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Jet-noise reduction via streak generation in the nozzle boundary layer

Filipe R. do Amaral, Petrônio A. S. Nogueira, Igor A. Maia, André V. G. Cavalieri, Peter Jordan

Abstract

We study the hydrodynamic and acoustic fields of turbulent jets issuing from nozzles modified by the addition of cylindrical tabs on the inner surface, one diameter upstream of the exit. The tabs are designed to promote significant growth of steady streaks in the nozzle turbulent boundary layer. A baseline smooth nozzle is also studied for comparison. Acoustic measurements are made using an azimuthal array for Mach numbers in the range 0.4 $\leq M_j \leq$ 0.9. The tabs are found to reduce the emitted sound levels by up to 3 dB/St. In terms of overall sound pressure levels (OASPL), reductions of up to 3 dB are observed at all measured polar angles in the range 20 deg $\leq θ\leq$ 90 deg. Time-resolved particle image velocimetry (TR-PIV) experiments are conducted to measure the three components of velocity for a series of cross-stream planes at $M_j =$ 0.7. A Floquet-based Fourier decomposition is applied for the azimuthally periodic flow field, and spectral proper orthogonal decomposition (SPOD) is then employed to extract coherent structures. Comparison of the structures obtained for nozzles with and without tabs shows an enhancement of the streaky structures by the tabs and a damping of Kelvin-Helmholtz (K-H) wavepackets. A linear model based on the one-way Navier-Stokes equations (OWNS) is employed to explore the underlying amplification mechanisms and how these are impacted by the tabs. The model reproduces the growth-attenuation mechanism observed in the data, showing that the changes in the mean flow induced by the streaks work to reduce the amplification of the noise-generating coherent structures associated with linear spatial growth mechanisms.

Jet-noise reduction via streak generation in the nozzle boundary layer

Abstract

We study the hydrodynamic and acoustic fields of turbulent jets issuing from nozzles modified by the addition of cylindrical tabs on the inner surface, one diameter upstream of the exit. The tabs are designed to promote significant growth of steady streaks in the nozzle turbulent boundary layer. A baseline smooth nozzle is also studied for comparison. Acoustic measurements are made using an azimuthal array for Mach numbers in the range 0.4 0.9. The tabs are found to reduce the emitted sound levels by up to 3 dB/St. In terms of overall sound pressure levels (OASPL), reductions of up to 3 dB are observed at all measured polar angles in the range 20 deg 90 deg. Time-resolved particle image velocimetry (TR-PIV) experiments are conducted to measure the three components of velocity for a series of cross-stream planes at 0.7. A Floquet-based Fourier decomposition is applied for the azimuthally periodic flow field, and spectral proper orthogonal decomposition (SPOD) is then employed to extract coherent structures. Comparison of the structures obtained for nozzles with and without tabs shows an enhancement of the streaky structures by the tabs and a damping of Kelvin-Helmholtz (K-H) wavepackets. A linear model based on the one-way Navier-Stokes equations (OWNS) is employed to explore the underlying amplification mechanisms and how these are impacted by the tabs. The model reproduces the growth-attenuation mechanism observed in the data, showing that the changes in the mean flow induced by the streaks work to reduce the amplification of the noise-generating coherent structures associated with linear spatial growth mechanisms.

Paper Structure

This paper contains 23 sections, 23 equations, 41 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Nozzle design
  3. Acoustic measurements
  4. Experimental setup
  5. Data post-processing
  6. Results
  7. Time-resolved velocity measurements
  8. Experimental setup
  9. Data post-processing
  10. Mean fields and fluctuation statistics
  11. Coherent structures
  12. Modal energy maps
  13. Eigenfunctions
  14. Wavepacket modelling
  15. One-way Navier--Stokes
  16. ...and 8 more sections

Figures (41)

  • Figure 1: Sketch of the nozzle with tabs and coordinate system (in red colour). Left frame: longitudinal section view (flow is directed from left to right). Right frame: frontal section view (flow is directed towards the reader, leaving the page). Dimensions in millimetres.
  • Figure 2: Picture of the two studied nozzles. Left: nozzle with the cylindrical tabs. Right: baseline nozzle.
  • Figure 3: Pictures of the internal walls tabbed nozzle after the stereo PIV experiments, where it is possible to verify the flow footprints left on the nozzle walls by the seeding particles.
  • Figure 4: Sketch representing the microphones distribution over the circular antenna (right frame) and the cylindrical surface covered by the acoustic measurements (left frame). Coordinate system in red colour.
  • Figure 5: Acoustic spectra for polar angle $\theta =$ 90 deg (nozzle exit, sideline) for both tripped ($-$) and untripped ($--$) configurations. Frames, from top to bottom: axisymmetric mode ($m =$ 0), 1st helical mode ($|m| =$ 1) and 2nd helical mode ($|m| =$ 2). Frames, from left to right: Mach 0.4, 0.7 and 0.9. The top frames display the full sound radiation without azimuthal decomposition, shown as thick curves.
  • ...and 36 more figures