Tham khảo tài liệu 'practical ship hydrodynamics episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Propellers 51 flow computations are able to deliver accurate flow details in the tip region of the propeller blade. Typical propeller geometries require careful grid generation to assure converged solutions. The warped propeller geometry makes grid generation particularly difficult especially for high-skew propellers. By the late 1990s most RANSE applications for propellers were still for steady flow open-water case but first unsteady computations in the ship wake appeared. However the excessive effort in grid generation limited the calculations to research projects. Cavitation High velocities result in low pressures. If the pressure falls sufficiently low cavities form and fill up with air coming out of solution and by vapour. This phenomenon is called cavitation. The cavities disappear again when the pressure increases. They grow and collapse extremely rapidly especially if vapour is filling them. Cavitation involves highly complex physical processes with highly non-linear multi-phase flows which are subject to dedicated research by specialized physicists. We will cover the topic only to the extent that any naval architect should know. For a detailed treatment of cavitation for ship propellers the reader is referred to the book of Isay 1989 . For ship propellers the velocities around the profiles of the blade may be sufficiently high to decrease the local pressures to trigger cavitation. Due to the hydrostatic pressure the total pressure will be higher on a blade at the 6 o clock position than at the 12 o clock position. Consequently cavitating propellers will then have regions on a blade where alternatingly cavitation bubbles are formed near the 12 o clock position and collapse again. The resulting rapid succession of explosions and implosions on each blade will have various negative effects vibration noise especially important for navy ships like submarines material erosion at the blade surface if the bubble collapse occurs there thrust reduction Fig.
