Analysis and prediction of noise from installed subsonic chevron jets

TL;DR

This study addresses installed jet noise for subsonic jets at $M=0.5$ by comparing round (SMC000) and chevron (SMC006) nozzles near a flat plate, across vertical separations $H$ and observer angles. A semi-analytical near-field instability-wave scattering model, fed by LES-derived spectra for the axisymmetric ($m=0$) and first helical ($m=1$) modes and a frequency-dependent convection velocity $\overline{U}_c(\omega)$, is used to predict low-frequency amplification from plate trailing-edge scattering and is added to the isolated jet spectrum to form installed predictions. Experiments show that installation strongly amplifies low-frequency noise, with chevrons increasing this effect due to enhanced jet mixing, while also increasing high-frequency content on the shielded side due to plate reflection. The model demonstrates good agreement with measured spectra across observer angles for both nozzle types, supporting the instability-wave scattering mechanism as a key driver of installed jet noise and offering a predictive tool for installed-noise mitigation in practical configurations.

Abstract

An experiment is conducted to investigate the effects of chevrons on installed subsonic jet noise at a Mach number of 0.5 using the NASA SMC000 (round) and SMC006 (chevron) nozzles. The jets are of a diameter D=16.93 mm and placed near a flat plate, with a horizontal separation distance L=6.5D between the plate's trailing edge and the nozzle exit. The vertical separation distance H varies between 1.5D, 2D, 2.5D and 3D. Far-field sound is measured at various observable angles ranging from $θ=60^\circ$ to $150^\circ$ to the downstream jet axis on the reflected side. The measured sound spectra are compared to the near-field scattering model developed by Lyu et al, together with the isolated near-field pressure spectra inputs from corresponding large eddy simulations for both nozzles. Results show that jet installation results in a strong noise amplification at low frequencies for both nozzles due to the scattering of the near-field pressure fluctuations and a mild noise increase at high frequencies due to surface reflection (reflected side). The low-frequency amplification is strongest at H=1.5D and has a dipolar directivity. A secondary spectral hump appears within this low-frequency amplification hump, which is hypothesised due to the interference between the sound generated by the large coherent structures directly and that by their scattering at the plate's trailing-edge. The use of chevrons reduces the low-frequency noise for isolated jets, but leads to even stronger noise amplification for installed jets; this is likely due to enhanced jet mixing resulting in stronger near-field pressure fluctuations at a fixed radial distance. Results show that the scattering model can predict the low-frequency noise amplification well at various observer angles for both nozzles, suggesting the validity of the instability-wave scattering mechanism and modelling for both round and chevron jets.

Figures (13)

• Figure 1: Schematic of the round and chevron nozzle configurations used in the present study.
• Figure 2: A schematic of the experimental setup, showing the positions of the far-field microphones for the isolated and installed jet. For representational clarity, the microphones and nozzles depicted in the schematic are not drawn to scale.
• Figure 3: Comparison of SPL for isolated round convergent nozzle (dashed line) with experimental data available in the literature Brown and Bridges brown2006small (solid lines) for select polar angles ($\theta = 60^\circ, 90^\circ, 120^\circ$ and $150^\circ$) obtained at acoustic Mach number $M = 0.5$.
• Figure 4: Sound pressure level comparison for round and chevron nozzles measured at various polar angles $\theta=60^\circ,90^\circ,120^\circ$ and $150^\circ$ for the isolated and installed configurations at acoustic Mach number of $M=0.5$.
• Figure 5: Sound spectral difference $\Delta$SPL for various plate heights and observer angles.