Acoustic characteristics of wall jets in the developing regime

Abstract

The present study is an experimental investigation of the acoustic characteristics of developing wall jets in the jet Reynolds number in the range 2.5 × 10⁴ ≤ Re ≤ 7.6 × 10⁴. The length of the wall jet is varied in the range of 5 ≤ L/h ≤ 20, and acoustic measurements are taken at different streamwise locations of the wall jet. The acoustic spectra of L/h = 5 and 10 show a higher noise level at a Strouhal number (St _h ) of 0.18. In the mid-frequency range, the noise levels are higher for larger plates (L/h = 20) due to the mixing of inner and outer shear layers of the wall jet. This is confirmed by the scaling of acoustic spectra in 0.2 ≤ St _h ≤ 0.6 using inner layer thickness (y _m ) and maximum jet velocity (U _m ). An increase in noise level up to 5 dB is obtained within 1.5 ≤ St _h ≤ 3 due to the higher turbulence of flow structures in the inner layer, which is depicted by the scaling of acoustic spectra using the inner layer half-width ${\left(y_{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}\right)}_{in}$ and streamwise velocity of ${0.5U}_m$ . Further, the high-frequency acoustic spectra are scaled with friction velocity (u _τ ) and wall shear stress (τ _w ). In addition, the autocorrelation and probability density function of wall jets demonstrate the behaviour of flow structures in the potential core and developing regimes on noise generation.