On the Mach number and temperature dependence of jet noise: Results from a simplified numerical model

Abstract

Numerical simulations of sound radiation from perturbed round jets are used, firstly to
explore the structure of the sound sources and then to carry out a parametric study of
the effect of jet Mach number and jet temperature. The simplified model problem
includes a steady base jet flow, maintained in the absence of disturbances, superimposed
with instability waves that are free to interact nonlinearly. Simulations over a
range of subsonic jet Mach numbers show that a nonlinear mechanism dominates over
a linear mechanism for low-frequency sound radiation, while for supersonic Mach
numbers the linear mechanism is dominant. Additional insight is gained from a
frequency-wavenumber analysis, including a transformation in the radial direction.
With this decomposition, the acoustic field is located by the arc of a circle in plots of
radial against streamwise wavenumber for discrete frequencies. The transformation is
applied to both the pressure field, showing the sound directivity, and to selected source
terms, showing characteristic directivity patterns for the streamwise and radial
quadrupole terms. Decreasing the Mach number leads to a reduction in amplitude of
the sources and of the sound radiation. Simulations with broadband forcing show that
the qualitative effects of Mach number and jet heating are captured by this approach,
which requires less resolution than a direct numerical simulation. A significant increase
in the strength of the acoustic radiation for cold jets is observed, which is worthy of
further investigation.

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Identifiers

ORCID for N. D. Sandham: orcid.org/0000-0002-5107-0944

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