Computational Modelling for Cavitation and Tip Vortex Flows
Abstract: Cavitation often brings negative effects, such as performance degradation, noise, vibration, and material damage, to marine propulsion systems, but for optimum performance, cavitation is almost inevitable. Therefore, it is necessary to better understand cavitation in order to maximize the performance without encounter- ing severe problems. Experimental tests can only provide limited information about this complex phenomenon. This thesis deals with improving computational methodologies that can offer a more complete picture of the cavitation process, making it possible to investigate the flow in more details with a higher level of confidence, which eventually enables an improved design. The study describes cavitation behaviour in the early stage of the formation, i.e. cavitation inception and its interaction with tip vortex structures, as well as in the developed form, i.e. sheet and cloud cavitation. The analysis of the tip vortex flows is associated with the spatial mesh resolution, the sub-grid scale and the turbulence modelling, as well as the cavitation-vortex interaction. For inception prediction, different inception methods are investigated to char- acterize tip vortex flows around an elliptical foil and high skewed low noise pro- pellers. The adopted inception models cover different levels of complexity in- cluding wetted flow analysis, Eulerian cavitation simulations, and simplified La- grangian Rayleigh-Plesset bubble dynamics models. For simulations of developed sheet/cloud cavitating flows, a homogeneous two-phase mixture method is adopted along with the Schnerr-Sauer mass transfer model. A manual calibration of the mass transfer model coefficients may signifi- cantly affect both accuracy and stability of the numerical predictions. In order to avoid this issue, an approach is suggested and tested to compute the mass transfer rate based on the flow local time scale during the solution procedure. Comparison between high speed videos and numerical results clearly shows the capability of the developed method in predicting the cavitating structures. It is shown that in addition to the well-captured difference in e.g. the amount of cav- itation, the simulation is capable of correctly predicting the small though crucial differences in flow features and cavitation inception characteristics of different propellers designs. The strong dependency of the inception on the initial nuclei sizes are demonstrated, and it is shown that for weaker propeller tip vortices this dependency becomes even more significant.
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