Attached cavitation in laminar separations within a transition to unsteadiness
Résumé
Attached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable
conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry
to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about
single phase flows are performed with steady simulations for a Reynolds number range Re > [50; 1400] and with unsteady simulations for
Re > [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition
to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography,
and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers
Re B 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas
typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the
laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized
by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Rebc
616. This regime
transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating
air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation.
Domaines
Sciences de l'ingénieur [physics]
Origine : Fichiers produits par l'(les) auteur(s)
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