Abstract: Compared with fixed wings, flapping wing has a significant aerodynamic performance advantage at low speed and low Reynolds number, which draws more and more attentions. Due ti the most studies focusing on rigid airfoil, the aerodynamic performance of flexible airfoil of flapping wing is still unclear. In this paper, a fluid-solid coupling model for a flexible elliptical airfoil is developed to analyze the flow field around the airfoil, deformation of airfoil, as well as the aerodynamic characteristics of airfoils, at different wind speed and attack angle. Compared with the rigid airfoil, the flexible airfoil can delay the shedding time of the wake vortex and reduce the oscillation frequency of the disturbance of lift force. The flexible airfoil significantly suppresses the disturbance of wake flow and reduces the oscillation amplitude of disturbance. Even, the disturbance oscillation can be completely eliminated at the appropriate Young's modulus airfoil. These results provide theoretical guidance for the design of soft aircraft.
Abstract: Vibration suppression performance of a damping composite structure depends on the material layout and the properties of damping material. This paper proposes a topology optimization method for damping material microstructure with varied volume constraints, which aims to obtain the damping material microstructure with the desired property under the smallest material consumption. Based on the homogenization method, a three-dimensional finite element model of the damping material is established, and the effective elastic matrix of damping material is formulated. The Hashin-Shrilman bounds theory is used inversely to estimate the volume fraction bound of the damping material corresponding to the desired effective modulus, and a movement criterion of volume constraint bounds for damping material is constructed. Then the optimization problem aiming to the desired property of damping material with microstructure is transformed into another problem of maximizing the desired modulus under volume constraints, and topology optimization model of the damping material microstructure is established. Optimality criteria method is employed to update the design variables, and the optimized topology configurations of damping material microstructure is obtained. The feasibility and effectiveness of the proposed method are verified by several numerical examples, and the influence of the initial configurations, mesh density and Young's modulus on the microstructure configurations of damping material are also discussed.
Abstract: The computation consumption of finite element analysis for structural optimization design of holding pole is large, and it is difficult to determine the feasible region. The response surface method (RSM) was used to simulate the real response of the holding pole, and an improved arithmetic optimization algorithm (IAOA) was proposed to optimize the holding pole. Fractional-order calculus was introduced into arithmetic optimization algorithm (AOA) to improve the exploitation ability of AOA. Latin hypercube sampling was applied to select the test samples of each member of the holding pole, and the least square method was employed to analyze the sample points. Then, the second-order response surface surrogate model of the stress and displacement of the holding pole on the cross-sectional size of each member was established. An optimization model was constructed with the minimum mass as the optimization objective and the allowable stress and displacement as constraints, and the IAOA was implemented to solve the model. The results show that the second-order response surface model can accurately predict the response value of the holding pole. The solution accuracy of the IAOA is significantly improved. The surrogate model can greatly decrease the calculation cost of finite element analysis. The mass of the holding pole is reduced 8.2% after optimization. The RSM and the IAOA can be combined to solve the optimization design problem of large spatial truss structures effectively.
Abstract: The radial basis function partition of unity method (RBF-PU) is applied to obtain the numerical solution of two-dimensional nonlocal diffusion and peridynamic problems. The main idea is partitioning the original domain into several patches and using the RBF approximation on each local domain, and then weight to obtain the global approximation of the unknown function. The radial basis function method based on the strong form of the equation has many advantages, such as avoiding an additional layer of integral calculation, no need to deal with intersections of neighborhoods with the mesh, and it is easy to implement. The numerical results show that this method can solve nonlocal diffusion equations and peridynamic equations accurately and efficiently.
Abstract: The circular membrane solar array has attracted extensive attention due to its high storage ratio and strong power supply capability. In order to adjust the tension of large film structure, a tension adjusting device composed of rope and spring is usually introduced. Its mechanical characteristics are highly nonlinear. But its influence has not been studied yet. In this paper, In order to study the influence of tension, a mechanism model is proposed in this paper. The nonlinear dynamics equation of the system with two-degree-of-freedom is established by using Lagrange energy method. Taking an engineering prototype as an example, the response of a tension mechanism with unsymmetrical ribs under resonance excitation is studied. The results show that the change of excitation amplitude has an important influence on the characteristics of the beat response of the system. It makes the response of the system appear chaotic, almost periodic and multifold periodic phenomena. These results have an important reference to the parameter design of tension mechanism.
