实验室中研究大尺度涡气旋反气旋对称性

We performed an experimental study of large-scale wakes in a rotating shallow-water layer.
Standard particle image velocimetry was used to measure the horizontal velocity field, while a
laser-induced fluorescence technique was used to measure the geopotential deviation i.e., the
interface deviation. According to these measurements, we were able to quantify the dynamics in a
wide region of parameter space beyond the quasi-geostrophic regime. For obstacles larger than the
deformation radius and with small Rossby numbers, a significant asymmetry occurs in the wake
between cyclonic and anticyclonic vortices. These parameters correspond to a frontal geostrophic
regime with the relative interface deviation being larger than 0.10.2. In this case, anticyclones
remain coherent and circular, whereas cyclones tend to be elongated and distorted. More
surprisingly, for some extreme cases, coherent cyclones do not emerge at all, and only an
anticyclonic vortex street appears several diameters behind the obstacle. The transition from a
quasi-geostrophic to a frontal geostrophic regime is characterized by a strong increase in the
Strouhal number, which can reach a value up to 0.6. Hence, we found that a large-scale wake could
differ strongly from the classical Karman street when the relative geopotential deviation becomes
larger than the Rossby number. 德国LaVision PIV/PLIF粒子成像测速场仪 自适应粒子成像测速场仪（PIV）