We experimentally investigated the effect of fluid–structure interaction on unsteady cavitating flows around flexible and stiff National Advisory Committee for Aeronautics 0015 hydrofoils in a low-pressure cavitation tunnel. We analyzed the cavitating dynamics by capturing the cavitation dynamics using two high-speed cameras at different cavitating regimes on the surface of the hydrofoils, made of polyvinyl chloride, brass, and aluminum. We then measured the associated structural deformations in specific cavitation regime such as cloud and partial cavitation dynamics, using a digital image correlation technique. The hydrofoil's angle of attack was set to 10°, and the flow's Reynolds number was adjusted to 0.6 × 10⁶. Results showed that the cavity's shedding frequency on the flexible hydrofoil shifted faster to a higher frequency than on the stiff hydrofoils under similar cavitating conditions. The flexible hydrofoil underwent strong structural oscillations at the low cavitation number for the cloud cavitation regime. The associated amplitudes of the vibration were about 20 times higher than those of the hydrofoil made of brass. It was observed that the fluid–structure interaction can significantly affect the cavitation-induced vibration of the flexible hydrofoil.

Within the framework of Space@Sea project, an articulated modular floating structure was developed to serve as building blocks for artificial islands. The modularity was one of the key elements, intended to provide the desired flexibility of additional deck space at sea. Consequently, the layout of a modular floating concept may change, depending on its functionality and environmental condition. Employing a potential-flow-based numerical model (i.e., weakly nonlinear Green function solver AQWA), this paper studied the hydrodynamic sensitivity of such multibody structures to the number of modules, to the arrangement of these modules, and to the incident wave angle. Results showed that for most wave frequencies, their hydrodynamic characteristics were similar although the floating platforms consisted of a different number of modules. Only translational horizontal motions, i.e., surge and sway, were sensitive to the incident wave angle. The most critical phenomenon occurred at head seas, where waves traveled perpendicularly to the rotation axes of hinged joints, and the hinge forces were largest. Hydrodynamic characteristics of modules attached behind the forth module hardly changed. The highest mooring line tensions arose at low wave frequencies, and they were caused by second-order mean drift forces. First-order forces acting on the mooring lines were relatively small. Apart from the motion responses and mooring tensions, forces acting on the hinge joints governed the system’s design. The associated results contribute to design of optimal configurations of moored and articulated multibody floating islands.

We experimentally investigated a passive cavitation control approach to control partial and cloud cavitation on a National Advisory Committee for Aeronautics 0015 hydrofoil. For this approach, we implemented two different kinds of mesoscale bio-inspired riblets, known as scalloped riblets and sawtooth riblets, on the suction side of the hydrofoil near its leading edge. We studied the dynamics of partial and cloud cavitation on the hydrofoil with and without two kinds of riblets using a high-speed camera. In addition, we performed hydro-acoustic measurements to analyze the effects of this passive control on the cavitation induced noise in the wake of the hydrofoils. We considered flows at Reynolds numbers 0.5 × 106 and 0.6 × 106 with the hydrofoils at angles of attack of 8° and 10°. Results revealed that the cloud cavitation shedding on the hydrofoil suction surface was mitigated significantly due to the reduction in the re-entrant jet momentum. Also, the influence of tip vortex cavitation was reduced on the modified hydrofoils. Furthermore, the large-scale cavitation induced noise for the unmodified hydrofoil was located mostly at lower frequency in the range of 30 Hz, which was similar to the cavity shedding frequency. The small-scale vortex-induced noise concentrated at relative higher frequency between 100 and 120 Hz. The noise reduction in this study was achieved by scalloped and sawtooth riblets at low frequency domain and for some cases at higher frequency range by manipulating of the cavitation oscillation mechanism and elimination of a large-scale cavitation structure. This study provided new insight into controlling the destructive effects of cavitation using bio-inspired riblets, which should be relevant for various engineering applications.

Extensive water entry experiments were performed to identify uncertainties associated with measured impact-induced loads acting on flat bottom ship structures. Of primary concern was the influence of air trapping on elastic structural deformations. The experimental measurements supplied the benchmark data suitable to validate computational fluid dynamics (CFD) predictions. Two bodies were tested. One body was fitted with stiffened, rigid bottom plating, and the other body was fitted with thin elastic plating. A large number of 30 repetitive runs recorded bottom pressures and forces acting on the flat bottom plating and monitored impact-induced elastic bottom strains. For each test case, high-speed videos of water entry sequences were evaluated. The resulting average peak values standard deviations quantified the uncertainty of these measurements.