Acoustic analysis of starting jets in an anechoic chamber

Explosive volcanic eruptions emanate complex acoustic signals. They
are influenced by several parameters, most of most of which are highly
unconstrained in volcanic setting.

We investigate the acoustics of starting jets analogous to
volcanic jets at high Mach numbers and with different nozzle
geometries, in a controlled environment. For the first time in
volcanic analog studies, an anechoic chamber is used to eliminate
contamination of the signals by reflections from any wall or
obstacle.  The analysis concentrates on the identification of the
principal jet noise components including: compression waves, vortex
ring noise, turbulent jet mixing noise,  broadband shock noise and
screech. We employed a shock tube apparatus and signals were recorded
using a microphone array.  Employing wavelet analysis, we have
identified the noise sources in both time- and frequency-space.

We have identified the principal sound sources of the jet in
time-frequency space and have analyzed their behaviour with respect to
changes in pressure ratio $p/p_\infty$ ,non-dimensional mass supply
L/D and exit-to-throat area ratio.

We find that at higher pressure ratios the peak frequency of the
broadband shock noise is noticeably lower whereas the amplitude is
higher. The non-dimensional mass supply controls whether a jet forms
and its blowing duration and maximum velocity. The nozzle geometry has
a markable effect on delay of the shock-shear layer-vortex ring
interaction with respect to the compression wave.

Changes in parameters of the starting jet leave a clear and
interpretable trace in the observed sound pattern. This quantitative
parametrisation of these effects is essential for utilizing these
findings as well as field observations for the solution of the inverse
problem in the lab and in nature.