Directivity of noise radiated by a vortex ring
Victor Feliksovich Kopiev, Mikhail Yu. Zaitsev, Sergey A. Chernyshev · The Journal of the Acoustical Society of America · 1998
It is shown theoretically that the vortex ring noise is to consist of three quadrupoles with axis along the ring velocity vector. These are axisymmetrical modes and two-mode families with dependency on the azimuthal angle ‘‘fi’’ of exp(i*fi) and exp(2i*fi) kind. Each family consists of an infinite number of closely disposed modes with an identical accumulation point. This fact does not allow the separation of the quadrupoles from the spectrum analysis since all the modes are disposed in a coinciding narrow frequency interval. To reveal the noise directivity, the noise generated by a freely flying vortex ring in the anechoic chamber is measured with the use of an array of six microphones set around the movement axis. A special analysis of six measured signals which account simultaneously for azimuthal and transversal directivity of acoustic signals has allowed the separation of each quadrupole component and has proven that the vortex ring noise really consists of three equally excited quadrupoles perceived in the far field as narrow-band random noise. [The investigations were partly realized with support of CRDF according to award RE2-134.]