Figure 13-7. MTS hydraulic power supply 



between the water surface profile, measured by an electro-resistive wave 

 gauge at the center of the wave board, and the computed profile, calculated 

 from linear wavemaker theory. This error is minimized by changing the 

 velocity of the wave board. Any reflections returning to the wave board from 

 the beach or other structure are canceled via this secondary loop. The trans- 

 fer function between wave board velocity and the wave profile is dependent on 

 water depth and wave frequency. For random waves, the peak spectral fre- 

 quency is used to quantify the transfer function. Thus, reflected wave absorp- 

 tion for random waves is most effective for frequencies close to the peak, 

 where most of the wave energy is concentrated. 



Direct digital wave board control is provided by a Digital Corporation 

 VAX 3400 server, appearing in Figure 13-8, which downloads the command 

 displacement and velocity signals to a local transputer. The transputer com- 

 pares the measured and computed signals and transmits a corrected displace- 

 ment and velocity signal to the wave board, modifying the pressure drop 

 across and flow through the control valve. The wave board control signal is 

 updated at a rate of 20 Hz or higher. The wave board, power supply, direct 



264 



Chapter 13 Wave Generation and Data Collection Systems 



