The objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.International audienceThe objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation

The objective of this work is to present combined numerical and experimental studies of natural flow-induced vibrations of flexible hydrofoils. The focus is on identifying the dependence of the foil’s vibration frequencies and damping characteristics on the inflow velocity, angle of attack, and solid-to-fluid added mass ratio. Experimental results are shown for a cantilevered polyacetate (POM) hydrofoil tested in the cavitation tunnel at the French Naval Academy Research Institute (IRENav). The foil is observed to primarily behave as a chordwise rigid body and undergoes spanwise bending and twisting deformations, and the flow is observed to be effectively two-dimensional (2D) because of the strong lift retention at the free tip caused by a small gap with a thickness less than the wall boundary layer. Hence, the viscous fluid-structure interaction (FSI) model is formulated by coupling a 2D unsteady Reynolds-averaged Navier-Stokes (URANS) model with a two degree-of-freedom (2-DOF) model representing the spanwise tip bending and twisting deformations. Good agreements were observed between viscous FSI predictions and experimental measurements of natural flow-induced vibrations in fully turbulent and attached flow conditions. The foil vibrations were found to be dominated by the natural frequencies in absence of large scale vortex shedding due to flow separation. The natural frequencies and fluid damping coefficients were found to vary with velocity, angle of attack, and solid-to-fluid added mass ratio. In addition, the numerical results showed that the in-water to in-air natural frequency ratios decreased rapidly, and the fluid damping coefficients increased rapidly, as the solid-to-fluid added mass ratio decreases. Uncoupled mode (UM) linear potential theory was found to significantly over-predict the fluid damping for cases of lightweight flexible hydrofoils, and this over-prediction increased with higher velocity and lower solid-to-fluid added mass ratio.

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In this paper, the advantages, state-of-the-art, and current challenges in the field of adaptive composite marine propulsors and turbines are reviewed. Adaptive composites are used in numerous marine technologies, including propulsive devices and control surfaces for marine vessels, offshore platforms, unmanned surface and underwater vehicles, and renewable energy harvesting devices. In the past, most marine propulsors and turbines have been designed as rigid bodies, simplifying the design and analysis process; however, this can lead to significant performance decay when operating in off-design conditions or in spatially or temporally varying flows. With recent advances in computational modeling, materials research, and manufacturing, it is possible to take advantage of the flexibility and anisotropic properties of composites to enable passive morphing capabilities to delay cavitation and improve overall energy efficiency, agility, and dynamic stability. Moreover, active materials can be embedded inside composites to enable energy harvesting, in situ health and condition monitoring, mitigation and control of flow-induced vibrations, and further enhancements of system performance. However, care is needed in the design and testing of adaptive composite marine propulsors and turbines to account for the inherent load-dependent deformations and to avoid potential material failures and hydroelastic instabilities (resonance, parametric excitations, divergence, flutter, buffeting, etc.). Here, we provide a summary of recent progress in the modeling, design, and optimization of adaptive composite marine propulsors and turbines, followed by a discussion of current challenges and future research directions.