ABSTRACT

When identifying hydrodynamic behaviour of a structure through wave tank testing, transfer functions in frequency domain are usually computed. Input-output system is then considered where the output is the characteristic parameter to be identified and the input is the incident wave. Since the considered body generates diffracted waves as well as radiated waves if free to move, identification of incident waves is not straightforward and the choice of the signal to be considered as the system input remains a problem. Wave measurement in the vicinity of the structure is disturbed by these parasitic waves and should not be taken as an input for the transfer function computation. Use of time series recorded without structure could be considered if the wave maker was ideally reproductive. And even if so, phase identification would remain uneasy. Identification of scattered and radiated wave remains difficult since their propagation directions depends on the shape of the tested body as well as on its motions. An approach to solve this problem is then to introduce as input lbr identification of transfer functions the free surface elevation measured upstream, far enough from the structure for the signal not to be disturbed. Wave propagation according to dispersion relations is then taken into account, introducing FFT techniques. An original method is introduced for correction of wave numbers according to Stokes third order developments. This method is based on properties of the Hilbert translbrm, applied to transient wave groups. In order to validate the procedure, it is applied to time series recorded during tests carried out at the IFREMER wave basin. Hydrodynamic forces on a fixed rigid cylinder in waves are measured and free surface elevation is controlled at different locations along the basin and in the vicinity of the cylinder.

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