Savitsky 



segments passing over the incident wave may develop distortions In 

 wave height time histories such as observed in the experiments. 

 The wave height irregularity at any point in the wake thus precludes 

 a reliable evciluation of the dissipative effects of the grid-produced 

 turbulence since only two wave probes were used in this study. 



IV. RECOMMENDATIONS FOR FURTHER STUDIES 



The original objective of the present study was to investigate 

 experinnentally the interaction between gravity waves and turbulence 

 fields generated in the wake of a towed grid. Unfortunately, the 

 longitudinal mean flow velocity gradients in the wake had a dominating 

 effect on wave deformation and thus precluded a direct evaluation of 

 turbulence effects alone. Although, in a realistic ocean environment, 

 turbulence fields can be generated by, and exist simultaneously with, 

 velocity gradients in ocean currents, it is nevertheless of fundamental 

 scientific interest to study separately the effects of turbulence fields 

 with no mean flow interacting with gravity waves. The results of 

 such an elemental turbulence study can then be combined with velocity 

 gradients to represent wave passage through realistic ocean currents. 

 Also, the results can be used alone to study the wave interaction 

 with Isolated turbulence fields such as exist, for example, in regions 

 of "splash" turbulence developed by breaking waves. 



It is thus recominended that the present study be continued 

 but with an experimental apparatus designated to produce loczilized 

 turbulence areas with no mean flow. The experimental procedure 

 should be capable of generating turbulence fields of controlled eddy 

 size, turbulence Intensities, depth of penetration below the free 

 surface, and length and width of turbulence patch. It is further 

 reconnmended that the turbulence generator be capable of developing 

 vortices with either a horizontal cLxIs or a vertical axis or a combina- 

 tion of both. 



The control of the vortex direction will be important in the 

 study of the eddy viscosity interaction In which energy Is transferred 

 from the wave motion to the turbulence. As discussed by Phillips 

 [ 1959] > the passage of the wave results In straining the elements of 

 the fluid near the surface in a manner periodic In time. The mean 

 strain per cycle of the Incident wave is of second order, nannely 

 (a/\)^ , where a is the amplitude and \ the wave length of the 

 Incident wave. The wave motion thus provides a mechanism for 

 stretching the vortex lines that operates in addition to the stretch- 

 ing Inherent In the turbulence itself, and so tends to increase co^, 

 the mean square vorticlty associated with the turbulence. It Is ex- 

 pected that this possible nnechanlsm for transfer of wave energy 

 will be for waves Interacting with vertical vortex fields. In this 

 case, the vertical velocity gradient in the long-crested wave stretches 

 the vertical vortices In the turbulence field, but should not effect the 



442 



