10 



\ 



^ 



1 



1 1 



PEAK 

 FAM 



1 1 



FREOUENCT OF NEW 

 LY OF SPECTRA 



1 









\ 

















NEUMANN SPECTRUM SIMULATION 

 IN MASK FACILITY 

 RMS AMPLITUDE OF WAVES 

 VERSUS FREQUENCY OF PEAK 

 ENERGY AS FUNCTION OF SCALE 

 RATIO K OR SEA STATE 



MAXIMUM 

 OBTAINED 



^ 





r— MAXIMUM OBTJ 



INEO 













^^ 





SEA 



STATE 

















^ 



6 



5:^ 

















,'-' 



N 



^ 



PEAK FREQUENCY IN CPS 



Figure 16 — Relation among Peak Frequency of Neumann Spectrum, 



RMS Wave Height, Model Scale Ratio A, and Sea State for 



Simulated Long-Crested Seaways in the Test Basin 



the useful frequency range of the basin with a set of six spectra, which will be measured so 

 as to perform a spatial averaging. The peak frequencies of these spectra, as shown in 

 Figure 16, are spaced so that a model of arbitrary scale ratio (within a large range) can be 

 tested in successively higher sea states with each program. A typical model with A = 30, 

 for example, would be able to test in a valid Neumann spectrum in States 4 through 7 seas» 



APPROACH TO THE GENERATION OF DIRECTIONAL SPECTRA 



With the present installation, there are eight wavemakers which can be independently 

 controlled from eight channels of a magnetic tape recorder. The proper programming of these 

 units to create an adequate simulation of a directional seaway is a challenging problem. One 

 approach is outlined here. 



If a voltage sine wave of constant amplitude is used to excite each actuator but the 

 phase lag between adjacent units is nonzero but constant, then in general a sinusoidal 

 wave traveling at an oblique angle from the bank of wavemakers will result, apart from some 

 second-order waves. The phase lag in degrees needed to produce a wave traveling at a fixed 

 angle is a function of frequency. If, instead of a sinusoidal wave, it is desired to generate a 

 program for an oblique but unidirectional random wave train, this could in theory be obtained 



18 



