Broadband noise performance of subsonic jet with different impingement distances to a deflector

To study the broadband noise characteristics of jet impingement on defector with different impact distances, an experimental study is carried out by using far-field microphones in a full anechoic chamber. Comprehensive data of the nozzle-plate distance (L) range between 30D (D is the diameter of nozzle exit) and 2D are obtained and the spectral characteristics are discussed. Reducing the impact distance, the results indicate: (1) The spectra increase significantly in all frequency bands, leading to an increment of the overall sound pressure level by 10–17 dB in the direction of polar angles (α) of α = 120° when L < 10D; (2) The noise energy of downstream directions transfers to lower frequency and the spectra of α = 30° tend to collapse at different impingement distances when L < 10D; (3) Since medium and high frequency impingement noise has a little effect on the sideline directions, the spectra of polar angle of 90° attenuate rapidly which form a steep peak when L < 10D. The quantitative analysis prove that the proportion of sound power of defector increases linearly with the decrease of L in the direction of strongest noise (α = 120°). Most noise are produced by deflector for impingement distance less than the length of potential core, such as 80% at L = 7D–5D. Moreover, the directivity of deflector noise is obvious. Impingement noise or trailing edge separation noise is dominated in different directions. Correspondingly, the low frequency and high frequency noise plays a dominant role in the upstream and downstream directions respectively.