It is known that the use of frequency dependent coefficients to represent the radiation forces associated with non-linear motions is not correct. Thus it is interesting to understand what is the magnitude of the error if the frequency domain expressions were applied directly for the calculation of the time domain non-linear motions and loads associated harmonic incident waves. Figure 11 represents the radiation contribution for the vertical shear force at 25%Lpp and the radiation contribution for the vertical bending moment at midship. The wave conditions are the same as those presented in Fig. 8.
Contrary to the results presented in Fig. 8, these calculations were carried out considering non-linear hydrostatic and FcoudeKrylov. The continuous and dashed lines labeled as l&, & represent the radiation contribution to the VSF and VBM and the procedure followed was
- first, the non-linear time domain solution is obtained and the motion and loads calculated [equations (18) and (19) for the motions and equations (29)-(32) for the loads]. Time domain responses &, & and &, ;cs are plotted (continuous lines).
- second, the previously calculated non-linear motions are forced in the expressions which give the frequency domain radiation contributions for the loads [equations (33) and (34)]. Results are represented by the dashed lines.
- third, the curves are superimposed in order to show that in fact the frequency domain coefficients cannot be used if the motions are non-harmonic (this simplified approach is used in some non-linear time domain procedures). The definition of directions for loads is such that in the sagging condition the vertical shear force is positive and the vertical bending moment is negative.