Gravity Waves and Finite Turbulent Flow Fields 



horizontal vortices. An experimental setup designed to control the 

 direction of the turbulent vortices can be most instructive in under- 

 standing this dlsslpatlve process. 



The experimental procedure proposed to develop controlled 

 turbulence fields with no mean flow Is to slnusoldally oscillate a 

 series of grids in a physically confined area in still water. The 

 barrier confining the turbulent field can be constructed of four thin, 

 vertical plates housing a rectangular box penetrating through the 

 water surface to a depth below the lower ends of the oscillating grids, 

 After oscillating the grids, the rectajigular barrier can be lifted 

 above the water surface just as the waves approach so that there Is 

 an Interaction between waves and turbulence. The dimensions of 

 this rectangular container can be varied to represent various sizes 

 of turbulence areas. The use of an oscillating grid in a confined 

 area has been Investigated by Murray [ 1968] In his laboratory 

 studies of horizontal turbulent diffusion. In his work, the grids 

 which filled a 50 cm wide channel were composed of several rods 

 1 cm in diameter and 5 cm apart. The array consisted of 3 grids, 

 each 30 cm apart, and had a stroke of 40 cm. The following con- 

 clusions concerning the generated turbulence are described by 

 Murray. 



1. There Is no mean flow within the confined area of turbu- 

 lence generation. This Is precisely the objective of the proposed 

 experimental procedure. 



2. The oscillating array of grids produces turbulence fields 

 which are essentially homogeneous and stationary. 



3. The turbulent velocity distributions are Gaussian, 



4. Taylor's statistical theory of turbulence effectively de- 

 scribes the variance, scsde time, and scale length of the generated 

 turbulence field. 



In summary then, the proposed turbulence stimulation tech- 

 nique appears to be adequate for generating controlled and mathe- 

 matically definable localized turbulence fields. 



The experimental procedure will consist In mounting the 

 turbulence generator over a section of the 75 ft square tank away 

 from the side wedls. The grid array would be oscillated to generate 

 the turbulence field and nnechanlcally generated gravity waves would 

 approach this field. Just prior to the waves reaching the area of 

 turbulence, the rectangular barrier surrounding the oscillating 

 grids would be lifted clear of the water surface so that the waves 

 would interact only with the turbulence. It is expected that the 

 lateral diffusion of the turbulence area will be very slow compared 

 to group velocity of the gravity waves so that, at least for the pas- 

 sage of several wave lengths, the turbulence properties may be 

 assumed to be stationary. 



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