Froude number dependence of the flow separation line on a sphere towed in a stratified fluid

In this paper experimental results on the near field of the flow past a sphere in a linearly stratified medium are presented. Emphasis is placed on the variation of the flow separation line with internal Fraude number F = U/NR and also with Reynolds number Re = 2RU/v, where U and R are, respectively, the velocity and the radius of the sphere, N is the Brünt- Väisälä frequency (rad sec^-1), and v is the cinematic viscosity. It is shown that in the Reynolds number range 200 < Re < 30 000 the flow is primarily conditioned by the Froude number when F≤1. The condition F = 1 defines a resonance state between the sphere and the internal wave field. In this case the waves create a strong depression behind the sphere that keeps the flow from separating. When F < 0.8 the flow is two dimensional in a layer confined between the upper and the lower wave. When F > 1.5 the flow starts to recover its three-dimensionality.