Turbulences as sound sources

The aerodynamic sound source of a turbulent flow with a high Reynolds' number is composed of two types of Reynolds' stress components containing turbulent vortices as elements: perturbation-perturbation solenoidal velocity interaction (self-noise) and perturbation-static velocity interaction (shear-noise). The vortices act as quadrupole sources. Near a scattering surface, a high turbulent velocity imparts high fluctuating forces to it and couples to sound causing the scattered sound to be even stronger than the original one. The high-Reynolds'-number subsonic cold-air jet structure, beginning from a nozzle and developing gradually to solenoidal and turbulent, consists of a mixing region, a transition region, and a fully developed region. Most of the sound power originates from the mixing region. The spectrum of the jet noise is of broadband character. The typical radiation pattern of a quadrupole distribution is totally masked by convectional effects of the jet flow, which tend to enhance the sound greatly in the flow direction. The sound propagating in the jet flow direction will be refracted sidewards, causing a cone centered on the downstream jet axis wherein the far field sound is greatly reduced (zone of silence), and tending to decline the lobe of the radiation pattern. When a flow attacks a plate parallel to its surface at Reynolds' numbers high enough, there begins to form unsteady vortices, forming a turbulent boundary layer. The wavenumber spectrum in the turbulent boundary layer has two maxima: the convective peak at a high subsonic wavenumber and the sonic peak at the acoustic wavenumber. The sonic and supersonic spectral components at the surface generate active propagating sound, most of the acoustic energy propagating at grazing incidence downstream. Downstream propagating sound due to the wavenumber components near the sonic peak refracts towards the surface, and upstream propagating sound refracts outwards from the surface, enhancing the sonic peak in the downstream radiation and possibly eliminating it in the upstream radiation. The convective peak at the surface leads to the hydrodynamic coincidence, causing sound transmission and radiation at frequencies below the hydrodynamic coincidence frequency.