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It is known for many years that the vorticity and the thickness fields insoap films are coupled and that the thickness wave propagates at the Marangoniwave speed. Based on the two observations, we propose a hypothesis that thevorticity field propagates at the Marangoni wave speed in flowing soap filmchannels. Using the hypothesis, we modify the point vortex model and find thatthe periodic array of vortices will have an expanded periodicity in thelongitudinal direction, similar to the redshift of the Doppler effect. Thetheoretical prediction is compared with the experiments and we find that thehydrodynamic Doppler effect is present in soap film flows.

Hydrodynamic Doppler effect in flowing soap films