The overall objective of this workpackage is to develop and validate a limited number of promising codes that are able to predict the flow about both non-cavitating (necessary for cavitation inception) and cavitating propulsors, and are in particular able to compute the adverse effect it has on the propulsion characteristics (such as radiated pressure fluctuations and cavitation erosion).
To achieve reliable predictions of pressure fluctuations, an accurate prediction of the time-dependent cavitation volume is a prerequisite. This is because the pressure fluctuations are mainly dependent on the second time derivative of the cavity volume. The time-dependent sheet cavity volume may be computed by a viscous RANS code, but may also be computed with a Boundary Element Method (or panel code); the advantage of the BEM code being its robustness and relative simplicity. To explore the possibilities and limitations of these codes, both will be implemented to yield cavitation induced pressure fluctuations. The higher frequency content of pressure fluctuations that are ascribed to tip vortex cavitation can only be obtained from a RANS code with a sufficient resolution in space and in time.
Cavitation erosion is a phenomenon which is eventually determined by micro-scale physics which needs a resolution in space and in time that is currently outside the scope of this project. The EU FP5 EROCAV project has however provided empirical relations between these micro-scale phenomena and the behaviour of large cavity structures, which are easier to compute. The predictive capability of RANS codes for cavitation erosion will thus be explored and further developed.
This workpackage aims at producing a selection of CFD tools (both RANS and BEM) to address practical problems as vibrations and cavitation erosion in a way that it can be used by the industry. This is an ambitious plan and for some of the Tasks it is expected that delivery of practical RANS tools is outside the scope and duration of the project. It is for this reason that a distinction is made between the long term, higher risk developments (such as cavitation erosion risk prediction by RANS or LES codes) and the more practical tools such as a hybrid BEM-RANS code for the prediction of hull pressure fluctuations. These tasks are examples of the results from a search for a balance between the long and the short term, where both solutions offer their own specific advantages. Practical use of the deliverables is stimulated and it is expected that this will greatly stimulate further technological developments.
Examples of the link with a practical environment are the validation studies that are defined for each task, and the design exercise, defined by Task 5.