Abstract
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.
Highlights
During the processes of violent dynamic shedding and unstable collapse, cloud cavitation is a severe problem for the safety of fluid machinery1–3 due to severe structural vibration, material erosion, and noise generated in the process.4–7 Complicated two-phase flows generate cavitation instability
Pham et al.16 and Kawanami et al.17 verified the presence of the re-entrant jet in the adverse pressure region, predicting that the re-entrant jet has a prominent role in cloud-cavitation generation
It can be observed that compared with the smooth NACA 0015 hydrofoil, for cases IIB(i)–(iii), the maximum velocities significantly decrease in the leading edge of the leeward hydrofoil surface while the turbulence kinetic energy (TKE) intensities become weaker, with significant shrinking of the minimum pressure regions
Summary
During the processes of violent dynamic shedding and unstable collapse, cloud cavitation is a severe problem for the safety of fluid machinery due to severe structural vibration, material erosion, and noise generated in the process. Complicated two-phase flows generate cavitation instability. To obstruct the re-entrant jet, they all placed an obstacle on a hydrofoil to find that auto-oscillations vanished, which revealed the direct relationship between the re-entrant jet and cloud cavitation They further investigated the necessary conditions for the development of a re-entrant jet and subsequent periodic shedding around canonical two-dimensional geometries. Foeth et al. used a high-speed camera to observe a three-dimensional twisted hydrofoil with an attached cavity closely related to propellers, finding that the span-wise component of the re-entrant jet caused the pinching off of water-vapor clouds. According to these earlier investigations, the re-entrant jet is popularly accepted as an important influence to the periodic shedding of cloud cavitation.
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