Cavitation Models for ship propellers

The research activity reported in this thesis concerns the numerical study of cavitating flows. Academic benchmark cases have been considered, to analyze the effectiveness of existing cavitation models. Cavitation is a phenomenon to be avoided given its many negative effects. and the vibrations and local damage to the structure can lead to total structural breakage and are a very important source of noise. An analysis of the Rayleigh-Plesset equation and the stability of the vapor bubble was initially performed analytically and numerically. Having in mind the final application to cases of engineering interest, the best approach in terms of computational cost to model the cavitation is the mixture model, where the processes of condensation and vaporization are treated through the two source terms m_v, m_c respectively, for which a lot of different models exist in literature. We compared the results obtained using four different cavitation models, finding some differences among them. Then we propose a normalization method for the evaluation of the coefficients Cc and Cv by comparing the integral time scale Tref associated with vaporization and condensation processes; and the models were compared again with the new normalized coefficients, finding some improvement in the comparison, especially considering the cavitation regime predicted. Since the tip vortex cavitation was found to be the main source of noise in ship propellers. We studied the tip vortex cavitation considering an isolated cavitating vortex. We analyzed the vortex forcing different natural modes considering different configurations of the mesh, different values of the coefficients Cc and Cv, and for two-dimensional and three-dimensional cases. The results obtained were in generally good agreement with the analytical solution available in literature [Bosschers (2018)]; the results obtained show that the coefficients maybe are not so influential in flow-driven cavitation, like that which occurs in a vortex; moreover, the geometry and the mesh strongly affect the results, inducing numerical instability and dissipation.

The research activity reported in this thesis concerns the numerical study of cavitating flows. Academic benchmark cases have been considered, to analyze the effectiveness of existing cavitation models. Cavitation is a phenomenon to be avoided given its many negative effects. and the vibrations and local damage to the structure can lead to total structural breakage and are a very important source of noise. An analysis of the Rayleigh-Plesset equation and the stability of the vapor bubble was initially performed analytically and numerically. Having in mind the final application to cases of engineering interest, the best approach in terms of computational cost to model the cavitation is the mixture model, where the processes of condensation and vaporization are treated through the two source terms m_v, m_c respectively, for which a lot of different models exist in literature. We compared the results obtained using four different cavitation models, finding some differences among them. Then we propose a normalization method for the evaluation of the coefficients Cc and Cv by comparing the integral time scale Tref associated with vaporization and condensation processes; and the models were compared again with the new normalized coefficients, finding some improvement in the comparison, especially considering the cavitation regime predicted. Since the tip vortex cavitation was found to be the main source of noise in ship propellers. We studied the tip vortex cavitation considering an isolated cavitating vortex. We analyzed the vortex forcing different natural modes considering different configurations of the mesh, different values of the coefficients Cc and Cv, and for two-dimensional and three-dimensional cases. The results obtained were in generally good agreement with the analytical solution available in literature [Bosschers (2018)]; the results obtained show that the coefficients maybe are not so influential in flow-driven cavitation, like that which occurs in a vortex; moreover, the geometry and the mesh strongly affect the results, inducing numerical instability and dissipation.