The objective of this investigation is to clarify reduction mechanism of cavity tone by using blowing jets in the incoming boundary layer and effects of spacing of the jets on noise reduction. To achieve this objective, direct aeroacoustic simulations based on the three-dimensional compressible Navier-Stokes equations were conducted with the various spacing s. The depth-to-length ratio of cavity was D/L = 0.5. The freestream velocity was U0=30 m/s and incoming boundary layer was laminar. The effects of spacing of jets on cavity flow and tone were investigated in the range of the spacing of s/L = 0.1, 0.5 and 1.0, where the cross-sectional average jet velocity was ten percent of the freestream velocity. As a result, the maximum reduction level was obtained for s/L = 0.5. For this control condition, longitudinal vortices are introduced by jets in the incoming boundary layer of cavity and interact with large-scale vortices which are related with cavity tone. As a result, cavity tone was reduced effectively. For s/L = 0.1, the distance of induced longitudinal vortices become narrow so that the neighboring vortices interfere with each other. As a result, the longitudinal vortices were not observed in the boundary layer. For s/L = 1.0, the large-scale vortices sustain two-dimensionality between the induced longitudinal vortices and cause still intense cavity tone.

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