Tezhuan Du

Study of Characteristics of Cloud Cavity Around Axisymmetric Projectile by Large Eddy Simulation

Cavitation generally occurs where the pressure is lower than the saturated vapor pressure. Based on large eddy simulation (LES) methodology, an approach is developed to simulate dynamic behaviors of cavitation, using k - mu transport equation for subgrid terms combined with volume of fluid (VOF) description of cavitation and the Kunz model for mass transfer. The computation model is applied in a 3D field with an axisymmetric projectile at cavitation number sigma = 0.58. Evolution of cavitation in simulation is consistent with the experiment. Clear understanding about cavitation can be obtained from the simulation in which many details and mechanisms are present. The phenomenon of boundary separation and re-entry jet are observed. Re-entry jet plays an important role in the bubble shedding.

Large Eddy Simulation of Unsteady Cavitating Flow Around a Highly Skewed Propeller in Nonuniform Wake

Unsteady cavitating flows around propellers become increasingly prominent on large-scale and high-speed ships, but large eddy simulations (LES) are limited in the literature. In this study, numerical simulation of an unsteady cavitating flow around a highly skewed propeller in a nonuniform wake is performed based on an explicit LES approach with k mu subgrid model. Kunz cavitation model, volume of fluid (VOF) method, and a mov-ing mesh scheme are adopted. The predicted evolution of the unsteady cavitating flow around a highly skewed propeller in a nonuniform ship wake is in good agreement with experimental results. An analysis of the factors affecting the cavitation on the propeller is conducted based on numerical simulation. Furthermore, the influences between cavitation structures and vortex structures are also briefly analyzed.

Cloud Cavitating Flow Over a Submerged Axisymmetric Projectile and Comparison Between Two-Dimensional RANS and Three-Dimensional Large-Eddy Simulation Methods

For the cloud cavitation around slender axisymmetric projectiles, a two-dimensional (2D) numerical method was based on the mixture approach with Singhal cavitation model and modified renormalization-group (RNG) k-epsilon turbulence model, and a three-dimensional (3D) method was established with large-eddy simulation (LES) and volume of fraction (VOF) approach. The commercial computational fluid dynamic (CFD) software FLUENT is used for the 2D simulation, and the open source code OpenFOAM is adopted for the 3D calculation. Experimental and numerical results were presented on a typical case, in which the projectile moves with a quasi-constant axial speed. Simulation results agree well with experimental results. An analysis of the evolution of cavitating flow was performed, and the related physical mechanism was discussed. Results demonstrate that shedding cavity collapse plays an important role in the generation and acceleration of re-entry jet, which is the main reason for the instability of cloud cavitation. The 2D Reynolds-Averaged Navier-Stokes (RANS) method can represent the physical phenomena effectively. The 3D LES method can give an efficient simulation on the shedding vortices, and considerable accurate shapes of shedding cavities are captured.

A numerical model for the evolution of internal structure of cavitation cloud

Bubble size distributions in cloud cavitation are important in cavitating flows. In this study, a numerical model was developed to study the evolution of the internal structure of cloud cavitation. The model includes (1) an evolution equation of bubble number density, which considers the bubble breakup effect and (2) the multiphase Reynolds-averaged Navier–Stokes equations with a modified cavitation model for background cavitating flows. The proposed model was validated with a flow over a projectile. Results show that the numerical model can predict the evolution of the internal structure of cloud cavitation. Comparisons of the proposed model and Singhal model were discussed. The effects of re-entrant jet and bubble number density on cavitating flows were also investigated.

Three stages of bubble formation on submerged orifice under constant gas flow rate

Bubble formation is involved in many engineering applications. It is important to understand the dynamics of bubble formation. This work reports experimental and numerical results of bubble formation on submerged orifice under constant gas flow rate. Compressible large eddy simulation combined volume of fluid (VOF) was adopted in simulation and results was validated by experiment. Bubble formation is divided into three stages in this paper, expansion stage, elongation stage and pinch-off stage. In expansion stage, The bubble grows radially due to the incoming gas flux, but the bubble base remains attached to the orifice. But as gas injected, the spherical bubble will go into the elongation stage when the downward resultant force is lager than upward resultant force. And when bubble necks length is bigger than √2Ro the bubble will go into pinch-off stage. Cylindrical Rayleigh-Plesset equation can be used to describe the pinch-off stage. Uncertain parameter r in it is given reference value in this paper.