Cavitation generation and inhibition. II. Invisible tail wing of cloud cavitation and non-cavitation control mechanism

The cloud-cavitation shedding mechanism was numerically investigated around the NACA 0015 hydrofoil of α = 7° and σ = 0.7, 0.67 under the identical computational conditions as in Paper I [W. Jin, AIP Adv. 11, 065028 (2021)]. We discovered the invisible tail wing and self-inhibition effects of cloud cavitation. As the invisible tail wing of cloud cavitation swings up, the generated re-entrant jet causes cavitation shedding or collapse by the “sweeping and ejection” processes and simultaneously moves away turbulence kinetic energy (TKE) from the near-wall flow fields of the leeward hydrofoil surface, stopping the cavitation generation. In low pressure regions, non-uniform TKE intensity distributions cause different water-vapor volume fractions, resulting in discontinuity of cavitation generation. The attached vortex accompanying an individual cavity is defined, which causes fluctuations and cavitation instability on the bottom of the cavity. The cavity-bubble truncation and stretching are two primary transition mechanisms from the sheet to cloud cavitation. Compared with the invisible tail wing of cloud cavitation, the fixed unilateral wing can more effectively inhibit the cloud shedding because it can redistribute energies to two hydrofoil surfaces and transfer the strong TKE intensity from the minimum to the high-pressure region, which inhibits flow boundary layer separation and achieves non-cavitation control of the hydrofoil. Energy transfer and balance are the most effective mechanisms for inhibiting cloud cavitation. Larger unilateral wing sizes result in weaker TKE intensity along the leeward hydrofoil surface as well as more significant cloud-cavitation inhibition. The TKE intensity in the leading edge of the leeward hydrofoil surface determines the fluid boundary layer separation and cloud-cavitation stability.