Motion of an Air-Cushion Vehicle in Transforming Nearshore Head Seas

Abstract

Pitching and heaving motions of an air-cushion vehicle, such as a surface effect ship (SES), in response to wave encounters as it transitions through a transforming wave-field such as in the near-shore zone are considered. The vehicle maintains a pressurized air cushion between its two side hulls and the deck hull. In head seas, the passage of waves across the length of the ship results in variations in the air cushion pressure and in associated compressibility effects. In view of the shallow draft of the vehicle, the primary forcing is considered to be due to the compressible dynamics of the air in the cushion in response to the evolving wave field. Through solution of equations governing the response of the vehicle to unsteady changes in the air cushion pressure due to encounter with transforming waves in head seas is described. Coupled unsteady equations for heave and pitch are solved to determine response to passage through transforming head seas. Complementary laboratory experiments are described. Wave transformation from deep waters to the beach is determined using the model of Dally et al. (1985) and the shallow-water SWAN model. The transformations of a regular wave over a beach slope and of an offshore JONSWAP spectrum are computed and used as incident wave fields to determine the response of the vehicle in transitioning at speed U in head seas. The heave and pitch motions for a specific SES are computed as functions of time and frequency in a coordinate system moving with the transitioning vehicle. The characteristics of the vehicle response to a regular transforming monochromatic wave and to transforming irregular waves are determined. Heave and pitch motion of a selected SES, driven by cushion dynamics suggest how the vibrational energy is spread over a range of frequencies as the vehicle proceeds through this evolving wave field. A test platform involving a scaled model SES instrumented with an inertial motion unit to measure six-degrees-of-freedom motions of the vehicle, a pressure gauge to record the air cushion pressure and a custom flex sensor to measure bow skirt deflections due to wave impacts is used to study the vehicle response to transforming waves in laboratory experiments. Results of low Froude number experiments show that the heave, pitch and air cushion excess pressure fluctuations increase as mean air cushion excess pressure increases. The results are compared with the computations described above.