Testing Ship Models in Transient Waves 



TEST TECHNIQUES 



Outline of Model Testing Technique Using Transient Waves 



Transient testing with a model in ahead waves is accomplished quite easily. 

 As currently conducted, the first waves to be generated are slowly progressing 

 high-frequency waves. When these first waves have traversed a portion of the 

 test basin, the model is brought up to speed in calm water and measurement of 

 all dynamic variables is commenced. The wave train passes, induces motion, 

 and then the water and model return to the quiescent condition where recording 

 is stopped. 



Each time- record is used to compute a Fourier transform from a common 

 time base. The wave height transform must be corrected to the location of the 

 model center of gravity by the transfer function q- }^\<^\><-/s^ where x is the dis- 

 tance to the ahead wave probe and c^ is the wave frequency. The ratio of motion 

 transform to corrected wave height transform defines the transfer function, 

 amplitude and phase for that motion. 



Programming for Transient Wave Generation 



The wavemaking system in the Harold E. Saunders Maneuvering and Sea- 

 keeping Facility, described recently by Brownell [3], is quite well suited for the 

 generation of transient waves. Eight electrohydraulic servo systems can be 

 used to control the flow of air to and from domes along the shorter side of the 

 basin, thus imparting energy to the water which travels away in the form of 

 waves. These servo systems can be driven in unison by an electrical signal 

 from either a low- frequency sine wave generator or a tape recorder. 



The actuator servo system has proved to be a considerable improvement 

 over the previous electromechanical arrangement for wave generation, which 

 provided a constant-amplitude variable-frequency, sinusoidal excitation to the 

 water. The actuator system can allow independent control of both amplitude 

 and frequency, or it can introduce transient or random disturbances of more 

 general form. Random wave generation has been described recently in Ref. 4. 



The transient waveform which has been used to date is characterized by a 

 linearly decreasing frequency, starting at the highest frequency of interest for 

 model testing— nominally 1.0 cps. The electrical signal that produces these 

 waves is recorded on magnetic tape by the crude but effective procedure of 

 linearly decreasing the frequency of a low-frequency sinusoidal source. 



The frequency response characteristics of the basin, relating wave height 

 to actuator motion, are such that frequency components near 0.4 to 0.5 cps are 

 greatly amplified. Two modes of transient waves have been used — one has the 

 program amplitudes weighted so as to counteract this frequency behavior; the 

 other has a constant amplitude with varying frequency. 



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