Gravity Waves and Finite Turbulent Flow Fields 



influenced the test results. The other series of experimental studies 

 involved towing a 3-ft wide grid in a 75-ft wide towing tank. The in- 

 tent of these tests was to allow any scattered wake system to be 

 defracted outside the turbulence patch. However, the finite width 

 grid also produced a pronounced longitudinal mean flow velocity 

 gradient in transverse sections through the wake. Thus, in these 

 latter tests, the generated waves were simultaneously subjected to 

 three modification effects: (1) dissipation due to eddy viscosity; 

 (2) scattering due to turbulent convective distortion of the wave front 

 and (3) deformation of the wave due to mean flow velocity gradients. 



Measurements were made of the wave deformation in the 

 wakes of both the one- and two-dimensional grids. An analysis of 

 these results indicated that the velocity gradients in the wakes had a 

 dominating effect on the wave deformation and thus, unfortunately, 

 precluded a reliable evaluation of the possible dissipative or scatter- 

 ing action of the turbulence field upon the incident wave. The studies 

 are, nevertheless, of importance since they provide unique results, 

 obtained under controlled laboratory conditions, describing the pro- 

 nounced distortion of a deep water wave when encountering sharp 

 current gradients, either naturally existing or artificially produced. 

 It is shown that the wave distortion can be such as to provide locally 

 areas of reduced wave motion which can be beneficial in launching 

 or retrieving small craft or submersibles from a mother ship at sea. 



The experimental results are described in some detail and an 

 elementary analytical model is developed which, using the combined 

 mechanics of wave refraction, defraction and superposition, at least 

 qualitatively reproduces the features of the test results and, perhaps 

 more important, describes a possible physical mechanism respon- 

 sible for the observed large wave deformations. 



These studies were supported by the Fluid Dynamics Branch 

 of the Office of Naval Research, Department of the Navy, under 

 Contract NR-062-254, Nonr263{36). They formed the basis for a 

 dissertation submitted to the Graduate Division of the School of 

 Engineering and Science in partial fulfillment of the requirements 

 for the degree of Ph.D. at New York University. 



II. EXPERIMENTAL PROCEDURES 



Turbulence-generating grids have been used with great suc- 

 cess in advancing the knowledge of turbulence in air flows, but have 

 been used only occasionally in hydrodynamics -- particularly in 

 towing tanks where a grid must be towed in quiet water to generate 

 a turbulence field. Taylor [ 1935] has shown that disturbances 

 generated in the wake of a grid transform rapidly into a quasi- 

 isotropic turbulent field whether the grid is towed in quiet air or an 

 airstream passes through the grid. 



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