The University of Southampton
Engineering and the Environment

Research project: Jet noise mechanisms

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In this work we demonstrate that a wavepacket driven by a nonlinear interaction between two unstable modes in the jet provides a very efficient mechanism of sound radiation.

Project Overview

The sound radiation from subsonic jets is less well understood than that from supersonic jets where there is a direct connection betwee jet vortical structures and the Mach waves they produce. In subsonic jets the sound originates with wavepackets in the shear layers, but the question of what drives the wavepackets remains unclear.

 

Showing how different modes may combine to force the sound radiaton
Difference mode amplitude plots
Comparing two sources: a longitudinal quadrupole (left) and a lateral quadrupole (right)
Sound directivity
Schematic showing frequencies in the jet core and the corresponding difference madein the acoustic field
Frequencies in the jet core

Theoretical predictions of the sound are made using the parabolised stability equations (PSE) and additional direct numerical simulations are used to solve the governing equations.  The efficiency of the nonlinear mechanism is clear from the simulations and theory.  The insight obtained enables the contributions of particular mode interactions to both the directivity and the spectrum of noise radiation to be assessed.  Axisymmetric mode interactions are shown to be particularly important, which has implications for jet noise control.

Pressure contours from a PSE calculation
Pressure contours
For azimuthal modes: (a) n=0, (b) n=1, (c) n=2, (d) n=3.  The growth factor N for each case was computed with a linear PSE method.  The curves corresponding to 30 different frequencies are plotted
Wave amplitude envelopes
Comparison of the effectiveness - nonlinear (top three) and linear (bottom three) mechanism, evaluated using DNS of the governing equations for a single jet.  Left to right: M=0.5, M=0.7, M=0.9.
Nonlinear mechanism effectiveness

Related research groups

Aerodynamics and Flight Mechanics

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