抄録
This work presents an experimental investigation for effects of the spanwise spacing of small-jets placed in the approaching boundary layer on cavity tone. The sound and velocity fields of a flow around a cavity were measured with a low noise wind tunnel. The freestream velocity was changed from U_0 = 10-45 m/s. The depth-to-length ratio of the cavity was D/L = 0.5. The upstream boundary layer is laminar, and the boundary layer thickness is δ_<99%>/L = 0.08 at U_0 = 43 m/s. The effects of the jets from the wall in the upstream boundary layer with various spanwise spacing p/L = 0.1, 0.25, 0.6, and 1.25 on the cavity tone were investigated. The jet velocity at the central axis of jet was changed from U_j = 2.2-4.0 m/s. The results show the cavity tone can be reduced by control of blowing jets. The control effect on the cavity tone depends on the spanwise pitch and velocity of blowing jets. In this experiment, the most effective pitch of jets for noise reduction was p/L = 0.25. The reduction level increases as the jet velocity becomes higher. The noise reduction effect becomes sufficiently large when the jet velocity is higher than 3 m/s. The three-dimensional spanwise velocity field was induced by blowing jets. As a result, the spanwise coherence of the vortices inducing intense sound became weak, and the radiating sound became weak.
