力学学报

RESEARCH ON PARTICLE MOTION CHARACTERISTICS UNDER HYPERSONIC MARS ENTRY ENVIRONMENT

Xing Haoyun, Liu Zhuo, Wang Qiu, Zhao Wei, Gao Liangjie, Liu Zhongchen, Qian Zhansen

Dust storms of varying degrees frequently transpire within the Martian atmosphere, and the dust particles present in the atmosphere will cause erosion on the surface of high-speed entering Mars vehicles, leading to increased wall heat flux. Consequently, the design of the vehicle's thermal protection system is confronted with a formidable challenge. In this paper, focusing on the two-phase flow problem in the hypersonic Mars entry environment, a non-equilibrium flow field and particle one-way coupling calculation method based on the Euler-Lagrange framework are established. Moreover, a Mars atmospheric particle distribution model with a modal radius of 0.35 μm is adopted to investigate the motion trajectories of particles with different sizes in the flow field. The effects of the high temperature phase change model on the particle motion and the impact energy distribution of particles with different particle sizes were obtained. The numerical simulation results show that particles are prone to melt or even vaporize during their moving in high-temperature flow fields, and it was confirmed that the high-temperature phase change model engenders a more pronounced effect on the trajectory of smaller particles due to their diminished dimensions. Conversely, particles with diameter above 3 μm exhibited a larger Stokes number, and their motion trajectory remained relatively unaffected by the surrounding flow field, and the radii of these particles remained relatively constant during motion. Particles with a diameter larger than 3 μm account for more than 95% of the impact fraction on the wall, which is the main source of wall impact. The results of the impact energy fraction indicate that particles with diameters between 3 and 10 μm are the main source of impact energy, accounting for approximately 80% of the total impact energy.

, Available online , doi: 10.6052/0459-1879-23-192

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COUPLED DYNAMICS AND GAIT OPTIMIZATION OF ROBOT ON-ORBIT ASSEMBLY STRUCTURE

Robotic assembly in orbit is one of the most promising ways to build large spacecraft, but there are serious dynamical coupling effects between the two when robots work on the surface of space structures, which brings new challenges to space structure construction. A robot-structure coupled dynamics modeling and gait optimization method is proposed for a coupled dynamics problem formed by a three-branch robot walking on a spatially flexible structure. First, a coupled dynamics model was established based on the Lagrangian equation and the Euler-Bernoulli beam model, which can be used to predict the coupled dynamics response of the robot when walking on the surface of the structure. Then, the relationship between robot motion and structural vibration is derived based on the coupled dynamics equation, and the effects of different walking modes of the robot on the dynamic response of the spatial structure are studied; and the optimization study of robot walking gait is carried out. Finally, the numerical simulation of the dynamic response of the space structure under the creeping gait motion of the robot is given with a three-branch robot walking on a cantilevered space structure as an example. The results show that the dynamic response of the space structure is closely related to the robot motion gait, and the faster the step frequency, the longer the step length and the higher the lifting height, the more significant the structural vibration will be. By optimizing the robot gait, the structural vibration can be effectively suppressed.

, Available online

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STUDY ON FRICTION BEHAVIOR OF FRACTAL ROUGH SURFACE BASED ON MOLECULAR DYNAMICS-GREEN’S FUNCTION METHOD

The surface friction between any object can be regarded as the friction between rough surfaces, and most rough surfaces have fractal characteristics. In order to study the friction behavior of fractal rough surface, the molecular dynamics-Green’s function method (GFMD) is used to establish the microscopic fractal rough surface. The contact and friction processes of fractal rough surface are controlled by displacement loading, and the contact cluster distribution is identified by breadth-first search algorithm. Then, the maximum friction coefficient and friction force at atomic scale, contact cluster scale and interface scale are calculated respectively. The influence matrix method is used to study the interaction between contact clusters in the friction process, and the influence of the distance between contact clusters and the area of contact clusters on the interaction is analyzed. The results show that the friction coefficient decreases from small scale to large scale during the friction process. The friction force fluctuates periodically with the displacement. Contact clusters don’t reach the maximum friction force at the same time, but local slip occurs. The friction force obtained by the global slip model is the upper limit of the molecular simulation results. The influence matrix method can simulate the interaction of contact clusters well. The friction force calculated by using the influence matrix is basically consistent with the result of GFMD model, while the friction force calculated by ignoring the influence of local slip is 20% larger than the result of GFMD model. The interaction between contact clusters is inversely proportional to the distance and proportional to the area. The results can provide theoretical basis for interface analysis and optimization of rough surface.

, Available online

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TOPOLOGY OPTIMIZATION METHOD FOR INTEGRATED THERMAL PROTECTION STRUCTURE CONSIDERING TRANSIENT TEMPERATURE AND STRESS CONSTRAINTS

Li Shuai, Zhang Yongcun, Liu Shutian

The integrated thermal protection structure is usually in a severe unstable thermal environment, and the time effect of thermal load, namely transient thermal effect, is obvious. In order to avoid huge calculation consumption of transient thermal analysis, previous optimization design studies of integrated thermal protection structures usually equivalent transient heat transfer to steady-state heat transfer under the same thermal boundary conditions, and take the temperature field of steady-state heat transfer analysis as the design thermal load. However, previous studies have shown that the steady-state heat transfer cannot accurately equivalent the effect of transient heat transfer, and the transient thermal effect has an important influence on the structural design results. In this paper, the optimization design problem of integrated thermal protection structure considering transient thermal effect is studied, and a topology optimization method of integrated thermal protection structure considering transient temperature and stress constraints is established. Based on the Solid Isotropic Material with Penalization (SIMP) method, two kinds of topology optimization models for integrated thermal protection structures are constructed: 1. The stiffness design model taking minimizing the structural strain energy as objective function, considering material volume fraction, maximum stress and maximum bottom-face temperature constraints. 2. The strength design model taking minimizing material volume fraction as objective function, considering maximum stress and maximum bottom-face temperature constraints. By solving the transient thermodynamic coupling equation, the thermodynamic coupling static analysis results of the structure are obtained. The maximum value of structural response in time domain is represented by the condensed integral function in space and time domains, which was taken as constraint and objective functions. The sensitivity expressions of objective function and constraint functions are derived by adjoint method. The effectiveness of the proposed method is verified by three numerical results. Numerical examples showed that the proposed method could accurately reflect the influence of transient thermal effects on the design results of integrated thermal protection structures under the condition of transient heat transfer. Compared with the design results based on steady-state thermal analysis, the design results considering transient thermal effects were significantly improved.

, Available online , doi: 10.6052/0459-1879-22-598

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