| In the process of underwater weapon development, to improve the navigation speed of underwater projectile and torpedo is always the biggest obstacle in front of design researchers. In the past, researchers had adopted many conventional methods to improve the speed of underwater navigation body, such as optimization of the navigation body structure, adding coating on the navigation body surface, increasing and improving the power system of propulsion. The result was shown that due to the great resistance of water, these methods were difficult to significantly increase the movement speed of the submerged navigation body. With the appearance of high-speed supercavitation torpedo developed in Russia, breaking the limitations of the conventional methods, successfully enhancing the speed of a submerged navigation body, and a very gratifying effect is completed. Presently, the contention of maritime military initiative becomes a hot topic in the word, and characteristics of military applications development prospect deeply attract the attention of the major military powers. Those have carried out lots of researches and made some certain results. However, cavitation is a very complex multiphase flow including phase change, viscous effect, turbulence flow, and compressibility of interface. That gives a greatly restricted on the cavitating flow research and development. Furthermore, relevant test research, numerical prediction and cavitation mechanism research are still research difficulties and hot in the multiphase flow field.In view of this situation, the self-developed LES program is used to carry out numerical simulation of supercavity drag reduction of underwater high-speed navigation body with cavitating flow. The navigation body model is designed by the numerical simulation results of cavitating flow, and the underwater measurement device is also developed. Based on it, the experimental study of underwater supercavity navigation body is carried out. The specific research contents are shown as follows.Firstly, based on the domestic and international research present situation and development trend on supercavity technology, combining the actual engineering application background, the research methods and technical ways that the problems needs to be solved and the solution of these problems have been put forward.Secondly, through the analysis of the domestic and international numerical simulation method on cavitation flow, combined with the characteristics of cavitation flow, the LES calculation solution method based on the three dimensional Navier-Stokes equation and considering phase change between gas-liquid is established in the homogeneous flow theory framework. The filter function characteristics of the LES common operation has been analyzed in detail, and also the advantages and disadvantages of different forms of subgrid stress models. The turbulent kinetic energy k-△model is deduced. The different cavitation models handling phase change between gas-liquid are discussed. On this basis, the numerical discretization method adopted in present is introduced in detail, which including discretization of LES control equation, pressure-velocity-density coupling algorithm, and the derived pressure correction equation in PISO algorithm. In addition, a detailed analysis of calculation domain and numerical boundary condition is completed. The advantages and disadvantages of different grid types are discussed. According to the problem that the complex multiphase flow needs a high calculation, a parallel algorithm is designed based on MPI.Thirdly, in order to validate the applicability of calculation method in present and explore the cavity flow characteristics of low-speed navigation body, numerical simulation research of cloud cavitation flow around the low speed NACA0015hydrofoil is carried out. The periodic shedding frequency and process of cavity forming have been calculated. Comparison of the experimental results with calculated results has been made, there is a well agreement, and applicability of calculation methods is also proved. On this basis, the analysis of unsteady periodic shedding process including the initial vortex, re-entrant jet vortex, cavitation separation and cavity regeneration is completed. The influence of Kunz cavitation model and Sauer cavitation model on cavity form is analyzed. Furthermore, the influence of different cavitation number on hydrodynamic characteristics is analyzed.Fourthly, in order to validate the correctness of calculation method in this paper and explore the cavitating flow characteristics of high-speed navigation body, numerical simulation research of supercavitating flow around the three dimensional navigation body model with different construction appearance is carried out, respectively in the two aspects of the cavity shape and resistance characteristics. The unsteady process that supercavity produces from warhead to surrounded completely navigation body is achieved through the calculation. Comparison of supercavity dimensionless characteristic length, thickness of the calculated results with Logvinovich experimental results has been made, there is a well agreement, and correctness of calculation methods is also proved. On this basis, the influence of the different cavitation number, cavitator diameter, cavitator shape, attack angle of navigation body, and tail shape of navigation body on the supercavity shape and resistance characteristics is analyzed in detail.Finally, in order to further verify the correctness of the numerical method in this paper, based on the analysis results of high-speed navigation body of cavity shape and resistance characteristics, combined with the disc cavitator has a higher drag reduction efficiency and better cavitation capability characteristic, designing a variety of construction appearance navigation body model with disc cavitator and tail, the experimental research that high-speed navigation body is from the air oblique into the water for forming supercavity is carried out. The forming image of underwater natural supercavity is achieved by underwater high-speed camera. The relationship between navigation body movement stability in air and forming supercavity has been discussed. The trajectory stability of supercavity navigation body is analyzed by punching position of navigation body on the underwater velocity measurement target. On condition that the different cavitation number and cavitator diameter, the average drag coefficient of supercavity navigation body is achieved. The influence of the different cavitation number and cavitator diameter on the average drag coefficient is also analyzed. Comparison of the calculated results with experimental results has been made, there is a well agreement, and correctness of calculation methods is also verified. On this basis, the drag reduction rate of the underwater supercavity navigation body is calculated and the effect of supercavity drag reduction is analyzed.A combination of the supercavity drag reduce problems of high speed navigation body about the numerical simulation and experimental study shows that numerical calculation method adopted in this paper can better simulate the supercavity flow of underwater high speed navigation body. In conditions of the cavitation number σ=5.9×10-4and the cavitator relative diameter Dn=2.3×10-1, the drag reduction rate of underwater supercavity navigation body can approximately get up to94.67%. |
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