A Lagally formulation of the wave drift force
Résumé
There are two well-known formulations of the wave drift force: the "far-field method", based on momentum considerations, introduced by Maruo (1960) and later extended by Newman (1967), and the "near-field method", based on direct pressure integration and first proposed by Pinkster & Van Oortmerssen (1977). The far-field method is considered as more accurate but it delivers the horizontal components only and it cannot provide individual drift forces in a multiple body configuration. The near-field method does not suffer from these restrictions but its numerical accuracy is poor in some cases. In the software Diodore the drift force is computed following another way, based on the Lagally theorem. This method was first proposed by Guével & Grekas (1981). In their paper the (lengthy) derivations deal with an unbounded fluid domain and the drift force expressions with a free surface are given ex abrupto, without justifications. Moreover they are ambiguous. Subsequently, in his 1986 monograph on wave energy recovery, Guével gives the same expressions (10) (11) as we obtain here (and as have been coded in Diodore for many years), but again without proof. In this paper we re-establish rigorously the Lagally formulation of the drift force. As compared with the classical formulations it appears to offer several advantages.
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