Effects of nozzle-lip thickness on the tones in the near-field pressure spectra and on the initial instability waves of jets

Abstract

The effects of the nozzle-lip thickness on the near-field pressure spectra of free jets are investigated by computing initially laminar isothermal jets at Mach numbers = 0.6, 0.9 and 1.3 for lip thicknesses equal to 0.014, 0.058, 0.023 and 0.093 times the nozzle radius. For a thicker lip, the jet mixing layers develop more rapidly and contain stronger low-frequency components, notably near the cut-off frequencies of the first radial modes of the free-stream upstream-propagating guided jet waves (GJW). More prominent or additional peaks are also generally found in the pressure spectra outside the mixing layers at the frequencies mentioned above. The variations of the peak properties as the nozzle-lip thickness increases depend on the Mach number and the location where the spectra are calculated. Inside the potential core, the tones in the spectra for = 0.9 do not change appreciably, whereas narrow peaks emerge by 3 dB for = 0.6 and 1.3, including one near the frequency of the least-dispersed waves of the first axisymmetric GJW mode in the subsonic case. Outside the jet, near the nozzle, the levels of the tones obtained for = 0.9 and 1.3 grow by at most 3 dB and 6 dB, while the broadband peaks appearing in the spectra for = 0.6 turn into 2 dB stronger, narrower peaks. Therefore, with one exception, a thicker nozzle lip promotes the establishment of tonal components in the pressure spectra of the jets, most likely by enhancing the reflection of the free-stream GJW at the lip. The exception, concerning the tones in the jet for = 0.9, can be explained by the fact that the upstream-propagating waves responsible for the latter are duct-like GJW.