Research On Multiphase Hydrodynamics Problems Of The Free Surface Penetration Of Bodies

Researchers in the areas of modern aviation, aerospace and navigation pay attention to problems that bodies penetrate a free surface from air into water or vice versa. The water-entry and water-exit phenomena are often linked with ship seakeeping and maneuvering, as well as airdrop of missiles or underwater launch. The free surface penetration will bring phenomena such as nonlinear free surface flows, unsteady cavitating flow, and it is a complex nonlinear multi-phase hydrodynamics problem. Apparently, research on this issue has great significance in many areas. Based on coupling solution of rigid body motion dynamics equations and the Navier-Stokes equations, this paper mainly used numerical methods to carry out multiphase hydrodynamics research on a series of typical free surface penetration problems. Some valuable conclusions were derived in this paper.For the water-entry problem of a horizontal circular cylinder with constant velocity, experiments were described briefly and the corresponding numerical water tank was established; slamming coefficient vs. nondimensional time curves were calculated; free surface variation during the water-entry process under different water-entry conditions were discussed. Numerical results and experimental results of slamming coefficient are in good agreement; viscosity has small influence on the water-entry process is also revealed by computation; the threshed value of Fr number causing cavity closure on the top of the water-entry cylinder is predicted to be a value between0.9247and1.1.For the water-entry problems of a horizontal circular cylinder and ship sections by free drop, experimental situation was introduced and numerical water tanks were set up respectively. In comparing with experimental results, important phenomena of the water-entry of the cylinder such as jet splash, air cushion effect and other free surface changes are well showed; the cylinder motion is also predicted accurately. Numerical results of the free drop problem of V-shaped and bow-flare sections are also accord with experimental records.6DOF hydrodynamic equations were deduced based on principles of Lagrangian dynamics and potential flow theory. Combined with Hess-Smith method, the equations were applied to simplified analysis on the water-exit problem of a horizontal cylinder with constant velocity. Based on an experiment of water-exit problem of a horizontal cylinder with constant velocity, a numerical tank was established to calculate this problem. Comparisons among numerical research, theoretical and experimental results indicate that potential theory can be used for guiding study on water exit problems of bodies, and numerical results of hydrodynamic forces are good, and the free surface variation also reflect the coupling of rigid body motion and fluid flow in a good way.A numerical water tank has been established for an experiment of water-exit of a horizontal cylinder with constant force; the original turbulent model was modified for describing viscous separation of the water-exit process exactly. Favorable agreement is obtained between simulation results and experimental photographs; vortex shedding phenomena are well presented; the numerical solutions of cylinder motion and experimental records are also basically in accordance.Research on cavity flow around water-exit axisymmetric bodies was conducted. At first, steady and unsteady cavitation flow of underwater vehicles set in certain depths were studied numerically. In comparing with experimental solutions, cavitation model was verified and the numerical method after modifying the original turbulent model used in this paper can be applied to solving unsteady cavitation flow commendably. Then, by coupling rigid motion model, VOF model and unsteady cavitation flow model, high-speed water-exit process of axisymmetric bodies with flat and hemispherical heads were numerically solved. The results showed that partial cavity caused during the water-exit process can keep existing after the bodies exit from the original still water level, and initially the cavity part near heads of axisymmetric bodies is squeezed to collapse, and then induces pressure pulse which lead to continuous collapsing and the initial collapse time of the hemispherical head form body appears earlier than the flat-nosed one.It is expected that research conducted in this paper on multiphase hydrodynamics problems of free surface penetration can provide some references for study on relevant fields.