Savitsky 



In the present task, vertical turbulence grids of finite drait 

 and two mesh sizes were towed at various constant speeds Ln Tank 

 No. 2 and 3 of the Davidson Laboratory in a direction normal to the 

 plane of the grid. Regular waves, generated by a plunger type wave- 

 maker in quiet water, traveled in the same direction as the grid tow 

 (with initial crest lines parallel to the grid) progressed through the 

 turbulent wake and grid into quiet water beyond the grid. Waves of 

 various constant length and height were generated such that the 

 group velocity of each regular wave was greater than the grid veloc- 

 ity. The wave lengths and water depth were such that deep water 

 gravity waves were generated. Wave amplitudes were measured 

 by resistance type wave wires which penetrated the fluid surface. 

 Several of the wave wires were towed ahead and behind the grid (at 

 the grid speed) while others were stationary and located both in and 

 outside of the grid wake. The outputs of these wave probes were 

 simultaneously recorded on a "Viscorder" oscillograph tape. 



The details of test procedure, grid characteristics, and test 

 conditions for the one -dimensional and two-dimensional turbulence 

 grid studies are described separately below. Common to both 

 studies was the observation that, for the grid sizes and grid veloc- 

 ities considered, the combination of physical grid and turbulent 

 wake in smooth water did not produce a measurable wave system of 

 its own. In fact, soon after passage of the grid and wake relative 

 to a fixed point in the test tank, the water surface appeared unusually 

 still. Further, the grid solidity was small enough that, when sta- 

 tionary, it did not noticeably affect the wave forms which passed 

 through the stationary grid. Neither was there a measurable wave 

 reflection from the grid. 



One -Dimensional Grid Studies 



The one -dimensional grid studies were conducted in Tank 

 No. 3 of the Davidson Laboratory. This tank is 300 ft long, 12 ft 

 wide and has a water depth of 6 ft. A plunger type wavemeiker is 

 located at one end of the tank and a slotted beach of 15° slope is 

 located at the opposite end to absorb the wave energy with minimum 

 reflection. 



Grid Characteristics: A turbulence grid 11,5 ft wide spanned 



the tank width, penetrated the water surface to a depth of 1 , 6 ft, 



was attached to a standard carriage and towed in a direction away 



from the wavemaker. Figure 1 shows the test setup. Two mesh 



sizes were tested; one had a mesh M = 0.36 ft and was made of 



crossed square wooden slats 0.80 inches wide; the other had a 



mesh M = 0.71 ft and was made of crossed square wooden slats 



1.60 inches wide. Thus, in both cases, the grid solidity was 



constant and equal to S = 0,40. The grid was towed at speeds of 



V = 1,0 and V = 1.7 ft/sec. The hydrodynamic drag and Reynolds 



No. of the grid (Re = VM/co) for these conditions are: 

 9 



392 



