TC3 Square/Cube 3

Abstract

A series of laboratory experiments was conducted to investigate the formation and evolution of large vortices generated in density stratified fluids by unsteadily moving (maneuvering) bodies at relatively high body Reynolds numbers. Flow visualization and particle image velocimetry (PIV) technique were used in these large-scale experiments to clarify vorticity dynamics in the late wakes and collect data on the flow evolution. Different body maneuvers were reproduced in the experiments, including starting motion from rest, nearly steady motion with strong/weak acceleration, passive towing along the arc trajectory, and steady motion. In all these experiments, the formation of large (compare to the body diameter), long living eddies was registered in the late wake flow, whence a significant momentum is imparted to the fluid. Some of the new effects discovered in the high-Re case are the influence of internal wave radiation on momentum balance in the wake, surface signatures of stratified wakes, and transverse-propagating vortex dipoles that form during the motion of bodies in curved paths. The flow phenomena and measurements were explained using dynamic arguments and phenomenological models.KEYWORDS: wake flow, stratified fluid, self-propelled body.1 INTRODUCTION Vortical flows that are commonly observed in various fluid systems can be considered to be generated by a forcing that acts in a localized area of the fluid. A typical example is the far wake behind a body. At late times the body is far from the observer, and for mathematical modeling it may be considered as a "point" source of vorticity. As such, the details of the flow near the body can be neglected and the late wake flow structure is expected to be dependent mostly on the dominant characteristic of body forcing (force, force doublet, etc.). Steady and starting flows induced by such forcing are of definite interest and have been intensively studied in the past while achieving success in obtaining some exact and approximate analytical solutions [1][2][3][4]. This fundamental problem has demonstrated applications in stratified and other quasi-twodimensional liquid systems (e.g., shallow-water flows and soap films) where the flow is effectively two-dimensional. Numerous studies show that such liquid systems pose a remarkable property -that they "remember" the forcing history, for example, a force generates a dipolar flow and a force doublet generates quadrupolar flow [5][6][7]. The morphology (spatial organization of vorticity) of the resulting flow patterns often depends on the forcing and determines the evolution of coherent eddies.The background stratification significantly increases the complexity of the problem of compact vortical flows, and hitherto there are no analytical solutions for this case. As such,