Savitsky 



mately 0.60 ft/sec. Trace No. 1 is for the moving wave probe 

 located 13. 25 ft. aft of the grid; trace No. 2 is for a moving wave 

 wire located 66. 5 ft aft of the grid; trace No. 3 represents the 

 moving wave wire 1 2 f t ahead of the grid, and trace No. 4 is for 

 the stationary control wave wire located approximately 20 ft aft of 

 the start of the turbulent wake. The times of start-up and stop of 

 the grid motion and the time of entry of the moving wave wires into 

 the wake are also indicated on this figure. Perhaps the most notable 

 feature on this typical test record is the pronounced oscillation of 

 the measured wave amplitude at all but the stationary wave wire. 

 It is seen that, for the specified test conditions, the measured wave 

 amplitudes varied from nearly zero to values somewhat larger than 

 the incident wave. Further, the time between successive minimum 

 values is approximately 9-10 seconds for the waves in the wake but 

 considerably longer, although not as clearly defined, for the wave 

 probe ahead of the grid. For longer wave lengths, the wave ampli- 

 tude variations were reduced and the apparent period between mini- 

 mum values increased. A reduction in grid speed reduced the wave 

 amplitude variation and increased the apparent period between mini- 

 mum values. There was no discernible effect of grid mesh size on 

 these general observations. 



It is to be noted from Fig. 5 that fluctuations in wave ampli- 

 tude continued for a long time after the turbulence grid was stopped. 

 This is, of course, due to the fact that the wake has a mean flow 

 defined in Fig. 2 and, consequently, moves past the stationary grid. 

 It is also interesting to note that wave deformation at wave wires 1 

 and 2 is first evident after approximately 3 seconds or, equivailently , 

 after a wave crest has traveled nearly 5 ft into the wake. 



Specific Behavior: The envelopes of wave height (h) variation 

 with time, normalized on the basis of incident wave height (h.) , are 

 plotted in Figs. 6 through 11 for a grid speed of approximately 1 ft /sec; 

 a grid draft of 20 inches; and a mesh size of 2.7 inches. Data are 

 presented for the 3, 4 and 8 ft wave lengths, each having a height of 

 approximately 1 inch. The data for the 2 ft wave length are not pre- 

 sented since the wave heights were most irregular even in the non- 

 turbulent flow area. The data for the 6 ft long wave were not unlike 

 those for the 4 and 8 ft test waves and, hence, are not included in 

 this paper. Two companion plots are presented for each wave length. 

 For example, the data for the 3 ft long wave are given in Figs. 6 and 

 7. The envelopes of the ratio h/hj for the three moving wave wires 

 are plotted in Fig. 6 along with the phase angle between wave crests 

 at the centerline and at a point 4 ft outboard of the centerline at a 

 longitudinal distance of 13.25 ft aft of the grid. A zero phase angle 

 represents a crest line parallel to the grid. The complementary 

 data plot for the 3 ft wave is given in Fig. 7 where, in addition to 

 the envelopes of h/h., the apparent wave length is plotted at a 

 longitudinal centerline position approximately 15 ft aft of the grid. 

 This wave length is computed from the data obtained at the two 

 centerline wave wires located at a distance of 13.25 ft and 16.5 ft 



402 



