The Hydrodynamic and Hydroelastic Responses of Rigid and Flexible Surface-Piercing Hydrofoils in Multi-Phase Flows

Abstract

Ventilation and vaporous cavitation are multi-phase flows with critical effects upon the performance, stability, and controllability of high-speed marine vessels. The entrainment of air from the free surface (ventilation) or the formation of water-vapor-filled voids (cavitation) can cause dramatic reductions in the efficiency of lifting surfaces, large dynamic loads, and strongly hysteretic flows. This thesis investigates the hydrodynamic and hydroelastic performance of surface-piercing hydrofoils through experiments on three hydrofoil models (one rigid and two flexible), which were tested in a towing tank and in a free-surface cavitation tunnel.

The results reveal four distinct flow regimes, which are defined by their parametric stability regions. The concept of flow stability is used to describe transitional flow and resulting hysteresis to yield a holistic description of ventilation on surface piercing hydrofoils. These concepts are used to develop scaling relations for the washout of ventilated cavities. Hydrodynamic loads are shown to vary as functions of the attack angle, immersion depth, forward speed, cavitation number, and flow regime. Flexibility of the hydrofoil model modifies the hydrodynamic loads and stability regions through hydroelastic coupling. Flow-induced vibration and lock-in are shown to result from coherent vortex shedding at all speeds tested. Fitted transfer functions are used to develop reduced-order models and to estimate modal parameters of a flexible hydrofoil, demonstrating that both modal resonance frequencies and modal damping ratios are dependent upon immersion depth, forward speed, and flow regime. A robust shape-sensing strut is also developed to measure the textit{in-situ} structural motions of deformable lifting-surfaces in real time.

The work presented in this thesis contributes significantly toward the study of multi-phase flows and fluid-structure interactions through the establishment of experimental methodologies, the construction of a versatile experimental platform with original instrumentation, and the collection and thorough interpretation of a large, rich dataset. The insights gained from the work significantly improve our understanding of ventilation, cavitation, and their interactions with structural dynamics, thereby aiding future researchers and designers to perform robust experiments, validate numerical solvers, and design safe, efficient, and controllable marine devices.