components demonstrates that the pads alleviate rear overpressure through array hindrance of the load, resulting in a potential reduction of up to 36% in the rear overpressure peak. The helmet shell exacerbates the rear overpressure peak through geometric restriction of the load after diffraction, leading to a remarkable 388% increase in rear overpressure. The prevailing impact of the geometric restriction imposed by the shell of the helmet leads to a significant 57% increase in overpressure when employing a complete helmet.

The strength and endurance of human limbs can be enhanced through equipping exoskeletons or other types of wearable devices. However, long-time use of such devices may cause musculoskeletal disorders (MSDs) or potential injuries due to external shocks and vibrations. Consequently, preventing potential risks and enhancing comfortability are crucial to the design of exoskeleton. This research introduces a novel hybrid rigid-soft knee joint exoskeleton, which is well flexible and supported by two curved beams. This design is friendly and comfortable for wearers. The stiffness of the curved beam is meticulously calibrated to match the natural need of the knee joint, which provides appropriate support under vibration and impact. We employ the analytical modeling, finite element method (FEM), numerical analysis, and experimental approaches to analyze the static and dynamic properties of the knee exoskeleton system. The results confirm that the exoskeleton system exhibits reduced vibration transmissibility in low-frequency environments, and present a new methodology for the design and mechanical analysis of exoskeleton systems.

AISI 430 ferritic stainless steel is popular in modern industry, while conventional welding methods with filler metals produce welded joints with tensile strength (586 MPa) and elongation (7.35%), which is insufficient to meet the growing engineering requirements. In this work, the elongation of the joint is doubled (15.11%) while yield strength remains unchanged after post-weld heat treatment (PWHT). Microstructural analysis of heat affected zone (HAZ) reveals the transformation process between equiaxed ferrite, intergranular martensite, and intragranular acicular martensite in the welded joint at 750 °C and 800 °C. Additionally, molecular dynamics simulations demonstrate the impact of various types of martensite on single crystals of ferritic stainless steel under tension. The results indicate that intergranular martensite and acicular martensite demonstrate transgranular fracture, while granular martensite exhibits intragranular fracture. Intergranular martensite and granular martensite are distributed near high-strain regions within the crystal, whereas acicular martensite is concentrated at the grain boundaries, away from the high-strain regions. The comparison of hardening parameters for different types of martensite reveals that granular martensite (58.98) has higher ductility than acicular martensite (97.40) and intergranular martensite (111.54). These findings are valuable for developing advanced stainless steel welded joints that balance high ductility and strength, meeting modern engineering demands.

Lumbar degeneration leads to changes in geometry and density distribution of vertebrae, which could further influence the mechanical property and behavior. This study aimed to quantitatively describe the variations in shape and density distribution for degenerated vertebrae by statistical models, and utilized the specific statistical shape model (SSM)/statistical appearance model (SAM) modes to assess compressive strength and fracture behavior. Highly detailed SSM and SAM were developed based on the 75 L1 vertebrae of elderly men, and their variations in shape and density distribution were quantified with principal component (PC) modes. All vertebrae were classified into mild (n = 22), moderate (n = 29), and severe (n = 24) groups according to the overall degree of degeneration. Quantitative computed tomography-based finite element analysis was used to calculate compressive strength for each L1 vertebra, and the associations between compressive strength and PC modes were evaluated by multivariable linear regression (MLR). Moreover, the distributions of equivalent plastic strain (PEEQ) for the vertebrae assigned with the first modes of SSM and SAM at mean ± 3SD were investigated. The Leave-One-Out analysis showed that our SSM and SAM had good performance, with mean absolute errors of 0.335±0.084 mm and 64.610±26.620 mg/cm³, respectively. A reasonable accuracy of bone strength prediction was achieved by using four PC modes (SSM 1, SAM 1, SAM 4, and SAM 5) to construct the MLR model. Furthermore, the PEEQ values were more sensitive to degeneration-related variations of density distribution than those of morphology. The density variations may change the deformity type (compression deformity or wedge deformity), which further affects the fracture pattern. Statistical models can identify the morphology and density variations in degenerative vertebrae, and the SSM/SAM modes could be used to assess compressive strength and fracture behavior. The above findings have implications for assisting clinicians in pathological diagnosis, fracture risk assessment, implant design, and preoperative planning.

The ground-based experimental tests are crucial to verify the related technologies of the drag-free satellite. This work presents a design method of the ground simulator testbed for emulating the planar dynamics of the space drag-free systems. In this paper, the planar dynamic characteristics of the drag-free satellite with double test masses are analyzed and non-dimensionalized. A simulator vehicle composed of an air bearing testbed and two inverted pendulums is devised on the basic of equivalent mass and equivalent stiffness proposed firstly in this paper. And the dynamic model of the simulator equivalent to the sensitive axis motion of the test mass and the planar motion of the satellite is derived from the Euler-Lagrange method. Then, the dynamic equivalence conditions between the space prototype system and the ground model system are derived from Pi theorem. To satisfy these conditions, the scaling laws of two systems and requirements for the inverted pendulum are put forward. Besides, the corresponding control scaling laws and a closed-loop control strategy are deduced and applied to establishing the numerical simulation experiments of underactuated system. Subsequently, the comparative simulation results demonstrate the similarity of dynamical behavior between the scaled-down ground model and the space prototype. As a result, the rationality and effectiveness of the design method are proved, facilitating the ground simulation of future gravitational wave detection satellites.

The anterior cruciate ligament plays a crucial role in maintaining stability within the knee joint, particularly for athletes who frequently experience its rupture. This study presents a novel approach using personalized three-dimensional (3D) parametric finite element modeling of the knee joint to simulate the treatment following anterior cruciate ligament reconstruction (ACLR) in both forward walking (FW) and drop landing (DL) tasks. The study encompasses two distinct cohorts: five healthy athletes and five ACLR patients. Biomechanical motion analysis was conducted on both cohorts, with the ACLR patient group evaluated at 6 and 9 months post-surgery. A comprehensive 3D parametric model of the knee joint was meticulously crafted. The findings reveal a notable reduction in stress on crucial knee structures such as the autograft, meniscus, and cartilages over time for both FW and DL tasks following ACLR, with a reduction in tissue tension of approximately 9.5% and 37% for FW and DL, respectively. This personalized model not only facilitates the investigation of knee joint tissue biomechanics post-ACLR but also aids in estimating the return-to-sports timeline for patients. By accommodating individual tissue geometries and incorporating patient-specific kinetic data, this model enhances our comprehension of post-ACLR biomechanics across various functional tasks, thereby optimizing rehabilitation strategies.