Abstract
Experimental studies of the influence of fluid–structure interaction on cloud cavitation about a stiff stainless steel (SS) and a flexible composite (CF) hydrofoil have been presented in Parts I (Smithet al.,J. Fluid Mech., vol. 896, 2020a, p. A1) and II (Smithet al.,J. Fluid Mech., vol. 897, 2020b, p. A28). This work further analyses the data and complements the measurements with reduced-order model predictions to explain the complex response. A two degrees-of-freedom steady-state model is used to explain why the tip bending and twisting deformations are much higher for the CF hydrofoil, while the hydrodynamic load coefficients are very similar. A one degree-of-freedom dynamic model, which considers the spanwise bending deflection only, is used to capture the dynamic response of both hydrofoils. Peaks in the frequency response spectrum are observed at the re-entrant jet-driven and shock-wave-driven cavity shedding frequencies, system bending frequency and heterodyne frequencies caused by the mixing of the two cavity shedding frequencies. The predictions capture the increase of the mean system bending frequency and wider bandwidth of frequency modulation with decreasing cavitation number. The results show that, in general, the amplitude of the deformation fluctuation is higher, but the amplitude of the load fluctuation is lower for the CF hydrofoil compared with the SS hydrofoil. Significant dynamic load amplification is observed at subharmonic lock-in when the shock-wave-driven cavity shedding frequency matches with the nearest subharmonic of the system bending frequency of the CF hydrofoil. Both measurements and predictions show an absence of dynamic load amplification at primary lock-in because of the low intensity of cavity load fluctuations with high cavitation number.
Highlights
Cavitation commonly occurs on marine lifting surfaces, such as propellers, hydrofoils, turbines, and energy harvesting and energy saving devices during high-speed operation near the free surface
To verify the system parameters used in the reduced-order models (ROMs) presented in § 3, the predicted system modal frequencies are compared with experimental measurements of the stainless steel (SS) and CF hydrofoils in fully wetted conditions in figure 5
The average power spectral density (PSD) was used, as the frequency at the peak of the PSD varied slightly with αo owing to changes in entrained fluid inertia, when stall develops for αo 10◦ (Zarruk et al 2014; Young et al 2018)
Summary
Cavitation commonly occurs on marine lifting surfaces, such as propellers, hydrofoils, turbines, and energy harvesting and energy saving devices during high-speed operation near the free surface. The interface between the fluid and body contains voids that can serve as a rupture point for bubbles to grow. This formation of rupture, as well as rupture around a solid contaminant particle in the fluid, is known as heterogeneous nucleation (Brennen 1995). Rupture can occur when there is a void in the fluid, which is known as homogeneous nucleation (Brennen 1995). There is much similarity between cavitation and ventilation, the formation and collapse mechanisms are different between the two phenomena owing to the differences in condensibility and compressibility of the vapour versus gas within the cavities.
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