Retardation of the field in a weakly compressible fluid

We discuss the retardation of the velocity field in a weakly two-dimensional fluid channel, in specific the flowing soap film channel. It is previously known that in this system the velocity field is strongly correlated to the thickness field and that the thickness field propagate at the Marangoni wave speed. We produce simple vortex streets in flowing soap films to show that the vortex structure changes depending on the speed of the mean flow $u$ and the speed of Marangoni wave $v_M$. The spacing ratio of the vortex street decreases as $u/v_M\rightarrow1$, and the dependence is characterized by the factor $1/(1+u/v_M)$.