In this study, unsteady cavitation and cavitation instabilities are numericaly analyzed around single and tandem cascades in water/air working fluid. Through the comparison of the results for several cascade arrangement, suppression effectivity of tandem arrangement to cavitation instabilities is considerd. The hydrofoil geometry is a flat plate with no thickness, and cyclic boundary condition is imposed to every three cascade passages in order to analyze propagating phenomena of cavitation as arising in rotating machinery. Additionally, an effect of tandem cascade with or without alternate arrangement between front and rear stages is investigated. From numerical results, it is confirmed that cavitation instabilities are suppressed in both tandem cascades with and without alternation Additionally, it is found that mutual interference between rear and front stages in tandem cascade without alternation is larger than that in cascade with half-pitch alternation. In tandem cascade without alternation, suction fluid through a slit between front and rear blades, which is induced by pressure gradient at the slit, plays effective role on suppression to cavitation instabilities.
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