Numerical And Physical Investigation Of Tip Leakage Vortex Cavitating Flows

Cavitation in strong vortical flows is becoming increasingly attractive in cavitation researches. As a typical vortex cavitation, the tip leakage vortex cavitation occurs in hydraulicmachinery, and may lead to performance delay and instability. It is meaningful to investigate the tip leakage vortex cavitation in both scientific field and engineering applications. To better understand the flow mechanics and the cavitation-vortex interactions in tip leakage vortex cavitating flows,experimental investigations are firstly conducted, numerical methods are then developed, and physical and numerical analysis are finally combined. Following main conclusions can be drawn:The tip leakage vortex cavitating flow patterns and evolutions are ontained.Firstly, based on the evolutions of tip leakage vortex cavitation, three kinds of cavitation, which are tip vortex cavitation, gap attached cavitation and the shear layer cavitation are defined. Interrelations between these kinds of cavitation are further indicated: as the decrease of cavitation number, gap attached cavitation develops beyond the gap and combines with the tip vortex cavitation to form the triangle shaped unsteady shear layer cavitation.Secondly, different cavitation evolutions in cases with various angles of attack and the gap sizes are discussed.With the angle of attack increasing from AOA= 2° to AOA= 6°, the tip leakage vortex cavitation change from wake-like to jet-like flows and the periodic breakup-reconstruction process of the shear layer cavitation becomes more distinct. With the dimensionless gap size increasing from =0.061 to =0.427, the tip leakage vortex cavitation change from jet-like to wake-like flows and the periodic breakup-reconstruction process of the shear layer cavitation becomes less distinct. Finally, the dynamics of the hydrofoil in cases with various angles of attack and the gap sizes are compared. With the angle of attack increasing from AOA= 2° to AOA= 6°, the time averaged value of the lift force increases and fluctuations with relatively small frequency distributions are enhanced. With the dimensionless gap size increasing from =0.061 to =0.427, lift fluctuations with relatively large frequency distributions are enhanced, and the maximum time averaged value can be found in the case with =0.244.A vortex cavitation model is developed for the simulations of the cavitation in strong vortical flows.Simulations of vortex cavitating flows indicate that the proposed model can be combined with different turbulence modeling methods and has better predicting capability compared with the conventional Zwart model. Validations of three cases are taken. Firstly, in the simulations of the steady tip leakage vortex cavitating flows, the vortex cavitation model can better model the interface mass transfer induced by the diffusion of the tip leakage vortex structures. This leads to the longer predicted vortex cavitation, which better matches with experimental results, compared with the Zwart model predictions. Secondly, in the simulations of the two-dimensional unsteady cloud cavitating flows, the vortex cavitation model can better model the interface mass transfer induced by the diffusion of the complex vortex structures in shedding cloud cavity. This leads to the better predicted cloud cavitation shedding process, compared with the Zwart model predictions. Finally, in the simulations of the unsteady tip leakage vortex cavitating flows, the vortex cavitation model can not only capture the time revolutions of all the tip vortex cavitation, gap attached cavitation and the shear layer cavitation in the tip region, but also the local and global cavitation shedding process near the suction side of the hydrofoil.The reasons for the unsteadiness of tip leakage vortex cavitation are indicated:the results show that the trailing edge vortex periodic shedding process is the main reason for the relatively braided spiral fluctuations with high frequency of the vortex cavitation. On the other hand, the breakup and shedding process of the leading edge cavitation would induce the periodic breakup-reconstruction process of the shear layer cavitation with relatively small frequency. Secondly, the evolutions of the vortex structures are further discussed. With the decrease of the cavitation number, the leading edge cavitation develops gradually to cover the whole suction side, resulting in the decrease of the pressure drop between the pressure and suction side of the hydrofoil in the tip region. This is the main reason for the decrease of the circulation of the tip leakage vortex. Finally, the vortex dynamics in cases with various angles of attack and the gap sizes are compared. With the angle of attack increasing from AOA= 2° to AOA= 6°, the circulation increases, its maximum location moves upstream, and the critical cavitation number for the distinct decrease of circulation increases. With the dimensionless gap size increasing from =0.061 to =0.244, the circulation increases, while it decreases as the gap size increasing from =0.244 to =0.427. The reasons can be explained as follows: as the increase of the gap size, the influence of the tip leakage flow on the pressure distributions on both suction and pressure sides become distinct. The pressure drop near the tip region researches its maximum value in the case with =0.244.