Multi-Domain 2.5D Method for Multiple Water Level Hydrodynamics

Abstract

Abstract:The mean water surface (interface) under the air cushion of a surface effect ship (SES) or an air cushion supported platform (ACSP) is generally lower than the outside water surface due to the overpressure of the air cushion. To precisely analyze the hydrodynamics under the air cushion, multiple water levels should be considered in numerical models. However, when using free surface Green's functions as numerical methods, the water level difference cannot be taken into account, because free surface Green's functions normally require users to set in the whole water domain a unique datum water surface that completely separates the air domain and the water domain. To overcome this difficulty, a multi-domain approach is incorporated into a 2.5D method that is based on a time domain free surface Green's function with viscous dissipation effects in this paper. In the novel multi-domain 2.5D method, the water domain is partitioned into inner and outer domains, and the interface is located in the inner domain while the outside water surface is placed in the outer domain. In each domain there exists only one unique water level, while water levels in different domains are allowed to be different. Benefited from this characteristic, the multi-domain 2.5D method is able to precisely consider the water level difference and its influence on hydrodynamics. The newly proposed multi-domain 2.5D method is employed to predict the hydrodynamics of an SES, and it is confirmed that the multi-domain 2.5D method can give better numerical results than the single-domain one for the given case.

Nonlinear air dynamics of a surface effect ship in small-amplitude waves

“…The distinctive nonlinear features of the air pressure and the motion response should result from the small displacement proportion of the air cushion (only 33% of the SES displacement), which reduces the influence of the fluctuating air pressure on the SES motions. Thereby, in our previous works [8][9][10][11] , only the linear models were applied to predict the seakeeping motions. Once the displacement of the air cushion contributes to a larger proportion, e.g., 80%, the motions of the SES show significant nonlinear effects, which requires more appropriate models for analysis.…”

“…For simplicity, the framework of air dynamic model and its linear components similar to Guo et al . [8] are given in Appendix A , and here we only present the nonlinear air leakage model that is different from the one in Guo et al [9][10][11] .…”

“…In the computational fluid dynamics (CFD) methods, the air pressure hydrodynamics and cushion air dynamics are coupled to each other and need to be simultaneously solved at each time step [1] . Alternatively, in potential theory the air pressure hydrodynamic problem could be deliberately decoupled from the air cushion dynamic problem, and specialized methods can be employed to individually solve the air pressure hydrodynamic problem, e.g., the analytical methods for pressure patches [ 9 , 20 ], 3D Green's function method for zero-speed air cushion supported platform [16] , a single-domain 2.5D method [10] or a multi-domain 2.5D method [11] for high-speed SES.…”

“…According to the experimental results from authors' SES model in small-amplitude waves [8] , the heave and pitch responses are almost linearly proportional to wave amplitudes while the fluctuating cushion air pressure, midship accelerations have significant nonlinear effects. It also suggests that the nonlinear effects of the SES should not be neglected and the existing linear models [8][9][10][11] are not appropriate for the nonlinear performance prediction. To this end, the source of nonlinearity for the SES will be examined and a novel efficient seakeeping motion prediction model will be developed to take into account the nonnegligible nonlinear effects of the seakeeping performance in this paper.…”