Laboratory Experiments on Dipole Structures in a Stratified Fluid

Abstract

Coherent flow structures (vortices, dipoles) as occurring in geophysical fluid systems have been investigated in the laboratory. This paper concentrates on dipole experiments, in which locally 3D turbulence is generated in a continuously stratified fluid, viz. by horizontal injection of a fixed volume of fluid during a short period of time. The cone-shaped region of 3D turbulent fluid collapses under gravity (due to the stratified ambient fluid), resulting in a nearly 2D flow confined within a thin layer of fluid. As can be expected from 2D-turbulence theory, one observes a spectral flux of energy towards larger scales, and the collapsed fluid eventually gets organized in a dipole-shaped structure. The formation process leading to these dipoles as well as their dynamics will be discussed. Two experiments were carried out In order to study interaction properties of dipoles, viz. a head-on collision between two dipoles, and a collision of a single dipole against a solid wall. Some preliminary results are presented.