# Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation
## 1 Introduction  
The presence of small separation bubbles close to the leading edge of a profile or to the summit of a wedge provides favorable conditions for the attachment of vaporous cavities when lowering the absolute pressure. The fundamentals of sheet cavitation inception can be found for instance in the classical books of [PERSON] [1] or [PERSON] [2]. The inception of such \"sheet\" cavitation has been for instance studied in Refs. [3; 4; 5; 6; 7] on smooth axi-symmetric bodies, on propellers blades or on foils. Moreover, a recent review of the different sheet cavitation inception mechanisms can be found in Ref. [8]. Once the inception of cavitation has occurred, for lower pressures the attached sheet cavities usually grow and become unstable, leading to periodical cloud shedding [9; 10; 11]. Partial and cloud cavitations are associated with a vapor region that extends over a part of the cavitating body [2]. At very low pressures, one can observe super-cavitation, consisting of a large and stable vapor cavity that extends beyond the body and closes in the liquid. In that case the pressure fluctuations are usually small [12; 13]. For turbomachinery applications, the unsteady dynamics of attached sheets may be responsible for an increase of noise and vibrations and is of crucial importance for system instabilities [14; 15]. The instabilities of attached sheet cavitation are attributed to two main phenomena: the formation of a re-entrant jet which is governed by inertia [16], and a bubbly shock propagation mechanism [17]. These two mechanisms have been recently subject to extensive studies, at leastfor very large Reynolds numbers, in water [18; 19; 20; 21; 22]. The cavitation in that case arises on a turbulent flow background. The current studies that are conducted in the LIFSE (Laboratory of Fluid Engineering and Energetic Systems) deal with cavitating flows at very low Reynolds numbers, i.e., in laminar or transitional regimes. To the best of our knowledge, only very few studies have been carried out on hydrodynamic cavitation [23; 24] or flow-induced degassing [25; 26] in laminar flows. The main goal of the present study is to identify and compare potential unsteady regimes in viscosity-dominated cavitating flows to the aforementioned regimes observed in turbulent flows. In the present study, viscous silicone oil is used in a well-documented Venturi-type geometry [10]. In a first study conducted in our laboratory ([PERSON] et al. (2018, 2019) [27; 28]), the main features of the multiphase structures that arise have been observed from a qualitative point of view, using silicone oil. Silicone oil can dissolve much more air than water under the same thermodynamical conditions [29; 30]. In the work of [PERSON] et al. (2018, 2019) [27; 28] the oil was saturated with air at atmospheric pressure. Single-phase numerical simulations have been performed additionally to illustrate the flow topology and to estimate the threshold for cavitation inception. As a result, different types of cavities are observed: tadpoles that attach on the lateral wall [27], sheets that attach on the Venturi slope and top-tadpoles that attach on the opposite side of the vein. The numerical simulations, pressure measurements and high-speed visualizations suggest to attribute the tadpoles to degassing, the main cavity sheet to cavitation, and the top-tadpoles to a secondary flow separation that captures air [28]. In the present article, experiments were performed with degassed silicone oil, to remove air bubbles and to get rid of degassing phenomena [25; 26; 31]. As a result, only central attached sheet cavities are present in the flow. Quantitative measurements based on high-speed visualizations are performed in order to characterize the evolution of the shapes, lengths and temporal features of the attached sheets with a decrease of the ambient pressure at constant Reynolds numbers \(Re\), both in steady laminar flow (at \(Re\leqslant 1200\)) and in transitional flows (up to \(Re\simeq 2000\)).