材料弹性对降落伞充气展开力学性能影响

Effect of material elasticity on the mechanics of opening a parachute

降落伞充气展开过程涉及柔性结构非线性变形与流场的高度耦合。为研究不同弹性的纬向加强带对降落伞充气展开过程中力学特性的影响,该文基于流固耦合(fluid-solid interaction, FSI)方法对3种弹性纬向加强带构型的带条伞进行充气展开过程模拟,获得了充气展开过程中气动力、纬向加强带张力和伞衣应力分布等关键参数,分析了织物材料弹性对降落伞开伞动载以及局部受力情况的影响,并通过风洞试验对带条伞充气展开过程的气动力变化进行测试,验证了利用FSI方法预测降落伞动态力学及局部受力特性的可行性。仿真和试验结果表明:纬向加强带弹性模量对降落伞充气展开过程中整体气动力影响甚微,对于伞衣和纬向加强带本身的应力分布影响显著;采用与伞衣相同材料的锦丝纬向加强带能显著降低局部应力水平,相较于高弹性模量的芳Ⅲ纬向加强带,纬向加强带和伞衣的最大应力分别降低83.3%和22.8%。

[Objective] Parachutes are aerodynamic devices widely used in the deceleration and landing stages of a spacecraft.The opening process is the most critical working scenario of a parachute, wherein structural damage often occurs due to a significantly high aerodynamic load. Generally, the peak aerodynamic load during inflation and deployment of a parachute can reach more than 1.5 times the load at its steady state.Because of the high elasticity and damping characteristics of the flexible fabric materials comprising the parachute, the peak dynamic load can be effectively reduced. Thus, it is extremely important to accurately predict the mechanical characteristics of fabric materials during the opening process. [Methods] In this paper, a numerical simulation method is used to study the mechanical characteristics of the fabric material in the inflation and deployment of a conical ribbon parachute. The opening process of a parachute involves a strong coupling effect of the nonlinear flexible structure and flow field. To investigate the influence of latitudinal reinforcing bands with different elasticities on the mechanics of a ribbon parachute, the opening processes of parachutes with different elastic and without latitudinal bands are simulated using the fluid-solid interaction(FSI) method. In the three parachutes, nylon and aramid fiber Ⅲ are used for latitudinal reinforcing bands in the first and second parachutes, respectively, the third has no latitudinal reinforcing band. The arbitrary lagrange-euler(ALE) method is applied to simulate the opening process, and the penalty-function method is used to demonstrate the force and displacement information between the canopy and flow field elements. The numerical simulation process is performed based on the LS-DYNA solver with the single-machine-distributed parallel computing strategy. Based on the simulation results of aerodynamics, latitudinal reinforcing band tension, and parachute canopies stress, the effect of the fabric material elasticity on the dynamic load of a parachute during the opening process is analyzed. Eventually, the aerodynamics of the parachute without latitudinal bands during the opening process is tested using the wind tunnel test, and the feasibility of predicting the mechanics of a parachute by the FSI method is verified.[Results] The simulation and experimental results showed that the elastic modulus of latitudinal bands had a nominal effect on the overall aerodynamics but had a significant effect on the stress of the canopies and latitudinal bands during the opening process. During the opening of a parachute, the maximum stress in the canopy appearaled soon after the reefing stage ends, and the projected area and aerodynamic load of the canopy increased exponentially at this time. Compared with aramid fiber Ⅲ and without latitudinal bands, the maximum stress of the canopy with a nylon configuration was reduced by 22.8% and 11.5%, respectively. Additionally, the maximum stress of the latitudinal bands and canopies made of nylon were reduced by 83.3% and 22.8%, respectively, compared to those made of aramid fiber Ⅲ. [Conclusions] Based on the finite element method, three dynamic models of conical ribbon parachutes with different latitudinal band configurations are established. Numerical analysis of the opening process is performed, and the part-delete method is introduced to simulate the disreefing process.The parachute opening process can be effectively simulated using the FSI method, which is later applied in the selection of materials and optimization of the parachute design.