An Anomalous Behavior in Vortex Shedding in a Flowing Soap Film

It is generally believed that von K\"arm\"an vortex street is characterized only by Reynolds number $Re=UD/\nu$, where $U$ is the mean flow speed, $D$ is the size of the body which generates the vortex street, and $\nu$ is the kinematic viscosity. In this study, we present experimental data in a flowing soap film showing that changing $U$ with fixed $D$ and changing $D$ with fixed $U$ are not equivalent to each other, suggesting that $Re$ alone is not sufficient to characterize vortex shedding by a bluff body. The velocity of eyes of the vortices relative to the mean flow, normalized by $U$, increases when we increase $D$, but decreases when we increase $U$. It is also found that the longitudinal spacing between the eyes is a linear function of $D$, but independent of $U$.