The investigation focuses on wave and inertia driven mechanisms of cavitating flows. We apply the CFD-Tool CATUM (CAvitation Technische Universität München) to simulate the 3-D flow around a 2-D hydrofoil with special emphasis on self-excited instabilities in spanwise direction. It is demonstrated that these instabilities are predictable by the assumption of inviscid flow and equilibrium thermodynamics. The importance of sufficient resolution in space and time is analyzed by a grid dependence study. Large scale characteristics are only weakly dependent on the resolution while small scale structures are strongly grid dependent. The simulation predicts various irregular break-up patterns, hairpin and horseshoe vortices. These delicate features of the flow vary from cycle to cycle, strong periodicity is not observed for the investigated set-up. The development of the re-entry jet as part of the shedding mechanism is analyzed. Here, we observe significant vorticity production during the growth and the collapse of the sheet cavity. It is demonstrated that the vorticity production is caused by a discontinuity at the end of the attached part of the cavity where condensation takes place. The discontinuity fulfills Rankine-Hugoniot conditions as known from gasdynamic shocks. Contrary to the discontinuity related with the formation and break-up of the sheet cavity, we observe shocks due to collapsing fragments of clouds. These shocks produce significant maximum loads of pmax nearly equal 2400 bar, particularly close to the trailing edge of the investigated hydrofoil. We conclude that the dynamics of sheet and cloud cavitation are essentially inertia controlled. Hence, the application of an inviscid flow model to simulate cavitating flows is justified - provided that it is ensured that the boundary layers of the corresponding single-phase flow remain attached.
Inertia controlled instability and small scale structures of sheet and cloud cavitation
Trägheitsgeregelte Instabilität und kleine Strukturen der Schicht- und Wolkenkavitation
2009
14 Seiten, 10 Bilder, 16 Quellen
Conference paper
English
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