INTERNAL WAVES GENERATED BY THE TURBULENT WAKE OF A SPHERE

Internal waves generated by the turbulent wake of a sphere travelling horizontally through a linearly stratified fluid were studied using sh adowgraph and particle-streak photography. The Reynolds and internal F roude number ranges considered were 2,000 greater-than-or-equal-to Re less-than-or-equal-to 12,900 and 2.0 less-than-or-equal-to Fi less-tha n-or-equal-to 28.0, respectively. Two quite distinct flow regimes base d on the structure of the turbulent wake were identified. In one, the wake is characterized by ''large-scale coherent structures''. In the o ther, the wake, as viewed on a side-view shadowgraph, grows in a rough ly symmetric fashion to a maximum height and then collapses slowly; su ch flows are termed the ''small-scale structures'' regime. Wave length s and maximum wave heights of the internal waves were measured as func tions of Nt and Fi, where N is the Brunt-Vaisala frequency and t the t ime. It was found that the wave lengths scale well with the streamwise dimension of the spiralling coherent structures. The maximum amplitud e of the internal waves were found to scale with the vertical dimensio n of the turbulent wake, upon varying the internal Froude number.