Reliability Algorithms And Structural Buckling Reliability Research Of Supercavitating Vehicle

The researches are less on structure problems of supercavitating vehicles, especially the researches about structural reliability analysis of supercavitating vechiles are few. Compared to traditional underwater vehicles which mainly suffer around hydrostatic pressure due to low velocity, it is different that super- cavitating vechiles which suffer high cavitator drag and engine thrust due to high velocity. To satisfy hydrodynamics requirements and be entirely enveloped by supercavity, generally it has to be designed as slender configuration. However, the high longitudinal force and circumferential pressure caused by ventilated cavity when operation depth is large, may cause structure buckling of supercavitating vehicles. When the uncertainty of supercavitating flow and structural own parameters is considered, it is necessary to perform structural reliability analysis of supercavitating vehicle. Structural buckling probability and non-probability reliability analysis of supercavitating vehicle is performed in this paper and the main contents are as follows:1. Iterative non-convergence problem of modified first-order second- moment method is discussed. To promote the robustness of limit step iteration method, golden section method and a new merit function are introduced into limit step iteration method for one-dimension step search and modified limit step iteration method is presented. Compared to the iterative results of another modified iteration method, modified limit step iteration method shows better convergence.2. In view of the insufficiency of both performance function ratio index and volume ratio index of super-ellipsoid convex sets under two conditions that stress sets and strength sets interfere or don't interfere with each other, super-ellipsoid convex sets reliability comprehensive index is presented by combining two above ratio definition index. Super-ellipsoid convex sets reliability comprehensive index is calculated by combined method of modified limit step iteration method and Monte-Carlo method. In view of difficulty to calculate the volume ratio definition index when limit state equation is multi-dimensional nonlinear equation, Monte-Carlo method is introduced to calculate super-ellipsoid convex sets reliability degree. The validity, feasibility and simpleness of calculation by Monte-Carlo method are proved by numerical example.3. As same as super-ellipsoid convex sets, non-probabilistic interval reliability comprehensive index is also presented and it is calculated by combined method of modified chaotic optimization method and Monte-Carlo method. Based on Skew-Tent mapping formula, four modified chaotic optimization methods such as ST-Geczb, ST-Powell, BST-Geczb and BST-Powell are presented. Compared to the search results of relative literature, modified chaotic optimization methods such as ST-Powell and BST-Powell show better global search optimization rate. The values and variety trends of non-probabilistic interval and super-ellipsoid convex sets reliability comprehensive index are compared under three uncertainty information described types such as interval sets, internal and external connect super-ellipsoid convex sets which are determined by interval sets.4. Critical buckling load of supercavitating projectile, which is simplified as variable cross-section beam, is calculated by Galerkin method. The partial matrixs of buckling safety margin implicit equation to each random variables are deduced, and structural buckling probabilistic reliability index of variable cross-section beam is calculated by combining with limit step iteration method. Structural buckling non-probabilistic interval reliability comprehensive index of super- cavitating projectile is calculated by combined method of modified chaotic optimization method BST-Geczb and Monte-Carlo method. Through the analysis of engineering numeric results, it is presented that the influence of the ratio which is defined as base diameter to the cavitation diameter and the mean value of initial launch velocity to buckling probabilistic reliability index and non-probabilistic interval reliability comprehensive index. Also it is presented that variation curves of the lower and upper bounds of buckling safety margin and non-probabilistic interval reliability comprehensive index with the variation of uncertainty degree of each interval variables.5. Critical buckling load of thin cylindrical shell compartment with stiffened rings is calculated by semi-analytical finite element method. The sensitivity expressions of buckling safety margin implicit equation to each random variables are presented, and structural buckling probabilistic reliability index of thin cylindrical shell compartment with stiffened rings is calculated by hybrid method of stochastic finite element and limit step length iteration method. The partial matrixs of element geometric matrix to each uncertainty variables are deduced when pre-buckling stress is calculated by semi-analytical finite element method, and the buckling load interval of thin cylindrical shell compartment with stiffened rings is analysed. Structural buckling non-probabilistic interval reliability comprehensive index of thin cylindrical shell with stiffened rings is calculated by combined method of modified chaotic optimization method BST-Geczb and Monte-Carlo method. Through the engineering numeric examples, it is analysed that structural buckling probabilistic reliability of thin cylindrical shell compartment of natural supercavitating torpedo, and structural buckling probabilistic and non-probabilistic reliability of thin cylindrical shell compartment with stiffened rings of ventilated supercavitating torpedo.