On some physics to consider in numerical simulation of erosive cavitation
Paper in proceedings, 2009
This paper discusses some hydrodynamic mechanisms in erosive cavitation, which are all important to capture, and consider, when assessing the risk of erosion, by numerical simulations or model tests in a cavitation tunnel. Focus is put on the visual appearance and underlying physics of the mechanisms with the aim to explain main developments towards erosive cavitation. From this knowledge it is possible to conclude about requirements on numerical and experimental prediction or analysis methods to be used in assessment of erosion risk, and to elucidate links between small-scale erosive collapses and global flow, the latter described by engineering parameters as pressure distributions etc. Based on physics as well as practical engineering aspects a decomposition of the cavitation process is discussed and in particular we introduce the concept of primary and secondary cavitation in order to put emphasis on a particular class of mechanisms: cavitation created in the secondary flow field, governed by e.g. a shedding or collapse of the primarily created cavity, typically in interaction with the global flow. These secondary cavities are usually erosive and have previously not been generally described in the literature. A basic example of secondary cavitation is a rebound controlled by compression of cavity content and liquid. Occurring in a flow close to a body asymmetry typically appears and the simple rebound becomes disturbed by vortex formation and a “generalized” rebound develops. An example of a generalized rebound is the “vortex group cavitation” for which a hypothesis is suggested. Viscous shear is assumed to redistribute vortex motion initiated by collapse asymmetry and acoustic interaction is assumed to control part of the development. The role of cloud cavitation is partly reconsidered. Despite the fact that cloud cavitation is demonstrated, by theory as well as experiments, including the present, to be able to generate the highest single collapse pulses, the focusing of collapse energy is often rather dispersed in space as well as time and the erosion is correspondingly reduced. In a number of cases it is demonstrated that the most severe erosion occurs when a small cloud is synchronized, and thus gains collapse energy, by the collapse of a mainly glassy sheet. Account of the glassy collapse seems very important. An underlying part of the discussion is how the described cavitation mechanisms influence numerical simulation of cavitation nuisance. Different numerical approaches, as LES, RANS or Euler may generate adequate results, a crucial factor being high resolutions.