Abstract2-D unsteady cavity flows through hydrofoils in cascade which is the most fundamental element of turbomachinery are numerically calculated. In particular, attention was paid to instability phenomena of the sheet cavity in transient cavitation condition and the mechanism of break-off phenomenon was examined. A TVD MacCormack's scheme employing a locally homogeneous model of compressible gas-liquid two-phase media was applied to analyze above cavity flows. The present method permits us to treat the whole cavitating/noncavitating unsteady flow field. By analyzing numerical results in detail, it became clear that there are at least two mechanisms in the break-off phenomena of sheet cavity; one is that re-entrant jets play a dominant role in such a break-off phenomenon, and the other is that pressure waves propagating inside the cavity bring about an another type of break-off phenomenon accompanied with cavity surface waves.

A numerical prediction method of cavitation erosion is proposed. In this method, the analysis of bubbles in cavitating flows is performed and the intensity of cavitation erosion is evaluated by the impact pressure induced by spherical bubble collapse. In the present study, two-dimensional cavitating flow around the Clark Y 11.7 % hydrofoil is used to examine the proposed numerical prediction method. The proposed numerical method predicts that the intensities of cavitation erosion in noncavitating, attached cavitating and pseudo-supercavitating flows are far weaker than the intensity of cavitation erosion in a transient cavitating flow, and the intensity in the vicinity of the sheet cavity termination is high. These results correspond well to experimental results, and it is confirmed that systematic erosion characteristics are generally captured by this method. Furthermore, the velocity dependence of cavitation erosion is examined, and it is found that the exponent n in the relation between the intensity I and main flow velocity U in (I ∝ U n in) becomes large when the bubble radius is large and ranges between 4.3 and 7.0 in the present study. According to the bubble dynamics, the ambient pressure and the rate of increases in pressure increase as the main flow velocity, and the maximum internal pressure increase. Therefore, it is thought that smaller bubbles cause cavitation erosion when the main flow velocity is large.

Bubble behavior in cavitating flow is analyzed for the development of practical erosion prediction method. CFD analysis with cavitation model is carried out for the flow field around a hydrofoil. Afterwards computation of bubble dynamics is carried out coupled with flow field CFD results by one way approach. For the bubble dynamic calculation, Rayleigh-Plesset equation is adopted. Bubble behaviors in the collapse of cloud cavitaion and in the break off of sheet cavity are analyzed. Bubble behavior at the trailing edge of sheet cavity is also calculated. It is observed that steep pressure change in the flow causes oscillation of the bubbles. Based on this qualitative information of bubble behaviors, numerical cavitation aggressiveness is simply defined. This numerical cavitation aggressiveness is a function of local void fraction and pressure over the solid surface and can be calculated directly from the cavitating flow field CFD results without concerning bubble dynamics.

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Cavitation in cryogenic fluids has a thermodynamic effect because of the thermal imbalance around the cavity. It improves cavitation performances in turbomachines due to the delay of cavity growth. The relationship between the thermodynamic effect and cavitation instabilities, however, is still unknown. To investigate the influence of the thermodynamic effect on rotating cavitation appeared in the turbopump inducer, we conducted experiments in which liquid nitrogen was set at different temperatures (74 K, 78 K, and 83 K) with a focus on the cavity length. At higher cavitation numbers, supersynchronous rotating cavitation occurred at the critical cavity length of Lc/h≅0.5 with a weak thermodynamic effect in terms of the fluctuation of cavity length. In contrast, synchronous rotating cavitation occurred at the critical cavity length of Lc/h≅0.9–1.0 at lower cavitation numbers. The critical cavitation number shifted to a lower level due to the suppression of cavity growth by the thermodynamic effect, which appeared significantly with rising liquid temperature. The unevenness of cavity length under synchronous rotating cavitation was decreased by the thermodynamic effect. Furthermore, we confirmed that the fluid force acting on the inducer notably increased under conditions of rotating cavitation, but that the amplitude of the shaft vibration depended on the degree of the unevenness of the cavity length through the thermodynamic effect.