NEUMANN'S 1948-1952 WORK ON WAVE GENERATION BY WIND 



331 



27,000 .... The workiufi; out of the material followed 

 the system in which the iii(li\idual series measurements 

 were divided into class intervals of 0.5 second, were 

 counted, and were presented in the form of distribution 

 of frequency of occurrence (period spectra). Altogether 

 about 250 such series were investigated. 



"In Fig. 3a to c, three such diagrams are presented as 

 examples of three series of ol)servations under ciuasi- 

 stationary wind conditions. . , . Individual 'bands' come 

 to attention as characteristic in the 'period spectra.' 

 The regular appearance of these concentrations of periods 

 in certain class intervals indicates possibly a certain 

 measure of law in a seaway at different wind velocities. 



"The series in Fig. 3(a) shows, besides the frequency 

 maximum in the class inter\'al 3.0-4.5 sec, three addi- 

 tional concentrations. This series gives observations at 

 a wind of 9 m/sec from a stf)pped ship (observations 

 in the sea region southwest of tlu' Azores in a steady wind 

 from an easterly direction). The arrows designated Ti 

 and 7^2 give the computed periods in accordance with 

 earlier theoretical results (Xeumami, 1-1950) for the 

 characteristic predominant wa\'es corresponding to the 

 wind speed. Ti designates the period for the 'sea' and T^ 



the period for the 'long wa\-e' The periotls in the 



interval 3.0-4.5 sec are ex'idently connected with meas- 

 urements of steeper 'sea' for which the theoretical 

 period was given as 3.9 sec. In the band of periods be- 

 tween 7.0 and 8.5 sec we find again the 'long wave' which 

 ought to have a period of 7.9 sec (G. Neumann, 1-1950). 



"The concentration of periods in an interval between 

 the above named main waves should be noted. In the 

 case of Fig. 3(a) the intermediate maximinii occurs in 

 periods from 5.0 to G.5 sec 



"Fig. 3(6) gives a series of measurements at a wind 

 speed of 13.5 m sec. The most conspicuous is the sea 

 with periods between 5 and 8 sec. The calculated period 

 is Ti = 6.6 sec. The 'long wave' should have the period 

 Ti = 11.8 sec, which could also agree with the concentra- 

 tion of periods in the interval 10.5-12.5 sec. As the third 

 concentration we find again the 'intermediate wave,' 

 clearly separated from the other main waves in the sea- 

 way, with periods in the inter\-al 8.5-9.0 sec. In Fig. 

 3(c) quite similar relationships also occur. The period of 

 the 'sea' rises most clearly, always with a certain scatter 

 of individual measurements. The diagram in Fig. 4 

 unites all measurements in the wind strength interval of 

 14.5-15.4 m/sec and shows a smoother 'spectrum' in 

 which the periods around 8 seconds strongly pre- 

 dominate." 



As an example, the following data are gi\-en for three 

 predominating waves in the fully developed sea in wind 

 strength of 16 m/sec (52.5 fps or 31 knots). The sub- 

 script 1 denotes "sea" 2 — "long wave/' and 3 — "inter- 

 mediate wave." 



