Gravity Waves and Finite Turbulent Flow Fields 



probes indicated a slight concavity to the wave front with the crest 

 along the grid centerline being in the lead by, at most, nearly 30 

 degrees for the 2 ft long wave at a grid speed of 1 ft/sec. It will be 

 recalled that, in the one-dimensional tests, the phase between two 

 transverse probes 4 ft apart continuously increased with time. In 

 general, the wave height time histories were characterized by either 

 a slow attenuation or amplification as the wave passed through the 

 wcike. 



Figures 14 and 15 present the envelope of wave height time 

 histories (normalized on the basis of incident wave height) along the 

 transverse line of stationary wave probes fixed in the tank for 2 ft 

 and 6 ft long waves respectively. The wave height was 1.0 inches 

 and the grid dimensions were: width = 3.0 ft; draft = 1 . 7 ft; 

 mesh =2.7 in; speed = 1 ft/sec. For the probe on the centerline 

 (No. 7), it is seen that there is a continuous decrease in amplitude 

 starting from a time when the probe was 13 feet upstream of the 

 grid. For the 2 ft wave (Fig. 14) the height is attenuated to approxi- 

 mately 10% of the incident wave height when the probe is 7 ft down- 

 stream of the grid and retains this reduced height for the entire time 

 of data collection (80 seconds). It will be noted from Fig. 3 that, 

 when the grid reaches the transverse wave probes, it has already 

 developed a wake 28 ft long moving at a mean longitudinal velocity of 

 approximately 35 per cent of the grid speed. 



The probe 2.4 ft from the centerline (No. 1) also shows a con- 

 tinuous reduction in wave height with increasing time, finally attain- 

 ing a value approximately 35% of the incident wave. Probe No. 2, 

 located 4.2 ft from the centerline, indicates only small variation in 

 wave height with time. The remaining outboard probes (No. 3, 4, 5) 

 all show Increases in wave height for the entire test run. These 

 probes also indicate the existence of mild "beats" in the envelope 

 of time histories although not as severe as for the one-dimensional 

 case previously discussed. The maximum wave height occurs be- 

 tween probes 3 and 4 attaining a value approximately 75% larger 

 than the incident wave. It is to be noted that, for all stationary 

 wave probes, the height modifications are initially noted when the 

 probes are still 13 ft upstream of the grid. In general, then, the 

 characteristics of wave deformation in the wake show a significant 

 reduction in wave height for approximately one grid width on either 

 side of the centerline and an amplification beyond this region. 



Figure 15 represents similar data for a 6 ft long wave -- all 

 other conditions being equal. The general characteristics of wave 

 deformation are identical to the 2 ft long wave except that the mag- 

 nitudes of the changes are reduced. For example, the minimum 

 wave height along the centerline is now 30 per cent of the incident 

 wave while the maximum wave height is 35% larger than the incident 

 wave. 



413 



