Van Oovtmevssen 



M_=/7* p (x) . 1 x x nl dA (18) 



The constant part of the wave force or drift force can be found from : 



A 

 Evaluation of this integral results in a constant term plus higher har- 

 monic components, which can be neglected. Although the constant for- 

 ce is a second order effect, Havelock [2] has shown that this force 

 may be determined, using a first order approximation for the veloci- 

 ty potential. In general, the constant force is small in comparison 

 with the oscillating wave force; for large structures, however, it may 

 become of interest. 



The wave pattern due to the diffraction of waves by the object 

 can also be found from Bernoulli's theorem. In the free surface, the 

 linearized pressure has to be zero, hence : 



P= -P gz+P^f =0 (20) 



Consequently we find for the surface elevation : 



y = _J_i.^l (21) 



g,l5t|z=0 



II. 5 Comparison of theoretical and experimental results. 



Model tests were performed at the Netherlands Ship Model 

 Basin in order to check the theoretical calculation of wave forces, 

 pressure and wave diffraction. 



In figures 2 and 3 the oscillating horizontal and vertical wave 

 forces on a circular cylinder, as calculated with the computer program 

 of the Netherlands Ship Model Basin, using the three-dimensional 

 source technique, are compared with experimental results. The ex- 

 perimental values, which are given in these figures, were obtained 

 from cross-fairing of the results of a great number of measurements, 

 which were performed with systematically varied cylinders. Also 

 given in these figures are the values according to the analytical solu- 

 tion of Garret. The results of the numerical calculations, which were 

 obtained using only 42 sources to represent the cylinder, closely ap- 

 proximate the analytical results of Garret, while there is also a good 

 agreement between the theoretical and experimental results. 



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