1 Abstract

In this paper we summarize the findings of an experimental investigation of the effect of a free surface on the development of a turbulent wake A fully sumberged sphere was towed at constant speed In the M I.T Towing Tank, and the velocity was recorded at several locations along Its wake using a series of constant-temperature hot film probes. It was found that, before the turbulent wake reached the free surface, the velocity distribution was almost axisymmetric, Similar to the one obtained in an unbounded fluid After the wake reached the free surface, however, Significant shear was found to be present all along the vertical from the axis of the wake to the ocean surface The velocity distribution after the wake reached the ocean surface was thus reminiscent of a two-dimensional wake. The effect IS attributed to the reduced transfer of momentum In the vertical direction because of the free surface, which results to an Increased transfer of momentum In the horizontal direction.

2 Introduction

Several problems of practical Interest Involve the Interaction of a turbulent wake With the ocean surface (figure 1) Examples Include Naval Hydrodynamics, In connection With the wake imaging problem, and Physical Oceanography, In connection with the modeling of geophysical flows past large-scale objects Traditionally, free surface flows have been seen mostly as Irrotational, and the research Interests have been directed mainly towards the study of gravity waves, and their interaction with structures and With other waves It is only In the last ten years, through satellite-taken pictures of the ocean, that It has been realized that the ocean surface IS in fact covered by vortices of different Sizes, the dynamics of which control such Important phenomena as ocean Circulation and regional climate.

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