A VORTEX SUBJECTED TO A SHEAR - AN EXPERIMENTAL-STUDY

The two-dimensional erosion of a vortex subjected to an external, adve rse shear is studied experimentally. The flow takes place in a thin st ratified layer; the vortex is produced by electromagnetic forcing, whe reas the shear is driven mechanically. The system thus allows the vort ex strength and the external shear to be controled independently. We o bserve the so-called 'erosion' process, i.e. the progressive decrease of the vortex area, leaving the vortex core unaffected. This process i s controlled by the ratio gamma = S/omega(max), where omega(max) and S are respectively the maximum vorticity of the vortex and the external shear. At small gamma, the erosion is weak and the vortex survives ov er the duration of the experiment. At large gamma, the vortex is first eroded, and then, after a critical time, becomes stretched and eventu ally breaks up into filaments. During the first period of time, the co mpensated maximum vorticity (with friction decay removed) is constant and the vortex area decreases, while beyond the critical time, both qu antities decrease with time. A critical value for gamma, defining the transition between these two regimes, is determined experimentally: ga mma(c) = 0.051+/-0.017. The breaking process itself, during which the vorticity of the vortex core decreases, is investigated. All the quali tative aspects of the erosion process, the onset of breaking and the b reaking process itself are found to be in excellent agreement with the theoretical and numerical description. The experimental value of gamm a(c) and the properties of the filamentation process are consistent wi th the numerical estimates.