Research On The Near-field Non-isentropic Flow Of Underwater Explosion Based On Finite Difference Method Of Characteristics

The underwater explosion(UNDEX)phenomenon widely exists in the civil engineering and defense industry.It is a basic problem for the applications of underwater blasting and dismantling,underwater explosion welding,damage of underwater weapons,and anti-explosion design of surface ships.With the rapid development of computer science and technology,it has gradually become an important means for the underwater explosion research through a large number of numerical simulations combined with a small number of key experiments.For specific problems and specific targets in underwater explosions,self-programming codes based on an algorithm or coupled multi-algorithm can provide a unique research perspective.The finite-difference method of characteristics(FDMOC)has the advantages of clear physical meaning,high numerical precision,high computational efficiency and backtracking capacity.However,the underwater explosion flow field is a typical non-isentropic flow field,in which the previous methods have great limitations.Under the above background,this paper aims to obtain an improved FDMOC and carry out research work from the following three aspects:Firstly,in order to improve the method substantially,this paper optimizes it at both the equation and the algorithm.At the equation aspect,aiming at the difficulty of solving the non-isentropic flow problem by the previous method of characteristics,this paper has proposed the concept that the acoustic wave is not accutally isentropic through analyzing the physical process of a non-isentropic compressible flow;and based on this,a "true" velocity of sound and a "quasi"velocity of sound are defined;then the "true "characteristic equations are deduced for a two-dimensional steady non-isentropic flow according to the standard method,and then a series of finite difference methods of the "quasi" characteritics are further deduced.Based on the"quasi" characteristic equations of the isentropic sound velocity,the contribution of the entropy variable to the equation is clarified,which embodies the physical essence of non-isentropic flow.At the algorithm level,in view of the difficulty of ensuring stability and strict dependency domain in the previous algorithms,this paper constructs a scanning-solving strategy that can achieve both of them;and based on this,this paper introduces an adaptive grid technology to avoid grid distortion problems caused by the intersections of the same family characteristic curves.Besides,the explicit solution of the isentropes under the framework of the Mie-Gruneisen equation of state is derived,so that the sound velocity and the characteristic curve can be accurately and quickly calculated.Based on the above optimization in equations and in algorithms,an improved FDMOC is finally obtained for solving the non-isentropic flow field.Secondly,in order to test the above improved FDMOC,this paper develops self-programming codes based on it for the spherical symmetry and axisymmetric non-isentropic flow.By simulating the near-field flow of underwater explosions for the spherical or cylindrical,ideal or non-ideal explosives,and by being compared with the experimental data with the commercial software AUTODYN results,the accuracy and time economy of the above improved FDMOC are verified.The obtained characteristic network is applied to analyze the underwater explosion flow field,and the expansion range of the detonation product which can determine the near-field shock wave is delineated.Based on an improved Miller model for the non-ideal detonation of an aluminized explosive,the enhancement of the water flow field caused by the post-combustion aluminum is traced through the characteristic network,and the influence range of the post-combustion aluminum on the shock energy output is clarified,which embodies the advantage of the method of characteristics.Finally,in order to demonstrate the practical application of the above improved FDMOC,based on the advantages of the backtracking capacity of it,this paper perfects a calibration scheme based on underwater explosion test to invert the equation of state of detonation products.In view of the incapacity in the low-pressure area of detonation products and the defects of cylindrical charge in the previous calibration scheme,this paper proposes an inversion calibration scheme combining "inverse FDMOC" combined with "genetic algorithm optimization".It is also applicable for the spherical explosives and cylindrical explosives.The required experimental data can be obtained from the near-field shock wave trajectory and the pressure-time history of a certain mid-field gauge point.Among them,the inverse FDMOC is used for the inversion calculation of the water flow field,and the genetic algorithm is used to optimize the parameters of the equation of state of the detonation product.From the final calibration results,this combination of shock wave data and pressure-time data can significantly broaden the acquisition range of the detonation product expansion information and thus the applicable range of the JWL equation of state.The error of isentrope pressure of 8 kinds of common explosives is less than 3%in the range between detonation pressure and 0.01 GPa.It is better the standard cylinder test which has a lower pressure limit of about 0.1 GPa,which means this calibration scheme can also determine the expansion law of the detonation product in the low-pressure area.