DESIGN PARAMETERS 



An actual meter of this type (fig. 17) was constructed in order to learn 

 more about the range of variables to be encountered. Data samples (figs. 18 and 

 19) were taken at the NEL Oceanographic Research Tower^'^ where the water 

 depth is 60 feet. Both records were taken at a depth of 40 feet; one shows the 

 north-south component and the other the east-west component. These records 

 will serve as examples of typical parameters that influence the accuracy of the 

 method. Keulegan's results can then be applied. 



A cylindrically shaped object attached to a cantilevered beam (fig. 20) 

 was used. The beam shape causes the cylinder to bend in only one plane and 

 therefore the meter is directional. The bending is measured by strain gages 

 cemented to each side of the beam. The hydrodynamic drag on the cylinder 

 causes the beam to bend and compress one strain gage and stretch the other. 

 This produces an equal resistance change in the gages, which in turn produces 

 a voltage output in a dc Wheatstone bridge circuit. 



The problems with the instrument include how to make a sensitive 

 electrical transducer with satisfactory, long-term stability and a low temperature 

 coefficient while retaining rugged mechanical construction and a high natural 

 frequency. However, consider now only the inherent hydrodynamic characteristics 

 and assume that a perfect electrical transducer can be built. Its voltage output 

 would be proportional to the drag force on the cylinder and would have perfect 

 stability without temperature coefficient. 



U T 



First the values of period parameter — ; — are computed. The approximate 



a 



range of period parameter during the 3 days when data were taken was 40 to 240 



for the north-south direction and 60 to 410 for the east-west direction, with d 



constant and equal to 0.51 centimeter. The ranges cover about 90 percent of the 



cases. A typical value for the period parameter was about 200 with T = 10 seconds 



and U^ = 10 centimeters per second. 



The easiest and most accurate quantity to find is the maximum velocity 



during a wave cycle since at this point — = 0. Figure 16, which shows the 



variation of the magnitude and phase of the maximum force as a function of period 

 parameter, can be used. These curves assume a sinusoidal velocity variation. 

 For example by using a typical value of 200 for the period parameter, 



^m approaches 0.6. Therefore F = 0.6 pVjd, which is the same as using 



^ m 



only the second term in equation (24) with a drag coefficient value of 1.2. This 

 is the steady-state drag coefficient value commonly used. Thus for high period 

 parameter values, the acceleration term is relatively unimportant in finding 

 maximum velocity. Note that the phase shift of the maximum force does not 

 approach zero even for large-period parameter values. 



Consider the case where it is desired to find the velocity throughout one 

 wave cycle. As shown in figures 13 and 15, if the period parameter is either 

 small (3) or large (44.7), the values of C^j and Cq are roughly constant over the 



entire cycle. The greatest variation of C\, and Cq is at^ = 0.25 and 0.75. 



30 



