Ventilated Cavitation around a sphere through surface air injection at Re = 10,000

Abstract

We perform DNS of ventilated cavitation over a sphere at subcritical (Re=10,000). The flow is modeled as a homogeneous mixture of water and air, with the interface tracked using the volume-of-fluid method. Unlike previous numerical investigations, surface tension is explicitly included. Air is injected through a circular strip on the sphere at three locations selected based on the bulk flow dynamics: front (FR025, (18^\circ\leqθ\leq63^\circ)), mid (MD025, MD050, (75^\circ\leqθ\leq104^\circ)), and back (BK025, (120^\circ\leqθ\leq180^\circ)), with (C_q=0.2) and (0.4). The injection location strongly governs cavity inception and stability and alters flow separation in the front- and mid-injection cases. Front injection produces an unsteady bubbly cavity due to vigorous puffing, which fragments the injected air and increases drag by (\sim56%) relative to the single-phase case. In contrast, mid- and back-injection produce stable cavities with distinct leading-edge dynamics. In the mid-injection cases, puffing dominates and, despite delayed flow separation, the cavity detaches from the leading edge of the injection patch. For back injection, the cavity detaches upstream of the injection patch owing to the adverse pressure gradient induced by cavity formation. Kelvin--Helmholtz instabilities and divot formation characterize the back-injection cavity. Stable cavities exhibit strong air entrainment and close through a re-entrant jet. The mid-injection cases achieve (35%) and (25%) drag reduction at (C_q=0.2) and (0.4), respectively, while back injection yields a maximum drag reduction of (46%) relative to the single-phase case.