Ventilation of Lifting Bodies: Review of the Physics and Discussion of Scaling Effects

Abstract

Ventilation is relevant to the performance, safety, and controllability of marine vessels, propulsors, and control surfaces that operate at or near the free surface. The objectives of this work are to (1) review the fundamental physics driving ventilation and its impact upon the hydrodynamic and structural response, and (2) discuss the scaling relations and its implications on the design and interpretation of reduced-scale studies. Natural ventilation occurs when the flow around a body forms a cavity that is open to the free surface. The steady flow regimes, hydrodynamic loads, and unsteady transition mechanisms of naturally ventilated flows are reviewed. Forced ventilation permits control of the cavity pressure and cavity shape, but can result in unsteady cavity pulsations. When a lifting surface is flexible, flow-induced deformations can increase the loading and the size of cavities, as well as lead to earlier ventilation formation. Ventilation tends to reduce the susceptibility of a lifting surface to static divergence. However, fluctuations of fluid added mass, damping, and disturbing forces caused by unsteady ventilation will change the structural resonance frequencies and damping, and may accelerate hydroelastic instabilities. Scaling relations are developed for both the hydrodynamic and hydroelastic response. Similarity in the three-dimensional (3D) ventilation pattern and hydrodynamic response requires simultaneous satisfaction of Froude number, cavitation number, and geometric similarity. However, Froude scaling complicates the selection of suitable model-scale material to achieve similarity in the dynamic hydroelastic response and material failure mechanisms between the model and full scale.