SPACE rocket inducers typically work under cavitating condi-tions that often lead to the development of flow instabilities, which in turn can seriously degrade the performance of the machine or even cause its rapid failure [1, 2]. As a rough initial approximation, the cavitating behavior of a rotating machine can be related to that of a static cascade of hydrofoils. Hence, the first step for understanding cavitation instabilities in inducers is typically represented by experimentation on test bodies in hydrodynamic tunnels. The onset of cavitation instabilities in twodimensional (2-D) hydrofoils is strongly related to the mean cavity length, which shows a strong dependence on the well-known= parameter (cavitation number divided to incidence angle [3, 4]). One of the best known intrinsic instabilities, as defined in [5], is the socalled cloud cavitation, which consists of violent periodical fluctuations of the cavity length, typically occurring at constant Strouhal numbers and followed by the release of a cavity cloud at the conclusion of each cycle [6]. The nature of this form of instability in cavitating hydrofoils has been studied in detail [7], proving the correlation between cloud cavitation and the formation of a reentrant jet at the cavity closure as a result of a critical adverse pressure gradient observed for cavity lengths greater than about 50% of chord [8–10].

In 2-D cascades, another driving parameter in the development of cavitation-induced flow instabilities is the dimension of the cavity with respect to the thickness of the blade passage. In particular, in rotating cascades, the blockage induced by cavitation can lead to rotating instabilities similar to rotating stall in compressors, such as rotating cavitation [11–14] and rotating choke [15, 16]. It has been found [17] that the onset point of rotating cavitation corresponds to a cavity length equal to about 70% of blade spacing and, furthermore, that four-bladed inducers are expected to show higher rotating cavitation frequency than three-bladed inducers. Another theoretical analysis [18], based on a 2-D cascade model with circular arc blades, showed that there is a relationship between the onset of rotating cavitation and the steady pressure gradient on the suction surface of a blade near the throat: the rotating cavitation typically occurs when this pressure gradient becomes smaller than a threshold value. Finally, the presence of the blade tip and the consequent clearance between the impeller and the casing introduces further complexities in the development of cavitation and cavitation-induced instabilities. The so-called backflow vortex cavitation, which is a rotating instability with multiple cells formed in backflow vortices, was detected for the first time in 1997 [19] for two separated ranges of values of the