Sound scattering by a Karman street consisting of large-scale vortices

The scattering of acoustic waves by a vortex street formed behind a cylinder in an air flow is studied both theoretically and experimentally for the case of the sound wavelength being much less than the vortex size. The theoretical calculations show that, at flow velocities well below the sound velocity, the vortex street can be considered as a moving phase screen. The spectrum of scattered sound in the far zone is shown to consist of harmonics whose frequencies differ by a multiple of the vortex rate. The computational results agree well with the experimental data obtained for the diffraction of ultrasound of the wavelength λ=3 mm by the Karman street formed behind a circular cylinder with an 8 mm diameter at a flow velocity of 7 m/s.