Savitsky 



tested. The grids were constructed of crossed square wooden slats 

 0.80 Inches wide. The solidity, towing speeds and Reynolds number 

 of the grids were the same as for the one-dimensional tests pre- 

 viously described. The hydrodynamic drag of the 0.36 ft mesh grid 

 was 3.2 lbs and 9.25 lbs at tow speeds of 1 . and 1.7 ft/sec re- 

 spectively at a grid draft of 1. 67 ft. At a grid draft of . 84 ft, the 

 hydrodynamic drags for this grid were 1. 5 lbs and 4. 33 lbs at 1 . 

 and 1. 7 ft/sec. It is to be noticed that the grid drag increased as 

 the square of the speed for all cases. 



Measurements were made of the mean values of the longitu- 

 dinal velocities at depths of 0.80 and 0.40 ft in the grid wake across 

 »«veral transverse sections aft of the 3 ft wide, 0.36 ft mesh grid 

 with a 1.67 and 0.80 ft draft. A plot of the ratio of wake velocity to 

 grid velocity Is given In Fig. 4. These velocity ratios were the 

 same for both grid drafts and towing speeds. 



It Is seen that there is a slow attenuation of velocity with 

 distance aft of grid. Further, there is also a slow lateral spreading 

 of the wake area. It Is interesting to note that the wake velocities 

 are all In the direction of grid tow. Surveys of the velocity field up 

 to 5 ft from the grid centerline did not indicate a reverse flow. 

 Visual observations did Indicate a reverse flow along the bottom of 

 the test tank. 



An empirical formulation was established to represent the 

 wake velocity V,,^ as a function of distance, x, aft of the grid and a 

 distance, y, measured from the grid centerline normal to the x- 

 dlrectlon. The wake equation is given by: 



.8 



= [0.45 - 0.00745x1 fe'^'-^'^^^"'" 

 V 



where x and y are in units of feet. 



Itf = [0.45 - 0.00745X] [e'^'-S^^OO"'" 



The above formulat was developed for use in the analysis 

 of wave deformation for reguxcrr waves running Into a velocity gradi- 

 ent. This analysis Is presented In a subsequent section of this 

 report. 



Wave Height Probes : The location of the wave amplitude 

 measuring probes are shown In Fig. 3. It is seen that four probes 

 were towed with the grid and 7 Inches off Its centerline; one 3 ft 

 forward and three others 1 ft, 3 ft and 6 ft aft of the grid. In addi- 

 tion, seven stationary probes were located In a transverse line 

 nornnal to the direction of ^rld tow and at distance 38 ft and then 

 20 ft ahead of the wavemaker. It will be noted that one of the 

 stationary wave wires was directly on the grid centerline. This 

 Installation was accomplished by mounting the probe on the floor 

 of the tank and providing a slot through the grid which passed over 



398 



