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The objective of this paper is to investigate cavitating flows around a pitching hydrofoil via combined physical and numerical studies. The aims are to (1) improve the understanding of the interplay between unsteady cavitating flow, hydrofoil motion, and hydrodynamic performance, (2) quantify the influence of pitching rate on subcavitating and cavitating responses, and (3) quantify the influence of cavitation on the hydrodynamic load coefficients and surrounding flow structures. Results are presented for a NACA66 hydrofoil undergoing controlled, slow $(\dot \alpha = 6^\circ /{\rm s})$(α̇=6∘/s) and fast $(\dot \alpha = 63^\circ /{\rm s})$(α̇=63∘/s) pitching motions from α = 0° to α = 15° and back to α = 0° for both subcavitating and cavitating conditions at a moderate Reynolds number of Re = 750 000. The experimental studies were conducted in a cavitation tunnel at the French Naval Academy, France. The numerical simulations are performed by solving the incompressible, multiphase Unsteady Reynolds-Averaged Navier-Stokes Equations via the commercial code CFX using a transport equation-based cavitation model; a modified k-ω SST turbulence model is used to account for the effect of local compressibility on the turbulent eddy viscosity. The results showed that increases in the pitching rate suppressed laminar to turbulent transition, delayed stall, and significantly modified post-stall behavior. Cavitation inception at the leading edge modified the pressure distribution, which in turn significantly changed the interaction between leading edge and trailing edge vortices, and hence the magnitude as well as the frequency of the load fluctuations. For a fixed cavitation number, increases in pitching rate lead to increase in cavitation volume, which in turn changed the cavity shedding frequencies and significantly modified the hydrodynamic loads. Inversely, the leading edge cavitation observed for the low pitching velocity case tends to stabilize the stall because of the decrease of the pressure gradient due to the formation of the cavity. The results showed strong correlation between the cavity and vorticity structures, which suggest that the inception, growth, collapse and shedding of sheet/cloud cavities are important mechanisms for vorticity production and modification.

Summary An energy dissipation factor was proposed to quantify the energy dissipation mechanism of particle dampers based on theoretical analysis and was further validated by free vibration tests and wind tunnel tests. The vibration energy of the main structure was consumed by impact and friction between particles and between particles and the container. An elastoplastic collision model and a simplified frictional‐elastic collision model were used to analyze the energy dissipation due to impact and friction, respectively. Then, an energy dissipation factor, reflecting the vibration energy consumption of a particle damper, was defined. Finally, free vibration tests and aero‐elastic wind tunnel tests of a benchmark model unattached or attached with particle dampers were conducted to validate the relationship between the vibration reduction performances and the energy dissipation factors, and the experimental results were in qualitative agreement with the theoretical results. Consequently, the energy dissipation factor indicated the energy dissipation mechanism of particle dampers and can be used to select the proper material of the particles, helping to maximize the vibration control effects from the material's perspective. It was shown that the material of higher kinetic friction coefficient, higher modulus of elasticity, and lower yield strength usually leads to better energy dissipation effects.

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The objective of this paper is to evaluate the predictive capability of three popular transport equation-based cavitation models for the simulations of partial sheet cavitation and unsteady sheet/cloud cavitating flows around a stationary NACA66 hydrofoil. The 2D calculations are performed by solving the Reynolds-averaged Navier-Stokes equation using the CFD solver CFX with thek-ωSST turbulence model. The local compressibility effect is considered using a local density correction for the turbulent eddy viscosity. The calculations are validated with experiments conducted in a cavitation tunnel at the French Naval Academy. The hydrofoil has a fixed angle of attack ofα=6° with a Reynolds number of Re = 750,000 at different cavitation numbersσ. Without the density modification, over-prediction of the turbulent viscosity near the cavity closure reduces the cavity length and modifies the cavity shedding characteristics. The results show that it is important to capture both the mean and fluctuating values of the hydrodynamic coefficients because (1) the high amplitude of the fluctuations is critical to capturing the extremes of the loads to ensure structural safety and (2) the need to capture the frequency of the fluctuations, to avoid unwanted noise, vibrations, and accelerated fatigue issues.

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