Current Velocity 

 Profile 



Vertical Velocity 

 Profile 



Figure 137. Qualitative description of current patterns produced by pneumatic 

 breakwater (after Straub, Bowers, and Tarapore, 1959). 



From equation (75) , an increase of air discharge by a factor of 8 is 

 necessary to double the maximum surface current. As the air discharge 

 increases at a given water depth, a condition is eventually reached where 

 further increases in discharge result in little change in attenuation. With 

 increasing submergence comes larger power requirements to overcome hydrostatic 

 pressure. Hence, an optimum depth for air release for a given wavelength and 

 water depth exists, but in practical applications the air-release mechanism 

 would probably be placed on the sea floor. 



Kurihara (1958) conducted theoretical and experimental studies of pneu- 

 matic breakwaters which qualitatively verify the Taylor (1955) theory. The 

 experimental laboratory work included three full-scale tests of prototype 

 conditions. It was found that the full-scale tests required much less power 

 than would be expected from a Froude extrapolation of the model data. The 

 power requirement was also much below that predicted by Taylor. Kurihara 

 reinterpreted the mechanism of attenuation to include the induced surface 

 current as the main attenuation factor, and developed a concept of turbulent 

 eddy viscosity which was defined as the product of current velocity and depth. 

 The surface current served as a catalytic agent affecting the turbulent diffu- 

 sion process. Kurihara also noted that the velocity of the induced current in 

 the model tests was proportional to the cube root of the discharge; however, 

 it deviated significantly from this relationship in actual field tests at 

 small discharge rates. A parameter was introduced which functionally related 

 the air discharge and the depth of the release pipe. A limiting value of this 

 bubble efficiency parameter was determined below which the velocity was no 

 longer proportional to the cube root of the discharge. This tended to explain 

 the deviation of the model studies from prototype field tests, and also con- 

 firmed the presence of scale effects because of the inability to scale the 

 model bubble growth in the laboratory. 



199 



