This paper presents an experimental investigation on drag characteristics and flow physics of ventilated supercavitating objects with different body shapes. The test model consists of a disk-type cavitator with two different forebodies (slender and blunt shape) and three different rear bodies (flat, shrinkage, and expanded shape). Experiments are conducted in a cavitation tunnel of the Chungnam National University. First, the drag forces acting on different body-combinations in fully wetted conditions are measured. The results show that the drag coefficients strongly depend on the body shapes. It explains in detail through particle image velocimetry measurements. Second, the drag characteristics are systematically examined over a broad range of ventilation rates. The formation and the drag characteristics of the foamy and clear supercavity flow are investigated for different Froude number conditions. The results reveal that the drag of the blunt forebody is smaller than that of the slender fore body in foamy cavity conditions. In ventilated supercavity conditions, supercavity shapes according to Froude numbers are examined and corresponding effects on drag characteristics are analyzed. The results show that the drag coefficients of the models with the expanded rear body are larger than that of models with the flat and shrinkage rear bodies until the cavity covers the body.

The applicability of leading-edge protuberances as a passive flow control approach inspired by humpback whale flippers has attracted significant research attention in aquatic and aeronautic systems because of their influence on critical hydrodynamic and aerodynamic aspects. An experimental investigation is conducted in a cavitation tunnel under various flow conditions to determine the effectiveness of leading-edge protuberances in controlling the detrimental effects of cavitation and suppressing flow-induced noise. The experiments are carried out on four National Advisory Committee for Aeronautics airfoil 0012 hydrofoils at 7° attack angles and free stream velocities up to 10 m/s. One of the four hydrofoils is considered the baseline, while the other models have wavy leading-edge modifications with different sinusoidal protuberances. These geometry modifications are defined by the amplitudes (A) (2% and 4% of the mean chord length) and wavelengths (λ) (12.5% and 25% of the mean chord length) of the sinusoidal protuberances. Investigations of flow over hydrofoils from top and side views at various Reynolds numbers exhibit that cavitation first appears in the modified hydrofoils' troughs and is restricted to just behind the protuberance troughs for the entire cavitating flow range. These results contrast the baseline geometry, where cavitation inception occurs at the flat leading edge, and the sheet cavity expands spanwise with extensive cloud shedding. Image processing under certain conditions reveals that the protuberances reduce cavitation by 25%–60%. The analysis of the sound pressure level demonstrates that the leading-edge protuberances effectively decrease flow-induced noise at higher flow velocities when cavitation is the dominant noise source. Finally, the direct comparison of cavitating flow characteristics, quantitative cavitation measurements, and noise production analysis between the baseline and modified hydrofoils, and their comparison among the modified geometries, provides a significant reference for future modeling of potential applications employing this passive flow control technique.

In many practical submerged objects, various types of cavitation such as bubble, sheet, and cloud cavitation occur according to flow conditions. In spite of numerous theoretical, numerical, and experimental studies, there are still many problems to be solved such as induced noise and damage potential due to cavitation. In this paper, an experimental investigation on coherent structures and induced noise characteristics of partial cavitation on a two-dimensional hydrofoil is presented. Experiments that focused on the dynamics of cavitation clouds were conducted in a cavitation tunnel. Using high-speed visualization, the series process consisting of inception, growth, and desinence of the partial cavity was investigated. The noise generated during the process was also measured, and the correlation with the cavity pattern was examined. The results show that the periodic behavior of cavitation clouds is directly reflected in the noise characteristics. In addition, the visualization of coherent structures within the sheet and cloud cavity provides a qualitative understanding of hairpin vortices and their packets, which play a dominant role in turbulent cavitating flows.

Cavitation affects engineering applications relating to aquatic operations. Tip vortex cavitation (TVC) leads to several technical problems, such as efficiency loss and noise. Hence, there is a need for a comprehensive understanding of the formation mechanism, shape distortions, and underlying physical phenomena of TVC. The dispersion relation of inertial waves on inviscid cavitating vortices is a valuable tool for predicting various TVC shapes. This study explains the patterns of flow around the tip of an elliptical foil and the cavity shape deformations under different flow conditions through experimental analysis. Experiments are conducted using a National Advisory Committee for Aeronautics hydrofoil in the cavitation tunnel at Chungnam National University. The appearance, development, and shapes of vortex cavitation are closely examined using high-speed imaging technology. There is good agreement between the vortex cavity shapes captured by this high-speed imaging and those derived in previous studies. Using laser Doppler velocimetry (LDV) measurements and analysis through image processing of high-speed images, we compare the vortex core trajectories in cavitating and non-cavitating conditions. There is a good match between the two, demonstrating the feasibility of predicting the flow behaviors around vortex cavities using LDV data. As the noise from the TVC is considered a significant source of underwater radiated noise we also measured sound pressure level, which exhibits abrupt changes at specific cavitation numbers, supporting the occurrence of the singing vortex phenomenon and highlighting its sensitivity to small variations in flow conditions. Furthermore, we obtain precise measurements of the instantaneous TVC diameter under different cavitation numbers, allowing the physical functionality of distinct TVC shapes to be determined. Our results significantly extend the scientific understanding of the flow around TVC and the fundamental causes of TVC distortions.