The generation of internal waves by a towed sphere and its wake in a thermocline

This report describes a combined experimental and numerical study of internal gravity waves generated by a sphere which is towed horizontally in a density stratified fresh water tank which exhibits a strong thermocline at mid-depth. The Reynolds number range covered extends from 2000 < Re = UD/v < 100000, and the Froude number range was 0.44 < Fr = U/N-max D < 22. Both the kinematics as well as the dynamics of the internal waves are studied. The internal wave field is shown to be generated by two very distinct types of sources, one stationary with respect to the body and one which is nonstationary. The stationary part of the internal wave field is shown to be generated by an ''effective'' body comprised of the sphere plus a stationary separation bubble which follows it. The nonstationary part of the internal wave field is shown to be generated by the nonstationary oscillations associated with the large-scale eddies of the turbulent wake. While such nonstationary internal waves have been observed in previous investigations, their physical origin has not been clearly identified. This paper presents a simple physical model for their generation. (C) 1997 American Institute of Physics.