the center of the test section, so that the wave records could be cor- 

 related directly with the resulting wave-induced force record. 



A Brush dual-strain gage amplifier was used in the experiments, 

 with one channel connected to the wave gage, and the other channel 

 connected to the force meter. The amplifier was connected to a Brush 

 two-channel rectilinear writing recorder which continuously recorded 

 the waves and corresponding wave-induced forces on the pipe section 

 (Fig. 16). 



An electronic digital data acquisition system (Paulling and Sibul, 

 1968) was used in the three-dimensional experiments. The digitizer was 

 connected in parallel with the strain gage amplifier to record simulta- 

 neously the wave and corresponding force data on magnetic tape, while 

 at the same time the data were being recorded continuously on the strip- 

 chart recorder (Fig. 17). The digitizer sampled alternatingly from 

 both the wave record and force record at a rate of 100 samples per 

 second, resulting in 50 samples per second from each of the two chan- 

 nels . 



2. Procedure for Two-Dimensional Experiments . 



a. Calibration . Both the wave gage and the force transducer were 

 calibrated before each set of experimental runs. The wave gage was 

 calibrated statically by raising and lowering the gage in increments of 

 u.us root (.i.i>z centimeters) and recording the output. The force meter 

 was also calibrated statically by hanging weights in increasing equal 

 increments from a system of pulleys connected to the force meter and 

 recording the output on the strip chart. The force transducer was cali- 

 brated in both the upward and downward directions by rearranging the 

 pulley system and repeating the above procedure. The calibration method 

 is shown in Figure 18. 



b. Procedure . After calibrating the force meter, the model pipe 

 section was lowered and fixed in a horizontal position at the desired 

 clearance above the bottom of the wave channel, with the long axis of 

 the test cylinder parallel to the approaching wave crests. A sliding 

 point gage was mounted to the wave channel above the pipe section and 

 was used to accurately set the model pipe to the desired bottom clear- 

 ance and aline the pipe section parallel to the wave crests. Once the 

 model was in the correct position, the mounting brackets and support 

 struts were clamped to the sides of the wave channel. The force trans- 

 ducer was mounted in such a way that it was sensitive only to forces 

 acting in the vertical direction. 



After the model pipe section was mounted in position, the wave gage 

 was lined up directly over the center of the pipe section with a plumb 

 bob and then clamped in position. The wave gage was then calibrated as 

 described above. The experimental arrangement is shown in Figure 19. 



45 



