Cavity induced vibration of flexible hydrofoils

Abstract

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

Parametric excitations and lock-in of flexible hydrofoils in two-phase flows

“…initial angle of attack and foil camber). As shown in Theodorsen (1935), Munch et al (2010), and Akcabay et al (2014a), all components of F fluid are proportional to the fluid density ρ f. , C f and K f depend on the flow velocity and the square of the flow velocity, respectively, and both terms have an additional dependency to the flow frequency due to fluid memory/radiation effect associated with the foil interactions with trailing vortices shed by the body. Eqs.…”

“…In a recent study, Akcabay et al (2014a) showed that for flexible hydrofoils in cavitating flow conditions, the cavityshedding frequencies may lock-in (collapse on) to one of the wetted natural frequencies of the hydrofoil ("primary lock-in") and/or their subharmonics ("secondary lock-in"). Lock-in, in general, is a resonant-type phenomenon that occurs if primary frequencies of different system physics (e.g.…”

“…Besides lock-in, it has also been shown in Akcabay et al (2014a) and Rajaomazava et al (2013) that unsteady hydrodynamic cavitation may also produce additional modulated frequencies in the vibration response; this is because the effective fluid-added mass fluctuates with changes in the fluid-mixture density. A major implication of the presence of modulated frequencies for cavitating systems is the added risk for primary parametric resonance: a case if the parametric excitations occur at twice of one of the system natural frequencies, for which vibrations may get much higher than for the case with classical resonance.…”

“…The ROM can also be used to assist the design of control strategies to mitigate cavity-induced vibrations. Akcabay et al (2014a), Akcabay and Young (2014), Chae et al (2015), and Chae et al (2013) modeled the spanwise bending and twisting deformations of a flexible, cantilevered, rectangular hydrofoil through Eq. (1), which was coupled to a viscous, incompressible, and multiphase flow solver (see Akcabay et al (2014a) for a detailed sketch of the geometry).…”

Numerical Study on Characteristics of 3D Cavitating Hydrofoil

“…But these methods are incapable of predicting the distribution and dynamic characteristics of the load on the solid structure that are essential for the utility of more flexible materials on hydrofoil designs. Most previous studies on interactions between hydrofoil and cavity fluid fields are grounded on experiments [3] [4] [5] [6] . Though it is indispensable, advancements in computer hardware and numerical methods open up new ways for cavitation study, which are much more detailed than any affordable experiment in the foreseeable future.…”