Gravity Waves and Finite Turbulent Flow Fields 



Increasing the mesh size from 2.7 to 5,4 inches has a negligi- 

 ble effect on the wave deformation. Increasing the width of the grid 

 increased the area of wave att-enuation and wave annplification. 



III. ANALYSIS AND DISCUSSION 



Viscous and Turbulent Effects 



The Initicil analysis of the experimental results was directed 

 to relating the observed wave deformations to possible physical 

 mechanisms associated with the turbulent and viscous nature of the 

 grid- generated wake. It was found that the large changes in wave 

 height could not be accounted for by these considerations -- parti- 

 cularly in the two-dimensional grid studies. In this case, the square 

 of the wave height was integrated along a crest line passing through 

 the wake to obtain a measure of the energy in the deformed wave. 

 This was compared with the energy in the incident wave for the same 

 length of crest line. The results of this comparison are presented 

 in Table 1 for wave and grid dimensions selected to be typically 

 representative of the total test program. The length of integration, 

 Y, along a given crest line was the distance between the grid center- 

 line and the point where the wave height was again equal to the inci- 

 dent wave height. The crest length thus includes both the attenuated 

 and anmplified wave height regions. For a given test condition, the 

 integrations were carried out for several transverse sections through 

 the weike, both ahead of and aft of the grid. It is seen from Table 1 

 that the integrated expression representative of wave energy pro- 

 duces nearly similar results with and without the towed grid. In 

 fact, for some test conditions, the integrated energy for waves in 

 the presence of the grid results in values somewhat higher than for 

 the case of no grid -- but this is attributed to experimental Inac- 

 curles. It thus appears that viscous dlsslpative effects were quite 

 small and, aithough they most certainly existed, their magnitude 

 could not be accurately detected because the limited number of 

 transverse wave probes were inadequate to trace the unexpected 

 large wave height deformation which developed along a crest line. 



A measure of the rms of the velocity fluctuations In the turbu- 

 lent field yielded substantially the same values with or without waves 

 passing through the wake. This was not surprising since the energy 

 Imparted by the grid to the fluid at a tow speed of 1 ft/sec was 

 nearly an order of magnitude larger than the wave energy In a 

 Crestline length equeil to the grid width. 



For the one-dlnnenslonal tests, It will be recalled that the 

 wave height at a given position In the wake exhibited large fluctua- 

 tions and was characterized by Irregularities In the recorded time 

 histories. These results could certainly not be accounted for by 

 dlsslpative mechanisms In the turbulent field. It appeared then 

 that for both the one and two-dimensional studies the principal 



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