Cavitation Research and its Applications
Cavitation may strictly be defined as the change of phase, from liquid to vapour, that occurs when the static pressure in the liquid about a body is reduced below vapour pressure. It is similar to boiling but the driving mechanism is pressure rather than heat. It is a complex phenomenon and places limitations on the performance of submerged machinery. It may cause thrust breakdown in propulsors and hydrofoils, loss of efficiency, metal erosion, noise, vibration and ultimately destruction of machinery.
Interest in cavitation has traditionally been in hydraulic and hydrodynamic applications such as in naval hydrodynamics, hydro-electric machinery, nuclear plant and rocket propulsion. Modern research is more diverse and is often focused on basic fluid dynamic phenomena with applications additionally in medicine, biomedical engineering and biology. Cavitation in this context is more appropriately defined as encompassing all phenomena involving the interaction between vaporous or gaseous volumes (or cavities) with flowing or non-flowing liquid volumes.
Research at the AMC, University of Tasmania involves classical work such as cavitation about marine propulsors and lifting surfaces but also more novel or basic problems. Fundamental to cavitation research is nucleation and inception controlled by the formation of ‘so-called’ nuclei and their transport and interaction with turbulent flow fields. Nuclei are typically microbubbles or other gas containing entities that provide sites of weakness for the initiation of liquid to vapour phase change. Their formation and dynamics is a complex multidisciplinary problem. These depend on the physical state of the liquid and the presence of contaminants such dissolved gases and other chemical and biological species. Dissolved gases and diffusion processes are particularly important in the formation and dynamics of microbubbles along with surface tension and turbulence.
Beyond inception complex macro-scale phenomena occur such as bubbly flows, free-surface effects, large-scale instabilities and shockwave formation in liquid and vapour phases. In summary, cavitation is a complex multidisciplinary phenomenon involving many physical and chemical processes that occur at temporal and physical scales ranging over several orders of magnitude.
To study such problems new laboratory facilities have been developed through funds from national and international competitive grant schemes and the Defence Science and Technology Group. These include an advanced variable pressure water tunnel with the capability for rigorous control of water properties, including the nuclei population, and several smaller tanks and bench facilities and instruments for basic research.