Abstract: The grazing-induced non-smooth dynamical behaviors of a single-degree-of-freedom cantilever beam system with bilateral elastic constraints are studied. Firstly, based on the dynamical equations of cantilever beam with soft impact and the definition of grazing points, the existence condition of bilateral grazing periodic motion is analyzed. Secondly, the zero-velocity Poincaré section is selected to derive the high-order discontinuous mapping with parameters near bilateral grazing orbits. Then a new composite piecewise normal form mapping is established by combining smooth flow mapping and high-order discontinuous mapping.Finally,the validity of the high-order mapping is verified by comparing the bifurcation diagram of the low-order mapping with the high-order mapping, and the grazing dynamics of the cantilever beam with soft impact are further revealed through numerical simulation.
Abstract: Considering the phenomenon of competing water resources between young vegetation and adult vegetation in arid and semi-arid areas, a vegetation-soil water dynamic model with intraspecies competition delay is established. We analyze the conditions for the existence of an unique vegetation survival equilibrium and the local stability of the vegetation extinction equilibrium. The generating conditions of Hopf bifurcating periodic solutions for non-spatial and spatial systems are given, respectively. The periodic oscillation pattern of vegetation evolutes with time is shown by numerical simulations. Through the parameter sensitivity analysis, it is found that the rainfall and vegetation growth rate have significant influences on generation, amplitude and period of this pattern, but the effect of evaporation is the least significant. These results indicate that rainfall and vegetation properties have profound effects on the evolution and development of vegetation in arid and semi-arid areas. It is found that the introduction of spatial diffusion inhibits the occurrence of this pattern, but doesn't affects on amplitude and period. The results obtained explain the phenomenon of vegetation periodic oscillation widely observed in nature, which provide theoretical support for the sustainable development of vegetation system.
Abstract: As a newly emerged light porous material, carbon aerogels are a class of carbonaceous solid material with high porosity, low density and excellent environmental stability. With the combination of high elasticity, high energy absorption, as well as special properties such as shock absorption, sound absorption and electromagnetic shielding, carbon aerogels are both functional and structural widely applied in the fields of flexible sensors, energy equipment, acoustic equipment and environmental protection. However, the existing general conflicts between mechanical robustness and intrinsic sparse network in porous aerogel materials have been a common challenge faced by researchers ranging from fields of material science, solid mechanics, design application and so on. Good robustness could ensure the structural integrity and performance stability of aerogels in the application process, while sparse network is the prerequisite to ensure the lightweight and porous structure of aerogels. Here, we discuss the recently emerged strategies for optimizing the mechanical robustness, including cell-wall strengthening, cell-wall orientation, pore topology controlling and joint reinforcement. Specially, we summarize the advanced design principles to realize the tensile elasticity in ultra-light all-carbon aerogels without intrinsic stretchable elastomers. In addition, we briefly overview the recent applications of robust carbon aerogels and outlook the problems to be solved in this field.
Abstract: Aiming at the active thermal protection with lattice sandwich structure, an unsteady heat transfer theoretical model which couples facesheet and core heat conduction and coolant convection in the sandwich structure was established, the model equations were discretized by finite volume method and solved iteratively in Matlab; the constriction thermal resistance between the facesheet and the lattice strut was considered for the first time in the model, and the approximate analytical solution of the constriction thermal resistance was obtained using the method of separation of variables; based on the unit-cell model and periodic boundary conditions, the heat transfer coefficients hb and hfin required by the model were first obtained by numerical simulation. Finally, a case study with a multi-cells structure was carried out to compare the numerical and theoretical results, and the influence of constriction thermal resistance on the prediction accuracy was discussed. The results show that the theoretical model can accurately predict the temperature variation of the sandwich structure and the internal fluid, and the maximum deviation between theory and simulation is less than 1%. As the external heat flux increases, the error of theoretical prediction increases when the constriction resistance is ignored. Compared with the numerical simulation, the theoretical model can significantly reduce the calculation time and save calculation resources, thus it is especially suitable for actively cooled lattice sandwich structure subjected to complex, non-uniform and unsteady thermal load.
Abstract: In order to analyze the load-bearing capacity and failure modes of carbon fiber reinforced polymer composite honeycomb sandwich structures with curved wall under three-point bending load, theoretical prediction, numerical simulation and test were carried out for structures with different core height and face panel thickness. According to the main failure modes of sandwich structure, different theoretical prediction formulas and failure mechanism diagrams were firstly made. Then, the numerical simulation model of the CFRP sandwich structure with honeycomb core was established to simulate its failure behavior under three-point bending load. Finally, different sizes of CFRP sandwich structures were fabricated by molding process, and the experimental results were compared with theoretical and simulation results. The results show that the bearing capacity of sandwich structure is positively correlated with the height of core and thickness of face panel, and the stiffness of core and face panel decreases with the decrease of size, which results in the structural failure mode changing from debonding between core and face panel to face crushing.