Savitsky 



Measurements were initially made of the mean value of the 

 longitudinal velocity (in the grid direction) of the grid wake at the 

 centerline of the grid and at a depth of 0.80 feet below the water 

 surface. At a distance of 10.0 ft sift of the grid, the wake velocity 

 was 0.40 V and decreased slowly with distance aft of the grid --at 

 a distance 20,0 ft aft of the grid, the mean velocity of the wake was 

 0.36 V. In these initial tests, a straight line of confetti was sprinkled 

 across the 1 2 f t width of the tank parallel to the plane of the grid. 

 Visual observations of this reference line after grid passage showed 

 that the confetti moved essentially in one straight line parallel to 

 the grid, thus indicating a lack of noticeable velocity gradients -- at 

 least on the free surface. An analysis of the wave distortion data in 

 this wake yielded anomalous results (these will be discussed in a 

 subsequent section) that could not be explained by the assumption of 

 a uniform longitudinal mean flow through transverse sections in the 

 grid wcike. Hence, a detailed survey was then miade of the mean 

 flow at distances of 10 ft and 20 ft aft of the grid. These results are 

 shown in Fig. 2 which presents a plot of longitudinal mean flow (V^,,) 

 versus transverse distance from the grid centerline at a probe depth 

 of 10 inches below the water surface. The wake velocities (V^,) are 

 normalized on the basis of grid speed (V). It is clearly seen that 

 the meeui flow in the wake is essentially constant for a distance of 

 approximately 5 ft from the grid centerline but then rapidly decreases 

 between this point and the tank wall. The significance of this local 

 velocity gradient will be subsequently discussed. 



The wind tunnel results of Dryden [ 1937] , who examined the 

 turbulence aft of a rectangular grid having a mesh size M = 0,41 ft 

 at a nearly similar Reynolds number, show that the turbulent veloc- 

 ity fluctuations u' as a function of distance, X, aft of the grid are: 



u M 



Thus, for a distance 10 ft aft of the 0,36' mesh grid x/M =27.7 

 and u' = V/37.7 or approximately 3% of the mean flow. At a distance 

 of 20 mesh lengths aft of the grid, wind tunnel experim.ents have 

 shown the establishment of quasi-isotropic turbulence. 



394 



