Experimental and Numerical Investigations on Control Methods of Cavity Tone by Blowing Jet in an Upstream Boundary Layer 2017-01-1786

The objective of this paper is to clarify the mechanism for the reduction of cavity tone with blowing jets aligned in the spanwise direction in the upstream boundary layer. Also, the effects of spacing of the jets on the reduction are focused. To achieve these objectives, direct aeroacoustic simulations were conducted along with wind tunnel experiments. The depth-to-length ratio of cavity was D/L = 0.5. The incoming boundary layer was laminar, where the boundary layer thickness was δ/L = 0.055. The predicted flow fields without control show that two-dimensional large-scale vortices are shed and become acoustic sources in the cavity. The effects of spanwise spacing of spanwise-aligned jets on the cavity flow and tone were clarified with computations and experiments with the different pitches of s/L = 0.1 - 1.0 (s/δ = 1.8-18.2). As a result, the largest reduction level was obtained for s/L = 0.5. For this control condition, the predicted results show that longitudinal vortices are introduced by the jets in the incoming boundary layer of cavity and suppress (break down) the two-dimensional large-scale vortices which are related with cavity tone. The measured effects of the control on the distributions of Coherent Output Power (COP) with reference to the sound support it. Meanwhile, for s/L = 0.1, the distance of induced longitudinal vortices become so narrow that the neighboring vortices interact with each other and intense longitudinal vortices are not developed. For s/L = 1.0, the large-scale vortices were clarified to sustain two-dimensionality between the induced longitudinal vortices due to the wide spacing of the introduced longitudinal vortices. As a result, the reduction of cavity tone was smaller for s/L = 0.1, 1.0. In summary, s/L = 0.5 is an adequate spacing for intense longitudinal vortices to be introduced into the cavity flow and to reduce cavity tone effectively.