Resumen de Numerical investigation and modelling of the unsteady behavior and erosion power of cloud cavitation
Cloud cavitation is a unwanted phenomenon taking place in many hydraulic machines which damages the surfaces of the solid walls due to the erosive aggressiveness induced by the collapse process. Therefore, it is necessary to accurately predict the occurrence of cloud cavitation and quantify its erosion intensity to improve the design and to extend the life cycle of existing machines and systems. The application of numerical simulation (CFD) offers the opportunity to predict unsteady cavitation. For that, it is of paramount importance to investigate how to select the most appropriate models to obtain more accurate results in an efficient way and how to relate the collapsing vapor structures with their erosion power. In the current study, the influence of the different turbulence models was assessed and the performance of cavitation models was improved. The relationship between the unsteady behavior and its erosion character was also considered by implementing an erosion model.
For the assessment of the turbulence models, three Unsteady Reynolds Average Navier-Stokes (URANS) turbulence models were employed to simulate the cloud cavitation around a NACA65012 hydrofoil at eight different hydrodynamic conditions. The results indicate that the Shear Stress Transport (SST) model can better capture the unsteady cavity behavior than the k-e and the RNG models if the near wall grid resolution is fine enough.
For the improvement of the cavitation models, the influence of the empirical constants of the Zwart model on the cavity dynamics was firstly investigated. The results show that the cavity behavior is sensitive to their variation, and thereby an optimal range is proposed which can provide a better prediction of the vapor volume fraction and of the instantaneous pressure pulse generated by the main cloud cavity collapse. Secondly, the original Zwart and Singhal cavitation models were corrected by taking into account the second order term of the Rayleigh-Plesset equation. The performances of the original and corrected models were compared for two different cavitation patterns. The results for a steady attached cavity demonstrate that the corrected model predicts better the pressure distribution at the cavity closure region and the cavity length in comparison with the experiment observations. The results for unsteady cloud cavitation also confirm that the prediction of the shedding frequency can be improved with the corrected Zwart model.
For the investigation of the cavitation erosion power, an erosion model based on the energy balance approach was employed. It has been found that the spatial and temporal distribution of the erosion aggressiveness is sensitive to the selection of the cavitation model and to the collapse driving pressure. In particular, the use of average pressure levels combined with the Sauer cavitation model permit to achieve reliable results. Then, two erosion mechanisms have been observed, one occurs at the closure region of the main sheet cavity characterized by low-intensity collapses but with high frequency, and the other is inducted by the collapse of the shed cloudy cavity which presents a high erosion intensity but with low frequency. Finally, it has been found that the erosion power follows a power law with the main flow velocity with exponents ranging from 3 to 5 depending on the erosion estimate being used.
