Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>800</mn></mrow></semantics></math></inline-formula> to <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>2000</mn></mrow></semantics></math></inline-formula>. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>1200</mn></mrow></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>1400</mn></mrow></semantics></math></inline-formula> for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>1400</mn></mrow></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>2000</mn></mrow></semantics></math></inline-formula>.