Spectral analysis of the von Kármán flow using ultrasound scattering

We report experimental evidence that the scattering of sound waves by a fluid may be used, as a nonintrusive and nonlocal method, to characterize the space-time structure of the flow. The experiment has been performed using, as a test flow, the von K\'arm\'an vortex street behind a cylinder at a low Reynolds number (Re=50). The results are in good qualitative agreement with earlier experiments on the von K\'arm\'an vortex street and with recent theoretical developments on sound-vorticity interaction.